This project is the first to investigate blood perfusion in the human hamstrings during isometric exercise with a near infrared spectroscopy (NIRS). A Kin Com dynamometer has been used to fix the knee positions and to measure torques during contractions. Both the NIRS optodes and the electromyography (EMG) electrodes were attached to the skin over the hamstrings. Previous studies used a NIRS to measure muscle blood flow in the forearm, quadriceps and calf muscles. The changes in haemoglobin concentrations were calculated using Spike 2 software. A total of 46 male volunteers participated in the four series of experiments described in this thesis. The following overall conclusions can be drawn: perfusion decreases in the hamstrings during contractions and then returns to normal levels after a period of time, changing the limb position at which the contractions are made does not affect the perfusion, warm-up exercises increase in blood perfusion for 8 minutes at 30 and 40% of MVC. The perfusion did not significantly change during an episode of DOMS or in the injured and non-injured limbs. These conclusions show the importance of warm-up before sports activities but not necessarily avoid injury. It can be concluded that there is no association between such conditions with hamstring injuries. The maintained perfusion at different conditions is a positive finding as the perfusion is not restricted indicating good delivery of oxygen despite muscle injury.