Accurate estimation of concentration changes in muscles by continuous wave near-IR spectroscopy for muscle measurements suffers from underestimation and crosstalk problems due to the presence of superficial skin and fat layers. Underestimation error is basically caused by a homogeneous medium assumption in the calculations leading to the partial volume effect. The homogeneous medium assumption and wavelength dependence of mean partial path length in the muscle layer cause the crosstalk. We investigate underestimation errors and crosstalk by Monte Carlo simulations with a three layered (skin-fat-muscle) tissue model for a two-wavelength system where the choice of first wavelength is in the 675- to 775-nm range and the second wavelength is in the 825- to 900-nm range. Means of absolute underestimation errors and crosstalk over the considered wavelength pairs are found to be higher for greater fat thicknesses. Estimation errors of concentration changes for Hb and HbO2 are calculated to be close for an ischemia type protocol where both Hb and HbO2 are assumed to have equal magnitude but opposite concentration changes. The minimum estimation errors are found for the 700/825- and 725/825-nm pairs for this protocol.
Near infrared spectroscopy (NIRS) is now a commonly accepted method
for the measurement of oxidative metabolism of brain and muscle
tissues. Nevertheless, this technology suffers from the error caused
by the homogeneous single layer assumption in the calculations of
concentration changes of light absorber chromophores using either
diffusion theory or modified Beer-Lambert law. Underestimation of
muscle oxidative metabolism for muscles having thicker fat layer
above is a particular case. Due to this uncertainty, statistical
analysis can be problematic in reviewing the results across subjects
having different fat thicknesses for muscle studies. In this study, partial pathlength method with two detectors based on modified
Beer-Lambert law extended for heterogeneous medium with homogeneous
layered regions is investigated. Using Monte Carlo simulations,
comparison between this technique and single homogeneous layer
assumption is done. Optical coefficients of fat and muscle layers
are chosen typical for muscle tissue measurements. In the
simulations, change of absorption coefficient in muscle layer was
made much bigger than in fat layer. It has been found that for
2-detector partial pathlength based method, fat and muscle layer
absorption coefficient change estimates are better than the
homogeneous medium based modified Beer-Lambert law estimates in all
In this paper, the changes in muscle deoxygenation trends during a sustained isometric quadriceps (chair squat/half squat) endurance exercise were evaluated among twelve male subjects and the relationship between muscle oxygenation and endurance times was investigated by means of functional near-infrared spectroscopy (fNIRS). Neuromuscular activation and predictions of muscle performance decrements during extended fatiguing task was investigated by means of surface electromyography (sEMG). The results of the study showed that in the subjects who maintained exercise longer than five minutes (group 1), mean Hb recovery time (33 [sec.]) was 37.4% less than the others (group 2, 52.7 [sec.]). Also mean HbO<sub>2</sub> decline amplitude (2.53 [a.u.] in group 1 and 2.07 [a.u.] in group 2) and oxy decline amplitude (8.4 [a.u.] in group 1 and 3.04 [a.u.] in group 2) in the beginning of squat exercise are found to be 22.6% and 176.9% bigger in these group. For the EMG parameters, mean slope of MNF and MDF decline are found to be 57.5% and 42.2% bigger in magnitude in group 2 which indicates higher degree of decrement in mean and median frequencies although their mean squat duration time is less. This indicates higher index of fatigue for this group. It is concluded that training leads to altered oxygenation and oxygen extraction capability in the exercising muscle and investigated fNIRS parameters could be used for endurance evaluation.
Monitoring the oxygenation of skeletal muscle tissues during rest to work transient provides valuable information about the performance of a particular tissue in adapting to aerobic glycolysis. In this paper we analyze the temporal relation of O<sub>2</sub> consumption with deoxy-hemoglobin (Hb) signals measured by functional Near Infrared Spectroscopy (fNIRS) technique during moderate isotonic forearm finger joint flexion exercise under ischemic conditions and model it with a mono exponential equation with delay. The time constants of fitting equation are questioned under two different work loads and among subjects differing in gender. Ten (6 men and 4 women) subjects performed isotonic forearm finger joint flexion exercise with two different loads. It is shown that under the same load, men and women subjects generate similar time constants and time delays. However, apparent change in time constants and time delays were observed when exercise was performed under different loads. When t-test is applied to compare the outputs for time constants between 0.41202 Watts and 0.90252 Watts, P value of 9.3445x10<sup>-4</sup> < 0.05 is observed which implies that the differences between the time constants are statistically significant. When the same procedure is applied for the time delay comparison, P value of 0.027<0.05 is observed which implies that also the differences between the time delays are statistically significant.
Functional Near Infrared Spectroscopy has been used to investigate changes in cerebral hemodynamics induced by hypercapnia challenges, such as carbon dioxide CO<sub>2</sub> inhalation and breath holding. The aim of this study was to investigate CO<sub>2</sub> pressure changes dependence of frequency spectrum of cerebral hemodynamic oscillations during breath holding task. Measurements of the relative changes in concentration of deoxy-hemoglobin ([Hb]) and oxyhemoglobin ([HbO<sub>2</sub>]) are performed on nine healthy subjects during three breath holdings of 30 seconds (s.) interleaved with 90 s. of normal breathing. Power spectra are computed by continuous wavelet transform and averaged for normal and hold episodes. The percent change values between hold and normal episodes are given for frequency peaks at (0.035 Hz), where a 17% higher increase was observed for PC of [Hb] on the right side compared to left side,while this value was at 64.8% for [HbO<sub>2</sub>] . Similarly, for a peak at 0.11 Hz these values were 54.5% and 9.5% for [Hb] and [HbO<sub>2</sub>] PCs, respectively. The smallest changes were observed for breathing freq. range (around 0.2 Hz) where the values are -72% and 55.8% for [Hb] and [HbO<sub>2</sub>], respectively.