Removing visual feedback for a single limb alters between-limb force tremor relationships during isometric bilateral contractions.

This study examined how force tremor and muscle activity are altered between limbs when a visual target is removed for one limb during bilateral index finger abduction. Isometric index finger abduction force was examined in healthy adults (23 ± 4 years) when both index fingers abducted simultaneously. Abduction forces ranged from 5 to 20% maximum voluntary contraction, and these target forces were displayed on a PC monitor in front of the subject. Force tremor and first dorsal interosseous (FDI) activity were first collected while subjects viewed visual targets for both index fingers and then when the visual target was removed for the non-dominant index finger. Subjects successfully matched the force amplitudes generated for both limbs regardless of visual condition. When the visual target was removed for one limb, force tremor increased in this limb (p < 0.01). Different power spectral profiles were evident for each FDI EMG when targets were available for both limbs (p < 0.05); however, when one target was removed, the pattern of FDI EMG for the limb without a visual target closely reflected FDI EMG for the limb which had the visual target. The CNS actively modulates muscle activity in each limb to perform visually guided isometric contractions. Given that the goal was to match force output with both limbs, the requirements of the task must be established from the limb that had a visual target, and a copy of those motor commands appears to have been sent to the FDI of the limb without a visual target.