Predictive modulation of muscle coordination pattern magnitude scales fingertip force magnitude over the voluntary range

Predictive modulation of muscle coordination pattern magnitude scales finge rtip force magnitude over the voluntary range. J. Neurophysiol. 83: 1469-14 79, 2000. Human fingers have sufficiently more muscles than joints such tha t every fingertip force of submaximal magnitude can be produced by an infin ite number of muscle coordination patterns. Nevertheless, the nervous syste m seems to effortlessly select muscle coordination patterns when sequential ly producing fingertip forces of low, moderate, and maximal magnitude. The hypothesis of this study is that the selection of coordination patterns to produce submaximal forces is simplified by the appropriate modulation of th e magnitude of a muscle coordination pattern capable of producing the large st expected fingertip force. In each of three directions, eight subjects we re asked to sequentially produce fingertip forces of low, moderate, and max imal magnitude with their dominant forefinger. Muscle activity was describe d by fine-wire electromyograms (EMGs) simultaneously collected from all mus cles of the forefinger. A muscle coordination pattern was defined as the ve ctor list of the EMG activity of each muscle. For all force directions, sta tistically significant muscle coordination patterns similar to those previo usly reported for 100% of maximal fingertip forces were found for 50% of ma ximal voluntary force. Furthermore the coordination pattern and fingertip f orce vector magnitudes were highly correlated (r > 0.88). Average coordinat ion pattern vectors at 50 and 100% of maximal force were highly correlated with each other, as well as with individual coordination pattern vectors in the ramp transitions preceding them. In contrast to this consistency of EM G coordination patterns, predictions using a musculoskeletal computer model of the forefinger show that force magnitudes less than or equal to 50% of maximal fingertip force can be produced by coordination patterns drasticall y different from those needed for maximal force. Thus when modulating finge rtip force magnitude across the voluntary range, the number of contributing muscles and the relative activity among them was not changed. Rather, the production of low and moderate forces seems to be simplified by appropriate ly scaling the magnitude of a coordination pattern capable of producing the highest force expected.