Reflex responses to unexpected stretches are well documented for selected muscles in both animal and human. Moreover, investigations of their possible functional significance have revealed that stretch reflexes can contribute substantially to the overall stiffness of a joint. In the lower extremity only the muscles spanning the human ankle joint have been investigated in the past. This study implemented a unique hydraulic actuator to study the contributions of the knee extensor stretch reflex to the overall knee joint torque. The quadriceps muscles were stretched at various background torques, produced either voluntarily or by electrical stimulation, and thus the purely reflex mediated torque could be calculated. The stretch had a velocity of 67 degrees /s and an amplitude of 20 degrees. A reflex response as measured by electromyography (EMG) was observed in all knee extensors at latencies of 26 - 36 ms. Both phasic and tonic EMG stretch responses increased with increasing background torques. Lines of best fit produced correlation coefficients of 0.59 - 0.78. This study is the first to examine the reflex contribution of the knee extensors to the total torque at background torques of 0 - 90% MVC. The contribution of the reflex mediated torque is initially low and peaked at background torques of 20 - 40% MVC. In terms of the total torque the reflex contributed 16 - 52% across all levels of background torque. It is concluded that during medium background torque levels such as those obtained during walking, the stretch reflex of the quadriceps muscle group contributes substantially to the total torque around the knee joint.