Task-oriented EMG activity recorded from partitions in human lateral gastrocnemius muscle

Twenty normal human subjects (mean age: 25.9, range: 22-40) performed specific lower extremity functional tasks while integrated electromyographic (IEMG) activity was recorded from the lateral gastrocnemius (LG) muscle. The electrode placements used corresponded with sites known to have distinct patterns of innervation based upon previous anatomical microdissections. These sites were defined as proximal medial, proximal lateral, distal medial, and distal lateral. Myoelectric activity from each site was normalized against maximal voluntary efforts. Quantitative analyses of records made from these distinct sites, which we call 'partitions', revealed significant differences both within and across specific tasks. The distal lateral site showed greatest activity during a step-up task with the test leg in knee flexion while the proximal lateral site tended to show the least activity for most tasks. No consistent pattern was observed across all eight leg tasks at any one site thus suggesting that motor units contributing to total electromyographic (EMG) activity in each partition did so differentially. Spike triggered averages were obtained from motor units within partitions of these muscles. Little synchrony could be demonstrated between units and averaged electrical activity in other partitions. This observation implies some degree of electromyographic isolation. Collectively these data suggest that partitions within a human two-joint muscle display selectively different levels of muscle activity depending upon the motor task. While these observations require further explanation, they also indicate that a more comprehensive understanding of the organization of muscles and their outputs is a necessary prerequisite to the establishment of therapeutic regimens.