Influence of afferent feedback on isometric fine force resolution in humans.

The influence of afferent feedback on isometric fine force resolution was studied in humans. Subjects performed the smallest possible isometric flexion force increments with the index finger while visual, cutaneous, and muscle spindle feedback conditions were varied. In the control conditions with visual feedback, isometric force resolution was finest and independent of cutaneous or muscle spindle feedback. In the absence of visual cues, force resolution was significantly coarser. When agonist muscle spindles were vibrated (100 Hz and 150 Hz), fine force resolution capabilities declined further. Diminution of cutaneous feedback per se did not affect fine force resolution. However, the effect of agonist vibration was attenuated when full cutaneous feedback was available. We conclude that in voluntary isometric contractions the degree of fine force resolution depends on the type of afferent feedback available for calibrating central motor commands. Visual feedback is more powerful than spindle feedback, which is more efficient than cutaneous feedback. The extent to which the central motor command itself contributes to the sensation of force is indirectly implied by reproducible, yet coarser force resolution levels when peripheral information is minimized.