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

Area of Science:

• Neuroscience
• Rehabilitation Medicine
• Biomechanics

Background:

• Stroke survivors often experience impaired hand function due to hemiparesis.
• Altered sensorimotor control, including abnormal muscle synergies, is a common challenge in post-stroke recovery.
• Understanding finger-thumb coupling is crucial for developing effective hand rehabilitation strategies.

Purpose of the Study:

• To investigate the nature of finger-thumb muscle coupling in individuals with chronic hemiparesis.
• To determine if passive muscle stretch or voluntary contractions reveal abnormal interactions between finger and thumb flexor muscles.
• To explore the potential sources and implications of altered finger-thumb coordination post-stroke.

Main Methods:

• Recruitment of stroke survivors with moderate/severe chronic hemiparesis and neurologically intact controls.
• Application of an external device to passively stretch finger flexor muscles via metacarpophalangeal (MCP) joint rotation.
• Measurement of isometric finger and thumb forces/torques and electromyographic (EMG) signals from relevant muscles during passive stretch and voluntary contractions.

Main Results:

• Stroke survivors demonstrated significant interactions between finger and thumb flexor muscles.
• Passive stretch of finger flexors evoked reflex activity in thumb flexors (heteronymous reflex) in stroke survivors.
• Voluntary finger flexion attempts resulted in increased, unintended thumb force production, and vice versa, showing a notable asymmetry.
• Coactivation of thumb and finger flexor muscles was frequently observed in stroke survivors, irrespective of activation method (passive stretch or voluntary).
• Intermuscular EMG coherence between finger and thumb flexors was low, suggesting the coupling is not primarily driven by direct common input at the level of the recorded motor units.

Conclusions:

• Altered finger-thumb flexor coupling is a characteristic impairment in chronic hemiparesis post-stroke.
• This coupling, potentially originating from subcortical or spinal pathways, leads to involuntary thumb muscle activation.
• The identified flexor coupling may negatively impact functional hand use and rehabilitation progress in stroke survivors.