STUDY DESIGN: Motor-evoked responses to transcranial magnetic stimulation of the motor cortex were recorded from erector spinae and deltoid muscles while the arm was abducted voluntarily in 10 normal subjects. OBJECTIVE: To understand the neuronal substrate for the activation of the contralateral erector spinae muscle when the opposite arm is abducted. BACKGROUND DATA: When a standing individual abducts an arm, the center of gravity is altered; to avoid falling, trunk muscles become activated on the contralateral side. METHODS: Surface EMG activity was recorded from the right deltoid and left and right erector spinae muscles. Subjects maintained abduction of their right arm to 90 degrees at five different levels of isometric force in standing and lying postures. Transcranial magnetic stimulation was delivered to the motor cortex, producing motor-evoked responses in the three muscles during arm abduction and while relaxed. RESULTS: EMG activity in the left erector spinae increased with the force of right arm abduction in both postures. EMG levels in right erector spinae showed no consistent change with right arm abduction force. As arm abduction force was increased, motor-evoked responses were facilitated in deltoid and the left erector spinae but not the right erector spinae in both postures. The pattern of motor-evoked potential facilitation with arm abduction force tended to plateau between 50% and 70% maximum voluntary contraction in the deltoid, whereas it continued to climb more linearly in the left erector spinae. CONCLUSIONS: Facilitation of erector spinae with arm abduction remains evident in the lying posture when spinal postural stabilization mechanisms are presumably reduced. Similar facilitation profiles have been seen previously with changing voluntary activation of erector spinae in a trunk extension task, supporting the notion that during arm abduction the drive to the contralateral erector spinae has a corticospinal origin.
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