The pattern of coupling dynamics between postural motion, isotonic hand movements and physiological tremor

• Performing rapid, alternating wrist movements affects postural motion.
• Tighter coupling of center of pressure motion (AP-ML) develops from rapid wrist movements.
• Physiological tremor in the upper limb is unrelated to that observed in the contralateral limb or to postural sway.
• Under postural tremor conditions, the body is able to effectively decouple oscillations within the upper limb from postural motion.

This study was designed to examine differences in the coupling dynamics between upper limb motion, physiological tremor and whole body postural sway in young healthy adults. Acceleration of the hand and fingers, forearm EMG activity and postural sway data were recorded. Estimation of the degree of bilateral and limb motion–postural sway coupling was determined by cross correlation, coherence and Cross-ApEn analyses. The results revealed that, under postural tremor conditions, there was no significant coupling between limbs, muscles or sway across all metrics of coupling. In contrast, performing a rapid alternating flexion/extension movement about the wrist joint (with one or both limbs) resulted in stronger coupling between limb motion and postural sway. These results support the view that, for physiological tremor responses, the control of postural sway is maintained independent to tremor in the upper limb. However, increasing the level of movement about a distal segment of one arm (or both) leads to increased coupling throughout the body. The basis for this increased coupling would appear to be related to the enhanced neural drive to task-specific muscles within the upper limb.