EMG activation patterns during force production in precision grip. I. Contribution of 15 finger muscles to isometric force.

Electromyographic (EMG) activity was examined in six normal subjects, producing low isometric forces between thumb and index finger in a visually guided step-tracking task. Target forces ranged between 0.5 and 3.0 N. EMG activity of all 15 muscles acting on thumb or index finger was screened with simultaneous recordings of up to 8 muscles. Linear regression was applied to quantify the EMG activity as a function of force. The intrinsic muscles and the long flexors of the index finger had a tight relation to force, as indicated by the high correlation coefficient, as did the adductor and short flexor of the thumb. In contrast, the long extensors of the index finger did not show force-related activity. The other muscles, including the long flexor and extensor of the thumb, had varying, on average moderate, correlations to force. The slope of the regression lines, a measure for the amount of EMG modulation with increasing force, revealed the same trends. Thus the majority of the intrinsic muscles were as closely related to force as the long flexors, suggesting a more important role in production of low isometric forces in the grip than previously believed, perhaps even a primary role. Systematic interindividual differences were rarely observed. Analysis of the trial-by-trial variability of EMG activity revealed that for most muscles the observed scatter was produced by varying background activity and was not a random fluctuation of relative increases in activity from one force level to the next.