Changes in muscle recruitment patterns during skill acquisition

The control of movement is predicated upon a system of constraints of muscu loskeletal and neural origin. The focus of the present study was upon the m anner in which such constraints are adapted or superseded during the acquis ition of motor skill. Individuals participated in five experimental session s, ill which they attempted to produce abduction-adduction movements of the index finger in time with an auditory metronome. During each trial, the me tronome frequency was increased in eight steps from an individually determi ned base frequency. Electromyographic (EMC) activity was recorded from firs t dorsal interosseous (FDI), first volar interosseous (FVI), flexor digitor um superficialis (FDS), and extensor digitorum communis (EDC) muscles. The movements produced on the final day of acquisition more accurately matched the required profile, and exhibited greater spatial and temporal stability, than those generated during initial performance. Tn the early stages of sk ill acquisition, an alternating pattern of activation in FDI and FVI was ma intained, even at the highest frequencies. Tn contrast, as the frequency of movement was increased, activity in FDS and EDC was either tonic or interm ittent. As learning proceeded, alterations in recruitment patterns were exp ressed primarily in the extrinsic muscles (EDC and FDS). These changes took the form of increases in the postural role of these muscles, shifts to pha sic patterns of activation, or selective disengagement of these muscles. Th ese findings suggest that there is considerable flexibility in the composit ion of muscle synergies, which is exploited by individuals during the acqui sition of coordination.