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

Compared with work-matched high-intensity continuous exercise, intermittent exercise dissociates pulmonary oxygen uptake (V)over dot(O2)) from the accumulated work. The extent to which this reflects differences in O-2 storage fluctuations and/or contributions from oxidative and substrate-level bioenergetics is unknown. Using pulmonary gas-exchange and intramuscular P-31 magnetic resonance spectroscopy, we tested the hypotheses that, at the same power: ATP synthesis rates are similar, whereas peak (V)over dot(O2) 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 (V)over dot(O2) 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 64:128s). Target work duration (576s) was achieved in all intermittent protocols; greater than continuous (252174s; P < 0.05). Mean ATP turnover rate was not different between protocols (similar to 43 mM min(-1) on average). However, the intramuscular phosphocreatine (PCr) component of ATP generation was greatest (similar to 30 mM min(-1)), and oxidative (similar to 10 mM min(-1)) and anaerobic glycolytic (similar to 1 mM min(-1)) components were lowest for 16:32 and 32:64s intermittent protocols, compared to 64:128 s (186, 2110 and 104 mM min(-1), respectively) and continuous protocols (86, 209 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 (V)over dot(O2) and intramuscular metabolism, dissociating exercise intensity from the power output and work done.