Impact of carpal tunnel syndrome on multidigit synergies and anticipatory synergy adjustments

This study investigates the impact of Carpal Tunnel Syndrome (CTS) on multidigit synergies and anticipatory synergy adjustments (ASA) during isometric force production tasks, using the Uncontrolled Manifold (UCM) framework. CTS, characterized by sensory feedback and motor impairment due to compression of the median nerve at the hand's carpal tunnel, was hypothesized to: 1) reduce the synergy index (normalized difference between within-manifold variance and task-relevant variance) due to increased task-relevant variability (V_ORT, variance affecting task performance) while preserving within-manifold variability (V_UCM, variance that does not affect task performance), and 2) modify the fundamental structure of finger pressing synergies (V_UCM > V_ORT). Thirty-six participants (23 CTS, 13 Controls) performed a steady-state four-finger force production task followed by a quick force change. Results showed that individuals with CTS had a lower steady-state synergy index compared to Controls due to increased V_ORT while maintaining V_UCM, confirming weakened but still present force-stabilizing synergies. During the initiation of quick actions, CTS participants showed further reductions in the synergy index, with the inequality V_UCM > V_ORT lost at the onset of action, indicating compromised stability during quick force adjustments. However, ASA characteristics, including the timing and magnitude of synergy reduction, didn't significantly differ between CTS and Controls. This suggests that feedforward adjustments are preserved in CTS, although subtle impairments cannot be excluded. These results highlight the value of multidigit synergy analysis in assessing sensorimotor impairments in CTS and suggest that examining synergy structure can help identify and quantify motor deficits in peripheral neuropathies more effectively.