Reversal of long-latency reflexes in first dorsal interosseous during a force-tracking task

Cutaneous reflexes in response to electrical stimulation of the index finger were recorded from first dorsal interosseous (1DI) at specified force levels under three conditions: during a static posture and during the incremental and decremental phases of a sinusoidal, force-tracking task. Under static conditions, while 1DI generated a steady force [10, 20 or 30% maximum voluntary contraction (MVC)], four reflex components could be identified: E1, I1, E2 and I2. The amplitudes of these components were measured as the subjects tracked a 0.5 Hz sine wave by gradually changing the force output of 1DI between zero and 40% MVC. E1 and I1 showed minimal changes from the static condition, while activity during the E2 period was inhibitory during the incremental phase and excitatory during the decremental phase of tracking. Abrupt switching from inhibition to excitation during the E2 period occurred around the transition from incremental to decremental force. Activity during the I2 period was less markedly modulated than that during E2. Reflex reversal during the E2 interval may function to reduce the rate of change of force under conditions requiring precise force-control.