Age-related changes in motor unit firing pattern of vastus lateralis muscle during low-moderate contraction
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.
KeywordsAging Multichannel surface electromyography Quadriceps femoris muscles
Maximal voluntary contraction
This work was supported by the Japanese Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP Project ID 14533567 Funding agency: Bio-oriented Technology Research Advancement Institution, NARO), by JSPS KAKENHI, a Grant-in-Aid for Young Scientists (B) (No. 26750309), and by the Slovenian Research Agency (No. L5-5550).
Compliance with ethical standards
The subjects in both groups gave written informed consent for the study after receiving a detailed explanation of the purposes, potential benefits, and risks associated with participation. All procedures used in this study were approved by the Research Ethics Committee of Chukyo University (2014–001) and were in accordance with the Declaration of Helsinki.
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