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

Evidence from human and animal studies suggests that motor neuron pool organization is not uniform for all motor tasks. Groupings of motor units within a muscle may be recruited differentially for a given task based on principles beyond anatomical or architectural features of the muscle alone. This study aimed to determine whether: (1) there was differential activation across locations of the first dorsal interosseous (FDI) muscle during a given task, (2) the differential activation was related to directional requirements and/or end goal of the task, and (3) there was an anatomical pattern to the differential activation. Twenty-six healthy right-handed participants carried out isometric finger/hand contractions in sitting while surface EMG was collected from 4 bipolar sites on the FDI muscle simultaneously. The tasks included: abduction, flexion, diagonal, 30% abduction + 30% flexion, 30% flexion + 30% abduction, key pinch, and power grasp. Mean peak integrated EMG for each task was normalized to site and task specific mean M waves. Differential activation was evident across FDI sites based on movement direction, order of directional components within a combination condition, and end goal of the task. There was greatest activation in the distal ulnar site for all tasks. Additionally there was a trend toward an ordering effect in the amount of activation at each site: distal ulnar > distal radial > proximal radial > proximal ulnar.