The role of the tonic stretch reflex during standing in man.

Relationships between shifts of the center of gravity and electromyographic activities (EMGs) were investigated in man during standing. The EMG was mainly obtained from the gastrocnemius and soleus muscles (GS), but other muscles, such as hamstring, tibialis anterior and quadriceps muscles were also included. The shift of gravity center was measured as a change in foot pressure. A forward shift of the gravity center by voluntary leaning produced an increase in EMG activity of the GS, whereas a backward leaning caused an opposite effect, Le., the EMG was suppressed during a backward shift of the gravity center. The same relationship was obtained when the gravity center was shifted by forward or backward pushing of the body. Rhythmic tilting of the pressure measuring plate, on which the subject was standing, also increased the EMG activity of GS during the forward shift of gravity center, at a tilting frequency of less than 0.5 Hz. However, this relationship was deteriorated when tilting frequency was increased to more than 1 Hz. When mechanical vibration (100 Hz) was applied to bilateral Achilles tendons, the gravity center was gradually shifted backward and oscillatory flucturation of the gravity center appeared. The EMG activity of GS was not increased by vibration, but decreased in parallel with a backward shift of the gravity center. The EMG activity of the GS during a forward shift of the gravity center is considered to be due to the tonic stretch reflex caused by stretch of the GS resulting from dorsiflexion of the ankle joint. However, the vibratory stimulus which increases the muscle tone if the GS is not engaged in standing is ineffective in causing the stretch reflex. The failure to observe the stretch reflex by vibration may be explained by assuming that during standing when many other muscle are sending informations on the posture, a functionally heterogenous information, such as the afferent impulses produced by vibration of one muscle, is suppressed at the spirnal cord by a supraspinal center.