MOTOR UNIT RECRUITMENT STRATEGY CHANGES WITH SKILL ACQUISITION

The modifications of motor unit recruitment strategy due to skill acqu isition was determined in the elbow flexor-extensor muscles of normal human subjects, The median frequency of the power density spectra of t he electromyograms recorded from the biceps and triceps muscles during a 3-s linear increase in flexion force in the range of 0-100% maximal voluntary contraction (MVC) was calculated for each subject, every 2 weeks over a total 6-week period during which subjects practiced linea r flexion force increase three times a week, Electromyograms were reco rded with two pairs of electrodes of different size and electrode spac ing. It was shown that skill acquisition due to the 360 practice trial s over the 6-week period caused an increase in the initial motor unit recruitment phase of the agonist's force generation cycle from about 0 -65% MVC to about 0-85% MVC. The increase in the recruitment range was gradual and statistically significant for the measurements made every 2 weeks. The recruitment range of the antagonist triceps demonstrated a minor, but statistically insignificant, decrease over the same trai ning period. There was a minor, but statistically insignificant, advan tage of using small electrodes and inter-electrode spacing. It was con cluded that skill acquisition, due to repeated functional use of a mus cle in the same contraction mode, results in a slower, prolonged recru itment. of mater units in the initial segment of the force generation cycle, thereby allowing a more precise and accurate control of the inc rements of force increase. Such conclusions reinforce the concept advo cating the plasticity of motor unit control according to the functiona l demands imposed on the muscle. The results have significant implicat ions in the design of various athletic, occupational and rehabilitatio n training modalities for optimal performance of various movement func tions.