Elsevier

Journal of Biomechanics

Volume 45, Issue 3, 2 February 2012, Pages 427-433
Journal of Biomechanics

Review
Muscle fatigue – from motor units to clinical symptoms

https://doi.org/10.1016/j.jbiomech.2011.11.047Get rights and content

Abstract

Reductionist approaches have provided little insight on the fatigue experienced by humans during activities of daily living. Some of the reasons for this lack of progress include the persistence of outdated concepts, the misinterpretation of experimental recordings, and a failure to embrace a global perspective on fatigue. This paper summarizes the three examples of these limitations that were discussed in the 2011 Muybridge Award lecture: motor unit types and muscle fatigue, myoelectric manifestations of fatigue, and fatigue and fatigability. Although the motor units in a population do exhibit a range of fatigability values, there are not distinct groups of motor units and the concept that some motor units are resistant to fatigue emerged from protocols in which motor units were activated by electrical stimulation rather than voluntary activation. The concept of distinct motor unit types should be abandoned. The second example discussed in the lecture was the use of surface EMG signals to assess fatigue-related adjustments in motor unit activity. The critical assumption with this approach is that the association between surface EMG amplitude and muscle force remains constant during fatiguing contractions. Unfortunately, the relation does not remain constant and a series of computational studies demonstrate the magnitude of the discrepancy, including the absence of an association with the activation signal emerging from the spinal cord and that received by the muscle. The third example concerned the concepts of fatigue and fatigability. It has long been recognized that fatigue involves both sensations and impairments in motor function, and the final part of the lecture urged the integration of the two constructs into a single scheme in which fatigue can be modulated either independently or by interactions between perceptions of fatigue and the mechanisms that establish levels of fatigability. The expectation is that such critical evaluations of the concepts and approaches to the study of fatigue will provide a more effective foundation from which to identify the factors that contribute to fatigue in health and disease.

Introduction

On the occasion of the 2011 Congress for the International Society of Biomechanics held in Brussels, I was recognized for career achievements in biomechanics and honored with the prestigious Muybridge Award. In recognition of the honor, I dutifully began the Muybridge Lecture by acknowledging the contributions to the field of biomechanics of the pioneer after whom the award was named, Eadward James Muybridge (1830–1904). The Society describes Muybridge as the “father of cinematography”. Although he did accumulate 20,000 images of animal and human locomotion over an 8yr period, he was more interested in the representation of motion from an artistic perspective than in analyzing its attributes. Rather, it was a contemporary of his who had the same initials and was born and died in the same years who contributed more than Muybridge to the scientific study of human motion. His name was Etienne-Jules Marey (1830–1904). The substantial contributions to the study of human motion by the venerable professor at the Collége de France and member of the Academy of Sciences are described in an excellent biography written by Marta Braun (1992). Marey and Muybridge were aware of each other's work, and their relatively brief interaction is described in the proceedings of a conference to celebrate the 100 yr anniversary of the beginning of cinema (Delimata, 1996). Accordingly, the lecture was dedicated to EJM (1830–1904).

The purpose of the lecture was not to provide an historical account of key developments in biomechanics, but rather to discuss the difficulties associated with changing the ideas that define a contemporary issue. The topic chosen for the occasion was muscle fatigue, and the approach was to proceed from ideas on muscle fatigue that should be discarded through to challenges for the future. This was accomplished by discussing motor unit types and fatigue, myoelectric manifestations of fatigue, and the distinction between fatigue and fatigability.

Section snippets

Motor unit types and fatigue

The motor unit denotes the basic functional element of the central nervous system and muscle that produces movement. It comprises a motor neuron in the ventral horn of the spinal cord, its axon, and the muscle fibers that the axon innervates (Duchteau and Enoka, 2011, Sherrington, 1925). The central nervous system controls muscle force by varying the activity of the motor units in the muscle. The force exerted by each motor unit depends principally on the number of muscle fibers that are

Myoelectric manifestations of fatigue

Voluntary contractions arise from the activation of a motor neuron pool by synaptic inputs that are provided by descending pathways, spinal interneurons, and peripheral afferent feedback. After these inputs have been integrated by the motor neurons, the activation signal sent from the spinal cord to muscle comprises trains of action potentials for the motor units that have been recruited for the action. The net motor unit activity, therefore, is reflected in the magnitude of the activation

Fatigue and fatigability

The fatigability of muscle is classically quantified as the decline in MVC force after performing some form of demanding physical activity (Enoka and Duchateau, 2008, Gandevia, 2001). At least since the observations of Mosso (1906), it has been known that the mechanisms responsible for the decline in MVC force depend on the characteristics of the task that induced the fatigue (Enoka and Stuart, 1992). Although the evidence clearly indicates that fatigue cannot be attributed to any single

Conflict of interest statement

None declared.

Acknowledgements

Among the many colleagues with whom I have been privileged to work, I would like to acknowledge the significant contributions of Professor Jacques Duchateau (Université Libre de Bruxelles) and Professor Dario Farina (Georg-August University of Göttingen) to the ideas and findings described in this paper.

References (49)

  • R.E. Burke et al.

    Physiological types and histochemical profiles in motor units of the cat gastrocnemius

    Journal of Physiology

    (1973)
  • J.S. Carp et al.

    Sag during unfused tetanic contractions in rat triceps surae motor units

    Journal of Neurophysiology

    (1999)
  • A. Carpentier et al.

    Motor unit behavior and contractile changes during fatigue in human first dorsal interosseus

    Journal of Physiology

    (2001)
  • A. Christie et al.

    Systematic review and meta-analysis of skeletal muscle fatigue in old age

    Medicine and Science in Sports and Exercise

    (2011)
  • Delimata, J., 1996. Actes du Colloque Marey/Muybridge, pionniers du cinema-recontre Beaune/Stanford: 19 mai 1995,...
  • J.L. Dideriksen et al.

    Neuromuscular adjustments that constrain submaximal EMG amplitude at task failure of sustained isometric contractions

    Journal of Applied Physiology

    (2011)
  • J.L. Dideriksen et al.

    An integrative model of motor unit activity during sustained submaximal contractions

    Journal of Applied Physiology

    (2010)
  • J.L. Dideriksen et al.

    Influence of fatigue on the simulated relation between the amplitude of the surface electromyogram and muscle force

    Philosophical Transactions of the Royal Society A

    (2010)
  • J. Duchteau et al.

    Human motor unit recordings: origins and insights into the integrated motor system

    Brain Research

    (2011)
  • R.M. Enoka et al.

    What, why and how it influences muscle function

    Journal of Physiology

    (2008)
  • R.M. Enoka et al.

    Neurobiology of muscle fatigue

    Journal of Applied Physiology

    (1992)
  • D. Farina et al.

    Adjustments differ among low-threshold motor units during intermittent, isometric contractions

    Journal of Neurophysiology

    (2009)
  • L.F. Ferreira et al.

    Muscle-derived ROS and thiol regulation in muscle fatigue

    Journal of Applied Physiology

    (2008)
  • R.H. Fitts

    The cross-bridge cycle and skeletal muscle fatigue

    Journal of Applied Physiology

    (2008)
  • Cited by (69)

    • Exploring the relationship between kinematic variability and fatigue development during repetitive lifting

      2023, Applied Ergonomics
      Citation Excerpt :

      In this study fatigue was determined based on RPE and number of completed sets. It could be argued that assessing fatigue using more objective measures such as median power frequency of EMG or isometric force measurements could improve the determination of fatigue, despite strong relations of RPE with the perception of fatigue (Bonato et al., 2003; Enoka, 2012; Vøllestad, 1997). However, in agreement with previous studies on variability-fatigue our RPE measurements demonstrated that participants reached volitional fatigue.

    • Fatigue monitoring techniques using wearable systems

      2020, Wearable Sensors: Fundamentals, Implementation and Applications
    • Force estimation in fatigue condition using a muscle-twitch model during isometric finger contraction

      2017, Medical Engineering and Physics
      Citation Excerpt :

      Muscle fatigue can be defined as the changes that occur in areas of the body as a result of sustained or repeated contractions of the muscles. The relationship between the amplitude of sEMG and the muscle force does not remain constant after the onset of muscle fatigue [5]. The manifestation of fatigue has been studied under limited conditions, such as isometric contraction or dynamic contraction with a constant force level (for details refer to [5–8]).

    • Muscle Fatigue Estimation based on Permutation Lempel-Ziv Complexity measure

      2022, Transactions of the Korean Institute of Electrical Engineers
    View all citing articles on Scopus
    View full text