Factors determining training-induced changes in V̇O _2max, critical power, and V̇O ₂ on-kinetics in skeletal muscle

Abstract

A mechanism of the training-induced changes in V̇O _2max, critical power, and V̇O ₂ on-kinetics in skeletal muscle reported in the literature is postulated. It involves the self-driving “P _i double-threshold” mechanism of muscle fatigue underlying exercise inefficiency, the slow component of the V̇O ₂ on-kinetics, and termination of exercise. It is proposed that an increase in OXPHOS activity and decrease in peak P _i at which exercise terminates are responsible for the training-induced changes in the muscle bioenergetic system.

Abstract

Computer simulations, using the “P _i double-threshold” mechanism of muscle fatigue postulated previously (the first threshold initiating progressive reduction in work efficiency and the second threshold resulting in exercise intolerance), demonstrated that several parameters of the skeletal muscle bioenergetic system can affect maximum oxygen consumption (V̇O _2max), critical power (CP), and oxygen consumption (V̇O ₂) on-kinetics in skeletal muscle. Simulations and experimental observations together demonstrate that endurance exercise training increases oxidative phosphorylation (OXPHOS) activity and/or each-step activation (ESA) intensity, the latter, especially in the early stages of training. Here, new computer simulations demonstrate that an endurance training-induced increase in OXPHOS activity and decrease in peak P _i (Pi _peak), at which exercise is terminated because of exercise intolerance, result in increased V̇O _2max and CP, speeding of the primary phase II of V̇O ₂ on-kinetics, and decreases V̇O ₂ slow component magnitude, consistent with their observed behavior in vivo. It is possible, but remains unknown, whether there is a contribution to this behavior of an increase in the critical P _i (Pi _crit), above which the additional ATP usage underlying the slow component begins, and a decrease in the activity of the additional ATP usage (k _add). Thus, we offer a mechanism, involving P _i accumulation, Pi _crit and Pi _peak, of the training-induced adaptations in V̇O _2max, CP, and the primary and slow component phases of V̇O ₂ on-kinetics that was absent in the literature.

NEW & NOTEWORTHY A mechanism of the training-induced changes in V̇O _2max, critical power, and V̇O ₂ on-kinetics in skeletal muscle reported in the literature is postulated. It involves the self-driving “P _i double-threshold” mechanism of muscle fatigue underlying exercise inefficiency, the slow component of the V̇O ₂ on-kinetics, and termination of exercise. It is proposed that an increase in OXPHOS activity and decrease in peak P _i at which exercise terminates are responsible for the training-induced changes in the muscle bioenergetic system.