Differential control during maximal concentric and eccentric loading revealed by characteristics of the electromyogram

Maximal eccentric loading has been associated with higher levels of spindle efferent activity but lower levels of integrated EMG as compared to maxima l concentric loading. Elbow flexor EMG was recorded from 17 subjects during concentric (CONC) and eccentric (ECC) elbow flexion at 70 degrees s(-1) us ing a Kin-Com dynamometer. We hypothesized that peak EMG amplitude would be more sensitive to fluctuations in facilitation by the spindle primary affe rents via the segmental stretch reflex pathway, and that the mean EMG would be more reflective of the ongoing level of muscle activation. A ratio of p eak to mean EMG (P/M EMG ratio) was predicted to be larger during maximal e ccentric loading than maximal concentric loading. The peak EMG (P<0.013) an d the P/M EMG ratio (P<0.001) were significantly greater during the ECC con dition than the CONC condition. In a subgroup of three subjects who underwe nt 3 weeks of eccentrically biased weight training, EMG, peak torque and to rque variability were assessed before and after training. P/M EMG ratio dec reased, while peak torque and torque variability increased following the tr aining. Differences in the PIM EMG ratio appear to reflect differences in t he way eccentric and concentric muscle actions are controlled and do not si mply represent less control during the eccentric task. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.