What This Means
Sports teams use wearable devices and tracking systems to measure things like distance run, speed, accelerations, and impacts. These are called external training load metrics. They are often interpreted as telling us how hard the body is working, how tired an athlete is, or how they are adapting. This review asked whether research actually backs up those biological interpretations.
The authors reviewed 46 studies, mostly small and mostly in men. Only about 28% (13 studies) compared the metric against a measure that directly matched the biological thing it was claimed to reflect. Most others relied on stand-in measures, comparisons between conditions or groups, or evidence that the device was simply reliable. Fewer than half clearly separated whether a device measures accurately from whether it means what people say it means, and about a quarter explained why they chose their comparison measure. Many studies also had weaknesses in how hypotheses were tested and analyzed.
This research suggests that external load metrics are best viewed as descriptions of movement, work, or exposure, not direct measures of metabolic, muscular, bone, fatigue, or adaptation responses. The authors argue that anyone using these metrics should start with the decision they want to inform, then check that there is a clear construct, a plausible mechanism, and a well-matched reference supporting the interpretation. Because the studies were small and varied, confidence in biological interpretations is limited.