Skeletal muscle biochemical origin of exercise intensity domains and their relation to whole-body V̇O(2) kinetics

This article presents the biochemical intra-skeletal-muscle basis of exercise intensity domains: moderate (M), heavy (H), very heavy (VH) and severe (S). Threshold origins are mediated by a ‘P(i) double-threshold’ mechanism of muscle fatigue, which assumes (1) additional ATP usage, underlying muscle V̇O(2) and metabolite slow components, is initiated when inorganic phosphate (P(i)) exceeds a critical value (Pi(crit)); (2) exercise is terminated because of fatigue, when P(i) reaches a peak value (Pi(peak)); and (3) the P(i) increase and additional ATP usage increase mutually stimulate each other forming a positive feedback. M/H and H/VH borders are defined by P(i) on-kinetics in relation to Pi(crit) and Pi(peak). The values of the ATP usage activity, proportional to power output (PO), for the M/H, H/VH and VH/S borders are lowest in untrained muscle and highest in well-trained muscle. The metabolic range between the M/H and H/VH border (or ‘H space’) decreases with muscle training, while the difference between the H/VH and VH/S border (or ‘VH space’) is only weakly dependent on training status. The absolute magnitude of the muscle V̇O(2) slow-component, absent in M exercise, rises gradually with PO to a maximal value in H exercise, and then decreases with PO in VH and S exercise. Simulations of untrained, physically active and well-trained muscle demonstrate that the muscle M/H border need not be identical to the whole-body M/H border determined from pulmonary V̇O(2) on-kinetics and blood lactate, while suggesting that the biochemical origins of the H/VH border reside within skeletal muscle and correspond to whole-body critical power.