Although commonly used, the physiological mechanisms underlying all-out exercise performance are still unclear and an in-depth assessment of skeletal muscle bioenergetics is lacking. Therefore, phosphorous magnetic resonance spectroscopy (31P-MRS) was utilized to assess skeletal muscle bioenergetics during a 5-min all-out intermittent isometric knee-extensor protocol in 8 healthy men. The metabolic perturbation, adenosine triphosphate (ATP) synthesis rates, ATP cost of contraction, and mitochondrial capacity were determined from intramuscular concentrations of phosphocreatine (PCr), inorganic phosphate (Pi), diprotonated phosphate (H2PO4⁻ ), and pH. Peripheral fatigue was determined by exercise-induced alterations in potentiated quadriceps twitch force (Qtw) evoked by supramaximal electrical femoral nerve stimulation. The oxidative ATP synthesis rate (ATPOX) attained and then maintained peak values throughout the protocol, despite a ~63% decrease in quadriceps maximal force production. Thus, the ATPOX normalized to force production (ATPOX gain) significantly increased throughout the exercise (1st min: 0.02 ± 0.01: 5th min: 0.04 ± 0.01 mM·min-1·N-1), as did the ATP cost of contraction (1st min: 0.048 ± 0.019: 5th min: 0.052 ± 0.015 mM·min-1·N-1). Additionally, the pre- to post-exercise change in Qtw (-52 ± 26 %) was significantly correlated with the exercise-induced change in intramuscular pH (r = 0.75) and [H2PO4⁻ ] (r = 0.77). In conclusion, the all-out exercise protocol utilized in the current study elicited a "slow component-like" increase in the intramuscular ATPOX gain, as well as a progressive increase in the phosphate cost of contraction. Furthermore, the development of peripheral fatigue was closely related to the perturbation of specific fatigue-inducing intramuscular factors (i.e. pH and [H2PO4⁻ ]).
- ATP synthesis
- ATP cost
- Magnetic resonance spectroscopy
- muscle metabolism
- neuromuscular fatigue
- Copyright © 2016, Journal of Applied Physiology