Stretch reflex of the biceps and brachioradialis muscles in patients with upper motor neuron syndrome.

The position of elbow flexion commonly observed in patients with upper motor neuron lesions is attributed by many clinicians to hyperactive stretch reflexes. The purpose of this study was to examine the passive stretch reflexes of the biceps and brachioradialis muscles in order to assess their possible relative contributions to the flexion posture. Twenty-one patients with upper motor neuron lesions were tested, using an automated system to produce controlled passive movement strech reflexes. Ratios derived from the electromyographic activity elicited during passive movement were used for quantitative comparisons. The stretch reflex response of the brachioradialis muscle was larger and earlier than the biceps response. A surprising finding was reciprocal activity in these two synergists following phasic stretch.