Quantitation of the stretch reflex. Technical procedures and clinical applications.

The stretch reflex should ideally be quantitated for better clinical use by standardizing the muscle stretch and measuring the resulting muscle contraction. Quantitation of muscle contraction can be done by force measurements or electromyographic recordings. The electromyographic response to stretch consists of one component (short latency response) for short stretches (less than 15 ms) and of 2 or 3 components (short and long latency responses) for longer stretches (greater than 40-50 ms). The magnitude of the phasic stretch reflex is reflected by the short latency response, whereas the magnitude of the tonic stretch reflex is reflected by both the short and the long latency responses. In clinical studies of upper motor neuron syndromes, the knee jerk and the muscle tone correlated with the magnitude of the short latency response. In patients with paralysis agitans an increased long latency response, which correlated to the rigidity, was found.