Discussion
The results of the present study demonstrated that an 8-week novel exercise intervention consisting of very brief but maximal efforts, dispersed over the day, is associated with improved cardiovascular fitness (or V̇O2peak) in females. This beneficial effect occurred, however, independently from changes in body mass and selected cardiometabolic health markers during the training period. Nonetheless, given that maximal aerobic power has been shown to be one of the strongest prognostic markers for cardiovascular health and premature death,13 24 the results indicate the potency of the new dispersed SIT exercise protocol, and more importantly, the cardiovascular fitness improvement was observed despite the time commitment of a mere 1.5 min per day of exercise time (excluding warm-up), for 3 days a week (see table 1).
The underlying mechanism(s) responsible for the observed improvement in V̇O2peak cannot be determined with the design of the present study. Nonetheless, previous reviews, however, have suggested that the enhanced cardiovascular fitness is likely due to enhanced central adaptations such as increases in stroke volume or blood volume25 and/or the result of peripheral adaptation of skeletal muscles by upregulating various mitochondrial enzymes activities such as citrate synthase, pyruvate dehydrogenase, 3-hydroxyacy CoA dehydrogenase and cytochrome oxidase26 27 and increasing mitochondria density.10 28 It has been suggested that increases in the central and peripheral adaptations were due to the very rapid glycogen depletion associated with the WAnT exercise.29 When the WAnT bouts are performed in the typical stack clustered protocol, only the first WAnT bout is fuelled predominantly by muscle glycogen with the subsequent bouts involving a greater contribution of fats oxidation.15 Therefore, in the present dispersed protocol, it may be argued that each bout of the WAnT over the day was performed with the highest usage of muscle glycogen during each 30 s effort causing a relatively greater muscle glycogen depletion over the three bouts.
The findings of the present study extend the previous literature of SIT’s efficacy10 13 by showing that the sprint repetitions can be separated over long durations, yet still maintaining the positive effects. One possible reason for the sustained training-induced effects is that when the WAnT bouts are separated by long periods of rest or recovery, or performed in a dispersed manner as in the present study, the exercise stimulus of each bout is maximised. This is supported by the finding that there was no significant difference in MPO of WAnT bout 1 (ie, performed early in the morning) relative to the MPO of WAnT bout 3 (performed in the late afternoon) observed during training week 3 and week 8. Indeed, this implies that each of the WAnT performed at three times of the day would have resulted in the maximal activation of the metabolic pathways or maximum increases in production of specific metabolites or higher signalling impulses between different kinases, transcriptional regulatory proteins and cells relative to a situation in which the three WAnT bouts were performed in a clustered fashion which likely lead to the enhanced training-induced adaptations within the sedentary females.
Considering the commonly reported barrier of lack of available time for exercise,2 there is also a need to design an exercise protocol that provides adequate fitness and health benefits and enhances motivation to perform exercise by overcoming key barriers. One possible alternative strategy could be to define the minimum volume of maximal sprint exercise required to improve both fitness and health indices whilst also increasing exercise adherence. To date, the exercise protocol utilised in the current study represents the smallest volume of exercise (when using total training time per week), relative to recent studies, that has been shown to induce positive effects on peak oxygen uptake, albeit in sedentary, low-level of fitness individuals (see table 1 for comparisons).
There is a dearth of sufficient evidence in current literature to indicate that a larger number of sprint repetitions or a greater total volume of sprint exercise will lead to superior cardiovascular or metabolic adaptations. Metcalfe and colleagues13 showed that very low volumes of supramaximal exercise–2×20 s exercise session–are associated with improvements in V̇O2max typically seen with much higher volumes of sprints.4 30 31 Recent studies10 13 have demonstrated that the SIT exercise repetitions may be as low as 2–3 bouts of 20 s per day to provide a positive stimulus, however, when reduced by Songsorn and colleagues14 to just a single bout of 20 s per day, performed three times per week on alternate days, this positive effect was lost. We hypothesised that there would be a significant deficit of stimulus if these 30 s bouts are performed only once per exercise day, especially when compared with a clustered 3×30 s WAnT protocol bouts. Therefore, we maintained the number of bouts as previous studies10 13 but eliminated the need for recovery time between bouts by integrating an extended 4-hour recovery interval period as part of the individual work day. This 4-hour duration provided full recovery from each bout and therefore, allowed our participants to undergo three sprint bouts per exercise day without appreciable accumulated fatigue. It should be noted that the absolute number of bouts performed per day in the present study is similar to the number of bouts per day employed in recent SIT protocols (table 1). Hence, these two key strategies: (1) completion of a greater absolute workload for the second and third Wingate bouts of the day through dispersion and (2) increasing the daily volume of SIT exercise with the 4-hour resting period which allowed 3 maximal sprint bouts per day of exercise, are likely responsible for the positive change in V̇O2peak observed in the present study.
Blood lipids are well-established metabolic risk factors that are associated with cardiovascular diseases.32 33 The present study’s dispersed WAnT bouts protocol showed no impact on blood lipids, which was similarly observed in other traditional or typical SIT protocols. For example, six sessions of 4–6 WAnT bouts (with 4.5 min recovery between bouts) performed over a 2-week period showed no change in blood lipids profile in sedentary overweight, obese men.34 Similarly, repeated cycle efforts of 8 s with 12 s recovery over a 20-min period, performed 4 days a week for 5 weeks, did not show any significant change in the blood lipid profile of obese females.35 However, the intensity of cycling in the latter study was estimated to be ~80% of VO2max and not of supramaximal. Interestingly, a recent study showed improvement in LDL but not in HDL and total cholesterol; however, the study’s SIT consisted of four clustered 20 s sprint at 175% VO2max, three times per week for 12 weeks.36 The combined findings of these studies suggest that a positive change in lipid profiles is dependent on a dose–response of energy expenditure, exercise duration and/or volume of exercise.35 Thus, the present study with its low volume exercise profile was unlikely to exert any positive effects on the individuals’ lipids profile.
High-intensity interval training has been shown to be effective in reducing body fat in overweight females.37 In the present study, however, there was no significant change in body mass and skinfolds measurements of the participants after 8 weeks of SIT training. The positive effects for body fat reduction were not achieved even though most of the participants were overweight females (mean BMI: 24.9±4.1 kg/m2, based on Asian population). However, there is substantial difference in the mean BMI between participants in the study of Mirghani and Yousefi37 and those in the present study (29.5±3.5 vs 24.9±4.1 kg/m2, respectively). Based on current weight loss concepts, individuals with high initial weight typically have faster initial weight loss due to higher caloric expenditure for similar intensity and duration of exercises and increased ease in initial diet caloric reduction. This is supported by the findings of Nackers et al,38 who demonstrated that individuals with higher BMIs have higher initial rates of weight loss. Moreover, our exercise protocol was relatively short with 8 weeks of training, compared with the usual training duration of 6 months or more, to achieve effective weight loss. These differences in initial weight and BMIs, in addition to the shorter exercise period and exercise volume of the present exercise protocol, may have resulted in a much slower rate of fat and weight loss. Another potential reason could be the low energy expenditure of the current dispersed exercise protocol. When SIT bouts are performed in a clustered fashion, the last few bouts are fuelled primarily by fat oxidation rather than via anaerobic glycolysis15 with greater levels of excess post-exercise oxygen consumption per exercise session, which can lead to a higher overall fat loss. Thus, it is very likely that the present dispersed protocol resulted in each WAnT bout utilising primarily glycogen as opposed to fats. This, in addition to the likelihood of lower excess post-exercise oxygen consumption for the entire day’s exercise may have led to the low body fat loss in our female subjects, although direct investigation is required to confirm our assertion.
Even though the present study dispersed exercise protocol provides a sufficient training stimulus for a robust increase in V̇O2peak, it did not reduce the incidence of non-response, similar to previously reported study.39 The present study reports an overall rate of 37.5% non-responders for V̇O2peak across all participants studied (6 out of 16 participants), with a single adverse responder. It was suggested that the incidence of non-response following SIT is likely due to individual variability in adaptive response to the training stimulus.40 41 Recently, however, Montero and Lundby42 showed the abolishment of all non-response to exercise training in individuals who trained an additional 240 min per week for 6 weeks, after they initially showed no training-induced adaptations after exercising between 60 min and 300 min per week for 6 weeks. Therefore, it is possible that a low training stimulus or insufficient training effort could be a significant factor resulting in the non-response, rather than individual genetic variation to exercise adaptation.42 While the exact dose of SIT required for more effective training-induced adaptations remain unclear and likely to be subjected to individual variability, it would appear that increasing the number of training days per week from 3 to perhaps 4–5 days is an important consideration, at least in the case of the present ‘dispersed’ WAnT bouts exercise protocol.
Our study is not free of limitations. Dietary intake outside the training intervention was not monitored. The present study employed a repeated measure design with no control group, which may introduce bias. However, many previous studies have already validated the positive effects of SIT on maximal aerobic power, and recent studies conducted were similarly conducted without a control group as in the present study (see table 1). Finally, the present study provides information on exercise training and cardiorespiratory fitness within a small sedentary female population (n=16), and therefore, there is clearly a need to conduct larger population-based studies to determine the effectiveness of this dispersing SIT protocol. Nonetheless, a major advantage of the present study’s novel ‘dispersal’ protocol for performing the WAnT bouts is that it is ideal for the practical ‘real world’ purpose of fitting the exercise easily into the daily schedule of an individual office worker, and concurrently, using his or her work time in between the bouts as a natural ‘built-in’ recovery period. With this new strategy, exercise duration was effectively shortened to 1.5 min per day (3 bouts of 30 s of exercise time) or a total time commitment of only 13.5 min per week (including warm-up).