Skeletal Muscle Biochemical Origin of Exercise Intensity Domains and their Relation to Whole-Body V̇O2 Kinetics

Training-Induced Increase in V·O2max and Critical Power, and Acceleration of V·O2 on-Kinetics Result from Attenuated Pi Increase Caused by Elevated OXPHOS Activity

Computer simulations using a dynamic model of the skeletal muscle bioenergetic system, involving the Pi-double-threshold mechanism of muscle fatigue, demonstrate that the training-induced increase in V·O2max, increase in critical power (CP) and acceleration of primary phase II of the V·O2 on kinetics (decrease in t0.63) is caused by elevated OXPHOS activity acting through a decrease in and slowing of the Pi (inorganic phosphate) rise during the rest-to-work transition. This change leads to attenuation of the reaching by Pi of Pipeak, peak Pi at which exercise is terminated because of fatigue. The delayed (in time and in relation to V·O2 increase) Pi rise for a given power output (PO) in trained muscle causes Pi to reach Pipeak (in very heavy exercise) after a longer time and at a higher V·O2; thus, exercise duration is lengthened, and V·O2max is elevated compared to untrained muscle. The diminished Pi increase during exercise with a given PO can cause Pi to stabilize at a steady state less than Pipeak, and exercise can continue potentially ad infinitum (heavy exercise), instead of rising unceasingly and ultimately reaching Pipeak 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 Pi increase (and other metabolite changes) results in, at a given PO (moderate exercise), the steady state of fluxes (including V·O2) and metabolites being reached faster; thus, t0.63 is shortened. This effect of elevated OXPHOS activity is possibly somewhat diminished by the training-induced decrease in Pipeak.

Mechanisms of slowed V̇O2 on-kinetics in second step of two-step-incremental exercise in skeletal muscle

Simulations using a computer model of the skeletal muscle bioenergetic system demonstrate that the slowed V̇O₂ on-kinetics of the second step in two-step incremental exercise (exercise initiated from elevated baseline metabolic rate) can be accounted for by a decrease in the stimulation of oxidative phosphorylation (OXPHOS) and/or increase in the stimulation of glycolysis through each-step activation (ESA) in working skeletal muscle. This effect can be caused by either a recruitment of more glycolytic type IIa, IIx and IIb fibers or metabolic regulation in already recruited fibers, or both. The elevated-glycolysis-stimulation mechanism predicts that the end-second-step pH in two-step-incremental exercise is lower than the end-exercise pH in constant-power exercise with the same work intensity (power output). The lowered-OXPHOS-stimulation mechanism predicts higher end-exercise ADP and P_i, and lower PCr in the second step of two-step-incremental than in constant-power exercise. These predictions/mechanisms can be verified or falsified in the experimental way. DATA AVAILABILITY STATEMENT: There are no additional data available.

V̇O2 (non-)linear increase in ramp-incremental exercise vs. V̇O2 slow component in constant-power exercise: Underlying mechanisms

A computer model of the skeletal muscle bioenergetic system involving the P_i double-threshold mechanism of muscle fatigue was used to study the V̇O₂ (non-)linear increase in time in ramp-incremental exercise as compared to the V̇O₂ slow component in constant-power exercise. The P_i double-threshold mechanism applies to both constant-power and ramp-incremental exercise. The additional ATP usage is initiated at a significantly higher ATP usage activity (power output), determining the moderate/heavy exercise border, in ramp-incremental, than in constant-power exercise. A significantly lowered additional ATP usage activity or elevated glycolysis stimulation at the highest power outputs in ramp-incremental exercise in relation to constant-power exercise can additionally explain the much smaller (or zero) V̇O₂ non-linearity in ramp-incremental exercise, than V̇O₂ slow component in constant-power exercise. The V̇O₂ (non-)linearity in ramp-incremental exercise and V̇O₂ slow component in constant-power exercise is a derivative of a balance between the additional ATP usage and ATP production by anaerobic glycolysis.