Quantification of Neuromuscular Fatigue: What Do We Do Wrong and Why?
Neuromuscular fatigue (NMF) is usually assessed non-invasively in healthy, athletic or clinical populations with the combination of voluntary and evoked contractions. Although it might appear relatively straightforward to magnetically or electrically stimulate at different levels (cortical/spinal/muscle) and to measure mechanical and electromyographic responses to quantify neuromuscular adjustments due to sustained/repeated muscle contractions, there are drawbacks that researchers and clinicians need to bear in mind. The aim of this opinion paper is to highlight the pitfalls inevitably faced when NMF is quantified. The first problem might arise from the definition of fatigue itself and the parameter(s) used to measure it; for instance, measuring power vs. isometric torque may lead to different conclusions. Another potential limitation is the delay between exercise termination and the evaluation of neuromuscular function; the possible underestimation of exercise-induced neural and contractile impairment and misinterpretation of fatigue etiology will be discussed, as well as solutions recently proposed to overcome this problem. Quantification of NMF can also be biased (or not feasible) because of the techniques themselves (e.g. results may depend on stimulation intensity for transcranial magnetic stimulation) or the way data are analyzed (e.g. M wave peak-to-peak vs first phase amplitude). When available, alternatives recently suggested in the literature to overcome these pitfalls are considered and recommendations about the best practices to assess NMF (e.g. paying attention to the delay between exercise and testing, adapting the method to the characteristics of the population to be tested and considering the limitations associated with the techniques) are proposed.
The authors would like to thank Chris Donnelly for editing the English and all co-authors who have worked with them on the topic of the present opinion, in particular S. J. Aboodarda, D. Bachasson, B. Kayser, R. Kruger, V. Martin, D. Neyroud, J. Rodriguez-Falces, T. Rupp, J. Temesi, S. Vergès, H. Westerblad.
Compliance with Ethical Standards
No funding was received for the preparation of this manuscript.
Conflict of Interest
Nicolas Place and Guillaume Millet declare that they have no conflicts of interest.
- 14.Simonson E, Weiser PC. Psychological aspects and physiological correlates of work and fatigue. Springfield, IL: CC Thomas; 1976. p. 336–405.Google Scholar
- 21.Bankole LC, Millet GY, Temesi J, Bachasson D, Ravelojaona M, Wuyam B, et al. Safety and efficacy of a 6-month home-based exercise program in patients with facioscapulohumeral muscular dystrophy: a randomized controlled trial. Medicine (Baltimore). 2016;95(31):e4497.PubMedPubMedCentralCrossRefGoogle Scholar
- 41.Aboodarda SJ, Mira J, Floreani M, Jaswal R, Moon SJ, Amery K, et al. Effects of endurance cycling training on neuromuscular fatigue in healthy active men. Part II: corticospinal excitability and voluntary activation. Eur J Appl Physiol. 2018;118(11):2295–305.PubMedPubMedCentralCrossRefGoogle Scholar
- 51.Neyroud D, Temesi J, Millet GY, Verges S, Maffiuletti NA, Kayser B, et al. Comparison of electrical nerve stimulation, electrical muscle stimulation and magnetic nerve stimulation to assess the neuromuscular function of the plantar flexor muscles. Eur J Appl Physiol. 2015;115(7):1429–39.PubMedPubMedCentralCrossRefGoogle Scholar
- 63.Bachasson D, Temesi J, Bankole C, Lagrange E, Boutte C, Millet GY, et al. Assessement of quadriceps strength, endurance and fatigue in FSHD and CMT: benefits and limits of femoral nerve magnetic stimulation. Clin Neurophysiol Off J Int Fed Clin Neurophysiol. 2014;125(2):396–405.CrossRefGoogle Scholar