Post-contraction variations in motor pool excitability

Previous evidence has shown a marked increase in motoneuronal excitability during muscle contraction and decreased post-contraction excitability. This post-contraction inhibition has been shown to persist up to 800 msec following the end of contraction (Gottlieb and Agarwal, 1973) and apparently continues with gradual recovery up to one minute (Enoka, Hutton and Eldred, 1980). These effects have been attributed in part to influences of muscle receptors on the homonymous motor pool. The purpose of the present study was to observe the time course of short term post-contraction effects, following a maximum isometric contraction, on excitability of the homonymous motoneuronal pool as measured by the Hoffmann reflex. Each of five subjects lay supine on a table with shoulder blocks to prevent movement during resisted plantar flexion. The subject's preferred foot was placed in a stirrup attached to a force gauge which measured tension. Recording electrodes were applied over the soleus muscle, and M- and H-responses were evoked by percutaneous electrical stimulation of the tibial nerve at the popliteal fossa. The stimulus strength was set to elicit a control H-reflex at rest of half the magnitude of the maximum H-reflex value. For each trial the subject performed a 6 s maximum contraction of the triceps surae muscle group, beginning on one visual signal and ending on a second visual signal. The H-reflex was elicited at semi-random intervals prior to and after the visual signal to relax. Increased H-reflex excitability was observed during muscle contraction followed by a marked decrease in evoked amplitudes with gradual recovery following the end of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)