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Vastus Lateralis Motor Unit Recruitment Thresholds are Compressed Towards Lower Forces in Older Men

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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.

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References

  1. Tomlinson, B., & Irving, D. The numbers of limb motor neurons in the human lumbosacral cord throughout life. Journal of the Neurological Sciences, 1977;34(2), 213–219.

    Article  CAS  Google Scholar 

  2. Jones, D. A., & Peters, T. J. Caring for elderly dependants: Effects on the carers’ quality of life. Age and Ageing, 1992;21(6), 421–428.

    Article  CAS  Google Scholar 

  3. Vespa, J., Armstrong, D. M., & Medina, L. Demographic turning points for the United States: Population projections for 2020 to 2060 US Department of Commerce, Economics and Statistics Administration, US Census Bureau, 2018.

  4. Kamen, G., & Knight, C. A. Training-related adaptations in motor unit discharge rate in young and older adults. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 2004;59(12), 1334–1338.

    Article  Google Scholar 

  5. Clark, B., & Taylor, J. Age-Related Changes in Motor Cortical Properties and Voluntary Activation of Skeletal Muscle. Current Aging Science, 2011;4(3), 192–199.

    Article  Google Scholar 

  6. Clark, B. C., Taylor, J. L., Hong, S. L., Law, T. D., & Russ, D. W. Weaker seniors exhibit motor cortex hypoexcitability and impairments in voluntary activation. Journals of Gerontology Series A: Biomedical Sciences and Medical Sciences, 2015;70(9), 1112–1119.

    Article  CAS  Google Scholar 

  7. Doherty, T. J., Vandervoort, A. A., Taylor, A. W., & Brown, W. F. Effects of motor unit losses on strength in older men and women. Journal of Applied Physiology (Bethesda, Md.: 1985), 199374(2), 868–874.

    Article  Google Scholar 

  8. Doherty, T. J., Vandervoort, A. A., & Brown, W. F. Effects of ageing on the motor unit: A brief review. Canadian Journal of Applied Physiology, 1993;18(4), 331–358.

    Article  CAS  Google Scholar 

  9. Contessa, P., De Luca, C. J., & Kline, J. C. The compensatory interaction between motor unit firing behavior and muscle force during fatigue. Journal of Neurophysiology, 2016;116(4), 1579–1585.

    Article  Google Scholar 

  10. Nawab, S. H., Chang, S., & De Luca, C. J. High-yield decomposition of surface EMG signals. Clinical Neurophysiology, 2010; 121(10), 1602–1615.

    Article  Google Scholar 

  11. Lanza, I. R., Larsen, R. G., & Kent-Braun, J. A. Effects of old age on human skeletal muscle energetics during fatiguing contractions with and without blood flow. The Journal of Physiology, 2007;583(3), 1093–1105.

    Article  CAS  Google Scholar 

  12. Zaheer, F., Roy, S. H., & De Luca, C. J. Preferred sensor sites for surface EMG signal decomposition. Physiological Measurement, 2012;33(2), 195.

    Article  Google Scholar 

  13. De Luca, C. J., Adam, A., Wotiz, R., Gilmore, L. D., & Nawab, S. H. Decomposition of surface EMG signals. Journal of Neurophysiology, 2006;96(3), 1646–1657.

    Article  Google Scholar 

  14. Cohen, J. Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale, NJ, USA: Lawrence Earlbaum Associates, 1988.

    Google Scholar 

  15. Brooks, S. V., & Faulkner, J. A. Skeletal muscle weakness in old age: Underlying mechanisms. Medicine and Science in Sports and Exercise, 1994;26(4), 432–439.

    Article  CAS  Google Scholar 

  16. Lexell, J., Henriksson-Larsén, K., Winblad, B., & Sjöström, M. Distribution of different fiber types in human skeletal muscles: Effects of aging studied in whole muscle cross sections. Muscle & Nerve: Official Journal of the American Association of Electrodiagnostic Medicine, 1983;6(8), 588–595.

    Article  CAS  Google Scholar 

  17. Lexell, J., Taylor, C. C., & Sjöström, M. What is the cause of the ageing atrophy?: Total number, size and proportion of different fiber types studied in whole vastus lateralis muscle from 15-to 83-year-old men. Journal of the Neurological Sciences, 1988;84(2-3), 275–294.

    Article  CAS  Google Scholar 

  18. Larsson, L. Histochemical characteristics of human skeletal muscle during aging. Acta Physiologica Scandinavica, 1983;117(3), 469–471.

    Article  CAS  Google Scholar 

  19. Lexell, J., Downham, D., & Sjöström, M. Distribution of different fibre types in human skeletal muscles: Fibre type arrangement in m. vastus lateralis from three groups of healthy men between 15 and 83 years. Journal of the Neurological Sciences, 1986;72(2-3), 211–222.

    Article  CAS  Google Scholar 

  20. Deschenes, M. R. Effects of aging on muscle fibre type and size. Sports Medicine, 2004;34(12), 809–824.

    Article  Google Scholar 

  21. Erim, Z., Beg, M. F., Burke, D. T., & de Luca, C. J. Effects of aging on motor-unit control properties. Journal of Neurophysiology, 1999;82(5), 2081–2091.

    Article  CAS  Google Scholar 

  22. Fling, B. W., Knight, C. A., & Kamen, G. Relationships between motor unit size and recruitment threshold in older adults: Implications for size principle. Experimental Brain Research, 2009; 197(2), 125–133.

    Article  Google Scholar 

  23. Kamen, G., & De Luca, C. J. Unusual motor unit firing behavior in older adults. Brain Research, 1989;482(1), 136–140.

    Article  CAS  Google Scholar 

  24. Watanabe, K., Holobar, A., Kouzaki, M., Ogawa, M., Akima, H., & Moritani, T. Age-related changes in motor unit firing pattern of vastus lateralis muscle during low-moderate contraction. Age, 2016;38(3), 48.

    Article  Google Scholar 

  25. Johnson, M., Polgar, J., Weightman, D., & Appleton, D. Data on the distribution of fibre types in thirty-six human muscles: An autopsy study. Journal of the Neurological Sciences, 1973;18(1), 111–129.

    Article  CAS  Google Scholar 

  26. Gabriel, D., & Kamen, G. Essentials of Electromyography Champaign, IL: Human Kinetics, 2010.

    Google Scholar 

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Authors and Affiliations

  1. Neuromuscular Plasticity Laboratory, School of Kinesiology and Physical Therapy, University of Central Florida, 12805 Pegasus Drive, HPA 1 — Room 258, Orlando, FL, USA

    R. M. Girts, K. K. Harmon, R. J. MacLennan & Matt S. Stock

  2. Department of Exercise and Sports Science, University of North Carolina at Chapel Hill, Chapel Hill, USA

    J. A. Mota

Authors
  1. R. M. Girts
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  2. J. A. Mota
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  3. K. K. Harmon
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  4. R. J. MacLennan
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  5. Matt S. Stock
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Corresponding author

Correspondence to Matt S. Stock.

Ethics declarations

This study was carried out in accordance with the ethical standards of the university Institutional Review Board.

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Conflict of interest

The authors declare no conflicts of interest.

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Girts, R.M., Mota, J.A., Harmon, K.K. et al. Vastus Lateralis Motor Unit Recruitment Thresholds are Compressed Towards Lower Forces in Older Men. J Frailty Aging 9, 191–196 (2020). https://doi.org/10.14283/jfa.2020.19

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  • DOI: https://doi.org/10.14283/jfa.2020.19

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