Following a brain injury (BI) in addition to all other systems, there can be a failure in the control of the autonomic system activity. Heart rate (HR) has its own normal variability. Heart rate is controlled by the Sympathetic and Parasympathetic systems. Therefore, monitoring HR variability (HRV) can help us evaluate the balance of the two systems and their efficiency.Decrease in HRV was found to be in correlation with death among patients in the acute stage following ABI. Decrease in HRV is a pre-stage of HR irregularity and ventricular fibrillation.This disturbance can have a great impact on the patients health condition. In addition there was found an inverse correlation between this situation and the rehabilitation outcomes. Based on this data there is a great importance in monitoring HRV during rehabilitation among patients following BI while the patients are required to perform physical activity.The aim of this work is to check whether we can replace the traditional way of measuring HR by EKG Holter (gold standard) with a more simple,accessible tool-the POLAR watch.