Endurance run performance is important for individuals of average cardiorespiratory fitness who aspire to improve their fitness and performance quickly and efficiently. Moreover, considering the importance of cardiorespiratory fitness and endurance performance in ball sports such as soccer, Australian Rules Football, and cricket (5,15,18), there is immense coaching emphasis on reducing training load to minimize injury risk but maximize training time to focus on other skills important to the sport. Sprint interval training (SIT) is an alternative mode of training to traditional constant-rate training as SIT improves cardiorespiratory fitness to a similar degree as constant-rate training but requires 90% less weekly energy expenditure and 66% less total training time (7).
Typically SIT consists of short, maximal efforts of 15–30 seconds, repeated several times interspersed with 3–5 minutes of recovery. Indeed, SIT may be more effective at improving endurance performance than both continuous running and other high-intensity interval training (HIT) programs. For example, 20 individuals who completed 7–12 efforts of 30-second sprints thrice weekly for 6 weeks, improved 3-km run performance greater than individuals who completed constant-rate running or four to six 4-minute efforts at 3-km average running velocity, over 6 weeks (4). Others have, however, demonstrated that recreationally active participants similarly improve 2-km run performance after either 6 weeks of SIT (4.6%) or constant-rate running (5.9%) (13). Therefore, SIT seems to enhance run performance over 2–3 km after 6 weeks of training, but whether these endurance benefits improve run performance in untrained participants in shorter time frames and over distances exceeding 3 km (a distance requiring greater reliance on aerobic metabolism) is unknown. Furthermore, the lack of inclusion of experimental controls in previous studies (4,6,13) obfuscates the genuine impact of SIT on endurance run performance.
Therefore, the purpose of this study was to determine whether a novel and relatively short-term (4 week), thrice weekly SIT program improves cardiorespiratory fitness and 5-km run performance. To that end, we assessed 5-km run performance and V[Combining Dot Above]O2max before and after either a novel 4-week SIT (four to eight 30-s maximal sprints, thrice weekly) program for cases (n = 20) or no training for controls (n = 10). We hypothesized that relative to the controls, the sprint interval–trained individuals would significantly improve 5-km run performance and V[Combining Dot Above]O2max.
Experimental Approach to the Problem
This study is a controlled trial where healthy participants performed either 4 weeks of SIT or no training, to evaluate the effect of SIT on 5-km run performance and V[Combining Dot Above]O2max. V[Combining Dot Above]O2max was measured during a maximal treadmill test conducted in the University Exercise Physiology Laboratory and the 5-km time trial was assessed at a public running track. These tests were performed on separate days within 1 week of commencing the initial SIT session and within 1 week after the final SIT session.
Thirty, apparently healthy young men (18–25 years) were recruited for this study. Participants were initially screened to ensure they were not currently engaging in any structured high-intensity aerobic exercise training. All participants had not completed any structured aerobic exercise training in the past year. Twenty participants were allocated to the SIT (cases) group, and 10 participants were allocated to the control group.
Participants gave written informed consent, and this study was approved by the University's Human Research Ethics Committee.
Participant V[Combining Dot Above]O2max was assessed during a maximal treadmill test, by pulmonary analysis conducted in an exercise physiology laboratory at the University. Before the V[Combining Dot Above]O2max test, participants were fitted with a 2-way breathing valve (Hans Rudolph, Lenexa, KS, USA), and expired air was collected into an online metabolic system (Moxus Modular, Pittsburgh, PA, USA) for gas (O2 and CO2) analysis. The metabolic system was calibrated before each test using ambient air and gas of known composition. Participants were given a standardized 3-min warm-up at 10 km·hour−1. The V[Combining Dot Above]O2max test was commenced at 10 km·hour−1, and treadmill speed was progressively increased by 1 km·hour−1 every second minute until volitional exhaustion. V[Combining Dot Above]O2max was determined as the highest O2 value averaged over 60-s. All laboratory testing was performed preprandially at the same time of day (8–10 AM). Participants were encouraged to hydrate the night before and morning of testing. Within a week of the V[Combining Dot Above]O2max assessment, each participant completed a supervised 5-km run time trial on a flat running track in a local park. Participants were familiar with 5-km circuit and were advised to hydrate before running. All testing was conducted in the afternoon (4–6 PM). Briefly, participants completed a short 10-min warm-up including some light aerobic exercise and dynamic stretches. Participants were supervised and instructed to run maximally at their own pace.
Cases completed a standardized SIT program performed 3 times a week over 4 weeks (total of 12 sessions). The sprint duration and recovery period was controlled at 30-s and 4-mins (passive), respectively. Participants were requested to run maximally without pacing for each 30-s sprint. All training was conducted on the University's sports oval, and an Accredited Exercise Physiologist (with Exercise and Sport Science Australia) provided participants with verbal encouragement and supervised each training session. The SIT volume increased from 4–8 sprints over the 4-week intervention (Table 1). Before each training session, participants completed a standardized warm-up entailing a 5-min aerobic warm-up, dynamic stretches, and some short (20 m) runs at approximately 70, 80, 90, and 100% effort.
All participants were instructed not to deviate from their current physical activity, exercise (if any), and dietary habits during the 4-week intervention period.
All statistical analyses were performed using IBM SPSS Statistics for Windows (Version 20, IBM Corp., Armonk, NY, USA). Data were tested for normality using the Kolmogorov-Smirnov and Shapiro-Wilk tests, and nonparametric data were log-transformed before further analysis. Paired samples t-tests were used to examine fitness and performance changes after SIT, relative to the control group. To ascertain the test-retest reliability of V[Combining Dot Above]O2max and 5-km time-trial performance, the intraclass correlation coefficient (2, k), coefficient of variation (log-transformed), SEM, and systematic bias (determined by paired samples t-test) were calculated. Significance was set at p ≤ 0.05.
Test-retest reliability data is outlined in Table 2. The 5-km time-trial test had excellent reliability with an intraclass correlation coefficient of 0.99 and a 3.4% coefficient of variation.
Figure 1 outlines the fitness and performance changes after 4 weeks of SIT. Relative to the control group who showed a marginal increase in 5-km time-trial performance (mean ± SD: 1,478 ± 350 to 1,447 ± 347, −2%, p = 0.20, Table 2 and Figure 1), cases had a significant improvement in 5-km time-trial performance by an average of 65 seconds (mean ± SD: 1,464 ± 298 to 1,368 ± 270, −4.4%, p < 0.001) after 4 weeks of SIT (Figure 1). Although absolute and relative V[Combining Dot Above]O2max increased after SIT in cases (mean ± SD: 3.8 ± 0.6 to 4.0 ± 0.6, 4.9%, p = 0.04 and 49.6 ± 4.6 to 51.4 ± 3.8, 4.5%, p = 0.045, respectively), absolute and relative V[Combining Dot Above]O2max were unchanged in controls (mean ± SD: 3.7 ± 0.6 to 3.7 ± 0.6, 0.6%, p = 0.73 and 49.6 ± 6.7 to 49.3 ± 6.7, –0.6%, p = 0.72, respectively, Table 2 and Figure 1).
The purpose of our study was to determine whether a novel and short-term (4 week), thrice weekly SIT program improves cardiorespiratory fitness and 5-km run performance in untrained men. To our knowledge, we are the first to demonstrate that 5-km run performance is significantly improved after short-term (4 weeks) low-volume SIT. Considering the marginal but not statistically significant improvement in the controls (31 seconds, 2%), the marked (96 seconds, 6.5%) improvement after 4 weeks of SIT translated to a mean 65 seconds (4.4%) faster 5-km run performance in cases. Notably, 5-km run performance was improved in conjunction with increased V[Combining Dot Above]O2max (4.5%).
Our data showing SIT enhances 5-km run performance corroborates others showing SIT enhances 2-km (13) and 3-km (4,6) run performance in recreationally trained individuals. We are the first to demonstrate 4 weeks of 3 times per week SIT, in the form of running, significantly improves 5-km run performance—a prestigious and Olympic running distance. Interestingly, the impact of SIT on 5-km endurance performance is established in other exercise modalities. A short-term (2-week) cycle SIT intervention improved 5-km cycle performance to a similar magnitude to our protocol (5.2%) (10). Whether SIT improves run performance over longer distances warrants attention. Additionally, whether SIT benefits already well-trained runners is yet to be fully understood, as data have, to date, been equivocal (1,11). Nevertheless, we verify SIT as an effective means of improving running endurance performance in untrained young men.
The increased 5-km run performance in our study is similar to previously observed improvements after traditional constant-rate endurance training. It was reported that an intervention of 3 running sessions per week at 75% of V[Combining Dot Above]O2max for 6 weeks improved 5-km run time by approximately 80 seconds (∼5%) in 39 untrained individuals (20). Additionally, others have shown that 5-km time trial improved by 78 seconds (5%) in a group that ran for 20 minutes 3 times per week for 6 weeks, initially starting at 0.8 km·hour−1 below their individual lactate threshold speed with progression to 0.8 km·hour−1 above their individual pretraining lactate threshold speed (14). Importantly, a control group was not included in these studies to establish the error of the 5-km time-trial test, which we established was 31 seconds. Collectively, it seems that SIT improves 5-km run performance to a similar extent as constant-rate training but in a quicker time frame (4 vs. 6 weeks) and a smaller training duration (249 minutes vs. 360 minutes) (14).
The improvement in run performance was accompanied by a corresponding improvement in V[Combining Dot Above]O2max. Although previous constant-rate training studies have demonstrated participants increased their V[Combining Dot Above]O2max to a similar magnitude observed in the our study (3.8–7.7%), they also showed marked improvements in their participants lactate threshold, which could have contributed to the improvements in run performance (14,20), particularly because not all cases had improvements to V[Combining Dot Above]O2max. Alternatively, an improvement in cardiac output may have facilitated the improvement in 5-km run speed, as opposed to improvements in muscle oxidative capability, based on the concepts that V[Combining Dot Above]O2max is chiefly governed by the maximal cardiac output and the lactate threshold by muscle oxidative capability (12). Other mechanisms by which SIT may enhance 5-km run performance include improved run economy, running mechanics, and potassium regulation. It was previously reported that moderately trained runners' run economy was improved by 7% after 4 weeks of SIT, but these subject did not improve their 10-km time-trial performance (11). Moreover, increases in knee flexor endurance, coupled with decreases in knee flexion torque and knee flexion/extension ratios are also adaptations associated with sprint training that could have contributed to the improvement in 5-km run performance observed in our study (19). Others have shown in well-trained runners, SIT enhanced key components of the Na+/K+ pump that would aid to minimize disturbances in nerve membrane potential and maintain sprint performance, and this was associated with an approximate 3% improvement in 3-km and 10-km run performance (1). The increase in muscle oxidative and anaerobic enzyme activity are adaptations gained from SIT and is another possible mechanism for the facilitated endurance performance observed in participants from our study (2,3,8,16).
A limitation of our study is that we did not include females. Some of the benefits gained from SIT seem to be dependent of gender. For example, males had greater muscle protein synthesis of proteins important for mitochondrial biogenesis after 9 sessions of SIT compared to their female counterparts (17). Given these muscle adaptations are vital for endurance performance (2,8,9), whether short-term SIT improves 5-km run performance in females is left for future investigations.
Future research could focus on optimizing training variables, such as the recovery time between sprints, number of sprint repetitions, time required of each sprint, number of sessions required per week, and the optimal length of SIT to improve fitness and performance. In fact, the efficiency of HIT in evoking positive adaptations may even be more potent than originally envisaged. It was recently revealed that 1 maximal 4-min effort at 90% of heart rate maximum on a cycle ergometer performed 3 times a week for 10 weeks improves V[Combining Dot Above]O2max by an average of 10% (21).
In conclusion, our data reveal that SIT is a highly effective strategy to improve endurance running quickly in previously untrained young men.
Physical conditioning professionals must be aware of efficient means of exercise training that augment cardiorespiratory fitness and endurance performance. The results from our study support the use of short-term low-volume SIT as an effective means for rapidly improving cardiorespiratory fitness and endurance performance. Therefore, team sport coaches should consider incorporating short-term SIT within periodized programs to improve athlete endurance performance quickly and also to prevent musculoskeletal overuse injuries—a risk associated with traditional forms of exercise that rely on large training volumes to improve endurance performance. In addition to this, SIT has an added advantage of simultaneously enhancing anaerobic qualities required in team sports. Finally, SIT should be encouraged to individuals who perceive time as an obstacle to exercising, as 4 weeks of thrice weekly SIT consisting of four to eight 30-s sprints (just over 4 hours of total exercise, including rest periods) quickly enhances cardiorespiratory fitness and endurance performance.
J. Denham and S. A. Feros are both supported by Australian Postgraduate Award (APA) Scholarships.
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