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 electromyog raphic (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 anatomica l microdissections. These sites were defined as proximal medial, proxi mal lateral, distal medial, and distal lateral. Myoelectric activity f rom each site was normalized against maximal voluntary efforts. Quanti tative analyses of records made from these distinct sites, which we ca ll 'partitions', revealed significant differences both within and acro ss specific tasks. The distal lateral site showed greatest activity du ring a step-up task with the test leg in knee flexion while the proxim al lateral site tended to show the least activity for most tasks. No c onsistent pattern was observed across all eight leg tasks at any one s ite thus suggesting that motor units contributing to total electromyog raphic (EMG) activity in each partition did so differentially. Spike t riggered averages were obtained from motor units within partitions of these muscles. Little synchrony could be demonstrated between units an d averaged electrical activity in other partitions. This observation i mplies some degree of electromyographic isolation. Collectively these data suggest that partitions within a human two-joint muscle display s electively different levels of muscle activity depending upon the moto r task. While these observations require further explanation, they als o indicate that a more comprehensive understanding of the organization of muscles and their outputs is a necessary prerequisite to the estab lishment of therapeutic regimens.