Role of Ia muscle spindle afferents in post-contraction and post-vibration motor effect genesis.

Experiments carried out on 14 human subjects showed that long-lasting involuntary tonic motor responses occurred after the offset of muscle vibration (70 Hz, 0.5 mm, duration 30 s). These post-vibratory biceps and triceps brachii motor responses were compared with the motor responses observed in the same subjects after performing an isometric contraction of the same duration, i.e., post-contraction responses, or the so-called 'Kohnstamm phenomenon'. The results show the existence of close similarities between these two types of motor after-effect, particularly as regards the muscle sites where they develop, their amplitudes and their temporal patterns (latencies and offset times). Neither type of excitatory post-effect can be elicited by co-contracting or co-stimulating two antagonist muscles at the same frequency. Lastly, visual stimulation can cause both types of motor response to switch from one muscle to its antagonist. Comparative analysis of the spindle proprioceptive activities recorded in response to either vibration or isometric contractions suggests that these motor after-effects may both result from the fact that the spindle afferents from agonist and antagonist muscles are asymmetrically activated in these two particular situations.