A phenomenological model of muscle fatigue and the power-endurance relationship

Abstract

The relationship between power output and the time that it can be sustained during exercise (i.e., endurance) at high intensities is curvilinear. Although fatigue is implicit in this relationship, there is little evidence pertaining to it. To address this, we developed a phenomenological model that predicts the temporal response of muscle power during submaximal and maximal exercise and which was based on the type, contractile properties (e.g., fatiguability), and recruitment of motor units (MUs) during exercise. The model was first used to predict power outputs during all-out exercise when fatigue is clearly manifest and for several distributions of MU type. The model was then used to predict times that different submaximal power outputs could be sustained for several MU distributions, from which several power-endurance curves were obtained. The model was simultaneously fitted to two sets of human data pertaining to all-out exercise (power-time profile) and submaximal exercise (power-endurance relationship), yielding a high goodness of fit (R(2) = 0.96-0.97). This suggested that this simple model provides an accurate description of human power output during submaximal and maximal exercise and that fatigue-related processes inherent in it account for the curvilinearity of the power-endurance relationship.

Physiological Evidence That the Critical Torque Is a Phase Transition, Not a Threshold

“…Subtle variations in motor unit recruitment, metabolism, and blood flow within trials, or between days, may result in the system being tripped into the severe domain when it would be expected to be in the heavy domain and vice versa. Indeed, James and Green ( 39 ) produced realistic power–duration curves during constant load and all-out exercise by modeling the effect of fatigue on the motor unit pool. The importance of this is that the aforementioned heterogeneities could increase the probability of motor units developing functionally significant levels of fatigue, leading to the recruitment of additional, and even less fatigue-resistant motor units.…”

A review of occupationally-relevant models of localised muscle fatigue

“…In contrast, dynamic contractions are more relevant to typical occupational demands, and MFMs would have more potential for practical use if they were applicable for such efforts. Indeed, recent authors of models applicable to dynamic tasks (James and Green, 2012;Marion et al, 2010;Sih et al, 2012;Ma et al, 2012) acknowledge this need, in part due to the failure of older models in predicting muscle fatigue in diverse conditions. Meanwhile, ergonomics approaches and tools are typically more focused on 'higher-level' aspects of biomechanical systems (e.g., muscle physical capacity), and for practical application they need to avoid excessive complexities (e.g., of physiological mechanisms).…”

Intensity-dependent effect of ageing on fatigue during intermittent contractions of the human calf muscle in males and females

“…The curvilinear effects of intensity on fatigue and endurance are accentuated by increasing the proportion of type II muscle fibres (James and Green 2012), suggesting that any effect of fibre type on fatigue is more apparent at higher intensities. For the gastrocnemius lateralis muscle, the cross-sectional area occupied by type II muscle fibres is greater in younger than older subjects for both sexes (Coggan et al 1992).…”

“…The well-established relationship between intensity and endurance implies that fatigue is a curvilinear function of intensity (James and Green 2012), but there is limited empirical evidence about the intensity-fatigue relationship. In a study of young males, rates of fatigue were assessed at seven intensities of calf exercise between 30 and 90 %MVC (Egana and Green 2007) and the findings suggested that fatigue was a curvilinear function of intensity.…”