Altering the rest interval during high-intensity interval training does not affect muscle or performance 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 ((V) over dot(O2peak)); (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 (V) over dot(O2peak), 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 (V) over dot(O2peak). However, manipulation of the rest period during intense interval training did not affect these changes.