Skeletal Muscle Force Production and Bioenergetics During All-out Exercise

Abstract

PURPOSE: To investigate the influence of group III/IV muscle afferent feedback on skeletal muscle force production and bioenergetics during all-out exercise. METHODS: Phosphorous magnetic resonance spectroscopy was performed during a 5-min all-out intermittent isometric single-leg knee-extensor exercise, consisting of 60 maximal voluntary contractions (MVC), with intrathecal fentanyl (FENT), to attenuate group III/IV leg muscle afferents, and control (CTRL) conditions in 8 healthy men (age: 28 ± 5 yrs, stature: 178 ± 4 cm, and body mass: 77 ± 8 kg). Peak, integrated, and mean forces were determined per MVC and critical force (CF) was determined as the mean force of the final 6 MVCs. The intramuscular metabolic perturbation and adenosine triphosphate (ATP) synthesis rates were determined from intramuscular concentrations of phosphocreatine (PCr), inorganic phosphate (Pi), diprotonated phosphate (H2PO4−), ATP, and pH. RESULTS: Peak force (FENT: 595 ± 113 vs. CTRL: 568 ± 126 N) and end-test force (FENT: 224 ± 50 vs. CTRL: 209 ± 52) were not significantly different between conditions. The cumulative integrated force was significantly greater for FENT than CTRL over the 1st min (17557 ± 2581 vs. 16154 ± 2825 N), but not thereafter (Figure 1). End-exercise [PCr] was not significantly different between conditions, while [Pi] and [H2PO4−] were significantly greater for FENT. The estimated total ATP synthesis rate was significantly greater for FENT than CTRL over the 1st min (66 ± 16 vs. 57 ± 13 mM), but not thereafter (Figure 1). The estimated total ATP synthesis rate at CF arose from a significantly greater oxidative ATP synthesis (FENT: 77 ± 15 vs. CTRL: 83 ± 13 %) than anaerobic ATP synthesis (FENT: 23 ± 15 vs. CTRL 17 ± 13 %). CONCLUSION: Attenuation of group III/IV muscle afferent feedback augmented force production during the 1st min of all-out exercise, for which the increased energy demand was met, en masse, by the creatine kinase reaction, glycolysis, and oxidative metabolism.