Distinct Motor Map Characteristics for Biceps and Triceps Muscles in Persons with Chronic Tetraplegia: Implications for Arm Function

Abstract

Following spinal cord injury (SCI), intact neural resources undergo widespread reorganization within the brain. Animal models reveal motor cortical representations devoted to spared muscles above injury expand at the expense of territories occupied by weaker muscles. In this study, we investigated whether motor representations are similarly reorganized between a relatively spared biceps muscle and a weakened triceps muscle in persons with chronic tetraplegia following traumatic cervical SCI in association with upper limb motor function. Twenty-four adults with cervical SCI and 15 able-bodied participants underwent motor mapping using transcranial magnetic stimulation. We determined following map characteristics: area, amplitude (maximal motor evoked potential and volume), and center of gravity. Maximal voluntary contraction (MVC) and motor function (Capabilities of the Upper Extremity Test or CUE-T) were also assessed. Findings reveal that participants with SCI had hyper-excitable biceps maps than triceps, and hyper-excitable biceps maps also compared to biceps maps in able-bodied participants. Higher amplitude of biceps and triceps maps was associated with better motor function (higher CUE-T) and more distal injury (i.e., more spared segments) in persons with SCI. Amplitudes of biceps but not the triceps maps were associated with higher muscle MVCs. In conclusion, over-excitable biceps than triceps map in SCI may represent deafferentation plasticity. For the first time, we demonstrate how map reorganization of spared and weaker muscles in persons with chronic cervical SCI is associated with upper limb motor status. Use-dependent mechanisms may shift neural balance in favor of spared muscles, supporting potential use as response biomarkers in rehabilitation studies. New & NoteworthyOur study reports evidence in humans with cervical SCI that motor representation for the relatively spared muscle becomes hyper-excitable compared to that for the weaker muscle to the extent that hyper-excitability is even higher compared to biceps maps in uninjured individuals. Use-dependent mechanisms likely favor such heightened excitability of spared maps. For the first time, we demonstrate clinical relevance of map excitability in humans with SCI, supporting potential use as a biomarker of recovery.