POSTURE-DEPENDENT CORTICOMOTOR EXCITABILITY DIFFERS BETWEEN THE TRANSFERRED BICEPS IN INDIVIDUALS WITH TETRAPLEGIA AND THE BICEPS OF NONIMPAIRED INDIVIDUALS

Neurorehabil Neural Repair, 2017 · DOI: 10.1177/1545968316680488 · Published: April 1, 2017

Spinal Cord Injury Neurology Neurorehabilitation

Simple Explanation

Following biceps transfer to enable elbow extension in individuals with tetraplegia, motor re-education may be facilitated by greater corticomotor excitability. Arm posture modulates corticomotor excitability of the nonimpaired biceps. If arm posture also modulates excitability of the transferred biceps, posture may aid in motor re-education. The purpose of this study was to determine whether multi-joint arm posture affects corticomotor excitability of the biceps after SCI and tendon transfer similarly to the nonimpaired biceps (nonimpaired data previously reported by Mogk et al.4). Corticomotor excitability of arms with biceps transfer was assessed using TMS. We also aimed to determine whether corticomotor excitability of the transferred biceps is related to maximum isometric elbow extension strength and muscle length.

Study Duration

Not specified

Participants

Seven arms of individuals with tetraplegia and biceps transfer, and twelve arms of nonimpaired individuals

Evidence Level

Not specified

Key Findings

1
Arm posture modulated corticomotor excitability of the transferred biceps differently than the nonimpaired biceps.
2
Elbow extension strength was positively related and muscle length was unrelated, respectively, to MEP amplitude across the arms with biceps transfer.
3
Corticomotor excitability of the transferred biceps is modulated by arm posture and may contribute to strength outcomes after tendon transfer.

Research Summary

We investigated the effects of multi-joint upper limb posture on the excitability of surgically transferred biceps at rest in individuals with tetraplegia. Posture-dependent changes in TMS-evoked responses were taken as evidence of posture-related modulation of overall corticomotor excitability. Our hypothesis was supported; MEP amplitude was greater in the transferred biceps relative to the nonimpaired biceps in overhead reach regardless of forearm orientation. Thus, motor re-education of the transferred biceps may be facilitated in an overhead reach posture should a facilitated pathway make the transferred muscle easier to activate. Corticomotor excitability was positively related to elbow extension strength across the individuals with tetraplegia and biceps transfer. Modulating arm posture to increase corticomotor excitability may contribute to improve strength outcomes after tendon transfer, and may be beneficial for promoting motor re-education during the rehabilitative period.

Practical Implications

Motor Re-education

Motor re-education of the transferred biceps may be facilitated in an overhead reach posture.

Strength Outcomes

Modulating arm posture to increase corticomotor excitability may contribute to improved strength outcomes after tendon transfer.

Rehabilitative Strategies

Rehabilitative strategies to increase corticomotor excitability (e.g., repetitive TMS, EMG biofeedback, anodal transcranial direct stimulation) may be beneficial for elbow extension strength.

Study Limitations

1
MEP amplitude, resting threshold, and maximum isometric moment in elbow extension varied across subjects with biceps transfer, and also between arms within subjects with biceps transfer.
2
The lack of relationship between normalized MEP amplitude and normalized elbow moment within individual arms across postures suggests that maximum strength is primarily determined by other posture-dependent biomechanical factors.
3
We assessed corticomotor excitability with the muscle relaxed as a baseline measure of posture-dependent excitability after biceps transfer. Future studies to investigate relationships between corticomotor excitability and posture during voluntary activation are needed.

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