Finger - thumb coupling contributes to exaggerated thumb 1 flexion in stroke survivors

38 Objective: The purpose of this study was to investigate altered finger-thumb coupling in 39 individuals with chronic hemiparesis post-stroke. 40 Methods: First, an external device stretched finger flexor muscles by passively rotating the 41 metacarpophalangeal (MCP) joints. Subjects then performed isometric finger or thumb force 42 generation. Forces/torques and electromyographic (EMG) signals were recorded for both the 43 thumb and finger muscles. Stroke survivors with moderate (n=9) and severe (n=9) chronic hand 44 impairment participated, along with neurologically intact individuals (n=9). 45 Results: Stroke survivors exhibited strong interactions between finger and thumb flexors. The 46 stretch reflex evoked by stretch of the finger flexors of stroke survivors led to heteronymous 47 reflex activity in the thumb, while attempts to produce isolated voluntary finger MCP flexion 48 torque / thumb flexion force led to increased and undesired thumb force/ finger MCP torque 49 production post-stroke with a striking asymmetry between voluntary flexion and extension. 50 Coherence between the long finger and thumb flexors estimated using intermuscular EMG 51 correlations, however, was small. 52 Conclusions: Coactivation of thumb and finger flexor muscles was common in stroke survivors, 53 whether activation was evoked by passive stretch or voluntary activation. The coupling appears 54 to arise from subcortical or spinal sources. 55 Significance: Flexor coupling between the thumb and fingers seems to contribute to undesired 56 thumb flexor activity after stroke and may impact rehabilitation outcomes. 57