Training Load: measuring external work is not enough to measure fatigue

An accurate measurement is not necessarily a measure of fatigue

External load describes the work performed. But its interpretation can go much further: metabolic demand, neuromuscular demand, tissue stress, fatigue or adaptation. According to the authors, these biological meanings require evidence separate from a device’s technical accuracy or reproducibility.

This is the question of construct validity: does the metric actually represent the phenomenon attributed to it? A reliable signal alone is not enough to establish that link.

What the researchers examined

The review searched five bibliographic databases for studies assessing an external load metric and the phenomenon it was intended to represent. Studies limited to technical accuracy, reliability, measurement error or descriptive monitoring were excluded.

The 46 included studies had main samples of 7 to 71 participants, sometimes with smaller subgroups for analysis. Most included only men. Differences in metrics, phenomena studied, comparators and methods prevented a meta-analysis that the authors considered meaningful. The synthesis is therefore narrative.

Biological benchmarks were often indirect

Only 13 studies, or 28%, used a direct criterion matching the target phenomenon. The others relied on proxy measures, comparisons between conditions or groups, or a combination of approaches. These approaches do not all directly validate the biological interpretation.

In addition, 19 studies explicitly distinguished technical performance from signal interpretation, and 12 explicitly justified their choice of comparator. The authors also note frequent limitations in hypothesis testing and analyses.

What this changes when interpreting a metric

This review does not make external load measurements useless. It calls for respecting their scope: describing work or exposure is not the same as directly quantifying fatigue, tissue stress or adaptation.

To support a biological interpretation, the authors call for a precisely defined phenomenon, a plausible mechanism, a hypothesis about the direction of the relationship and a suitable benchmark. A mechanism alone therefore does not constitute validation. The useful question remains: what does this metric actually measure, and what evidence justifies assigning it a biological meaning?

What we can take away

An external load figure primarily describes the work performed. Interpreting it as a measure of fatigue or adaptation requires specific validation, which remains limited in this review.