Fig 1.
Power-type vs. saturating-type AOX (ESA, each-step activation intensity)-AUT (relative ATP usage activity) dependence.
Simulated power-type and saturating-type AOX-AUT dependences (lines) are compared with the values of AOX and AUT extracted from experimental data for rest, moderate exercise and severe exercise (points) [1]. The power-type dependence, described by Eq 1, is postulated to be present in electrically-stimulated muscle, while the saturating-type dependence, described by Eq 2, is postulated to be present during voluntary exercise (cortically-stimulated muscle).
Fig 2.
Simulated (lines) and experimental (points) dependence of system variables on relative ATP usage activity AUT for the power-type AOX (ESA, each-step activation intensity)-AUT dependence in the absence of the ‘additional’ ATP usage.
A, dependence of , ADP and pH; B, dependence of PCr, Pi and ATP. Re-scaled (see sub-section 2.5) experimental data from [24] are presented (points). The power-type AOX-AUT dependence without additional ATP usage is postulated to be present in electrically-stimulated muscle.
Fig 3.
Simulated dependence of system variables on relative ATP usage activity AUT for the saturating-type AOX (ESA, each-step activation intensity)-AUT dependence in the absence of the ‘additional’ ATP usage.
A, dependence of , ADP and pH; B, dependence of PCr, Pi and ATP. The saturating-type AOX-AUT dependence without additional ATP usage is postulated to be present in voluntary exercise (cortically-stimulated muscle) below critical ATP usage activity (critical power).
Fig 4.
Simulated time courses of system variables during transition from rest to moderate muscle work (relative ATP usage activity AUT = 35) to recovery for the saturating-type AOX (ESA, each-step activation intensity)-AUT (relative ATP usage activity) dependence.
A, dependence of , ADP and pH; B, dependence of PCr, Pi and ATP; C, dependence of ATP usage (vUT) as well as of ATP supply by OXPHOS (+ aerobic glycolysis) (vOX), creatine kinase (vCK) and anaerobic glycolysis (vGL). The saturating-type AOX-AUT dependence without additional ATP usage is postulated to be present in voluntary exercise below critical ATP usage activity (critical power).
Fig 5.
Simulated time courses of system variables during transition from rest to heavy/severe muscle work (relative ATP usage activity AUT = 80) to recovery for the saturating-type AOX (ESA, each-step activation intensity)-AUT (relative ATP usage activity) dependence in the presence of the ‘additional’ ATP usage.
A, dependence of , ADP and pH; B, dependence of PCr, Pi and ATP; C, dependence of ATP usage (vUT) as well as of ATP supply by OXPHOS (+ aerobic glycolysis) (vOX), creatine kinase (vCK) and anaerobic glycolysis (vGL). The saturating-type AOX-AUT dependence with additional ATP usage is postulated to be present in voluntary exercise above critical ATP usage activity (critical power).
Fig 6.
Simulated (lines) and experimental (points) dependence of system variables on relative ATP usage activity AUT for the saturating-type AOX (ESA, each-step activation intensity)-AUT dependence in the presence of the ‘additional’ ATP usage above the critical ATP usage activity.
A, dependence of , ADP and pH; B, dependence of PCr, Pi and ATP. Re-scaled (see sub-section 2.5) experimental data for medial gastrocnemius from [14] are presented. The saturating-type AOX-AUT dependence with additional ATP usage is postulated to be present in voluntary exercise above critical ATP usage activity (critical power).
Fig 7.
Simulated relationship of the characteristic transition time τp of the principal phase of the muscle on-kinetics on relative ATP usage activity (AUT) for the power-type and saturating-type AOX (ESA, each-step activation intensity)-AUT dependencies.
The relative activation of OXPHOS during rest-to-work transition AOX was increased as a function of AUT according to Eq 1 for power-type dependence and to Eq 2 for saturating-type dependence. The power-type AOX-AUT dependence without ‘additional’ ATP usage is postulated to be present in electrically-stimulated muscle, while the saturating-type AOX-AUT dependence with ‘additional’ ATP usage is postulated to be present in voluntary exercise (cortically-stimulated muscle).
Fig 8.
Experimental dependence of skeletal muscle bioenergetic system variables on parameters / variables related to ATP usage activity.
A. Original (not re-scaled) dependence of PCr, Pi, ADP, ATP, pH after 8–12 min of stimulation on electrical stimulation frequency in rat skeletal muscle (Table I and II in [24]). B. Original (not re-scaled) dependence of PCr, Pi and ADP after 4 min of exercise on ATP turnover rate (% of maximal) in human calf muscle during voluntary constant-power exercise (pedal pressing) (extracted from Fig 6 in [14]). C. Dependence of the decrease in PCr and pH (in relation to rest) after 6 min of exercise on work intensity in human quadriceps muscles during voluntary constant-power exercise (bilateral knee extension) (closed symbols, [2]; open symbols, [35]).
Fig 9.
Experimental -power output dependence.
Experimental dependence of (oxygen consumption at the end of subsequent steps) on the power output (PO) in subsequent steps in step-incremental exercise (increase in PO by 60 W after each 6 or 8 min in two overlapping protocols ‘shifted in phase’ by 30 W, with a baseline of 20 W and 50 W) extracted from Table 1 in [19] is presented.
Table 1.
Experimental values of the characteristic transition time τp of the on-kinetics at different bilateral knee extension exercise intensities for the same group of individuals in each experiment.