Vastus Lateralis Motor Unit Recruitment Thresholds in Younger Versus Older Men

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.

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Circulating testosterone and dehydroepiandrosterone are associated with individual motor unit features in untrained and highly active older men

GeroScience ◽

10.1007/s11357-021-00482-3 ◽

2021 ◽

Author(s):

Yuxiao Guo ◽

Jessica Piasecki ◽

Agnieszka Swiecicka ◽

Alex Ireland ◽

Bethan E. Phillips ◽

...

Keyword(s):

Firing Rate ◽

Motor Unit ◽

Motor Nerve ◽

Vastus Lateralis ◽

Older Men ◽

Total Testosterone ◽

Motor Unit Potential ◽

Hormone Levels ◽

Athletic Status ◽

Age Related

AbstractLong-term exercise training has been considered as an effective strategy to counteract age-related hormonal declines and minimise muscle atrophy. However, human data relating circulating hormone levels with motor nerve function are scant. The aims of the study were to explore associations between circulating sex hormone levels and motor unit (MU) characteristics in older men, including masters athletes competing in endurance and power events. Forty-three older men (mean ± SD age: 69.9 ± 4.6 years) were studied based on competitive status. The serum concentrations of dehydroepiandrosterone (DHEA), total testosterone (T) and estradiol were quantified using liquid chromatography mass spectrometry. Intramuscular electromyographic signals were recorded from vastus lateralis (VL) during 25% of maximum voluntary isometric contractions and processed to extract MU firing rate (FR), and motor unit potential (MUP) features. After adjusting for athletic status, MU FR was positively associated with DHEA levels (p = 0.019). Higher testosterone and estradiol were associated with lower MUP complexity; these relationships remained significant after adjusting for athletic status (p = 0.006 and p = 0.019, respectively). Circulating DHEA was positively associated with MU firing rate in these older men. Higher testosterone levels were associated with reduced MUP complexity, indicating reduced electrophysiological temporal dispersion, which is related to decreased differences in conduction times along axonal branches and/or MU fibres. Although evident in males only, this work highlights the potential of hormone administration as a therapeutic interventional strategy specifically targeting human motor units in older age.

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Motor unit recruitment during vertebrate locomotion

Animal Biology ◽

10.1163/1570756053276880 ◽

2005 ◽

Vol 55 (1) ◽

pp. 41-58 ◽

Cited By ~ 12

Author(s):

James Wakeling ◽

Antra Rozitis

Keyword(s):

Motor Unit ◽

Animal Movement ◽

Vastus Lateralis ◽

Motor Units ◽

Mechanical Efficiency ◽

Motor Unit Recruitment ◽

Glycogen Depletion ◽

Mechanical Power Output ◽

Gastrocnemius Muscles ◽

Force Activation

AbstractVertebrate skeletal muscles act across joints and produce segmental accelerations and therefore animal movement when they contract. Different muscles and different motor units vary in their mechanical contractile properties. Early studies on motor unit recruitment demonstrated orderly recruitment of motor units from the slowest to the fastest during a graded contraction. However, many subsequent studies illustrate conditions when alternative recruitment strategies may exist. Motor unit recruitment during locomotion is thus multifactorial and more complex than typically thought.Different types of motor unit vary in their mechanical properties, including rates of force activation and deactivation, maximum unloaded shortening velocities and the shortening velocities at which maximum mechanical power output and maximum mechanical efficiency occur. In short, it would make mechanical sense to perform fast activities with the faster motor units and slow activities with the slower motor units. However, determining patterns of motor unit recruitment during locomotion has presented experimental challenges.Comparisons between distinct muscles have shown that fast fish swimming and the cat paw shake are activities which employ predominantly the fast and not the slower muscle. Glycogen depletion studies have showed that jumping in the bushbaby uses fast without slow motor units within the vastus lateralis and gastrocnemius muscles. Studies in man show that differential recruitment of the different types of muscle fibre occurs at different times within each running stride. It is suggested that vertebrates may have a strategy of recruiting the motor units that are most mechanically suited for the different locomotor demands. However, we have much to learn about motor recruitment patterns during locomotion.

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Influence of Sex and Cross-Sectional Area on Motor Unit Recruitment Patterns of the Vastus Lateralis

Medicine & Science in Sports & Exercise ◽

10.1249/01.mss.0000536955.20554.9f ◽

2018 ◽

Vol 50 (5S) ◽

pp. 566-567

Author(s):

Michael A. Trevino ◽

Trent Herda ◽

Jonathan Miller ◽

Adam Sterczala ◽

Hannah Dimmick

Keyword(s):

Motor Unit ◽

Vastus Lateralis ◽

Cross Sectional Area ◽

Motor Unit Recruitment ◽

Sectional Area ◽

Cross Sectional ◽

Recruitment Patterns

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Endurance training alters motor unit activation strategies for the vastus lateralis, yet sex-related differences and relationships with muscle size remain

European Journal of Applied Physiology ◽

10.1007/s00421-021-04622-7 ◽

2021 ◽

Vol 121 (5) ◽

pp. 1367-1377

Author(s):

Stephanie A. Sontag ◽

Michael A. Trevino ◽

Trent J. Herda ◽

Adam J. Sterczala ◽

Jonathan D. Miller ◽

...

Keyword(s):

Endurance Training ◽

Motor Unit ◽

Vastus Lateralis ◽

Muscle Size ◽

Motor Unit Activation

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Statistically rigorous calculations do not support common input and long-term synchronization of motor-unit firings

Journal of Neurophysiology ◽

10.1152/jn.00725.2013 ◽

2014 ◽

Vol 112 (11) ◽

pp. 2729-2744 ◽

Cited By ~ 11

Author(s):

Carlo J. De Luca ◽

Joshua C. Kline

Keyword(s):

Motor Unit ◽

Statistical Tests ◽

Vastus Lateralis ◽

Common Input ◽

Data Set ◽

Order Of Magnitude ◽

Score Method ◽

Surface Electromyographic Signal ◽

Measure Synchronization

Over the past four decades, various methods have been implemented to measure synchronization of motor-unit firings. In this work, we provide evidence that prior reports of the existence of universal common inputs to all motoneurons and the presence of long-term synchronization are misleading, because they did not use sufficiently rigorous statistical tests to detect synchronization. We developed a statistically based method (SigMax) for computing synchronization and tested it with data from 17,736 motor-unit pairs containing 1,035,225 firing instances from the first dorsal interosseous and vastus lateralis muscles—a data set one order of magnitude greater than that reported in previous studies. Only firing data, obtained from surface electromyographic signal decomposition with >95% accuracy, were used in the study. The data were not subjectively selected in any manner. Because of the size of our data set and the statistical rigor inherent to SigMax, we have confidence that the synchronization values that we calculated provide an improved estimate of physiologically driven synchronization. Compared with three other commonly used techniques, ours revealed three types of discrepancies that result from failing to use sufficient statistical tests necessary to detect synchronization. 1) On average, the z-score method falsely detected synchronization at 16 separate latencies in each motor-unit pair. 2) The cumulative sum method missed one out of every four synchronization identifications found by SigMax. 3) The common input assumption method identified synchronization from 100% of motor-unit pairs studied. SigMax revealed that only 50% of motor-unit pairs actually manifested synchronization.

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