Understanding Training Load as Exposure and Dose

Abstract

Various terms used in sport and exercise science, and medicine, are derived from other fields such as epidemiology, pharmacology and causal inference. Conceptual and nomological frameworks have described training load as a multidimensional construct manifested by two causally related subdimensions: external and internal training load. In this article, we explain how the concepts of training load and its subdimensions can be aligned to classifications used in occupational medicine and epidemiology, where exposure can also be differentiated into external and internal dose. The meanings of terms used in epidemiology such as exposure, external dose, internal dose and dose-response are therefore explored from a causal perspective and their underlying concepts are contextualised to the physical training process. We also explain how these concepts can assist in the validation process of training load measures. Specifically, to optimise training (i.e. within a causal context), a measure of exposure should be reflective of the mediating mechanisms of the primary outcome. Additionally, understanding the difference between intermediate and surrogate outcomes allows for the correct investigation of the effects of exposure measures and their interpretation in research and applied settings. Finally, whilst the dose-response relationship can provide evidence of the validity of a measure, conceptual and computational differentiation between causal (explanatory) and non-causal (descriptive and predictive) dose-response relationships is needed. Regardless of how sophisticated or "advanced" a training load measure (and metric) appears, in a causal context, if it cannot be connected to a plausible mediator of a relevant response (outcome), it is likely of little use in practice to support and optimise the training process.

Fig. 1

Diagram depicting the parallels exhibited between terms used in sports science and certain areas of epidemiology and pharmacology. The left and right diagrams represent classifications where the training load and exposure include two different but related subdimensions (external and internal load and dose). The central section provides the role of the two subcomponents in the causal pathway leading to a response. The internal dose and load are more proximal causes of the outcome of interest, i.e. they are mediators of (or mechanisms for) the effect of the external dose/load on the outcome

Fig. 2

Hypothetical exposure history from the start at, t₁, to the end, t₂₀ (adapted from Checkoway et al. [40]). The sum (∑) of the black rectangles, one for each time interval (from t₁ to t₂₀) corresponds to the measure of cumulative exposure (simplified representation of integral calculation)

Fig. 3

Directed acyclic graph representing a surrogate outcome (S) fully mediating the effect of A (e.g. training) on the response of interest, Y (e.g. performance), b surrogate outcome (S) with an unmeasured confounder (U) of the S-Y path and another causal path between A and Y, independent from S, and c hypothetical additional mediators (M₁ to M₃) of the A-Y causal path, where the surrogate outcome is actually just another mediator (M₄) that can be used as an intermediate outcome