INTRODUCTION: It is still unclear how central commands and afferent feedback interact to produce finely graded forces. To explore this question, we experimentally degraded afferent responsiveness using wrist vibration. METHODS: Subjects grasped an isometric joystick with their preferred hand and produced forces of different magnitudes and directions according to visually presented vectors. In one condition, the dorsal and palmar sides of the wrist were vibrated at 80 Hz to degrade proprioception; in another condition, the wrist was not vibrated. RESULTS: Response magnitude averaged 2.46 kg of force (kgf) without, and decreased to 1.90 kgf with vibration. The coefficient of magnitude variation was vibration-independent and averaged 0.30 kgf. The directional error of responses and its standard deviation were also vibration-independent, averaging -2.6 degrees and +/-17 degrees, respectively. Subjects' maximum voluntary force was 9.77 kgf without, and decreased to 8.83 kgf with vibration. CONCLUSION: Our findings clearly indicate that afferent feedback plays a role in the production of finely graded forces, probably by facilitating descending motor commands.
INTRODUCTION: It is still unclear how central commands and afferent feedback interact to produce finely graded forces. To explore this question, we experimentally degraded afferent responsiveness using wrist vibration. METHODS: Subjects grasped an isometric joystick with their preferred hand and produced forces of different magnitudes and directions according to visually presented vectors. In one condition, the dorsal and palmar sides of the wrist were vibrated at 80 Hz to degrade proprioception; in another condition, the wrist was not vibrated. RESULTS: Response magnitude averaged 2.46 kg of force (kgf) without, and decreased to 1.90 kgf with vibration. The coefficient of magnitude variation was vibration-independent and averaged 0.30 kgf. The directional error of responses and its standard deviation were also vibration-independent, averaging -2.6 degrees and +/-17 degrees, respectively. Subjects' maximum voluntary force was 9.77 kgf without, and decreased to 8.83 kgf with vibration. CONCLUSION: Our findings clearly indicate that afferent feedback plays a role in the production of finely graded forces, probably by facilitating descending motor commands.