Dissociating external power from intramuscular exercise intensity during intermittent bilateral knee-extension in humans

Abstract

Key points: Continuous high-intensity constant-power exercise is unsustainable, with maximal oxygen uptake (V̇O2 max ) and the limit of tolerance attained after only a few minutes. Performing the same power intermittently reduces the O₂ cost of exercise and increases tolerance. The extent to which this dissociation is reflected in the intramuscular bioenergetics is unknown. We used pulmonary gas exchange and ³¹ P magnetic resonance spectroscopy to measure whole-body V̇O2, quadriceps phosphate metabolism and pH during continuous and intermittent exercise of different work:recovery durations. Shortening the work:recovery durations (16:32 s vs. 32:64 s vs. 64:128 s vs. continuous) at a work rate estimated to require 110% peak aerobic power reduced V̇O2, muscle phosphocreatine breakdown and muscle acidification, eliminated the glycolytic-associated contribution to ATP synthesis, and increased exercise tolerance. Exercise intensity (i.e. magnitude of intramuscular metabolic perturbations) can be dissociated from the external power using intermittent exercise with short work:recovery durations.

Abstract: Compared with work-matched high-intensity continuous exercise, intermittent exercise dissociates pulmonary oxygen uptake (V̇O2) from the accumulated work. The extent to which this reflects differences in O₂ storage fluctuations and/or contributions from oxidative and substrate-level bioenergetics is unknown. Using pulmonary gas-exchange and intramuscular ³¹ P magnetic resonance spectroscopy, we tested the hypotheses that, at the same power: ATP synthesis rates are similar, whereas peak V̇O2 amplitude is lower in intermittent vs. continuous exercise. Thus, we expected that: intermittent exercise relies less upon anaerobic glycolysis for ATP provision than continuous exercise; shorter intervals would require relatively greater fluctuations in intramuscular bioenergetics than in V̇O2 compared to longer intervals. Six men performed bilateral knee-extensor exercise (estimated to require 110% peak aerobic power) continuously and with three different intermittent work:recovery durations (16:32, 32:64 and 64:128 s). Target work duration (576 s) was achieved in all intermittent protocols; greater than continuous (252 ± 174 s; P < 0.05). Mean ATP turnover rate was not different between protocols (∼43 mm min^-1 on average). However, the intramuscular phosphocreatine (PCr) component of ATP generation was greatest (∼30 mm min^-1 ), and oxidative (∼10 mm min^-1 ) and anaerobic glycolytic (∼1 mm min^-1 ) components were lowest for 16:32 and 32:64 s intermittent protocols, compared to 64:128 s (18 ± 6, 21 ± 10 and 10 ± 4 mm min^-1 , respectively) and continuous protocols (8 ± 6, 20 ± 9 and 16 ± 14 mm min^-1 , respectively). As intermittent work duration increased towards continuous exercise, ATP production relied proportionally more upon anaerobic glycolysis and oxidative phosphorylation, and less upon PCr breakdown. However, performing the same high-intensity power intermittently vs. continuously reduced the amplitude of fluctuations in V̇O2 and intramuscular metabolism, dissociating exercise intensity from the power output and work done.

Keywords: bioenergetics; exercise intensity; exercise tolerance; interval exercise.

Figure 1. Schematic of the intermittent exercise protocols and time‐bins used for V˙O2 and ³¹P MRS measures

Following a warm‐up at 5 W, intermittent exercise with work phases performed at 110% of ramp‐incremental peak power was initiated with work:recovery durations of either 16:32 s (top), 32:64 s (middle) or 64:128 s (bottom). The first 192 s of each test was eliminated (grey box) to exclude a kinetic transient phase that preceded the stabilization of ${\dot{V}}_{{\mathrm{O}}_2}$ and ³¹P MRS fluctuations, with like transitions in each time‐bin time‐aligned to exercise onset and data averaged to improve signal:noise.

Figure 2. Contributions from phosphocreatine breakdown (D), oxidative phosphorylation (Q) and anaerobic glycolysis (L) to the mean ATP turnover rate at 110% of ramp‐incremental peak power during continuous and intermittent exercise comprising work:recovery durations of 16:32, 32:64 and 64:128 s

Upper: absolute energetic system contributions to mean ATP turnover. Lower: relative energetic system contributions to mean ATP turnover.

Figure 3. V˙O2, PCr (top row) and pH_i (bottom row) responses to work:recovery durations of 16:32 s (first column), 32:64 s (second column), 64:128 s (third column) or continuous exercise (forth column)

Also shown is the lactate threshold (LT) from the ramp‐incremental exercise test (dotted line), as well as the ${\dot{V}}_{{\mathrm{O}}_2\max }$ (top row, dashed line) and pH_i (bottom row, dashed line) attained at the limit of tolerance of the continuous exercise protocol. Grey areas indicate the exercise period performed at 110% of ramp incremental peak power. Note, in the 16:32 s protocol, that ${\dot{V}}_{{\mathrm{O}}_2}$ never exceeds the LT, and there are only minor changes in pH_i, consistent with the 16:32 s intermittent protocol being moderate‐intensity. The peak ${\dot{V}}_{{\mathrm{O}}_2}$ amplitude exceeds the LT in the 32:64 and 64:128 s intermittent protocols and during continuous exercise, with this accompanied by a metabolic acidosis (decline in pH_i), consistent with a greater exercise metabolic strain in these protocols.