Neuromuscular fatigue and metabolism during high-intensity intermittent exercise

Purpose: To examine the degree of neuromuscular fatigue development along with changes in muscle metabolism during two work-matched high-intensity intermittent exercise protocols in trained individuals.

Methods: In a randomized, counter-balanced, crossover design, eleven endurance-trained men performed high-intensity intermittent cycle exercise protocols matched for total work and including either multiple short- (18×5 s; SS) or long-duration (6×20 s; LS) sprints. Neuromuscular fatigue was determined by pre- to post-exercise changes in maximal voluntary contraction force (MVC), voluntary activation level and contractile properties of the quadriceps muscle. Metabolites and pH were measured in vastus lateralis muscle biopsies taken before and after the first and last sprint of each exercise protocol.

Results: Peak power output (11±2 vs. 16±8%, P<0.01), MVC (10±5 vs. 25±6%, P<0.05) and peak twitch force (34±5 vs. 67±5 %, P<0.01) declined to a lesser extent in SS than LS, while voluntary activation level decreased similarly in SS and LS (10±2 vs. 11±4%). Muscle [PCr] prior to the last sprint was 1.5-fold lower in SS than LS (P<0.001). Pre- to post-exercise intramuscular accumulation of lactate and H⁺ was two- and three-fold lower, respectively, in SS than LS (P<0.001), whereas muscle glycogen depletion was similar in SS and LS. Rate of muscle glycolysis was similar in SS and LS during the first sprint, but two-fold higher in SS than LS during the last sprint (P<0.05).

Conclusion: These findings indicate that, in endurance-trained individuals, multiple long-sprints induce larger impairments in performance along with greater degrees of peripheral fatigue compared to work-matched multiple short-sprints, with these differences being possibly attributed to more extensive intramuscular accumulation of lactate/H⁺ and to lower rates of glycolysis during multiple long-sprint exercise.