• Tetsuro Muraoka and Isaac Kurtzer.
• College of Economics, Nihon University, Tokyo, Japan; Department of Biomedical Sciences, New York Institute of Technology - College of Osteopathic Medicine, Old Westbury, New York, USA. Electronic address: muraoka.tetsurou@nihon-u.ac.jp.
• Neuroscience. 2020 Apr 1; 431: 115-127.

AbstractInter-limb reflexes play an important role in coordinating behaviors involving different limbs. Previous studies have demonstrated that human elbow muscles express an inter-limb stretch reflex at long-latency (50-100 ms), a timing consistent with a trans-cortical linkage. Here we probe for inter-limb stretch reflexes in the shoulder muscles of human participants. Unexpected torque pulses displaced one or both shoulders while participants adopted a steady posture against background torques. The results demonstrated inter-limb stretch reflexes occurring at short-latency for both shoulder extensors and flexors; the rapid timing (36-50 ms) must involve a spinal linkage for the two arms. Inter-limb stretch reflexes were also observed at long-latency yet they were opposite to the preceding short-latency; when the short-latency stretch reflex was excitatory then the long-latency stretch reflex was inhibitory and vice versa. Comparing the responses to contralateral arm displacement to those during simultaneous displacement of both arms revealed that inhibitory inter-limb stretch reflexes are independent of within-limb stretch reflexes, but that excitatory inter-limb stretch reflexes are suppressed by within-limb stretch reflexes. Our results provide the first demonstration of short-latency inter-limb stretch reflexes in the upper limb of humans and reveal interacting spinal circuits for within-limb and inter-limb stretch reflexes.Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.

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