title: Age-related changes in motor unit firing pattern of vastus lateralis muscle during low-moderate contraction.
abstract: Age-related changes in motor unit activation properties remain unclear for locomotor muscles such as quadriceps muscles, although these muscles are preferentially atrophied with aging and play important roles in daily living movements. The present study investigated and compared detailed motor unit firing characteristics for the vastus lateralis muscle during isometric contraction at low to moderate force levels in the elderly and young. Fourteen healthy elderly men and 15 healthy young men performed isometric ramp-up contraction to 70 % of the maximal voluntary contractions (MVC) during knee extension. Multichannel surface electromyograms were recorded from the vastus lateralis muscle using a two-dimensional grid of 64 electrodes and decomposed with the convolution kernel compensation technique to extract individual motor units. Motor unit firing rates in the young were significantly higher (~+29.7 %) than in the elderly (p < 0.05). There were significant differences in firing rates among motor units with different recruitment thresholds at each force level in the young (p < 0.05) but not in the elderly (p > 0.05). Firing rates at 60 % of the MVC force level for the motor units recruited at <20 % of MVC were significantly correlated with MVC force in the elderly (r = 0.885, p < 0.0001) but not in the young (r = 0.127, p > 0.05). These results suggest that the motor unit firing rate in the vastus lateralis muscle is affected by aging and muscle strength in the elderly and/or age-related strength loss is related to motor unit firing/recruitment properties.
authors.affiliation: Laboratory of Neuromuscular Biomechanics, School of International Liberal Studies, Chukyo University, Yagotohonmachi, Showa-ku, Nagoya, 466-8666, Japan. wkohei@lets.chukyo-u.ac.jp.
authors.forename: Kohei
authors.initials: K
authors.lastname: Watanabe
authors.affiliation: Faculty of Electrical Engineering and Computer Science, University of Maribor, Maribor, Slovenia.
authors.forename: Aleš
authors.initials: A
authors.lastname: Holobar
authors.affiliation: Laboratory of Neurophysiology, Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan.
authors.forename: Motoki
authors.initials: M
authors.lastname: Kouzaki
authors.affiliation: Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya, Japan.
authors.forename: Madoka
authors.initials: M
authors.lastname: Ogawa
authors.affiliation: Research Center of Health, Physical Fitness and Sports, Nagoya University, Nagoya, Japan.|Graduate School of Education and Human Development, Nagoya University, Nagoya, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Akima
authors.affiliation: Laboratory of Applied Physiology, Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan.
authors.forename: Toshio
authors.initials: T
authors.lastname: Moritani
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D004576:Electromyography; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D018763:Muscle Fatigue / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D052097:Quadriceps Muscle / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology*
keywords: Aging; Multichannel surface electromyography; Quadriceps femoris muscles