Pi-induced muscle fatigue leads to near-hyperbolic power-duration dependence.

PURPOSE: Consequences of combining three ideas proposed previously by other authors: (1) that there exists a critical power (CP), above which no steady state in [Formula: see text]O2 (oxygen consumption) and metabolites can be achieved in voluntary constant-power exercise; (2) that muscle fatigue is related to decreased exercise efficiency (increased [Formula: see text]O2/power output ratio); and (3) that Pi (inorganic phosphate) is the main fatigue-related metabolite are investigated.
METHODS: A previously-developed computer model of the skeletal muscle bioenergetic system is used. It was assumed in computer simulations that skeletal muscle work terminates when cytosolic Pi (inorganic phosphate) exceeds a certain critical level.
RESULTS: Simulated changes in muscle [Formula: see text]O2, cytosolic ADP, pH, PCr and Pi as a function of time at various ATP usage activities (corresponding to power outputs) agreed well with experimental data. Computer simulations resulted in a fourth previously-published idea: (4) that the power-duration relationship describing the dependence of power output (PO) on the time to exhaustion of voluntary constant-power exercise at a given PO has a (near-)hyperbolic shape.
CONCLUSIONS: Pi is a major factor contributing to muscle fatigue, as such an assumption leads to a (near-)hyperbolic shape of the power-duration relationship, at least for exercise duration of ~ 1-10 min. Thus, a potential mechanism underlying the power-duration relationship shape is offered that was absent in the literature. Other factors/mechanisms, such as cytosol acidification, glycogen stores depletion and central fatigue can contribute to this relationship, especially in longer exercises.