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Exercise is a stressor that induces various psychophysiological responses, which mediate cellular adaptations in many organ systems. To maximize this adaptive response, coaches and scientists need to control the stress applied to the athlete at the individual level. To achieve this, precise control and manipulation of the training load are required. In 2003, the authors introduced a theoretical framework to define and conceptualize the measurable constructs of the training process. They described training load as having 2 measurable components: internal and external load. The aim of this commentary is to extend, clarify, and refine both the theoretical framework and the definitions of internal and external training load to avoid misinterpretation of this concept.

Impellizzeri and Coutts are with the Faculty of Health, Human Performance Research Centre, University of Technology Sydney, Sydney, NSW, Australia. Marcora is with the School of Sport and Exercise Sciences, University of Kent, Chatham, United Kingdom, and the Dept of Biomedical and NeuroMotor Sciences (DiBiNeM), University of Bologna, Bologna, Italy.

Impellizzeri (franco.impellizzeri@uts.edu.au) is corresponding author.
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