Metabolic Equivalents as a Foundational Physical Activity Metric: Origins, Validity, and Challenges

Metabolic equivalents (METs) are a long-standing physiological concept used to express the physical intensity and energy cost of human behavior. METs underpin many research methods, mostly employed in epidemiological studies, and they also are implemented in real‑life settings by health and sports professionals to guide individual lifestyle recommendations, exercise testing, and prescription. Therefore, METs can be conceived as a foundational physical activity metric supporting strategies to improve health and exercise performance. However, the origins and evolution of METs remain scarcely documented, and despite growing evidence questioning their validity across demographics, discussion of the challenges associated with their application remains limited. Given these concerns, this review aims to trace the origins and evolution of METs, interpret existing evidence regarding their validity, and discuss some of the challenges of their use in research, health, and training practice.