Muscle V˙O2-power output nonlinearity in constant-power, step-incremental, and ramp-incremental exercise: magnitude and underlying mechanisms

Abstract

A computer model of the skeletal muscle bioenergetic system was used to simulate time courses of muscle oxygen consumption (VO2), cytosolic metabolite (ADP, PCr, Pi, and ATP) concentrations, and pH during whole-body constant-power exercise (CPE) (6 min), step-incremental exercise (SIE) (30 W/3 min), and slow (10 W/min), medium (30 W/min), and fast (50 W/min) ramp-incremental exercise (RIE). Different ESA (each-step activation) of oxidative phosphorylation (OXPHOS) intensity-ATP usage activity relationships, representing different muscle fibers recruitment patterns, gave best agreement with experimental data for CPE, and for SIE and RIE. It was assumed that the muscle (VO2) -power output (PO) nonlinearity is related to a time- and PO-dependent increase in the additional ATP usage underlying the slow component of the (VO2) on-kinetics minus the increase in ATP supply by anaerobic glycolysis leading to a decrease in (VO2). The muscle (VO2) -PO relationship deviated upward (+) or downward (−) from linearity above critical power (CP), and the nonlinearity equaled +16% (CPE),+12% (SIE), +8% (slow RIE), +1% (moderate RIE), and −2% (fast RIE) at the end of exercise, in agreement with experimental data. During SIE and RIE, changes in PCr and Pi accelerated moderately above CP, while changes in ADP and pH accelerated significantly with time and PO. It is postulated that the intensity of the additional ATP usage minus ATP supply by anaerobic glycolysis determines the size of the muscle (VO2) -PO nonlinearity. It is proposed that the extent of the additional ATP usage is proportional to the time integral of PO - CP above CP.