New findings

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    What is the central question of this study?
    Are exercise-induced changes in metabolites and ions a crucial factor in the adaptation of contracting muscle? This was assessed by manipulating the rest period between high-intensity intervals.
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    What is the main finding and its importance?
    Our results suggest that the perturbation of muscle metabolites (specifically phophocreatine, lactate and H+) during high-intensity interval training is not a crucial factor regulating related adaptations of the contracting muscle, when training intensity and volume are matched. This has implications for understanding the mechanisms that regulate muscle adaptations.

It has been hypothesized that exercise-induced changes in metabolites and ions are crucial in the adaptation of contracting muscle. We tested this hypothesis by comparing adaptations to two different interval-training protocols (differing only in the rest duration between intervals), which provoked different perturbations in muscle metabolites and acid–base status. Prior to and immediately after training, 12 women performed the following tests: (1) a graded exercise test to determine peak oxygen uptake (inline image); (2) a high-intensity exercise bout (followed 60 s later by a repeated-sprint-ability test; and (3) a repeat of the high-intensity exercise bout alone with muscle biopsies pre-exercise, immediately postexercise and after 60 s of recovery. Subjects performed 5 weeks (3 days per week) of training, with either a short (1 min; HIT-1) or a long rest period (3 min; HIT-3) between intervals; training intensity and volume were matched. Muscle [H+] (155 ± 15 versus 125 ± 8 nmol l−1; P < 0.05) and muscle lactate content (84.2 ± 7.9 versus 46.9 ± 3.1 mmol (g wet weight)−1) were both higher after HIT-1, while muscle phosphocreatine (PCr) content (52.8 ± 8.3 versus 63.4 ± 9.8 mmol (g wet weight)−1) was lower. There were no significant differences between the two groups regarding the increases in inline image, repeated-sprint performance or muscle Na+,K+-ATPase content. Following training, both groups had a significant decrease in postexercise muscle [H+] and lactate content, but not postexercise ATP or PCr. Postexercise PCr resynthesis increased following both training methods. In conclusion, intense interval training results in marked improvements in muscle Na+,K+-ATPase content, PCr resynthesis and inline image. However, manipulation of the rest period during intense interval training did not affect these changes.