Task-related variations in the surface EMG of human first dorsal interosseous.

Abstract

Results from animal and human studies question the traditional view of a homogeneous organization of the motorneuron pool. Single muscles may be organized topographically into task groups that correspond with an intraspinal somatotopic organization. The aims of this study were to determine if: i.) there was differential activation in different locations of the first dorsal interosseous (FDI) muscle during a given task, ii.) the differential activation related to directional requirements and/or end goal of the task, and iii.) there was an anatomical pattern to the differential activation. Twenty-six healthy right-handed participants carried out 48 isometric finger/hand contractions [8 tasks x (3 M waves + 3 active contractions)] in sitting while surface EMG was collected from 4 bipolar sites on the FDI muscle simultaneously. Index finger abduction and flexion forces were collected using 2 orthogonally placed load cells. The tasks were: abduction pre, flexion, diagonal, 30% abduction + 30% flexion, 30% flexion + 30% abduction, pinch, power, and abduction post. Mean peak integrated EMG (IEMG; smoothed over 100ms and integrated over contraction period) for each task was normalized to site and task specific mean M waves. We found differential IEMG across sites for all tasks, which further differed based on task direction and end goal. The anatomical pattern of the differential IEMG was such that there was always greatest activation in the distal ulnar site. We conclude that there are task-related variations in activation across locations of the human FDI muscle. The organization of the nervous system at the level of the muscle is not necessarily an "all-or-none" phenomenon.