Relationship between force and electromyographic activity during rapid isometric contraction in power grip.

Force response and surface electromyographic (EMG) activity of extrinsic extensors and flexors of the hand were measured under 6 target force conditions during rapid pulse isometric contractions (power grip) targeted using an oscilloscope display of exerted and target forces. For target forces ranging from 16.7% to 50% of maximum voluntary contraction (MVC), the rate of force rise increased with the peak force, while the time to peak force remained almost constant. However, at target forces between 66.7% and 100.0% MVC, the rate of force rise leveled off and the time to peak force was prolonged. In association with these changes in force trajectories, modulation of the EMG activity of the flexor digitorum superficialis muscle was observed. At the lowest target force (16.7 MVC), the EMG of this muscle showed a single initial activity; the activity increased linearly up to the 50% MVC target force, while the duration was relatively constant. However, at target forces above 50% MVC, no further increase of the initial activity was observed, while the amplitude and duration of an additional activity progressively increased. These results indicate that the neural control of rapid isometric contraction at target forces at and below 50% MVC differs from that operating at larger target force levels.