Stretch reflex of quadriceps femoris in normal man: methodological considerations and normative data.

Short- and long-latency responses to stretching have been evoked in various muscles in the healthy subject. The quadriceps femoris, however, has never been investigated in this connection, despite its functional importance. We studied its stretch reflex in 17 healthy subjects with a torque motor to determine its morphological features in relation to the level of background EMG activity, the stretching velocity and duration and the instructions given to the subject. Two bursts of EMG activity were constantly evoked when the muscle was voluntarily activated. Response magnitude was proportional to the background activity and displacement velocity. Short stretching evoked only one EMG burst. These findings are similar to those from other muscles. Intra-individual variability was evaluated repeating the experiment in the same session without changing position of the electrodes, and then after two months. The long-latency response showed a good intra-individual reproducibility.