Identifying sex differences in neuromuscular fatigue: the challenge of normalizing exercise intensity and interpreting the results between populations

Hureau TJ, Hucteau E, Massamba A, Mallard J, Ducrocq GP

Excerpt

Exercise-induced neuromuscular fatigue is well known for being specific. This specificity, which determines both the magnitude and aetiology of fatigue, includes multiple parameters such as the nature of the task (i.e. contraction mode, muscle mass involved), the intensity–duration of the task, but also individual characteristics such as the participants’ sex (Hunter, 2016). However, similarly to other research areas, sex differences in neuromuscular fatigue are still under-investigated (Hunter, 2016). In a recent article published in The Journal of Physiology, Ansdell and colleagues aimed to clarify the sex differences in fatigue during whole-body exercise (Ansdell et al. 2020a).

How to normalize exercise intensity in order to compare male and female physiology?

A classic question when comparing independent groups in exercise physio- logy is the methodological approach to normalize exercise intensity. One approach is to use an absolute exercise intensity (e.g. 100 W on a cycle ergometer), which is particularly relevant for clinical outcomes since daily living activities such as climbing stairs or doing grocery shopping
are driven by the environment and are not relative to the individual’s maximal capacity. Alternatively, or additionally, relative intensity is often utilized to match the exercise demands between groups, which also provides important mechanistic insights. In this context, exercise intensity can be relative to the maximal voluntary contraction force (% of MVC) or the maximal oxygen uptake (% of V ̇O2 max ), for single-joint or whole-body exercise modalities, respectively. If this normalization is insightful, it is not devoid of confounding factors when comparing males and females. Indeed, in this scenario (matching exercise at a specific percentage of V ̇O2 max ), metabolic thresholds are neglected and the metabolic rate can be drastically different between individuals. As a result, males and females could experience substantial differences in time to task failure, which would be a serious limitation as the intensity and duration of the exercise task are known to influence the magnitude and aetiology of neuromuscular fatigue (Ducrocq et al. 2021). In order to circumvent this limitation, Ansdell et al. (2020a) normalized exercise intensity to critical power, a metabolic threshold derived from the power–time relationship, with the intention to match the metabolic rate between males and females.