Abstract
Finger–thumb coordination is crucial to manual dexterity but remains incompletely understood, particularly following neurological injury such as stroke. While being controlled independently, the index finger and thumb especially must work in concert to perform a variety of tasks requiring lateral or palmar pinch. The impact of stroke on this functionally critical sensorimotor control during dynamic tasks has been largely unexplored. In this study, we explored finger–thumb coupling during close–open pinching motions in stroke survivors with chronic hemiparesis. Two types of perturbations were applied randomly to the index with a novel Cable-Actuated Finger Exoskeleton: a sudden joint acceleration stretching muscle groups of the index finger and a sudden increase in impedance in selected index finger joint(s). Electromyographic signals for specific thumb and index finger muscles, thumb tip trajectory, and index finger joint angles were recorded during each trial. Joint angle perturbations invoked reflex responses in the flexor digitorum superficialis (FDS), first dorsal interossei (FDI), and extensor digitorum communis muscles of the index finger and heteronymous reflex responses in flexor pollicis brevis of the thumb (p < 0.017). Phase of movement played a role as a faster peak reflex response was observed in FDI during opening than during closing (p < 0.002) and direction of perturbations resulted in shorter reflex times for FDS and FDI (p < 0.012) for extension perturbations. Surprisingly, when index finger joint impedance was suddenly increased, thumb tip movement was substantially increased, from 2 to 10 cm (p < 0.001). A greater effect was seen during the opening phase (p < 0.044). Thus, involuntary finger–thumb coupling was present during dynamic movement, with perturbation of the index finger impacting thumb activity. The degree of coupling modulated with the phase of motion. These findings reveal a potential mechanism for direct intervention to improve poststroke hand mobility and provide insight on prospective neurologically oriented therapies.
Highlights
The dexterity of the digits of the hand is one of the hallmarks of human motor control and a central factor in the evolution of our species
The Cable-Actuated Finger Exoskeleton (CAFE) [12, 13] was employed to perturb joints of the index finger. This rigid exoskeleton structure has joints that are aligned with the flexion/ extension axis of each of the three index finger joints: metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP)
Stretch Reflex To find the shorter-latency reflex responses to the perturbations, we examined the EMG envelope during the 150-ms window following the onset of perturbation
Summary
The dexterity of the digits of the hand is one of the hallmarks of human motor control and a central factor in the evolution of our species. Motion [1] and force [2] independence are especially great in the index finger and thumb, the two most functionally important digits. Coordination between these digits is critical for proper execution of a number of important tasks. During object manipulation through pinch, for example, the thumb and index fingertip forces must create equal and oppositely directed forces to prevent slip of the object. Alteration in the force created by one digit, such as might arise due to perturbations or changing conditions such as sweat, requires immediate compensation by the other digit
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