| Summary: | Computer simulations using a dynamic model of the skeletal muscle bioenergetic system, involving the P<sub>i</sub>-double-threshold mechanism of muscle fatigue, demonstrate that the training-induced increase in <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo stretchy="false">·</mo></mover></mrow></semantics></math></inline-formula>O<sub>2max</sub>, increase in critical power (CP) and acceleration of primary phase II of the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo stretchy="false">·</mo></mover></mrow></semantics></math></inline-formula>O<sub>2</sub> on kinetics (decrease in t<sub>0.63</sub>) is caused by elevated OXPHOS activity acting through a decrease in and slowing of the P<sub>i</sub> (inorganic phosphate) rise during the rest-to-work transition. This change leads to attenuation of the reaching by P<sub>i</sub> of Pi<sub>peak</sub>, peak P<sub>i</sub> at which exercise is terminated because of fatigue. The delayed (in time and in relation to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo stretchy="false">·</mo></mover></mrow></semantics></math></inline-formula>O<sub>2</sub> increase) P<sub>i</sub> rise for a given power output (PO) in trained muscle causes P<sub>i</sub> to reach Pi<sub>peak</sub> (in very heavy exercise) after a longer time and at a higher <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo stretchy="false">·</mo></mover></mrow></semantics></math></inline-formula>O<sub>2</sub>; thus, exercise duration is lengthened, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo stretchy="false">·</mo></mover></mrow></semantics></math></inline-formula>O<sub>2max</sub> is elevated compared to untrained muscle. The diminished P<sub>i</sub> increase during exercise with a given PO can cause P<sub>i</sub> to stabilize at a steady state less than P<sub>ipeak</sub>, and exercise can continue potentially ad infinitum (heavy exercise), instead of rising unceasingly and ultimately reaching Pi<sub>peak</sub> and causing exercise termination (very heavy exercise). This outcome means that CP rises, as the given PO is now less than, and not greater than CP. Finally, the diminished P<sub>i</sub> increase (and other metabolite changes) results in, at a given PO (moderate exercise), the steady state of fluxes (including <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">V</mi><mo stretchy="false">·</mo></mover></mrow></semantics></math></inline-formula>O<sub>2</sub>) and metabolites being reached faster; thus, t<sub>0.63</sub> is shortened. This effect of elevated OXPHOS activity is possibly somewhat diminished by the training-induced decrease in Pi<sub>peak</sub>.
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