There are multiple components to fitness ranging from cardiovascular endurance to muscular strength (27,29). It is well established that aerobic endurance training (ET) causes significant improvements in numerous health and performance variables, especially cardiovascular endurance (27,29,38,43). While ET enhances some fitness components, several investigations manipulate the variables within this training modality to see if greater physical enhancements may be made, such as concurrently increasing aerobic capacity and anaerobic power (23).
To this end, the most commonly observed comparisons are between ET and interval training (IT). Interval training protocols consist of periods in which individuals exercise at increased intensity followed by periods of recovery, which can be either passive or active (2,5,10,42). Previous investigations asking whether 1 style of training is superior to the other have demonstrated equivocal results for multiple reasons. For example, some conflicting findings might be attributed to differences in work matching; IT is compared to ET at similar (21,34) or different (19,23) total workloads. Moreover, there are broad variations in training protocols, such as exercise mode (17,23,26,32,38,39), training duration (7–9,19,23,32), training intensity (7,8,17,32,37–39), work-to-rest ratio (7–9,17,26), and populations studied (16,30), that all may subsequently affect the comparisons.
A specific type of IT called “sprint interval training” (SIT), defined by an exercise intensity of V̇o2max ≥130%, is mainly implemented in a training program to improve power output, anaerobic capacity, and sprint ability (20,31). However, SIT with active recovery (SITAR) is rarely used, and even fewer studies compare it with ET. Interestingly, several studies have demonstrated that the addition of active recovery during SIT may give an advantage over passive recovery with this training modality (10,42). Therefore, the purpose of this study was to compare the effects of 10 weeks of SITAR to ET in time- and work-matched protocols on subsequent aerobic and anaerobic power, muscular strength, and sprint time in college-aged adults to help answer the question, does altering intensity with SITAR elicit greater training adaptations compared with ET?
Experimental Approach to the Problem
The current experimental study was designed to examine and compare the effects of SITAR and ET on aerobic and anaerobic power, muscular strength, and sprint time. Our aims were (a) to choose a field training protocol that would be easy to administer to large groups across training and competition levels, (b) to design a SITAR program that would meet athlete energy demands involving mainly ATP-PC and anaerobic glycolysis during work intervals and aerobic metabolism during active recovery, and (c) to design the SITAR protocol to produce similar total work when compared with ET at approximately 75% V̇o2max on a 400 m Olympic track. Therefore, several pilot studies were performed before experimental procedures to determine the work-to-active-rest ratio of the SITAR. One to 3 (1:3) work (200 m sprint) to active recovery (200 m rapid walk/light jog) intervals in the SITAR group produced a similar training distance as ET at approximately 75% V̇o2max for 30 minutes. To test the experimental design's effectiveness, we chose dependent variables that measured the transfer effect of training on isokinetic strength at 60 and 300°·s−1, 50 m sprint, anaerobic running test, and V̇o2max.
Forty-eight healthy recreationally active male and female college students were recruited for this study. The 2 groups initially consisted of 24 subjects each, but 2 subjects from the ET group and 4 from SITAR did not complete the full protocol and were excluded from statistical analysis. Results are presented for 22 participants (12 female, 10 male) in the ET group (mean ± SD, age = 24.0 ± 5.2 years (age range: 18–40 years), height = 167.4 ± 7.4 cm, body mass = 71.8 ± 14.4 kg) and 20 participants (11 female, 9 male) in the SITAR group (mean ± SD, age = 22.2 ± 3.8 years, height = 169.8 ± 11.0 cm, body mass = 68.0 ± 15.3 kg). None of the subjects had been involved in organized endurance or SIT before the study. They had no current injuries and had not undergone any rehabilitation for an injury within the 6 months before the first session. All subjects were informed of the purpose, procedures, benefits, and risks of the investigation before signing the institutional review board–approved informed consent form and medical history questionnaire. The California State University Fullerton Institutional Review Board approved the use of human subjects.
All subjects participated in pretraining and posttraining tests; the procedures were identical for both. Subjects were familiarized with testing protocols 1 week before pretraining testing to eliminate learning effects. None of the subjects showed a contraindication to the tests or regular training. The subjects were asked to abstain from moderate to heavy exercise for at least 48 hours and meals for at least 3 hours, before their visits to the laboratory for testing. Both the pretraining and posttraining test procedures required collecting data on 2 separate laboratory visits, with at least 48 hours rest between the visits. All testings were performed in the morning hours before noon.
The first laboratory visit measured body mass, height, and composition, and maximal oxygen consumption (V̇o2max). Body mass was measured, with each participant in shorts and a t-shirt, using a digital platform scale to the nearest 50 g (Mettler-Toledo International, Inc., Columbus, OH, USA) and height without shoes to the nearest cm using a laboratory-constructed wall stadiometer. Body composition was determined using a 3-site Jackson and Pollack (J-P) skinfold method (25) using Harpenden Skinfold Calipers (HaB International Ltd., Warwickshire, United Kingdom). The right side of the participant's body was marked and measured at 3 sites in sequence: for males—chest, abdomen, and thigh; for females—triceps, suprailium, and thigh. The sequence was repeated and recorded 3 times for each person. The average result from each site was calculated and used to determine the percentage of fat. The maximal oxygen consumption (V̇o2max) test was performed using a treadmill (Trackmaster Treadmills, Inc., Newton, KS, USA), according to the Bruce protocol (3). Subjects ran to exhaustion wearing a Polar heart rate monitor (Polar, Oulu, Finland). Heart rate (HR) and ratings of perceived exertion were recorded every 3 minutes, 30 seconds before advancing to the next stage. At the end of the test, participants cooled down until HR returned to 120 b·min−1. During testing, oxygen uptake, carbon dioxide production, minute ventilation, and respiratory exchange ratio were continuously monitored using a ParvoMedics TrueOne 2400 metabolic cart (ParvoMedics, Inc., Sandy, UT, USA).
In the second laboratory visit, subjects performed two 50 m sprints on a 400-m outdoor track, an isokinetic strength test, and finally, 2 trials of an anaerobic treadmill test, with 30 minutes rest between the different tests. Each subject warmed up with light jogging followed by three 50-m runs at a moderate pace before the 50-m sprint trials. Subjects then ran a 50-m sprint at maximal effort using a specific starting position (one foot in front of the other, knees bent) with a low center of gravity and forward lean on the rubberized Olympic track (3). A stopwatch was started as soon as the subject made the first movement to sprint and stopped as soon as the runner's foot or head broke the plane of the finish line. Trial times were recorded to the closest 100th of a second with the faster time used for analysis. Subjects rested for 3 minutes between the 2 trials.
Next, isokinetic strength was assessed on a Biodex System 3 (Biodex Medical Systems, Inc., Shirley, NY, USA) by measuring subjects' right leg extension and flexion peak torque at 60 and 300°·s−1. Each subject sat in an upright position with the hip flexed at 90°, the knee joint's axis of rotation was identified and the input shaft of the dynamometer was visually aligned with this axis at the lateral epicondyle. Straps were used to stabilize the torso, hip, and thigh, and the range of motion of each subject's extension and flexion was determined. Subjects performed 1 light warm-up set followed by the extension-flexion trials. For these trials, each subject performed 7 maximal concentric repetitions first at 300°·s−1 and then at 60°·s−1, with 3 minutes passive seated recovery between the 2 sets. During the test repetitions, subjects were given continuous verbal encouragement to work as hard and fast as possible. The 60 and 300°·s−1 test results were computer-generated and stored in the Biodex System 3, and peak torque values were used for analysis.
Finally, the anaerobic treadmill test was performed on a Trackmaster treadmill at 12.8 km·h−1 (8 mph) at 20% incline (3,11). Subjects began with a 5-minute walk for warm-up at 20% incline and practiced getting on and off the treadmill at a slow speed. Subjects began the trials with a running start at 12.8 km·h−1 (8 mph) at the same 20% incline, holding the handrails. As the subjects released their grip, the stopwatch was started; it was stopped once they grabbed the handrail at the point of exhaustion. Verbal encouragement was provided throughout the test. Each subject was given 2 trial opportunities, recorded to the closest 100th of a second, with the faster time used for analysis. Not all participants completed the second trial within 15 minutes of recovery, because of extreme exhaustion. The first trial time was used in such cases.
After pretesting, subjects were assigned randomly to 1 of 2 training groups: SITAR or ET. SITAR and ET groups trained 3 times per week for 10 weeks under researcher supervision on a rubberized 400-m Olympic track. Training for ET was approximately 75% of V̇o2max for a duration of 30 minutes for the first 4 weeks, increased at week 5–35 minutes, and at week 8–40 minutes. During training, ET subjects wore HR monitors to control their pace and adjust exercise intensity. They ran without rest for the full duration of the training session at the appropriate HR calculated from pretraining V̇o2max values. At the end of each training session, each participant's average HR, determined by the HR monitor, was recorded.
The SITAR protocol consisted of a 1:3 work-to-active-rest ratio in an attempt to produce a similar running distance (workload). Heart rate monitors were used to determine peak and recovery HR each sprint and recovery, and average HR responses each session. For work intervals, SITAR subjects sprinted as fast as possible at an intensity approximately 130–150% of their V̇o2max until the 200-m mark. Active recovery intervals required rapid walking or light jogging to cover the second 200 m to the finish line in 3 times the original sprint time. For example, if a subject ran the first 200 m in 30 seconds, the subject would walk or jog the second 200 m in 90 seconds, and the 400 m lap would take a total of 2 minutes. When the 200 m active recovery was complete, subjects resumed work intervals. This design allowed half of the total distance (workload) to be covered by active rest intervals by the SITAR group during the study. The SITAR exercise duration increased parallel to the ET increase, at weeks 5 (35 minutes) and 8 (40 minutes).
Both ET and SITAR training took place in 2 iterations to meet the study's goal for number of subjects. The study took place over an academic year in Southern California; the Fall group trained September through December and the Spring group trained February through May. Subjects trained from 7 am to 10 am in temperate weather in 1 setting as a group or as individuals. For the duration of the study, all subjects refrained from other endurance and resistance training and all sports events. Subjects were asked to maintain their current nutritional status for the entire study, and they were encouraged to hydrate with water during training. After 10 weeks of training, the tests given at the beginning of the study were re-administered.
All data were analyzed using the statistical package SPSS 22 (SPSS, Inc., Chicago, IL, USA). Subject characteristics are presented as descriptive statistics for age, height, body mass, body composition, treadmill time, V̇o2max, sprint time, and isokinetic strength measurements. Measures of central tendency and variations were calculated for all variables, and outliers (>2 SD) were identified and analyzed for confounding factors. The effects of the 2 training programs on treadmill time, V̇o2max, sprint time, and isokinetic strength at 60 and 300°·s−1 were assessed with a 2-way repeated-measure analysis of variance with factors group (ET and SITAR) and time (before and after). In the event of a significant F score, Fisher's LSD or paired t-test was used post hoc to determine pair-wise differences. A t-test was used to see the effect of training within groups. The effect size (ES) was calculated for all dependent variables between pre- and post-training (24) using Cohen's d formula: ES = (Mpost − Mpre)/SDpooled, where Mpost is the mean post-training measure, Mpre is the mean pretraining measure, and SDpooled is the pooled SD of the pre- and post-measurements. The magnitude of the difference was considered trivial (ES <0.2), small (ES >0.2), moderate (ES >0.6), or large (ES >1.2). The accepted level of statistical significance was set at p ≤ 0.05 and the results are presented as mean ± SD.
At the end of the training period, total training sessions, total training time, training time per session, total training distances, and average training HR were similar between the SITAR and ET groups (Table 1), only average training HR and number of laps per session differed. At the beginning of training, body mass was 68.0 ± 15.3 and 71.8 ± 14.4 kg, for SITAR and ET, respectively. Only the body mass of the SITAR group significantly decreased at posttraining, from 68.0 ± 15.3 to 66.7 ± 15.0 kg. However, statistical analysis revealed that both groups showed significant reductions in percent body fat (Table 2). The ES was trivial for change in body mass for both groups and percent fat for the ET group, whereas the ES was small (ES = 0.25) for change in percent fat with SITAR.
The relative V̇o2max values significantly increased at posttraining 15.6 and 8.7% for SITAR and ET, respectively. SITAR had moderate ES (ES = 0.77) and ET had small ES (ES = 0.42). After 10 weeks of training, the SITAR group demonstrated significantly greater improvement in V̇o2max compared with ET (Figure 1 and Table 3).
The training resulted in significant improvement in anaerobic treadmill and 50 m sprint time in both groups. Anaerobic treadmill time improved 32.3% in SITAR and 17.0% in ET (Figure 1 and Table 3). At the end of training the results were statistically different between SITAR and ET. Training resulted in a large ES (ES = 0.90) in SITAR and a small ES (ES = 0.29) in ET. Both groups improved their 50 m sprint time with training. Sprint time improved 7.58 and 2.94% with SITAR and ET, respectively. Sprint IT with active recovery's improvement was significantly greater than ET's (Figure 1 and Table 3). Posttraining, SITAR showed a moderate ES (ES = 0.61) and ET had a small ES (ES = 0.22).
The results of the isokinetic test showed no significant differences between the groups at 60 or in 300°·s−1 leg extension and leg flexion (Table 4). No isokinetic strength values changed with training, except for significant improvements in the SITAR group 300°·s−1 leg extension of 7.6% and flexion of 12.8%. Similarly, SITAR and ET showed a trivial ES in leg extension at 60 and at 300°·s−1 and flexion at 60°·s−1. Training in both groups resulted in a small ES in leg flexion at 300°·s−1.
This study compared the effects of 10 weeks of SITAR and ET on aerobic and anaerobic power, muscular strength, and sprint time in college-aged females and males. Because previous studies recognized the importance of similar workloads for effective comparison of training modes (21,34,37), across the multiple components of fitness, this study aimed for and accomplished similar training sessions, training time, training time per session, and total training distance (workloads) for SITAR and ET over the 10 weeks (Table 1). Yet, these similar workloads for SITAR and ET drew on different metabolic processes for energy utilization (9,14,18,28,37).
The main question of this study was whether altering intensity with SITAR could elicit the same aerobic benefits as ET, with additional benefits to anaerobic power, muscular strength, and sprint time. As expected, both training groups showed significant improvement in aerobic fitness with improvements in V̇o2max of 15.6% with a moderate ES (ES = 0.77) and 8.7% with a small ES (ES = 0.42) in the SITAR and ET groups, respectively (Figure 1). Sprint IT with active recovery and ET results fall within the range of improvement previously reported for these training modes, 6–23.2% (16,21,34,37). Although similar results between SIT and ET groups have been previously reported (16,34), in the current study the SITAR group had significantly greater relative V̇o2max compared with ET. The significant reduction in body mass in the SITAR group may have contributed to this finding because there were no group differences in absolute V̇o2max results.
Previous studies have shown that ET improves oxidative capacity through an increased rate of oxygen delivered, an increased rate of oxygen extraction by the muscle (14,37), an increase in oxidative enzymes (15,16,40), and an increase in mitochondrial density and capillaries (1). The increases in oxidative capacity from intense SIT seem to be much more complicated than ET (17,32,37), because of having different values for V̇o2 at work intervals and at active or passive rest intervals. The response mechanisms by which IT enhances V̇o2max are still not clearly known (17,37). The improvement in oxidative capacity seen with SIT could be the result of (a) hypoxic effects by reducing the O2 level in the muscle, and increased mitochondrial respiration, or (b) improved muscle buffering capacity. Earlier studies demonstrated that the hypoxia of intense work intervals, similar to that stimulated by hypoxia in high altitude, may increase V̇o2max because of the increase in mitochondrial volume, in capillarization of muscle, and in mitochondrial oxidative enzymes (26,32,36,37). A recent study by Granata et al., however, demonstrated improvement in aerobic capacity of skeletal muscle through mitochondrial respiration, because neither mitochondrial content nor enzymes had changed. Another possible mechanism in improved oxidative capacity might be an improved muscle buffering capacity (19) through enhanced lactate threshold with SIT (12,17,26). Recent studies demonstrated that lactate (La−) might be an important substrate in cell respiration that is oxidized within skeletal muscle cells, and increased La− transporter proteins (MCT 1 and 4) in mitochondria could be the evidence (6,35). Active recovery after sprint intervals in the current study might additionally upregulate La− oxidation and rate of La− clearance by active muscle, which in turn may explain why increased exercise intensity improves aerobic capacity through buffering capacity (19).
Anaerobic power is measured here using a treadmill run (12.8 km·h−1 at 20% incline), representing approximately 125% of V̇o2max intensity (3,11). In this study, the average treadmill running time was between 27 and 38 seconds for the SITAR and ET groups. This time frame represents the short intense exercise that demands energy contributions mainly from the glycolytic and ATP-PC pathways (17,32,36). Average running time to exhaustion after SITAR was significantly greater than after ET, increasing 32% with a large ES (ES = 0.90) and 17% with a small ES (ES = 0.29) for SITAR and ET groups, respectively. A previous study reported similar improvements (23%) in a short exhaustive treadmill test (12.8 km·h−1 with 20% incline) after mixed concurrent (both interval and continuous) training. The study did not attempt to separate the contributions of SIT and ET (11). Jacobs et al. found no improvement in peak or mean power in a 30-second anaerobic Wingate test after 6 weeks of IT, although they had improvements in % fast-twitch (FT) oxidative fibers and both glycolytic and oxidative enzyme activity. They concluded that Wingate testing is less sensitive in measuring anaerobic capacity. Because SITAR participants trained by sprinting the 200 m until completion (25–45 seconds) as fast as they could, their significant improvement compared with ET may be due to the specificity of their training, which may have improved (a) ATP-PC and glycolytic energy systems (26,32,36), (b) buffering capacity and tolerance to La− (12,17,19,35,36), or (b) % FT oxidative fibers (26).
Performance in the 50-m sprint test is highly dependent on the capacity and rate of activating ATP-PC pathways and on the rate of myosin ATPase activity in skeletal muscle fibers (41). Both groups improved 50-m running time at posttraining by 7.58% with moderate ES (ES = 0.61) and 2.94% with a small ES (ES = 022) in SITAR and ET, respectively. However, SITAR subjects showed significantly greater improvement in their sprint times than ET. Similar improvements were found in 40 m sprint time in professional soccer players with SIT in season (13) and in 25 m sprint time after 8 weeks of sprint training in untrained participants (41). This study's significant sprint time improvement with SITAR compared with ET may point to an increase in % FT muscle fiber (26) or increased ability to utilize the ATP-PC energy system (16,26,32,36,41). It should be noted that the kinetics of running during SITAR mimics that of sprinting as opposed to submaximal running and may have contributed to the SITAR group's sprint results because of improved running economy (4).
Isokinetic strength testing was used to measure muscular strength at 60°·s−1 and at 300°·s−1. Ten weeks of training did not elicit improvements in 60°·s−1 (SITAR) and 60 and 300°·s−1 (ET) isokinetic leg extension and leg flexion. Similarly, no significant improvement in muscular strength was found in isometric knee extension torque (33) and 1RM strength in squat (29) in previous ET studies. Thorstensson et al. showed improvement in maximal isometric contraction after sprint training. However, their study was limited by the subject pool; they studied only 4 young individuals younger than 18 years. In the current study, the only significant improvements compared with pretraining values were seen in 300°·s−1 isokinetic leg extension and flexion with a small ES (ES = 0.19 and ES = 0.24, respectively) with SITAR. The specific training in SITAR more closely resembles skeletal muscle shortening velocity with the faster 300°·s−1 test than the slower 60 test. Muscle strength is usually considered to be closely related to type of muscle fibers and amount of muscle mass (29). Because SITAR did not elicit increased lean body mass, the improvements in isokinetic strength at 300°·s−1 after SITAR may be attributed to an increased rate of muscle fiber recruitment and rate of myosin ATPase activity (41), improved capacity of the ATP-PC system (36), or increased % FT muscle fiber area (26,41).
This carefully controlled 10-week training study yielded significant improvements in V̇o2max, anaerobic power, and sprint time in both SITAR and ET groups in training adaptation. The SITAR group, however, had significantly greater improvements in sprint, and anaerobic power than ET. Our speculation is that the improvements in the SITAR group were more closely associated with changes in the skeletal muscle properties such as improved ATP-PC and glycolytic enzyme activity, buffering capacity, fiber type, contractility, and running kinetics.
Selecting the right exercise mode and intensity in a workout program is vital for improving an athlete's aerobic and anaerobic capacity and athletic performance in sports. For that purpose, the most common modality is combining ET early in season and sprint training later, both accompanied with a resistance training program. These may be taxing when the athlete is also seeking to improve sport-specific skills. The current study uses recreationally active but nontrained subjects to mimic preseason conditions and test whether SITAR would produce greater training adaptations in anaerobic power, sprint ability, and muscular strength than ET yet similar aerobic benefits. Our findings suggest that SITAR (1:3 work-to-recovery [time] ratio using 200 m sprint and 200 m active recovery distances) is the superior training program compared with ET. Using it early in the preseason targets multiple fitness components, including relative V̇o2max and performance in the anaerobic treadmill test and 50 m sprint time. Authors recommend that SITAR might have a greater transfer effect of training to sports settings such as soccer, basketball, tennis, badminton, lacrosse, ice and field hockey that depend on high intensity intermittent work interrupted with light recovery. Sprint IT with active recovery training distance could be increased or decreased depending on (a) athlete's current fitness level, (b) targeted energy system, (c) time in season, and (d) physiological and fitness demand of the sport. For example, using a SITAR training program with 1:3 work-to-recovery ratio with 100, 50, or 20 m sprints with equal recovery distances specifically in middle or late season would additionally improve sprint ability. Our findings suggest that SITAR is a time-efficient strategy to induce rapid adaptations in V̇o2max comparable to ET with added improvements in anaerobic power, isokinetic strength and sprint time not observed with ET.
The authors thank all the participants in the study, and they acknowledge the kind help of Dr. Guillermo Noffal, and the help of Birgitta Grothues and John Hampton during data collection. The results of the present study do not constitute endorsement by the National Strength and Conditioning Association. The authors have no conflicts of interest to disclose.
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