title: Spatial variation of compound muscle action potentials across human gastrocnemius medialis.
abstract: The massed action potential (M wave) elicited through nerve stimulation underpins a wide range of physiological and mechanical understanding of skeletal muscle structure and function. Although systematic approaches have evaluated the effect of different factors on M waves, the effect of the location and distribution of activated fibers within the muscle remains unknown. By detecting M waves from the medial gastrocnemius (MG) of 12 participants with a grid of 128 electrodes, we investigated whether different populations of muscle units have different spatial organization within MG. If populations of muscle units occupy discrete MG regions, current pulses of progressively greater intensities applied to the MG nerve branch would be expected to lead to local changes in M-wave amplitudes. Electrical pulses were therefore delivered at 2 pps, with the current pulse amplitude increased every 10 stimuli to elicit different degrees of muscle activation. The localization of MG response to increases in current intensity was determined from the spatial distribution of M-wave amplitude. Key results revealed that increases in M-wave amplitude were detected somewhat locally, by 10-50% of the 128 electrodes. Most importantly, the electrodes detecting greatest increases in M-wave amplitude were localized at different regions in the grid, with a tendency for greater stimulation intensities to elicit M waves in the more distal MG region. The presented results indicate that M waves recorded locally may not provide a representative MG response, with major implications for the estimation of, e.g., the maximal stimulation levels, the number of motor units, and the onset and normalization in H-reflex studies.
authors.affiliation: Laboratorio di Ingegneria del Sistema Neuromuscolare, Dipartimento di Elettronica e Telecomunicazioni, Politecnico di Torino, Turin, Italy; Escola de Educação Física e Desportos, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil;
authors.forename: Taian M
authors.identifier: http://orcid.org/0000-0002-6239-7301
authors.initials: TM
authors.lastname: Vieira
authors.affiliation: Laboratorio di Ingegneria del Sistema Neuromuscolare, Dipartimento di Elettronica e Telecomunicazioni, Politecnico di Torino, Turin, Italy; alberto.botter@polito.it.
authors.forename: Alberto
authors.initials: A
authors.lastname: Botter
authors.affiliation: Division of Endocrinology, Diabetology and Metabolism, Department of Medical Sciences, University of Turin, Turin, Italy; Division of Physical Medicine and Rehabilitation, Department of Surgical Sciences, University of Turin, Turin, Italy; and.
authors.forename: Marco A
authors.initials: MA
authors.lastname: Minetto
authors.affiliation: School of Healthcare Sciences, Manchester Metropolitan University, Manchester, United Kingdom.
authors.forename: Emma F
authors.initials: EF
authors.lastname: Hodson-Tole
mesh_terms: D000200:Action Potentials; D000328:Adult; D005260:Female; D006181:H-Reflex; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*
keywords: M wave; electrical stimulation; medial gastrocnemius; motor units; surface EMG