Corticospinal contribution to arm muscle activity during human walking

• Published in: The Journal of physiology 2010-03, Vol.588 (6), p.967-979
• Main Authors: Barthelemy, Dorothy, Nielsen, Jens Bo
• Format: Article
• Language: English
• Subjects: Adult; Arm - innervation; Electromyography; Ergonomics; Evoked Potentials, Motor - physiology; Female; Gait - physiology; Humans; Male; Muscle Contraction - physiology; Muscle, Skeletal - innervation; Muscle, Skeletal - physiology; Nervous system; Neuroscience; Pyramidal Tracts - physiology; Transcranial Magnetic Stimulation; Walking; Walking - physiology
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2009.185520

When we walk, our arm muscles show rhythmic activity suggesting that the central nervous system contributes to the swing of the arms. The purpose of the present study was to investigate whether corticospinal drive plays a role in the control of arm muscle activity during human walking. Motor evoked potentials (MEPs) elicited in the posterior deltoid muscle (PD) by transcranial magnetic stimulation (TMS) were modulated during the gait cycle in parallel with changes in the background EMG activity. There was no significant difference in the size of the MEPs at a comparable level of background EMG during walking and during static PD contraction. Short latency intracortical inhibition (SICI; 2 ms interval) studied by paired‐pulse TMS was diminished during bursts of PD EMG activity. This could not be explained only by changes in background EMG activity and/or control MEP size, since SICI showed no correlation to the level of background EMG activity during static PD contraction. Finally, TMS at intensity below the threshold for activation of corticospinal tract fibres elicited a suppression of the PD EMG activity during walking. Since TMS at this intensity is likely to only activate intracortical inhibitory interneurones, the suppression is in all likelihood caused by removal of a corticospinal contribution to the ongoing EMG activity. The data thus suggest that the motor cortex makes an active contribution, through the corticospinal tract, to the ongoing EMG activity in arm muscles during walking. During normal human walking, the arms are moving rhythmically out of phase with the corresponding leg. Recent evidence has suggested that neural circuitries in the spinal cord may be responsible for this rhythmic activity. In this paper, we show that the motor cortex also makes an active contribution to the ongoing activity of the arm muscles during walking. These results increase our understanding of the mechanisms that control locomotion in healthy people, and are essential in order to assess the changes that occur after a lesion to the central nervous system.