V?O2 (non-)linear increase in ramp-incremental exercise vs. V?O2 slow component in constant-power exercise: Underlying mechanisms

A computer model of the skeletal muscle bioenergetic system involving the Pi double-threshold mechanism of muscle fatigue was used to study the VO2 (non-)linear increase in time in ramp-incremental exercise as compared to the VO2 slow component in constant-power exercise. The Pi 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) VO2 non-linearity in ramp-incremental exercise, than VO2 slow component in constant-power exercise. The VO2 (non-)linearity in ramp-incremental exercise and VO2 slow component in constant-power exercise is a derivative of a balance between the additional ATP usage and ATP production by anaerobic glycolysis.