In order to learn more about stretch reflex behaviour of triceps surae, normal human subjects sat in a chair with one foot on a platform attached to a torque motor that produced phasic dorsiflexion displacements on the ankle. EMG activity was recorded from triceps surae and responses were obtained for various conditions. When the subjects's foot was relaxed, stretch of triceps surae produced a single EMG component at short-latency which increased in magnitude with increasing velocity of stretch. The response was not altered if the subject was asked to plantarflex or dorsiflex the ankle voluntarily when he felt the perturbation. It was reduced by vibration of the Achilles tendon. If the triceps surae was stretched while the subject plantarflexed his ankle, the short-latency response was followed by one and sometimes two long-latency responses. Like the short-latency reflex when the foot was relaxed, none of these responses was altered by the subject's planned movement after feeling the perturbation. All of the responses were suppressed to a similar degree by vibration. The long-latency reflexes depended on long-duration of stretching and relatively slow acceleration of stretch. The reflexes persisted after anaesthesia to the foot suggesting that muscle afferents were responsible. Interactions between H-reflexes and stretch-reflexes revealed that the afferent volley producing a stretch reflex acted like the afferent volley producing a small H-reflex. Responses at an interval of 30 ms to both an electrical stimulus for an H-reflex and a stretch stimulus were possible if the electrical stimulus produced only a small H-reflex and if the subject had been plantarflexing the ankle. The short-latency reflex when the foot was relaxed or exerting a background force appears to be the monosynaptic, Ia mediated stretch reflex. The physiological properties of the long latency reflexes are similar to those of the short-latency reflex, and they may represent, at least to a certain extent, response of the motor neuron pool to successive Ia bursts.
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