UCLA

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.