Vastus Lateralis Motor Unit Recruitment Thresholds are Compressed Towards Lower Forces in Older Men
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ORIGINAL RESEARCH
VASTUS LATERALIS MOTOR UNIT RECRUITMENT THRESHOLDS ARE COMPRESSED TOWARDS LOWER FORCES IN OLDER MEN R.M. GIRTS1, J.A. MOTA2, K.K. HARMON1, R.J. MACLENNAN1, M.S. STOCK1 1. Neuromuscular Plasticity Laboratory, School of Kinesiology and Physical Therapy, University of Central Florida, Orlando, FL, USA; 2. Department of Exercise and Sports Science, University of North Carolina at Chapel Hill, USA. Corresponding author: Matt S. Stock, Ph.D., School of Kinesiology & Physical Therapy, University of Central Florida, 12805 Pegasus Drive, HPA 1 - Room 258, Orlando, FL 328162205, Phone: (407) 823-0364, Fax: (407) 823-2596, E-mail: [email protected]
Abstract: Background: Aging results in adaptations which may affect the control of motor units. Objective: We sought to determine if younger and older men recruit motor units at similar force levels. Design: Cross-sectional, between-subjects design. Setting: Controlled laboratory setting. Participants: Twelve younger (age = 25 ± 3 years) and twelve older (age = 75 ± 8 years) men. Measurements: Participants performed isometric contractions of the dominant knee extensors at a force level corresponding to 50% maximal voluntary contraction (MVC). Bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis. A surface EMG signal decomposition algorithm was used to quantify the recruitment threshold of each motor unit, which was defined as the force level corresponding to the first firing. Recruitment thresholds were expressed in both relative (% MVC) and absolute (N) terms. To further understand age-related differences in motor unit control, we examined the mean firing rate versus recruitment threshold relationship at steady force. Results: MVC force was greater in younger men (p = 0.010, d = 1.15). Older men had lower median recruitment thresholds in both absolute (p = 0.005, d = 1.29) and relative (p = 0.001, d = 1.53) terms. The absolute recruitment threshold range was larger for younger men (p = 0.020; d = 1.02), though a smaller difference was noted in relative terms (p = 0.235, d = 0.50). These findings were complimented by a generally flatter slope (p = 0.070; d = 0.78) and lower y-intercept (p = 0.009; d = 1.17) of the mean firing rate versus recruitment threshold relationship in older men. Conclusion: Older men tend to recruit more motor units at lower force levels. We speculate that recruitment threshold compression may be a neural adaptation serving to compensate for lower motor unit firing rates and/or denervation and subsequent re-innervation in aged muscle. Key words: Motor unit, recruitment threshold, aging, electromyography. J Frailty Aging 2020;in press Published online April 16, 2020, http://dx.doi.org/10.14283/jfa.2020.19
Introduction
example, motor unit recruitment thresholds progressively decrease as a muscle fatigues as a compensatory mechanism to offset decline in twitch force amplitude (9). To further our understanding of the aging neuromuscular system, we set out t
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