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Low-volume ‘sprint’ interval training (SIT) stimulates rapid improvements in muscle oxidative capacity that are comparable to levels reached following traditional endurance training (ET) but no study has examined metabolic adaptations during exercise after these different training strategies. We hypothesized that SIT and ET would induce similar adaptations in markers of skeletal muscle carbohydrate (CHO) and lipid metabolism and metabolic control during exercise despite large differences in training volume and time commitment. Active but untrained subjects (23 ± 1 years) performed a constant-load cycling challenge (1 h at 65% of peak oxygen uptake before and after 6 weeks of either SIT or ET (n= 5 men and 5 women per group). SIT consisted of four to six repeats of a 30 s ‘all out’ Wingate Test (mean power output ∼500 W) with 4.5 min recovery between repeats, 3 days per week. ET consisted of 40–60 min of continuous cycling at a workload that elicited ∼65% (mean power output ∼150 W) per day, 5 days per week. Weekly time commitment (∼1.5 versus∼4.5 h) and total training volume (∼225 versus∼2250 kJ week−1) were substantially lower in SIT versus ET. Despite these differences, both protocols induced similar increases (P < 0.05) in mitochondrial markers for skeletal muscle CHO (pyruvate dehydrogenase E1α protein content) and lipid oxidation (3-hydroxyacyl CoA dehydrogenase maximal activity) and protein content of peroxisome proliferator-activated receptor-γ coactivator-1α. Glycogen and phosphocreatine utilization during exercise were reduced after training, and calculated rates of whole-body CHO and lipid oxidation were decreased and increased, respectively, with no differences between groups (all main effects, P < 0.05). Given the markedly lower training volume in the SIT group, these data suggest that high-intensity interval training is a time-efficient strategy to increase skeletal muscle oxidative capacity and induce specific metabolic adaptations during exercise that are comparable to traditional ET.
Prolonged sessions of moderate-intensity exercise (e.g. ≥ 1 h at ∼65% of peak oxygen uptake ), performed repeatedly for at least several weeks, increases skeletal muscle oxidative capacity and alters substrate utilization during matched-work exercise, resulting in improved endurance capacity (Gollnick et al. 1973). Although less-widely appreciated, numerous studies have shown that brief, repeated sessions of ‘all out’ high-intensity or sprint-type interval training (SIT) elicits changes in skeletal muscle energy metabolism that resemble traditional endurance training (ET) (Henriksson & Reitman, 1976; Saltin et al. 1976; Gibala et al. 2006a). The relatively few studies that have directly compared skeletal muscle metabolic adaptations to interval and continuous training have yielded equivocal results, and in all cases the total volume of work performed was similar between groups (Henriksson & Reitman, 1976; Saltin et al. 1976; Eddy et al. 1977; Fournier et al. 1982; Gorostiaga et al. 1991; Edge et al. 2006). Recently, we (Gibala et al. 2006a) examined molecular and cellular adaptations in resting human skeletal muscle after six sessions of SIT or ET performed over 2 weeks. By design, total training time commitment and exercise volume was markedly lower in the SIT group, yet we found similar improvements in the maximal activity of cytochrome c oxidase (COX) and the protein content of COX subunits II and IV after training in both groups. Although previously speculated by others (Coyle, 2005), to our knowledge this was the first study to demonstrate that SIT was indeed a very ‘time-efficient’ strategy to improve skeletal muscle oxidative capacity and exercise performance (Gibala et al. 2006a).
In the present study, we sought to confirm and extend the findings of our previous work showing similar increases in muscle oxidative capacity after 2 weeks of SIT or ET (Gibala et al. 2006a). This previous study was limited in that the duration of training was relatively short and it could be argued that the very intense nature of SIT might stimulate rapid skeletal muscle remodelling (possibly due to altered fibre recruitment), whereas adaptations to lower-intensity ET accrue more slowly. In addition, our previous study involved a single marker of muscle oxidative capacity in resting biopsy samples, and thus provided limited insight regarding the potential for changes in substrate metabolism during exercise. In consideration of these issues, the unique purpose of the present study was to compare changes in markers of skeletal muscle carbohydrate (CHO) and lipid metabolism and metabolic control during matched-work exercise, before and after 6 weeks of low-volume SIT or high-volume ET. The SIT protocol was modelled on previous work in our laboratory and consisted of four to six 30 s ‘all out’ cycling tasks performed three times per week for 6 weeks (Burgomaster et al. 2006; Gibala et al. 2006a). By contrast, the ET protocol was modelled on public health guidelines (American College of Sports Medicine, 1998) and consisted of 40–60 min of continuous cycling at ∼65% 5 days per week for 6 weeks. Whereas previous studies have examined adaptations in resting skeletal muscle after matched-work interval and continuous training (Henriksson & Reitman, 1976; Saltin et al. 1976; Eddy et al. 1977; Fournier et al. 1982; Gorostiaga et al. 1991; Edge et al. 2006), the present study was unique in that we assessed changes during exercise and, by design, the total weekly exercise volume was ∼90% lower in the SIT group (i.e. 225 versus 2250 kJ week−1 in the SIT and ET groups, respectively). We hypothesized that 6 weeks of SIT and ET would induce similar adaptations in muscle oxidative capacity and selected measures of whole-body and skeletal muscle substrate metabolism during exercise despite large differences in total training time commitment and exercise volume.