Impaired corticomuscular coherence during isometric elbow flexion contractions in humans with cervical spinal cord injury

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Abstract
After spinal cord injury (SCI), the reorganization of the neuromuscular system leads to increased antagonist muscles’ co-activation – i.e., increased antagonist vs. agonist muscles activation ratio – during voluntary contractions. Increased muscle co-activation is supposed to result from reduced cortical influences on spinal mechanisms inhibiting antagonist muscles. The assessment of the residual interactions between cortical and muscles activity with corticomuscular coherence (CMC) in participants with SCI producing different force levels may shed new lights on the regulation of muscle co-activation. To this aim, we compared the net joint torque, the muscle co-activation and the CMC ˜10 Hz and ˜20 Hz with both agonist and antagonist muscles in participants with SCI and healthy participants performing actual isometric elbow flexion contractions at 3 force levels. For all participants, overall CMC and muscle co-activation decreased with the increase of the net joint torque but only CMC ~10 Hz was correlated with muscle co-activation. Participants with SCI had greater muscle co-activation and lower CMC ~10 Hz, at the highest force levels. These results emphasize the importance of CMC as a mechanism that could take part in the modulation of muscle co-activation to maintain a specific force level. Lower CMC ~10 Hz in SCI participants may reflect the decreased cortical influence on spinal mechanisms leading to increased muscle co-activation. Though, plasticity of the corticomuscular coupling seems to be preserved after SCI to modulate the force level. Clinically, the CMC may efficiently evaluate the residual integrity of the neuromuscular system after SCI and the effects of rehabilitation.

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