Physiological models to understand exercise fatigue and the adaptations that predict or enhance athletic performance

A popular concept in the exercise sciences holds that fatigue develops duri ng exercise of moderate to high intensity, when the capacity of the cardior espiratory system to provide oxygen to the exercising muscles falls behind their demand inducing "anaerobic" metabolism, But this cardiovascular/anaer obic model is unsatisfactory because (i) a more rigorous analysis indicates that the first organ to be affected by anaerobiosis during maximal exercis e would likely be the heart, not the skeletal muscles. This probability was fully appreciated by the pioneering exercise physiologists, A. V. Hill, A. Bock and D. B. Dill, but has been systematically ignored by modern exercis e physiologists; (ii) no study has yet definitely established the presence of either anaerobiosis, hypoxia or ischaemia in skeletal muscle during maxi mal exercise; (iii) the model is unable to explain why exercise terminates in a variety of conditions including prolonged exercise, exercise in the he at and at altitude, and in those with chronic diseases of the heart and lun gs, without any evidence for skeletal muscle anaerobiosis, hypoxia or ischa emia, and before there is full activation of the total skeletal muscle mass , and (iv) cardiovascular and other measures believed to relate to skeletal muscle anaerobiosis, including the maximum oxygen consumption (VO2 max) an d the "anaerobic threshold", are indifferent predictors of exercise capacit y in athletes with similar abilities. This review considers four additional models that need to be considered when factors limiting either short durat ion, maximal or prolonged submaximal exercise are evaluated. These addition al models are: (i) the energy supply/energy depletion model; (ii) the muscl e power/muscle recruitment model; (iii) the biomechanical model and (iv) th e psychological model. By reviewing features of these models, this review p rovides a broad overview of the physiological, metabolic and biomechanical factors that may limit exercise performance under different exercise condit ions, A more complete understanding of fatigue during exercise, and the rel evance of the adaptations that develop with training, requires that the pot ential relevance of each model to fatigue under different conditions of exe rcise must be considered.