Muscle V ˙ O 2 ‐power output nonlinearity in constant‐power, step‐incremental, and ramp‐incremental exercise: magnitude and underlying mechanisms

A computer model of the skeletal muscle bioenergetic system was used to simulate time courses of muscle oxygen consumption ( V ˙ O 2 ), cytosolic metabolite ( ADP , PC r, P i , 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 muscleV ˙ O 2 ‐power output ( PO ) nonlinearity is related to a time‐ and PO ‐dependent increase in the additional ATP usage underlying the slow component of theV ˙ O 2on‐kinetics minus the increase in ATP supply by anaerobic glycolysis leading to a decrease inV ˙ O 2 . The muscleV ˙ O 2 ‐ 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 PC r and P i 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 muscleV ˙ O 2 ‐ PO nonlinearity. It is proposed that the extent of the additional ATP usage is proportional to the time integral of PO ‐ CP above CP .