This paper presents a wearable human movement monitor designed within the context of a telehealthcare system for the elderly. One major characteristic of this device is the capability for an on-line personalization to the user. This capability compels to a trade-off among processing capacity, portability, low cost and power consumption, which are necessary to assure its feasibility. We have performed a preliminary laboratory study to assess the influence of the customization capacity in the reliability of the device for capturing falling events. The study was based carried out over 8 voluntaries and demonstrated that the device is able to distinguish true falling events from normal activities like fast walking or going up/downstairs. Moreover, our outcomes indicate that the subject and the environment have a critical influence on the reliability of the falling detection. This result underlines the importance of providing personalized support in telehealthcare.