Intermittent and continuous high-intensity exercise training induce similar acute but different chronic muscle adaptations

New Findings: What is the central question of this study? How important is the interval in high-intensity interval training (HIIT)? What is the main finding and its importance? The intermittent nature of HIIT is important for maximizing skeletal muscle adaptations to this type of exercise, at least when a relatively small total volume of work is performed in an all-out' manner. The protein signalling responses to an acute bout of HIIT were generally not predictive of training-induced outcomes. Nonetheless, a single session of exercise lasting <10min including warm-up, performed three times per week for 6weeks, was sufficient to improve maximal aerobic capacity. High-intensity interval training (HIIT) performed in an all-out' manner (e.g. repeated Wingate tests) is a time-efficient strategy to induce skeletal muscle remodelling towards a more oxidative phenotype. A fundamental question that remains unclear, however, is whether the intermittent or pulsed' nature of the stimulus is critical to the adaptive response. In study1, we examined whether the activation of signalling cascades linked to mitochondrial biogenesis was dependent on the manner in which an acute high-intensity exercise stimulus was applied. Subjects performed either four 30s Wingate tests interspersed with 4min of rest (INT) or a bout of continuous exercise (CONT) that was matched for total work (67 +/- 7kJ) and which required approximate to 4min to complete as fast as possible. Both protocols elicited similar increases in markers of adenosine monophosphate-activated protein kinase (AMPK) and p38 mitogen-activated protein kinase activation, as well as Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1) mRNA expression (main effects for time, P0.05). In study2, we determined whether 6weeks of the CONT protocol (3days per week) would increase skeletal muscle mitochondrial content to a similar extent to what we have previously reported after 6weeks of INT. Despite similar acute signalling responses to the CONT and INT protocols, training with CONT did not increase the maximal activity or protein content of a range of mitochondrial markers. However, peak oxygen uptake was higher after CONT training (from 45.7 +/- 5.4 to 48.3 +/- 6.5mlkg(-1)min(-1); P<0.05) and 250kJ time trial performance was improved (from 26:32 +/- 4:48 to 23:55 +/- 4:16min:s; P<0.001) in our recreationally active participants. We conclude that the intermittent nature of the stimulus is important for maximizing skeletal muscle adaptations to low-volume, all-out HIIT. Despite the lack of skeletal muscle mitochondrial adaptations, our data show that a training programme based on a brief bout of high-intensity exercise, which lasted <10min per session including warm-up, and performed three times per week for 6weeks, improved peak oxygen uptake in young healthy subjects.