1 August 2017 Functional near-infrared spectroscopy in movement science: a systematic review on cortical activity in postural and walking tasks
Author Affiliations +
Neurophotonics, 4(4), 041403 (2017). doi:10.1117/1.NPh.4.4.041403
Safe locomotion is a crucial aspect of human daily living that requires well-functioning motor control processes. The human neuromotor control of daily activities such as walking relies on the complex interaction of subcortical and cortical areas. Technical developments in neuroimaging systems allow the quantification of cortical activation during the execution of motor tasks. Functional near-infrared spectroscopy (fNIRS) seems to be a promising tool to monitor motor control processes in cortical areas in freely moving subjects. However, so far, there is no established standardized protocol regarding the application and data processing of fNIRS signals that limits the comparability among studies. Hence, this systematic review aimed to summarize the current knowledge about application and data processing in fNIRS studies dealing with walking or postural tasks. Fifty-six articles of an initial yield of 1420 publications were reviewed and information about methodology, data processing, and findings were extracted. Based on our results, we outline the recommendations with respect to the design and data processing of fNIRS studies. Future perspectives of measuring fNIRS signals in movement science are discussed.
Herold, Wiegel, Scholkmann, Thiers, Hamacher, and Schega: Functional near-infrared spectroscopy in movement science: a systematic review on cortical activity in postural and walking tasks



Safe locomotion is indispensable for human daily living and requires good functionality of motor control processes. The efficiency of motor control processes of daily motor activities such as walking1,2 and standing3,4 relies on complex neuronal networks encompassing subcortical and cortical brain structures. Studies show that a smaller gray matter volume is associated with lower gait performance indicated by increased gait variability56.7 or slower gait velocity.8,9 Moreover, lower whole-brain gray matter volume goes along with worse postural balance performance irrespective of age,10 whereas the increase of gray matter volume is associated with balance improvements.1112.13 In older age, however, shrinking of those cortical structures14,15 might diminish motor control capabilities.16 The substantial body of literature suggests that cortical structures play an important role for the motor control of daily motor tasks. Therefore, the assessment of cortical activity while subjects are moving is a key factor to foster a better understanding of neuromotor control which, in turn, could help to improve rehabilitation strategies.17

Brain activity can be measured by the following neuroimaging methods: functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG), positron-emission-tomography (PET), electroencephalography (EEG), and functional near-infrared spectroscopy (fNIRS). While fMRI is considered as gold standard for the assessment of activity in cortical and subcortical structures, it suffers from the vulnerability for movement artifacts and the restricted range of motion in the scanner.1819.20.21 Likewise, MEG exhibits a high vulnerability for motion artifacts18 while the use of PET does not allow repeated measurements due to the injection of radioactive tracers.20 EEG puts out not only signals with high temporal resolution but also signals with a relatively weak spatial resolution.18,22 Furthermore, EEG is vulnerable to artifacts, time consuming in preparation,18,22,23 and the signals are hard to interpret for nonexperts.24 Hence, fMRI, MEG, PET, and EEG suffer from specific restrictions that hamper a time-efficient evaluation of cortical activation in moving subjects.

fNIRS is a relatively new optical neuroimaging technique that uses the theory of neurovascular coupling.19,2526.27 Neurovascular coupling results from the neuronal activity or glia activity that provokes an enhanced blood flow in an active brain region to satisfy energetic demands of the neuronal tissue.2728.29 Based on these hemodynamic responses of neuronal cortical tissues, the fNIRS technology allows an indirect evaluation of brain activation (such as fMRI).18,19

Therefore, light with different wavelengths in the near-infrared spectrum is emitted through the skull and undergoes some scattering and absorption processes inside the neuronal tissue.27,30,31 In the neuronal tissue, the chromophores such as oxygenated (oxyHb) and deoxygenated hemoglobin (deoxyHb) absorb light at different spectra19,20,32,33 whereas the nonabsorbed components of the scattered light leave the brain in a banana-shaped course. Those components are recorded by a detector on the head surface.30,31,34 Based on the described neurovascular coupling, an enhanced brain activation induces an intensified blood flow in the active brain regions leading to an increase in oxyHb and decrease of deoxyHb.27,30 As a consequence of the different absorption spectra of the chromophores, the activity-dependent concentration changes in oxy- and deoxyHb can be quantified with the modified Beer–Lambert law and used as an indicator of regional brain activation.19,20,27,30,32

The advantage of optical neuroimaging using fNIRS is the possibility to measure cortical activity (quantified as changes in tissue oxygenation and blood perfusion, associated with neural activity) noninvasively25,27,35,36 with a relatively good spatial and temporal resolution.1920.21.22 This benefit makes fNIRS systems suitable for the usage in special cohorts, such as children.18,20,22,3637.38.39.40 Moreover, fNIRS systems are applicable even during outdoor activities41 and could be used as a monitoring tool in neurorehabilitation settings.18,4243.44 From this point of view, fNIRS is a promising tool to understand the contribution of cortical areas in the neuromotor control of gross motor skills, such as posture and walking.17 However, the fNIRS technology also has some disadvantageous including a limited depth sensitivity that restricts the measurements of brain activity to cortical layers36 and the vulnerability to systemic vascular changes that may contaminate the signal during strenuous physical tasks.27,45 In addition, no standardized procedures regarding the usage of fNIRS with respect to measuring cortical activity in moving subjects exist17,42 which clearly limits the comparability across existing studies.

This systematic review elucidates the application of fNIRS in neuromotor research and concentrates on two crucial motor tasks, namely locomotion and postural stability. In this context, we aim to give an overview about (a) the methodological approach of fNIRS and (b) the main findings of the fNIRS measurements reported in the literature.


Systematic Literature Search and Data Extraction

Two independent researchers performed a systematic literature search to identify all relevant studies applying fNIRS to investigate hemodynamic brain responses during walking and postural tasks on February 4, 2017. Therefore, we used the following search terms: gait OR walking OR posture OR “postural control” OR balance OR balancing OR sway AND NIRS OR fNIR OR fNIRS OR “functional near-infrared spectroscopy” OR “near-infrared spectroscopy” OR “functional near-infrared spectroscopic” OR “optical imaging system.” All studies that used brain–computer interfaces, examined orthostatic regulation or animals, provided insufficient statistical methods, or used non-English language were excluded. During this procedure, six articles were excluded due to the lack of statistical analyses,4647.48 ineligible measurement condition,49 and non-English language.50,51 The search and screening process is shown in Fig. 1. From the included studies, data about cohort characteristics, fNIRS methodology, and main findings were extracted.

Fig. 1

Search process and identification of relevant studies.



Results: Methodology Employed in the Studies

In the following, we will provide information about the methodological approaches of the reviewed studies. We focused on general aspects regarding the application, data processing, and data analyzing of fNIRS (e.g., study design, used filter methods, and statistical analysis). Further information about the cohorts, tasks, sampling frequencies, wavelengths, and numbers of channels can be requested by e-mail from the corresponding author or is available in Ref. 52.


Baseline Condition and Duration


Treadmill walking

Fifteen studies investigating cortical activation during treadmill walking assessed baseline brain activation during quiet standing.5354. In contrast, Eggenberger et al.68 chose slow walking (2  km/h) for 1 min as the baseline condition. The duration of baseline cortical activation used for further analyses varied between 2.555,56 and 20 s5960.61 (for an overview see Table 1).

Table 1

Overview about the population, study designs, and data processing steps of reviewed fNIRS studies (note that the number of trials is reported per condition).

First author– Population (n=number of participants; age in years±SD)1. Baseline condition1. DPF
• Conditions2. Baseline duration2. Data processing (filtering)
3. Number of trials and duration3. Final data processing
4. Rest phase duration4. Activation parameters
5. Time used for analysis
Al-Yahya et al.65– Stroke patients (n=19; 59.61±15.03)1. Quiet standing1. Age-dependent value (4.99+0.067×age0.814)
– Healthy old adults (n=20; 54.35±9.38)2. 25 to 45 s (randomized order)2. LPF at 0.67 Hz
• DTW vs. NW3. 5×; 30 s3. Baseline correction; averaging
4. 25 to 45 s (between trials / randomized order)4. Oxy- and deoxyHb
5. 6 to 16 s after task begin
Atsummori et al.69– Healthy young adults (n=6; 29.7±3.3)1. Quiet standing1. Constant value (no details reported)
• DTW vs. NW2. 5 s before task begin2. Not reported
3. 5× (DTW) / 6× (NW); 10 s3. Baseline correction; averaging
4. 20 s at beginning4. Oxy- and deoxyHb
5. 6 s after task begin/ending
Basso-Moro et al.98– Healthy young adults (n=16; 29±4.8)1. Quiet standing1. Age-dependent value (4.99+0.067×age0.814)
• Perturbations in semivirtual reality with increasing difficulty2. Last 30 s (of 2 min)2. LPF at 0.1 Hz
3. 7×; 45 s3. Averaging
4. 2 min after block4. Oxy- and deoxyHb
5. Last 10 s of perturbation
Beurskens et al.105– Healthy young adults (n=15; 24.5±3.3)1. Sitting on chair1. Constant value (6.0)
– Healthy old adults (n=10; 71.0±3.8)2. 30 s2. HRF-filter; wavelet MDL detrending algorithm
• DTW vs. NW3. 2×; 30 s3. Moving standard deviation and spline interpolation, baseline correction, canonical HRF
4. Not reported4. Oxy- and deoxyHb
5. Entire task time
Caliandro et al.70– Patients with ataxic gait (n=14; 27 to 71)1. Quiet standing1. Constant value (5.93)
– Healthy controls (n=20; 32 to 65)2. Last 10 s of standing2. LPF at 0.1 Hz
• Patients vs. HC3. 1×; 10 m3. Baseline correction; averaging
4. Not relevant4. OxyHb
5. Entire task time expect of first 5 s
Caliandro et al.71– Patients with ataxic gait (n=19; 31 to 70)1. Quiet standing1. Constant value (5.93)
– Healthy controls (n=15; 36 to 73)2. Last 10 s of standing2. LPF at 0.1 Hz
• Patients vs. HC3. 2×; 10 m3. Baseline correction; averaging
4. 30 min between trials4. OxyHb
5. Entire task time expect of first 5 s
Clark et al.66– Older persons with mobility and somatosensory deficits (n=14; 77.1±5.56)1. Quiet standing1. N/A
• Walking in normal shoes vs. texture insoles vs. barefoot vs. DTW2. 10 s immediately before task2. No filter
3. 5× walking laps with 18 m (overground); 60 to 120 s (treadmill)3. Averaging
4. 2 min after task4. TOI
5. Entire task phase
Clark et al.86– Older adults with mild mobility deficits (n=16; 77.2±5.6)1. Quiet standing1. N/A
• NW vs. DTW2. 10 s immediately before task2. No filter
3. 5× walking laps with 18 m3. Averaging
4. 2 min after task4. TOI
5. 10 s before task begin (preparation phase) and in steady phase/transition phase excluded
Doi et al.72– Adults with mild cognitive impairment (n=16; 75.4±7.2)1. Quiet standing1. N/A (arbitrary unit)
• NW vs. DTW2. 10 s before walking2. LPF at 0.05 Hz; linear fitting on baseline data
3. 3×; 20 s3. Averaging
4. 30 s between trials4. OxyHb
5. Entire task period
Eggenberger et al.68– Healthy old adults (dancing: n=19; 72.8±5.9; balance: n=14; 77.8±7.4)1. Walking at 2  km/h1. N/A (absolute values)
• Dancing vs. balancing (before and after intervention)2. Middle 40 s (of 1 min)2. 60 s moving average: motion artifact correction (oxyHb: >2.5 and <2.5  μM/deoxyHb: >1.5 and <1.5  μM excluded); visual inspection of data
3. 8×; 30 s3. Averaging
4. 30 s between trials (walking at 2  km/h)4. OxyHb
5. 1 to 7 s = acceleration phase; 10 to 25 s = steady state walking phase; 26 to 34 s = deceleration phase; 35 to 46 s = drop phase
Ferrari et al.99– Healthy, young adults (n=22; 26.5±4.0)1. Quiet standing1. Age-dependent value (4.99+0.067×age0.814)
• Balancing in semivirtual reality2. Last 30 s (of 2 min)2. LPF at 0.1 Hz
3. 2×; 9 min3. Averaging
4. 2 min after block4. Oxy- and deoxyHb
5. 30 s per task
Fraser et al.63– Healthy young adults (n=19; 21.83±1.92)1. Quiet standing1. Constant value (no details reported)
– Healthy old adults (n=14; 66.85±5.26)2. 5 s2. No filter
• NW vs. single cognitive task vs. easy DTW vs. hard DTW3. Walking: 2×; 2 min; single cognitive task: 4×; 75 s; DTW 4×; 2 min (for each dual-task condition)3. Averaging
4. 30 to 60 s between trials4. Oxy- and deoxyHb
5. Entire task period
Fujimoto et al.102- Patients with subcortical stroke (n=20; 60.2±9.5)1. Quiet standing1. N/A (arbitrary unit)
• Postural test before/after rehabilitation2. Time before next perturbation (ERD)2. HPF at 0.01 Hz; PCA
3. 15×; 1 s3. Two parameter gamma HRF
4. 5 to 15 s between trials (randomized)4. Oxy- and deoxyHb
5. Around perturbations
Fujita et al.101– Healthy, young adults (low span group: n=13; 24.0±3.1 / high span group: n=16; 22.5±3.6)1. Quiet standing1. N/A (arbitrary unit)
• Single- and dual-task mono- or bipedal standing2. 10 s2. LPF at 0.5 Hz; HPF at 0.01 Hz; 5 s moving average
3. 3×; 20 s3. Baseline normalization, baseline correction, averaging
4. 10 s between trials4. OxyHb
5. Entire task time
Harada et al.53– Healthy, old adults (n=15; 63±4)1. Quiet standing1. N/A (arbitrary unit)
• Low vs. high gait capacity group at different speeds2. 10 s before walking2. HPF at 0.03 Hz
3. 3×; 60 s3. Baseline normalization; averaging
4. 40 s between trials4. OxyHb
5. 20 s after target speed
Helmich et al.108– Young, concussed adults with persistent postconcussive symptoms (n=7; 29±15)1. N/A1. Constant value (6.0)
– Young, concussed adults with minor postconcussive symptoms (n=13; 26±7)2. N/A2. LPF at 0.1 Hz; HPF at 0.001 Hz; spline interpolation; visual inspection
– Healthy, young adults (n=10; 27±8)3. 10×; 10 s3. Normalization; averaging
• Comparison of three groups during standing on different surfaces (stable vs. instable) and sensory conditions (eyes closed vs. eyes open vs. blurred vision)4. No rest between trials4. Oxy- and deoxyHb
5. Entire task time
Hernandez et al.82– Healthy old adults (n=8; 61±4)1. Quiet standing1. Constant value (6.0)
– Patients with multiple sclerosis (n=8; 57±5)2. 10 s before walking (counting silently in steps of 1)2. LPF at 0.14 Hz; noisy channels excluded (dark current condition or saturation); visual inspected
• Comparison of healthy adults and patients with multiple sclerosis during NW and DTW3. 3× walking loops (= 6× straight walks a 14 ft.)3. Baseline normalization; averaging
4. At least 10 s after trial4. OxyHb
5. Entire task time
Herold et al.100– Healthy young adults (n=10; 25; 21 to 47)1. Quiet standing1. N/A (arbitrary unit)
• Standing vs. balancing on balance board2. 30 s before task2. 5.0 s moving average; LPF at 0.5 Hz; HPF at 0.01 Hz; PCA (r=0.25)
3. 3×; 30 s3. Averaging
4. 30 s after trial4. Oxy- and deoxyHb
5. Middle 20 s
Holtzer et al.73– Healthy, young adults (n=11; 19 to 29)1. Quiet standing1. Constant value (6.0)
– Healthy, old adults (n=11; 69 to 88)2. 5 s before walking2. LPF at 0.14 Hz; combined principal and independent component analysis
• DTW vs. NW vs. standing/comparison between cohorts3. 3× walking loops (= 6× straight walks a 15 ft.)3. Baseline normalization; averaging
4. Not reported4. OxyHb
5. Old 4 s / young 3.5 s
Holtzer et al.74– Nondemented older adults (n=318; 76.66±6.7)1. Quiet standing1. Constant value (6.0)
• DTW vs. NW vs. standing2. 10 s (counting silently forward in steps of 1)2. LPF at 0.14 Hz; noisy channels excluded (dark current condition or saturation); visual inspected
3. 3× walking loops (= 6× straight walks a 14 ft.) / standing for 30 s3. Baseline normalization; averaging
4. “Short break” reported4. OxyHb
5. Entire task time
Holtzer et al.84– Nondemented older adults (n=348; 76.8±6.8)1. Quiet standing1. Constant value (6.0)
– Older adults with low perceived stress (n=147; 76.72±6.87)2. 10 s (counting silently forward in steps of 1)2. LPF at 0.14 Hz; noisy channels excluded (dark current condition or saturation); visual inspected
– Older adults high perceived stress (n=171; 76.58±6.37)3. 3× walking loops (= 6× straight walks a 14 ft.) / standing for 30 s3. Baseline normalization; averaging
• DTW vs. NW vs. standing/comparison between cohorts4. “Short break” reported4. OxyHb
5. Entire task time
Holtzer et al.75– Nondemented older adults (total: n=236; 75.5±6.49)1. Quiet standing1. Constant value (6.0)
– Healthy older adults (n=167; 74.43±6.04)2. 10 s (counting silently forward in steps of 1)2. LPF at 0.14 Hz; noisy channels excluded (dark current condition or saturation); visual inspected
– Older adults with peripheral NGA (n=40; 77.03±6.27)3. 3× walking loops (= 6× straight walks a 14 ft.) / standing for 30 s3. Baseline normalization; averaging
– Older adults with central NGA (n=29; 79.59±7.38)4. “Short break” reported4. OxyHb
• DTW vs. NW vs. standing/comparison between cohorts5. Entire task time
Holtzer et al.85– Older adults with low perceived fatigue (n=160; 76.20±6.64)1. Quiet standing1. Constant value (6.0)
– Older adults with high perceived fatigue (n=154; 77.41±6.66)2. 10 s (counting silently forward in steps of 1)2. LPF at 0.14 Hz; noisy channels excluded (dark current condition or saturation); visual inspected
• DTW vs. NW vs. standing/comparison between cohorts3. 3× walking loops (= 6× straight walks a 14 ft.) / standing for 30 s3. Baseline normalization; averaging
4. “Short break” reported4. OxyHb
5. Entire task time
Huppert et al.91– Healthy young adults (n=10; 21 to 47)1. Quiet standing1. Not relevant (image reconstruction)
• Stepping reaction task2. Time before next trial (4 to 8 s, random order)2. Discrete cosinus transform term (01/120  Hz); visual inspected
3. 8× blocks a 32× trials3. Gamma-variant HRF; averaging
4. 4 to 8 s between trials (random order) / few minutes after 2 to 3 scans4. Oxy- and deoxyHb
5. Entire task phase
Karim et al.97– Healthy young adults (n=9; 18 to 42)1. Quiet standing1. Not relevant (image reconstruction)
• Video game with balance task2. 60 s (pre- and posttask)2. Cosinus transform term (0 to 1/120  Hz); visual inspected
3. 6× beginner / 8× advanced level; 30 to 60 s3. Boxcar HRF; averaging
4. 30 s between trials4. Oxy- and deoxyHb
5. Entire task phase
Karim et al.92– Healthy young adults (n=15; 28±9)1. Quiet standing1. Not relevant (image reconstruction)
• SOT conditions2. 45 s before trial2. Cosinus transform term (0 to 1/120  Hz)
3. 2×;45  s3. Gamma-variant HRF; averaging
4. 60 s after trial / 2 min after two scans4. Oxy- and deoxyHb
5. Entire task phase
Kim et al.106– Healthy young adults (n=14; 30.06±4.53)1. Not reported1. Not reported
• Stepping (ST) vs. Treadmill walking (TW) vs. robot-assisted walking (RAW)2. Not reported2. Gaussian smoothing; wavelet MDL algorithm
3. 5×; 30 s (ST, TW); 60 s (RAW)3. Canonical HRF
4. 15 s at begin and end; 30 s between trials (ST, TW) / 60 s at begin and end; 45 s between trials (RAW)4. OxyHb
5. Entire task time
Koenraadt et al.54– Healthy, young adults (n=11/23±4)1. Quiet standing1. N/A (arbitrary unit)
• Precision walking vs. NW2. 25 to 35 s2. LPF at 1.25 Hz; HPF at 0.01 Hz; superficial interference with LPF at 1 Hz; short separation channels (1 cm)
3. 10×; 35 s3. Baseline normalization; averaging
4. 25 to 35 s before/after trial / 3 min after 10 trials4. Oxy- and deoxyHb
5. 12.5 s in task phase
Kurz et al.55– Healthy, young adults (n=13; 23.7±1.4)1. Quiet standing1. N/A (arbitrary unit)
• Forward vs. backward walking2. 2.5 s before walking2. HPF at 0.01 Hz; 5 s moving average; PCA (r=0.25)
3. 10×; 30 s3. Baseline correction; averaging
4. 30 s between trials4. Oxy- and deoxyHb
5. Entire task phase
Kurz et al.56– Children with spastic diplegic cerebral palsy (n=4; 11.0±4)1. Quiet standing1. N/A (arbitrary unit)
– Healthy children (n=8; 13.2±3)2. 2.5 s before walking2. HPF at 0.01 Hz; 5 s moving average; PCA (r=0.25)
• Patients vs. HC3. 10×; 30 s3. Baseline correction; averaging
4. 30 s between trials4. OxyHb
5. Entire task phase
Lin et al.103– Healthy middle-aged adults (n=15; 46±11)1. Quiet standing1. N/A (image reconstruction)
– Healthy old adults (n=15; 73±5)2. 40 s before trial2. Autoregressive model with prewhitened iterative reweighted least squares algorithm
• Middle-aged vs. old adults (different balance conditions)3. 4×; 40 s3. HRF; averaging
4. 1 min between trials4. Oxy- and deoxyHb
5. Entire task phase
Lin and Lin79– Healthy young adults (n=24; 20 to 27)1. Quiet standing1. Age-dependent value (4.99+0.067×age0.814)
• DTW vs. NW2. 20 s2. LPF at 0.2 Hz
3. 1×; 60 s3. Baseline correction
4. 20 s before/after task / 2 min after two trials4. OxyHb
5. Entire task phase
Lu et al.76– Healthy young adults (n=17; 23.1±1.5)1. Quiet standing1. Constant value (6.0)
• DTW vs. NW2. 5 s before walking2. Bandpass filter (LPF at 0.01 Hz; HPF at 0.2 Hz); PCA; spike rejection (channels with > CV 15% rejected/channels with CV > 10% for further analysis)
3. 3×; 60 s3. Averaging
4. 60 s between trials4. Hbdiff (oxyHb–deoxyHb)
5. Early phase (5 to 20 s after task begin); late phase (21 to 50 s after task begin)
Mahoney et al.93– Healthy, nondemented older adults (n=126; 74.41±6.12)1. Quiet standing1. Constant value (6.0)
– Older adults wild mild Parkinson symptoms (n=117; 77.50±6.72)2. First 2 s2. LPF at 0.14 Hz; visual inspected
– Patients with Parkinson disease (n=26; 81.23±5.93)3. 10 s3. Baseline normalization; averaging
• Patients vs. HC (standing while counting silently in steps of 1)4. “Short break” reported4. OxyHb
5. Entire task phase
Maidan et al.90– Parkinson patients with FOG (n=11; 66.2±10.0)1. Walking1. Age-dependent value (4.99+0.067×age0.814)
– Healthy controls (n=11; 71.2±6.0)2. 6 s before FOG2. LPF at 0.14 Hz
• Patients vs. HC (walking; turning)3. 6 s walking with 180 deg turn3. Baseline correction; averaging
4. 2 min between tasks4. OxyHb
5. Defined time period around FOG event (prior=6 to 3  s / before=3 to 0 / during=0 to 3 s)
Maidan et al.80– Healthy, older adults (n=38; 70.4±0.9)1. Quiet standing1. Age-dependent value (4.99+0.067×age0.814)
– Parkinson patients (n=68; 71.7±1.1)2. 5 s before task2. Bandpass filter (LPF at 0.01 Hz and HPF at 0.14 Hz), wavelet filter; CBSI
• DTW vs. NW vs. obstacle negotiation3. 5×; 30 s3. Baseline correction; averaging
4. 20 s after trial / between trials on individual needs4. OxyHb
5. Entire task phase
McKendrick et al.88– Healthy, young adults (n=13; 22; 19 to 31)1. Sitting (for sitting condition) and standing (for walking condition)1. Constant value (5.94)
• Sitting vs. walking indoors vs. walking outdoors (all conditions while performing secondary task)2. 10 s2. LPF at 0.1 Hz; visual inspected
3. 16×; 120 s (sitting) / 8×; a 120 s (per walking condition)3. Baseline correction
4. 5 min between walking conditions4. Oxy- and deoxyHb
5. Entire task time
Meester et al.57– Young, healthy adults (n=17; 27.8±6.3)1. Quiet standing1. Age-dependent value (4.99+0.067×age0.814)
• DTW vs. NW2. Middle 10 s of rest2. LPF at 0.67 Hz; 4 s moving average; visual inspected
3. 5×; 30 s3. Averaging
4. 20 to 40 s between trials4. OxyHb
5. Middle 10 s of task
Metzger et al.64– Healthy young adults (n=12; 27.6; 19 to 39)1. Quiet standing1. N/A (arbitrary unit)
• DTW vs. NW2. 10 s at begin2. 5 s moving average; CBSI
3. 4×; 45 s3. Averaging; baseline correction
4. 15 s after trial4. Oxy- and deoxyHb
5. Entire task time
Mihara et al.58– Stroke patients with ataxic gait (n=12; 52.7±16.9, 12 to 74)1. Quiet standing1. N/A (arbitrary unit)
– Healthy controls (n=11; 42.6±11.6, 30 to 70)2. 6 s before walking2. Not reported
• Patients vs. HC3. 3×; 60 s (HC); 30 s (patients)3. Baseline correction; averaging
4. 15 s before/after walking4. OxyHb
5. Acceleration phase = 6 s after starting treadmill; steady phase = 6 s during steady speed
Mihara et al.94– Healthy young adults (n=15; 29.4±6.7)1. Quiet standing1. N/A (arbitrary unit)
• Warned before perturbations vs. baseline; unwarned before perturbations vs. baseline; warned vs. unwarned2. Time before next perturbation (ERD)2. HPF at 0.05 Hz
3. 20 to 30×; 1 s3. Gaussian HRF; averaging
4. 5 to 20 s between trials (randomized) / 4 to 5 min after block4. OxyHb
5. Around perturbation
Mihara et al.95– Stroke patients (n=20; 61.6±11.9)1. Quiet standing1. N/A (arbitrary unit)
• Balance perturbations2. Time before next perturbation (ERD)2. HPF at 0.03 Hz
3. 15×; 1 s3. Two-parameter gamma HRF
4. 5 to 15 s between trials (randomized)4. OxyHb
5. Around perturbations
Mirelman et al.77– Young, healthy adults (n=23; 30.9±3.7)1. Quiet standing1. Age-dependent value (4.99+0.067×age0.814)
• Standing vs. DTS vs. NW vs. DTW2. 20 s before walking2. LPF at 0.14 Hz; continuous wavelet transform
3. 5×; 30 m3. Baseline correction; averaging
4. 20 s before/after trial4. OxyHb
5. Entire task phase
Miyai et al.107– Healthy young adults (n=8; 35±8, 24 to 56)1. Quiet standing1. N/A (arbitrary unit)
• NW vs. standing2. 30 s2. HPF at 0.03 Hz
3. 5×; 30 s3. Linear interpolation; averaging
4. 30 s between trials4. Oxy- and deoxyHb
5. Entire task phase
Miyai et al.61– Stroke patients (n=6; 57±13)1. Quiet standing1. N/A (arbitrary unit)
• Walking with mechanical assistance vs. walking with facilitation technique2. Middle 20 s2. HPF at 0.03 Hz
3. 4×; 30 s3. Linear interpolation; baseline correction; averaging
4. 30 s between trials4. OxyHb
5. Last 20 s of task phase
Miyai et al.60– Stroke patients (n=8; 57±12)1. Quiet standing1. N/A (arbitrary unit)
• Before/after 2 months rehabilitation2. Middle 20 s2. HPF at 0.03 Hz
3. 4×; 30 s3. Linear interpolation; baseline correction; averaging
4. 30 s between trials4. OxyHb
5. Last 20 s of task phase
Miyai et al.59– Stroke patients with hemiparesis (n=6; 57±6)1. Quiet standing1. N/A (arbitrary unit)
– Healthy controls (n=6, 53±11)2. Middle 20 s2. HPF at 0.03 Hz
• Walking with weight support vs. walking without weight support3. 4×; 30 s3. Linear interpolation; baseline correction; averaging
4. 30 s between trials4. OxyHb
5. Last 20 s of task phase
Nieuwhof et al.81– Parkinson patients (n=14; 71.2±5.4)1. Quiet standing1. Constant value (6.0)
• DTW (with different tasks)2. Last 5 s of standing2. LPF at 0.1 Hz; visual inspected
3. 5×; 40 s3. Baseline correction; averaging
4. 20 s between trials / 1 to 2 min between blocks4. OxyHb and deoxyHb
5. Entire task phase
Osofundiya et al.87– Obese old adults (n=10; 80.6±6.79)1. Quiet standing1. Constant value (6.0)
– nonobese old adults (n=10; 80.6±7.50)2. 10 s2. Not reported
• Quiet sitting vs. NW vs. precision walking vs. DTW3. 8× a 30 s3. Baseline correction; averaging
4. 10 s between trials4. OxyHb and HbT
5. Entire task phase
Saitou et al.78– Hemiplegic stroke patients (n=44; 66±9.3)1. Quiet standing1. Constant value (5.9)
• Different tasks (e.g., calculation, pulley, we only consider walking vs. baseline)2. 5 min2. Not reported
3. 1×; 5 min3. Averaging
4. 5 min4. OxyHb; CBV; COV
5. Entire task phase
Suzuki et al.62– Healthy, young adults (n=9; 28.1±7.4, 22 to 46)1. Quiet standing1. N/A (arbitrary unit)
• Walking at different speeds2. First 13 s2. HPF at 0.03 Hz
3. 3×; 90 s3. Linear interpolation; baseline correction; averaging
4. 30 s between trials4. Oxy- and deoxyHb; regional cortical activation ratio (oxy Hb of the specific channel divided by oxyHb of all 42 channels multiplied by 100)
5. 13.5 s in task phase
Suzuki et al.67– Healthy, young adults (n=7; 31.3±4.8, 24 to 45)1. Quiet standing1. Not relevant (arbitrary unit)
• Walking with vs. without verbal preadvice2. 10 s before walking2. HPF at 0.03 Hz
3. 4×; 40 s3. Baseline normalization; averaging
4. 10 to 25 s between trials (randomized order)4. Oxy- and deoxyHb
5. First 10 s of task phase
Takeuchi et al.89– Young healthy adults (n=16; 25.9±4.4, 20 to 33)1. Walking1. Constant value (no details reported)
– Healthy older adults (n=15; 71.7±3.3, 65 to 78)2. 30 s2. Spike rejection (artifact with more than 3 SD); 5 s moving average; bandpass filter (LPF at 0.5 Hz; HPF at 0.01 Hz)
• Walking vs. walking with smartphone3. 15×; 10 s3. Baseline normalization; averaging
4. No rest4. OxyHb
5. Entire task phase
Takakura et al.96– Healthy young adults (n=11; 33.4±7.4)1. Quiet standing1. Constant value (1.0)
• SOT conditions2. 20 s before task2. Bandpass Fourier filter (0.01 to 0.1 Hz)
3. 3×; 20 s3. Averaging
4. Few minutes after 3 trials4. OxyHb
5. Entire task phase
Verghese et al.83– Older adults (n=166; 75±6.1)1. Quiet standing1. Constant value (6.0)
– NW vs. DTW vs. standing2. 10 s (counting silently forward in steps of 1)2. LPF at 0.14 Hz; noisy channels excluded (dark current condition or saturation); visual inspected
3. 3× walking loops (= 6× straight walks a 14 ft.) / standing for 30 s)3. Baseline normalization; averaging
4. “Short break” reported4. OxyHb
5. Entire task phase
Wang et al.104– Healthy young adults (n=22; 24.4±1.6)1. Sitting (eyes closed)1. Age-dependent constant value (WL: 760  nm=5.91; WL: 850=5.40)
– Healthy older adults (n=39; 70.5±7.7)2. 20 min2. Bandpass filter (0.005 to 2 HZ)
• Standing connectivity differences healthy young and healthy old adults3. 1×; 10 min3. Wavelet phase coherence analysis
4. No rest4. OxyHb
5. Entire task time

Abbreviations: deoxyHb, deoxygenated hemoglobin; DTS, dual-task standing; DTW, dual-task walking; ft., feet; HC, healthy controls; HPF, high-pass filter; HRF, hemodynamic response function; LPF, low-pass filter; MDL, minimum description length; MRI, magnetic resonance imaging; N/A, not applicable; NW, normal walking; NGA, neurological gait abnormalities; oxyHb, oxygenated hemoglobin; PCA, principal component analysis; RAW, robot assisted walking; SOT, sensory organization test; ST, stepping; TOI, tissue oxygenation index; and vs., versus.


Overground walking

Twenty-one studies conducting overground walking, quantified baseline brain activation in a standing position.66,6970. In contrast, two studies assessed baseline brain activation while walking89 or during a predefined time period prior to a freezing of gait event (FOG; a sudden, brief inability to start movement or to continue rhythmic, repeated movements despite the internal intention to move).90 The duration to assess baseline brain activity ranged between 5 s69,73,76,81 and 5 min.78 Most studies used 10 s to quantify baseline brain activity.66,7071.72,74,75,8283. Interestingly, Holtzer et al.74,75,8384.85 asked their participants to conduct a simple counting task (in steps of 1) during the baseline condition (for an overview see Table 1).


Postural tasks

In postural research, 13 studies assessed baseline brain activity during quiet standing.9192. The temporal duration to quantify baseline brain activity ranged from 293 to 60 s.97 In most studies, data of 30 s9899.100 or a few seconds before starting the next trial91,94,95,102 were used to assess baseline brain activation. In addition, Wang et al.104 used 20 min quiet sitting to measure baseline connectivity (for an overview see Table 1).


Number and Duration of Trials and Rest Phases


Treadmill walking

The studies that used a treadmill for the walking condition5354.,105106.107 are shown in Table 1. Per task, a minimum of 2 trials105 and a maximum of 10 trials5455.56 were performed. Most studies used three to five trials to assess task-relevant cortical activity.53,57,5960.61.62,64,64,65,67,106,107 Task phases were set to 30 s in the majority of the studies,5556.,65,68,105106.107 but Harada et al.,53 Kim et al.,106 and Mihara et al.58 used 60 s, Koenraadt et al.54 used 35 s, Metzger et al.64 used 45 s, Suzuki et al.67 used 40 s, Suzuki et al.62 used 90 s, and Fraser et al.63 used 120 s. The time of the rest phases ranged in most studies between 25 and 60 s.5354.55.56.57,5960.61.62,65,68,106,107 Additionally, rest times of 15 s prior to58,64 and after each walking trial58 were reported while Suzuki et al.67 implemented 10 to 25 s between trials (for an overview see Table 1).


Overground walking

Twenty-three studies investigated cortical hemodynamic responses during overground walking.66,6970. For each condition, 1,70,79 3,76 4,87 5,66,69,71,77,80,81,86 6,7374.75,8283.84.85 and 15 walks were used.89 Either the time for each task phase ranged between 1069 and 120 s66 or the participants were asked to walk a predetermined distance ranging between 47374.75,82,83,85 and 90 m.86 The resting phases prior to and after each trial lasted 20 s77,79 or 60 s76 and 10 s87 or 30 s between the trials.72 Two studies used 20 s of rest between successive trials and 1 to 2 min of rest between successive task blocks.80,81 Furthermore, in three studies, a rest of 2 min was used66,86,90 while one study allowed participants to rest 588 or 30 min between tasks71 (for an overview see Table 1).


Postural tasks

Regarding the examination of brain activity during a sensory organization test (SOT; a balance test using quantitatively different visual, proprioceptive, and vestibular cues to assess the quality of postural stance stability), two trials,92 three trials,96 or four trials were conducted103 which lasted 45,92 40,103 or 20 s.96 The participants of the three studies using mechanical perturbations performed 1595,102 to 30 trials94 with a randomized perturbation duration of 5 to 20 s.94,95,102 In semivirtual reality, seven trials with a task phase duration of 45 s were used.98 The rest between task phases depended on the conducted tasks (see Table 1) and ranged between 4 and 20 s.91,94,95,101 In other studies, a rest of 1103 or 2 min was included.92,98,99 To avoid fatigue, resting times after some trials that lasted a few minutes were common91,92,94 (for an overview see Table 1).


Source–Detector Separation

The closest distances between the optodes (source and detector) were reported to be 1  cm, which was used as a short separation channel54 and was followed by an interoptode distance of 2.5 cm.7374.75,8283.84.85,93,105 Three studies used 3.2 cm,91,92,97 and two studies used 3.5 cm.80,81,90 Another seven studies used 4 cm.70,71,7879.80.81,104 One study used a different distance between source and detector (1, 3, and 4 cm)54 and another one used multidistance measurement (2.0, 2.5, 3.5, and 4.0 cm).68 The remaining 36 studies set the interoptode distances at 3 cm.53,5556.,6465.66.67,69,72,76,77,80,81,8687.88.89,9495.96,9899.,106107.108 An overview on used source–detector is shown in Fig. 2(a).

Fig. 2

Overview on (a) used source–detector separation and (b) DPF values in the reviewed studies.



Placement of the Optodes

The majority of studies used the international “10-20 EEG system” for the placement of the optodes.5354.55.56.57,5960.,6768.,79,8182.83,85,8788. In some studies, an additional three-dimensional (3-D)-digitizer was applied69,76,9495.96,106,108 or an MRI scan was conducted5960.61.62,67,90,9495.96,102,107 to coregister optode positions on the head. Other placement strategies (placing optodes on the forehead) were applied in four studies.58,66,80,86


Differential Path Length Factor

The differential path length factor (DPF) is a scaling factor that specifies how many times the detected light has traveled farther than the source–detector separation through the brain.109,110 In 21 studies, constant DPF values were used63,6970.71,7374.75.76,78,8182.83.84.85,8788.89,93,96,105,108 whereas nine studies used age-dependent DPF values.57,65,77,79,80,90,98,99,104

For constant DPF, values of 1.0,96 5.9,78 5.93,70,71 5.94,88 and 6.07374.75.76,8182.83.84.85,87,93,105,108 were used, while age-dependent DPF values were calculated according to the formula (DPF=4.99+0.067×age0.814).57,65,77,79,80,90,98,99 An overview on used DPF values is provided in Fig. 2(b). In other studies, arbitrary units,5354.55.56,5859.60.61.62,64,67,72,94,95,100101.102,107 tissue oxygenation index [TOI; the ratio of oxyHb to total hemoglobin (sum of oxy- and deoxyHb)],66,86 image reconstruction,91,92,97,103 or absolute values68 were used, which are not dependent on specific DPF values.


Data Processing: Signal Filtering and Movement Artifact Removal

Twenty-one studies applied a low-pass filter (LPF) to their data,54,57,65,7172.73.74.75,77,79,8182.83,85,88,90,93,9899.100.101 14 studies used a high-pass filter (HPF)53,55,56,5960.61.62,67,94,95,100101.102,107 and 5 studies used a bandpass filter.76,80,89,96,104 Most studies applied an LPF with a cut-off frequency around 0.1 Hz71,7374.75,77,8081.,88,90,93,98,99,108 whereas some studies used an LPF with a cut-off frequency at 0.05,72 0.67,57,65 0.5,100,101 and 1 Hz54 (for an overview see Table 1). Eight studies applied an HPF with a cut-off frequency at 0.03 Hz,53,5960.61.62,67,95,107 six studies at 0.01 Hz,5455.56,80,100,101 and one study at 0.0594 or 0.001 Hz.108 Furthermore, eight studies used the moving average method5556.57,64,68,89,100,101 to smooth their data. Filter methods based on principal component analysis (PCA) were conducted in six studies55,56,73,76,100,102 and a spike artifact correction was used in three studies.68,76,89 Few studies applied HRF filter,105 an autoregressive model with prewhitened iterative reweighted least square algorithms,103 wavelet filter,80,105,106 Gaussian smoothing,106 and correlation-based signal improvement (CBSI).64,80 A visual inspection of data was reported in 13 studies.68,74,75,8182.83.84.85,88,93,97,108,111


Data Processing: Correction for Physiological Artifacts

One study applied short separation channels54 and one study used multidistance measurements68 to correct for superficial blood flow. For multidistance measurements or short separation channels, normally lower source–detector separation (<1.5  cm) is chosen, which is used to probe extracerebral noise. Furthermore, the following additional physiological parameters were measured to take into account systemic physiological artifacts: (1) heart rate,53,57,5960.61.62,71,87,98,99,107 (2) blood pressure,53,54,57,5960.61.62,71,107 and (3) arterial oxygen saturation.5960.61.62,107 The usage of filter methods based on PCA, which can be useful for the correction of motion and physiological noise, was used in six studies.55,56,73,76,100,102


Data Processing: Final Data Processing and Statistical Analysis

Twenty-three of the reviewed studies used a baseline correction53,55,56,5859.60.61.62,64,65,6970.71,77,7980.81,87,90,96,100,101,105 and 14 studies conducted a baseline normalization.53,54,67,7374.75,8283.84.85,89,93,101,108 Furthermore, almost all studies computed an average of (1) all trials and (2) across the channels of a specific ROI.5354.,6869.,8081.,87,8990.91,9394.95.96,9899.100.101,103,107

In addition, linear interpolations were used in the studies of Miyai et al. and Suzuki et al.5960.61.62,107 A method based on moving standard deviation and spline interpolation was applied by Beurskens et al.105 Three studies applied discrete cosine transform terms.91,92,97

Canonical hemodynamic response function was conducted in two studies105,106 that examined cortical activation during walking. Studies researching postural tasks used either a gamma hemodynamic response function91,92,95,102,103 or a Gaussian hemodynamic response function.94 A wavelet coherence analysis was used in one study.104

Five studies divided their task phase in different time periods,58,68,76,86,90 18 studies used predetermined time intervals inside the task phase,53,54,57,5960.61.62,65,67,6970.71,73,93,94,9899.100 and 24 studies used the entire task phase for analysis.55,56,63,64,72,74,75,77,80,82,83,85,87,89,91,92,96,97,101102.103,106107.108

The statistical analysis was performed in 47 studies with parametric53,54,5657.,7374.75.76.77,79,80,8283.,9192.,103104. and in one study with nonparametric methods.81 Eight studies used parametric and nonparametric methods55,71,72,78,90,100101.102 (see Table 1).


Markers for the Assessment of Cortical Activation

The majority of reviewed studies used changes of oxyHb to assess brain activation.53,5758.,68,7071.,7778.79.80,8283.84.85,89,90,9394.95.96,101,104,106 Furthermore, 21 studies used both oxyHb and deoxyHb to quantify the activation of the region of interest.5455.56,6364.65,67,69,81,88,91,92,9798.99.100,102,103,105,107,108 Only Clark et al.66,86 used the TOI, which is the ratio of oxygenated to total tissue hemoglobin, to evaluate cortical activation. In addition, Lu et al.76 used Hb diff (oxyHb – deoxyHb) for the quantification of cortical activation. Furthermore, one study used a cortical activation ratio62 to measure brain activation (for an overview see Table 1).


Results: Main Findings of the Studies

In the following sections, we will provide an overview about the main findings of the reviewed studies. The results section is divided into outcomes of walking and postural tasks.



Walking was associated with a higher activation of prefrontal cortex (PFC),53,54,57,58,67 presupplementary motor area,53,67 premotor cortex (PMC),53,106 supplementary motor area (SMA),5354.55,58,67,106,107 and sensorimotor cortex (SMC).53,58,67,106 A higher PFC activation was observed in persons with low gait capacity,53 high perceived stress,84 high perceived fatigue,85 high risk of falling,83 ataxic gait,70,71 and patients with Parkinson’s disease80,90 during walking. Moreover, higher activation of precentral gyrus (PrG), postcentral gyrus (PoG), and superior parietal lobule (SPL) was observed in children with cerebral palsy56 and in stroke patients in the nonaffected hemisphere in the PFC,58 SMA,58,61 and SMC.61 During dual-task walking (e.g., walking and solving an additional cognitive or motor task), the PFC exhibited an enhanced activation in stroke patients,65 patients with multiple sclerosis,82 patients with Parkinson’s disease,81 obese adults,87 older adults with mild cognitive impairments,72 old adults with mobility deficits,66,86 and healthy older63,65,7374.75,82 and young adults.57,63,64,73,76,77 In comparison to young adults, older adults exhibited a higher73 or similar63 PFC activation during dual-task walking. The activation of PFC during dual-task walking was associated with the performance in motor tasks,75,77,89,105 cognitive tasks,75,77,89 and neuropsychological tests.72 In single task walking, PFC activation positively correlated with the neuropsychological performance in healthy older persons68 and with motor performance in neurologically diseased persons.70,71 A decrease in PFC activation was observed in younger adults while walking and solving a working memory task79,88 and in healthy seniors while solving a complex visual task.105 Interestingly, the activation of PFC in older adults is decreased after a motor intervention68 and when textured insoles were used or barefoot walking was conducted.66 In contrast, the inpatient intervention in stroke patients enhanced PMC activation during walking.60 Additionally, an increase of motor complexity due to the increase in walking speed led to a pronounced activation of PFC,62 SMA,53 and Broca area,64 whereas a decrease of motor complexity due to body weight support induced a decrease in SMC activation.59


Postural Tasks

In balance tasks, the activation of PFC,91,98,99 SMA,101,102 and superior temporal gyrus97 was modulated by task difficulty and by age-related processes.104 Furthermore, an increased PFC activation was observed during standing in young adults with postconcussion symptoms,108 in patients with Parkinson’s disease93 or in stroke patients’ in the affected95,102 and unaffected hemisphere.95 Furthermore, stroke patients showed a stronger activation in PMC and parietal areas concerning the unaffected hemisphere.95 After the rehabilitation program, the same patients showed a decreased activation of PMC and parietal areas but a bilateral increase in PFC and SMA activations.102

During the SOT, different sensory information changes the functional connectivity of brain areas96,103 and induced activation changes especially in superior marginal gyrus,92,96 operculum,96 temporal–parietal areas,103 and occipital regions.103 Additionally, correlation between balance performance and the activation of PFC95,102 and SMA was observed.95,100,102



fNIRS is a relatively new neuroimaging technique that has attracted attention in scientists who examine neuromotor control. This resulted in a considerable magnitude of published studies. However, a summarization and evaluation that can help to improve future experimental protocols was still required. In the first part of the discussion section, we will discuss the findings about study designs, fNIRS configurations and data processing steps to come closer to more standardized protocols that are not available at this moment.27,112 In the second part, the main findings of the reviewed studies are discussed.


Discussion: Methodology


Baseline condition and duration

The majority of studies with walking or postural tasks assessed baseline brain activation in quiet standing. Interestingly, Holtzer et al.74,75 used a silent counting task to avoid mind wandering. Mind wandering occurs up to 50% of the waking hours113 for instance during driving114,115 especially when perceptual requirements are low.116 Moreover, the wandering of the mind is characterized by the processing of task unrelated thoughts such as worrying about the past or future,117 which evokes a stronger activation of default networks118 and hence changes the activation in PFC areas.119,120 In addition, it was shown by Durantin et al.120 that fNIRS is sensitive to detect mind wandering. Based on these assumptions, it is possible that mind wandering influences the cortical activation during baseline (and maybe motor control) affecting further analyzation processes. Hence, it might be advantageous to use the approach of Holtzer et al.,74,75 which eventually minimizes the detrimental effect of mind wandering on cortical activation and leads to a more standardized baseline assessment. However, before the usage of this simple counting task can be recommended, further research should investigate its influence on cortical activation patterns including examination of enhanced reproducibility.


Number and duration of trials and rest phases

Our results revealed that the number of trials and their durations varied across the studies evaluating walking or postural tasks. The most common time interval was set to 30 s. However, we are unaware of a study investigating the influence of measurement strategy (e.g., required number of trials to achieve a sufficient reproducibility). Hence, further methodological investigations to optimize fNIRS measurement protocols are needed. Moreover, the duration and number of the trials depend on the aim of the study. Longer measurement durations may be useful to study the contribution of different areas in the temporal course of movement execution. In contrast, longer measurement durations could result in motor fatigue. Motor fatigue does diminish performance for example in postural tasks121122. and would hence change underlying motor control processes. This again could potentially evoke altered hemodynamic responses, which were observed after cognitive fatigue.127 However, research examining the interplay between a specific gross motor task and hemodynamic responses as a function of physical fatigue level has not been conducted yet.

Another interesting point influencing the trial duration is the combination of analysis methods. From a movement scientific view, the analysis of gait features (especially gait variability and stability) gives an insight in the central organization of motor control processes128129.130.131 and those are useful to detect risk groups such as fallers.132,133 To reliably assess gait variability or stability, a larger number of strides is required134,135 and as a consequence, a sufficiently long time period (in which an adequate number of strides can be undertaken) of the trial duration has to be recorded. The rest phase durations in included studies have varying temporal ranges. In general, empirical evidence suggests that refraction time or time with reduced responsiveness lasted for almost the same duration as stimulus time.136 Hence, we recommend to include intertrial rest intervals with at least the same duration as the task period, especially in block design studies.


Source–detector separation

The separation of source to detector is one important aspect for penetration depth27,34 and the influence of extracerebral signals.34,137 Our results indicated that 3 cm was the most commonly used distance in the reviewed studies. In the literature, different recommendations about optimal source–detector separation exist. While some authors recommend 4 cm,34 other collectives recommend 3 cm.138,139 In addition, especially in children or infants shorter interoptode distance (>2.0  cm) is recommended for usage.22,139 The issue of the optimal separation between source and detector is a controversial debate because different third variables such as different colors of the participant’s skin and/or hair used wavelengths and head size could influence penetration depth.34,140 Furthermore, the varying thickness of scalps, skulls, and cerebrospinal fluids in individuals and cortical regions141142.143 could influence the penetration depth and the sensitivity to hemodynamic changes in cortical layer.142143.144

Remarkably, a longer source–detector separation leads to a greater contribution of cerebral than extracerebral layer to obtain hemodynamics signals.145146.147.148 The penetration depth of light is less than half of the interoptode distance147 causing short channel distances to cover only signals from noncerebral compartments.137,141 For instance, at the source–detector separation of 3 cm, the contribution of the gray matter to the light absorption is estimated to range from about 20% to 30%.149 Moreover, Kohri et al.150 observed that at source–detector separation of 2, 3, and 4 cm, the cerebral tissue contributes to 33%, 55%, and 69% to the optical signal. Hence, we recommend that the source–detector separation should be greater than 3 cm to enhance the contribution of cerebral cortical layer to the optical signal.


Placement of optodes

The majority of the studies used the 10 to 20 EEG systems to place the optodes. This standardized location system ensures the comparability among the different studies. The additionally used 3-D digitizer or individual MRI scan improves the registration of channels to specific brain areas. Based on the data we recommend for optode placement the usage of the 10 to 20 EEG systems to ensure the comparability among studies.


Differential path length factor

Our results show that most studies used constant DPF with a value of 6. The usage of a constant DPF value seems not always appropriate because the brain undergoes age-related changes of gray and white matter,151,152 intracranial volume,153 and cerebral volume as well as blood flow154, which may affect DPF.155 Furthermore, methodological studies show that DPF values are (1) age-dependent and subject-specific,110,155,156 (2) wavelength-dependent,110,155,157 and (3) cortex region-dependent.110,155,158159.160 Hence, it seems favorable to calculate specific DPF values to enhance the measurement accuracy in age-groups in which formulas to calculate age-specific DPF values are available (adults under 50 years).110,155 Otherwise, “arbitrary units,”161 TOI,162163.164 or absolute values137,163,165 could be used since those do not depend on a specific DPF value. In addition, it is suggested that the calculation of effect sizes is useful to deal with the DPF issue.166 However, additional research is strongly needed that provides a formula to calculate DPF values for specific age-groups (adults older than 50 years) dependent on wavelength and cortex region. In our opinion, the optimal approach to quantify DPF, taking the dependency of DPF regarding subject, age, wavelength, and cortex region into account, is the direct quantification of DPF using frequency or time-domain NIRS.


Data processing: signal filtering and movement artifact removal

In sum, either LPFs or HPFs were commonly applied in the reviewed studies to remove noise and drifts. Most of the studies used a cut-off frequency for LPF around 0.1 Hz and HPF around 0.01 Hz. The reviews of Brigadoi et al.,167 Cooper et al.,168 and Gervain et al.40 recommended to use a bandpass filter (consisting of both LPFs and HPFs) with cut-off frequencies at 0.5 (LPF) and 0.01 Hz (HPF). The bandpass filtering should be used carefully to avoid accidental removal of stimulus-dependent hemodynamic response signals.111 Hence, a higher cut-off frequency at 0.5 Hz (LPF) in conjunction with other more sophisticated filter methods is recommended to be used for the removal of movement and physiological noise.111,167,168 Different methods such as PCA,169170.171 task-related component analysis,172173.174 CBSI,175 wavelet-based filters,171,176177.178.179 autoregressive algorithm-based filters,180 Kalman filter,181 and Wiener filter182 are proposed for the filtering of fNIRS data. Interestingly, Nozawa et al.183 suggested that effectiveness of motion correction filter methods depends on subject and task. However, reviews comparing a variety of filter methods recommend the additional application of wavelet filter167,168 or spline technique.168 These filter methods were occasionally applied in reviewed studies80,105,106 leaving potential to optimize the filtering processes in further studies. Based on these assumptions, we recommend the usage of a bandpass filter and wavelet filter to reduce motion artifacts. If there are sudden shifts in the data (baseline shift), the approach developed by Scholkmann et al.184 can be useful to remove them.


Data processing: correction for physiological artifacts

Twelve studies recorded physiological signals such as heart rate, blood pressure, or arterial oxygenation saturation parallel to the fNIRS signals. Task-related systematic changes in heart rate, respiration rate, or blood pressure are known to influence the fNIRS signal and may cause false-positive results.45 For instance, often unconsidered factors such as adding of speech as a task (e.g., in dual-task paradigms) lead to changes in partial pressure of end-tidal carbon dioxide, which influences cerebral hemodynamics and masked neuronal-induced activity changes.185,186 Hence, to improve the accuracy of fNIRS, the recording and elimination of systemic physiological changes seems necessary.45,187,188 The signals of additional physiological measures could be useful for filtering of fNIRS signal189,190 or to ensure the absence of systematic physiological differences among the experimental conditions.87 In addition, some measures such as heart rate variability could be used to study the interplay between the central (fNIRS) and the autonomic (e.g., heart rate variability) nervous system.120,191 Furthermore, filter methods based on PCA and independent component analysis, which were applied in six studies,55,56,73,76,100,102 could be used to remove movement-related167 or physiological artifacts.169,170,192193.194.195 In addition to the other filter methods,196,197 a more “direct” approach to reduce extracerebral noise is the use of short separation channels or multidistance technique198199.200, which were applied in only two of the reviewed studies.54,68 Short separation channels have a small distance between source and detector to record extra cerebral signals, such as superficial blood flow.141,198,201 These extracerebral signals are used to filter the remaining fNIRS data. Previous studies revealed that the application of short separation channels is powerful in reducing extracerebral noise141,145,200,202203., which contaminates fNIRS signals.45,199,201,209210. The optimal distance between short separation channels varied across different cortex regions141,202 but should be generally <1  cm for measurement on the head of adult humans. Hence, further development and implementation of short separation channels (multidistance technique) could enhance the accuracy of fNIRS measurements and have to be considered whenever technically possible.


Data processing: final data processing and statistical analysis

Most studies used baseline normalization and baseline correction to circumvent the influence of different path lengths factors.166 Furthermore, averaging of channels across trials and in specified ROIs was common practice in the reviewed studies.

Some studies divided their task phase in different time periods, which seems useful for studying the contribution of cortical areas in different temporal periods during task execution. Therefore, attention should be paid to the temporal delay of 2 to 5 s in hemodynamic response.69,107,139

The majority of the reviewed studies used simple statistics based on processing mean values over the task period. This approach, however, tends to result in a loss of acquisition of information because it does not consider the temporal shape of the fNIRS signal.192 Hence, some authors suggest that the analysis of fNIRS data with general linear models is more favorable.192,215 However, the choice of the statistical analysis methods should depend on the research question and the experimental design.216 For instance, in an event-related design, the application of a general linear model is a valid technique216 whereas simple statistics might also be appropriate (and commonly used192) especially in studies utilizing block designs.55,56,5960.61.62,107,217 The majority of reviewed studies used parametric methods for statistical data analysis. In fNIRS studies, the assumptions for parametric tests are sometimes violated (e.g., normal distribution due to small sample size); therefore, nonparametric tests are a considerable option.218,219 Moreover, nonparametric tests are more robust and less influenced by outliers or nonnormal distributed data220221.222 and are recommended to use in fNIRS studies. From another point of view, in neuroscience, multiple experimental conditions (crossed) or multiple observations per condition (nested) were used.223,224 Furthermore, different categorical or continuous confounding variables have to be considered (e.g., gait speed, education, and gender) and/or data were unbalanced or incomplete, which makes it necessary to use advanced statistical methods.223,225 To account for those problems, linear mixed-effect models can be used.10,224225.226 However, statistical methods should be chosen carefully considering the experimental design and distribution of recorded data. A further description of statistical methods for fNIRS data is given in the reviews of Tak and Ye192 and Kamran et al.227


Markers for the assessment of cortical activation

The majority of reviewed studies used only oxyHb for the quantification of cortical activation since a change in oxyHb is assumed to be a more robust marker of changes in regional cerebral blood flow than changes in deoxyHb.160,228,229 However, this procedure seems questionable because neuronal activity is not just mirrored in an increase of oxyHb but also in a decrease in deoxyHb in healthy adults.30,230 Furthermore, an enhanced level of physiological noise is more prominent in oxyHb signals30 and the decrease in deoxyHb is related to an increase in BOLD contrast obtained in fMRI231,232, which supports the validity of the evaluation of deoxyHb changes. In pathological states, neurovascular coupling might perhaps be impaired, which results in altered concentration changes in deoxyHb during neural activity.230 Lindauer et al.230 assumed that in some pathological states, an increase in deoxyHb may reflect neural activity. Based on the mentioned assumptions, it seems favorable to report at least oxyHb as well as deoxyHb to assess task-dependent activity.30,45,165


Discussion: Main Findings



Evidence from neuroimaging studies point out that two distinct supraspinal locomotor networks are responsible for the control of walking and standing1,233234.235.236.237 (see Fig. 3). The direct locomotor network consists of the primary motor cortex (M 1) and the cerebellar locomotor region and is potentially activated in the absence of pathologies or challenging situations.235 In the indirect locomotor pathway, the neuronal commands are transmitted via PFC and SMA to the basal ganglia and subthalamic as well as mesencephalic locomotor regions.233234.235.236.237 The indirect locomotor pathway becomes activated when the automatic execution of walking is impaired (e.g., in challenging situations) and compensatory mechanisms are necessary.44,238 This assumption is supported by findings of our reviewed fNIRS studies, which reported more pronounced activation in prefrontal structures in (1) in adults during dual-task walking,57,63,63,66,69,7273.74,77,86,89 (2) in adults during fast walking,53,64 (3) in obese persons,87 (4) in individuals with low gait capacity during fast walking,53 (5) in older adults with high level of perceived fatigue85 or stress,84 (6) in old adults with increased fall risk,83 and (7) in neurological patients.58,70,71,75,80,82,90 Remarkably, the PFC activation in neurological patients correlates with their step widths,71 which again (1) is associated with balance control239 and (2) serves as a predictor of falls.240 Furthermore, correlations between cortical activation and motor performance,55,56 especially obvious in dual-task walking conditions,76,77,89,105 was observed. This reinforces the important role of cortical areas in motor control. Moreover, the reduction of PFC activity after a motor-cognitive intervention program (lasting 8 weeks)68 perhaps originated from the shift toward a more automatic control of locomotion relying on the enhanced usage of direct locomotor pathway via M1, cerebellum, and spinal cord.1,233,234,238

Fig. 3

Schematic illustration of the indirect and direct locomotor pathways as a function of the degree of automaticity in motor control.


However, premotor areas and the SMA play a role in different cognitive processes241242.243 and were activated as a function of task difficulty in a variety of cognitive domains.244245.246 Hence, the phenomenon of a more pronounced activation of premotor areas (as part of indirect locomotor pathway) in diseased cohorts (or during challenging motor tasks) is perhaps not fully attributable to motor task complexity but partly also to general task complexity.

However, the decrease in PFC activity in a complex visual task105 or difficult working memory tasks during walking79,88 may not be induced by the shifts in locomotor pathways but rather originate from the prioritization of task-relevant areas as consequence of the limited resources of the brain.247 While those three studies focused only on PFC activity, it is difficult to draw a final conclusion about potentially underlying cortical processes in other areas. Hence, to elucidate the mechanisms with respect to task prioritizations, we require further research248,249 including the simultaneous assessment of more cortical structures (e.g., motor areas).

For the design and monitoring of rehabilitative interventions, fNIRS could be a promising tool.42 For instance, the SMC activity decreases during weight-supported walking in stroke patients59 and could be a hint that weight supports lower task complexity.250 Interestingly, a verbal preadvice67,94 or the usage of mechanical assistance during walking61,106 increases central nervous load. These findings could be useful to create tailored rehabilitation programs that consider mental load as variable for workload assessment.


Postural tasks

As pointed out for walking, neural control of posture is realized via direct or indirect pathway251 which are shown in Fig. 3. Our results reveal that the PFC activation is enhanced in (1) neurological patients during standing93 or during postural perturbations95,102 and (2) healthy adults during challenging balance tasks.91,98,99 These findings and the observations that PFC activity and SMA are associated with balance measures95,100,102 support the notion that indirect locomotor pathway is crucial for neuromotor processes in nonautomatized challenging situations.

Additionally, altered sensory information evoked by the execution of SOT induces a higher activation especially in STG.92,96 The STG is associated with (1) the control of more difficult balance tasks,97 (2) the integration of vestibular information,252253.254 and (3) the spatial orientation.255 So far, the mentioned studies did include only young participants.92,96 While aging changes the contribution of somatosensory, vestibular, and visual system in balance tasks,256 it seems necessary to enlarge existing knowledge about cortical sensory integration processes.


Key Studies

In the following, we highlight one key study in the area of walking and balance. Those studies are of high practical relevance and cannot be performed in an fMRI since motor imagery is suggested not to be a satisfactory indicative of brain activation during motor execution.257



The usage of a smartphone during walking causes serious injuries.258,259 Hence, the understanding and the analysis of underlying motor control processes of walking while texting on a smartphone seems to be of high practical relevance.260 The investigation of smartphone usage while recording the kinematics of gait is not possible in an fMRI-scanner but could be conducted with fNIRS. In the study of Takeuchi et al.,89 the influence of using a smartphone while walking was investigated in healthy old and young adults. Takeuchi et al.89 observed that in young adults, the activation magnitude of left PFC is associated with dual-task cost (change between single- and dual-task performances) of gait acceleration and right PFC is related to the dual-task cost of the conducted cognitive smartphone task. In contrast, in the older adults middle PFC was associated with dual-task costs of step time and the activation of the left PFC is associated with dual-task costs of gait acceleration.89 Furthermore, younger adults have lower dual-task costs in kinematic parameters.89 In sum, these results point toward the effective lateralization in young adults, while in older adults more resources are needed to maintain gait performance which is in accordance with the theories of hemispheric asymmetry reduction261 and compensational recruitment.262


Postural Tasks

While fMRT is sensitive to motion artifacts,1819.20.21 the simultaneous recording of brain activity and the quantification of kinematic parameters of gross motor skills (e.g., dynamic whole-body balance task) are impossible. Remarkably, it is assumed that to increase our knowledge about neuromotor control processes, the simultaneous assessment of brain activity and kinematic parameters is necessary.263 Furthermore, gross motor skills are, for example, an essential part of rehabilitative interventions (e.g., balancing on wobble board264265.266). The study of Herold et al.100 used fNIRS to investigate the contribution of motor areas in online neuromotor control of balance performance on a wobble board and recorded simultaneous sway parameters via an inertial sensor. They observed (1) a pronounced activation of PrG, PoG, and SMA during balancing and (2) a strong negative correlation between the magnitude of SMA activation and sway in mediolateral direction during balancing.100 The results of Herold et al.100 allow a deeper understanding of the role of the SMA in online neuromotor control of balance movements and may be helpful to design tailored intervention programs or to monitor the intervention progress.



In sum, neuroimaging with the fNIRS technology seems to be a promising tool to shed light on the functioning of cortical areas in motor control. However, the absence of standardized study protocols limits the comparability among studies. Based on our findings, we deduce recommendations and potential future directions, which are shown in Table 2. Hopefully, those recommendations will lay foundations to improve the study protocols and data processing of fNIRS methodology encouraging further research to extend our existing knowledge about neuromotor control processes. This increase in knowledge might be helpful to develop tailored rehabilitation programs for clinical settings in, e.g., orthopedics and neurology.42 Furthermore, combining the information we can derive from fNIRS signals with kinematic parameters which are risk factors for falls132,267 or for cognitive decline268 could perhaps support a more sensitive and effective early detection of persons with a high likelihood for falls or with a high risk to develop cognitive diseases. This, in turn, may allow an early onset of therapeutic interventions, an effective monitoring of intervention programs and it would support the decision making in health care units. Those potential applications could be beneficial for patients and the resources of the health care system.

Table 2

Recommendations for future fNIRS studies.

• Report all technical configuration details (source-detector separation, wavelengths, sampling frequency, number of measurement channels, DPF values with selection process, etc.) and design-related details (e.g., duration of task and rest phases).
• Optode placement should be based on the 10 to 20 EEG system.
• Additional measures (e.g., heart rate, blood pressure, respiration, skin conductance, etc.) should be used to monitor systematic changes.
• In order to process data, the use of bandpass filters and wavelet filters is recommended.
• DPF values should be calculated depending on age and cortex region or directly quantified via frequency- or time-domain NIRS.
• Physiological cofounders (e.g., scalp blood flow) should be reduced with the aid of PCA/ICA analyses or the usage of short separation channels.
• Baseline correction or baseline normalization should be applied.
• Averaging across channels of a ROI and trials seems to be favorable.
• The relative changes of both, oxyHb and deoxyHb, should be reported and used in the statistical analysis.



For further information about search strategy, cohort characteristics, study protocols, number of fNIRS channels, used wavelengths and sampling frequencies in the reviewed studies, we provide supplemental content which is available in Ref. 52, or can be requested by e-mail from the corresponding author.


The authors declare no conflict of interest.


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Fabian Herold received his BA degree in sport science from Otto von Guericke University in 2014. Currently, he is working in the Department of Sport Science at Otto von Guericke University as a research assistant, where he finished his MA thesis. His research interests include analyzing neuromotor control processes of gait and the application of functional near-infrared spectroscopy in motor control experiments.

Patrick Wiegel received his BA degree from Otto von Guericke University Magdeburg in 2014 and his MSc degree from Albert-Ludwigs-University Freiburg in 2016. Currently, he is a PhD student at Albert-Ludwigs-University Freiburg. His research focuses on the underlying neural processes of human motor control and motor learning.

Felix Scholkmann received his PhD at the University of Zurich, Switzerland, in 2014. As a postdoc at the Biomedical Optics Research Laboratory of the University Hospital Zurich and a research associate at the University of Bern, his research focuses on biomedical signal processing, biomedical optics (development and application of fNIRS for human optical neuroimaging), neuroscience, integrative physiology, and biophysics.

Angelina Thiers received her master’s degree in information technology from the University of Applied Science in Brandenburg in 2014. She is a researcher at the Chair Health and Physical Activity at Otto von Guericke University Magdeburg, Germany. Her research interests include biosignal analyses and the development of new applications.

Dennis Hamacher received his doctorate from Otto von Guericke University in 2017. At the Chair Health and Physical Activity, he is working as a research associate, where he analyzes the underlying mechanisms of gait stability in old and diseased cohorts.

Lutz Schega is a professor (full) and chair of the Department of Health and Physical Activity at Otto von Guericke University in Magdeburg. He received his PhD in sports science in 1994 and his habilitation (postdoctoral qualification) in sports and rehabilitation science in 2003 from the University of Leipzig. A major focus of his work is on investigating various aspects of human walking performance.

© The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
Fabian Herold, Patrick Wiegel, Felix Scholkmann, Angelina Thiers, Dennis Hamacher, Lutz Schega, "Functional near-infrared spectroscopy in movement science: a systematic review on cortical activity in postural and walking tasks," Neurophotonics 4(4), 041403 (1 August 2017). https://doi.org/10.1117/1.NPh.4.4.041403 Submission: Received 4 March 2017; Accepted 23 June 2017
Submission: Received 4 March 2017; Accepted 23 June 2017

Near infrared spectroscopy

Data processing

Process control

Gait analysis

Bandpass filters




Optics In Automated Inspection
Proceedings of SPIE (October 20 1975)

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