Excess body fat and the metabolic problems that coincide with obesity are an increasing problem worldwide. Being overweight is associated with long-term health problems and lower quality of life, making effective strategies for reducing body fat necessary. Exercise interventions are one proposed strategy for combatting excess body fat. Typically, steady-state aerobic exercise at a moderate intensity is recommended to reduce body fat and improve body composition (9). The American College of Sports Medicine recommends more than 250 minutes a week of moderate-intensity physical activity to produce clinically significant weight loss in the general population (9).
Unfortunately, participants are rarely able to meet public health recommendations for exercise (16). Aerobic exercise programs with a lower time commitment have little effect on body fat loss (4,24). In one trial of young women, only 4.7% of the participants achieved the prescribed exercise volume of 150 minutes weekly for the entire study (1). Mean adherence to the exercise intervention was 108.3 minutes per week, with completion rates being relatively high (78%), suggesting that a less time-intensive exercise protocol might be preferable if it were able to induce equal or greater fitness and body composition adaptations.
Sprint interval training is one option that requires less total training time, has been found to be more enjoyable than steady-state aerobic exercise, and may have a greater impact on improving body composition outcomes (3). A study by Tremblay et al. (33) showed that incorporating high-intensity intervals into an exercise program produced significantly greater reductions in skinfold body fat measurements than a moderate-intensity continuous aerobic training (MCT) program. Interestingly, the total energy cost of the MCT program over the duration of the study was 120.4 MJ (28,661 kcal), whereas that of the high-intensity training (HIT) program was roughly half that at 57.9 MJ (13,614 kcal). When the difference in the total energy cost of the two programs was taken into account, the reduction in the sum of 6 skinfolds was 9-fold greater with the HIT program.
Subsequent interval training studies have supported the favorable effect on body fat (14,32); however, there is some evidence that women may not respond to sprint training in the same way as men (20). A 6-week study compared MCT with sprint interval training (SIT) using 20 healthy, recreationally active college-age men and women. The SIT group experienced a mean decrease in body fat of 12.4% compared with a more modest 5.8% decrease in body fat in the MCT group as measured by Bod Pod (20). However, a sex-based analysis of body fat changes revealed that the decrease in body fat in the SIT group occurred only in the men (body fat decreased by 3 kg, going from 13.7 ± 2.4 pre-training to 10.7 ± 2.3 kg post-training) and not the women (body fat increased from 13.7 ± 1.7 kg fat mass pre-training to 14.0 ± 1.1 kg post-training). In contrast, the women in the MCT group reduced body fat by 1.4 kg from 18.7 ± 2.6 kg to 17.3 ± 2.5 kg. It was suggested that there may be an interaction between sex and training mode whereby women have a different response than men. Because of the small sample size (only 4 women in each group) and a higher initial body fat in the women in the MCT group (MCT = 30.6% versus SIT = 22.3%), it is impossible to draw definitive conclusions. Therefore, the purpose of this article is to review the literature on the use of sprint interval training for improving body composition in women. Practical recommendations are provided for trainers to develop programs geared at reducing body fat in women.
IMPACT OF SPRINT TRAINING ON BODY FAT IN WOMEN
A series of studies have shown that sprint interval training leads to favorable body composition changes in both normal-weight and obese women. Reductions in body fat are greater than when a comparable or greater volume of moderate-intensity aerobic exercise is performed. What follows is a summary of the studies testing sprint interval training on body fat loss in women. Table 1 presents a summary of all the interval training studies with women as subjects that measured body composition.
A 2008 study compared the effect of a sprint interval protocol on a stationary bike (60 bouts of 8-second sprints followed by 12 seconds of active rest) with moderate-intensity aerobic training (40 minutes of steady-state aerobic exercise at 60% of V̇o2 max) in young, healthy, inactive women with normal body mass index (BMI) (BMI of 23.6, and body fat of 33% as measured by DEXA) (32). Despite exercising for half the time, SIT subjects lost 11.2% of total fat mass (−2.5 ± 0.83 kg loss of body fat) with MCT subjects experiencing a small increase in fat mass of 0.44 ± 0.88 kg. Fat loss and initial adiposity levels were moderately correlated (r = −0.58, p < 0.02), indicating that subjects possessing greater initial fat levels tended to lose more fat. Within the SIT group, the leanest women lost the least body fat. Four women who had an average weight of 52.9 kg (compared with 63.3 kg in the whole group) and BMI of 21.1 lost less than 3% total fat. When these 4 women were removed from calculations, the mean fat loss in the SIT group was 3.94 kg, equating to a 4.3% decrease in body mass and 14.7% decrease in total fat mass. The SIT group also produced a significant decrease in central abdominal fat (−0.15 ± 0.07 kg), whereas the MCT group had a nonsignificant increase in abdominal fat (+0.1 ± 0.08 kg). There were no significant changes in lean body mass.
Other cycle-training protocols have also produced significant fat loss in female subjects. A 2016 study compared the effect of a 6-week SIT cycling program (5 increasing to 7 sprint repeats of 30-second maximal efforts with 4 minutes rest) with an MCT aerobic program (20–30 minutes of moderate cycling at an intensity of 60–70% of heart rate reserve) in untrained, obese (BMI 30.3), young women (15). The SIT and MCT programs were matched for energy expenditure. Results showed that SIT induced a significantly greater loss of total fat (−3.6%) compared with the MCT group (−0.6%) as measured by DEXA. Reductions in android mass were also significant with subjects in the SIT group decreasing android fat mass by 6.6% and no change in the MCT group. Leg fat–free mass increased similarly in both groups (+2%).
A 2017 study investigated the effect of high-intensity cycle intervals of short (1-minute intervals at 90% of V̇o2peak repeated 10 times interspersed with 1–minute rest) and longer (2-minute intervals at intensities of 80–100% of V̇o2peak repeated 5 times with 1-minute rest) duration on body fat in overweight women over 3 weeks (28). After only 9 training sessions, fat mass decreased by 1.96 ± 0.99 kg in both groups as measured by Bod Pod. There was no change in lean mass in either group. Abdominal fat thickness decreased by a nonsignificant 11.29 ± 18.4 cm when the results from the training groups were combined.
To explore the impact of training mode on body fat changes in women, a 2014 study tested the effect of 6 weeks of running sprint interval training on body composition in young, normal weight, recreationally active women (12). The training program was 4–6 repeats of 30-second maximal intensity sprints with 4 minutes rest on a self-propelled Woodway treadmill. Results showed that the training program produced an 8% decrease in body fat, a 3.5% decrease in waist circumference, and a 1.3% increase in fat-free mass. Specifically, body mass decreased from 60.8 ± 5.2 to 60.3 ± 4.8 kg, fat mass from 15.1 ± 3.6 to 13.9 ± 3.4 kg, body fat percentage from 24.7 ± 4.9 to 23.0 ± 4.6%, and waist circumference from 80.1 ± 4.2 to 77.3 ± 4.4 cm. There was an increase in fat free mass from 45.7 ± 3.5 to 46.3 ± 2.9 kg. The study authors note that this evidence shows reductions in body fat with sprint training are not sex specific, as was considered a possibility following the trial by Macpherson et al.(20). Nonetheless, the 3-kg fat loss observed among the men in that study is more than the 1.2 kg observed in this trial, suggesting that further research is needed to explore possible sex differences in the magnitude of fat loss in response to interval training.
Most of the studies performed on women show that SIT is an efficient mode of reducing body fat and improving body composition. The mechanisms underlying the fat reduction associated with SIT are incompletely understood at this time and complicated by the lack of precision in food diaries and measures of energy expenditure. That is, it is possible that the change in fat mass that occurs in SIT may be partly influenced by unreported changes in diet. Previous studies show interval training may suppress appetite or decrease attraction to energy dense foods in some populations (8,35); however, the literature is inconclusive in women (11). Trapp et al. (32), Hazell et al. (12), and Higgins et al. (15) all performed 3-day self-reported food diaries that were taken pre-and post-training. No significant changes in either the energy or macronutrient intake of subjects were recorded. Poor reliability of self-reported food diaries makes it impossible to draw definitive conclusions.
A second proposed mechanism is increased lipid utilization during and after sprint interval exercise. Interval training increases plasma glycerol levels acutely and chronically, suggesting that lipids may serve as a greater source of energy during and after training (31). Concentration and activities of enzymes and proteins involved in beta-oxidation (6,33) as well as transport of fat into the skeletal muscle cell and mitochondria (29) are also increased in response to SIT. An elevation in and greater sensitivity to catecholamines in response to SIT is also thought to increase lipolysis (32). The catecholamines have been shown to elevate lipolysis and are largely responsible for fat release from both subcutaneous and intramuscular fat stores for oxidation. It is also possible that SIT enhances B-adrenergic sensitivity, which would promote fat oxidation in the abdominal region and lower central fat stores (4). Though undemonstrated in response to SIT, there is evidence that endurance training enhances B-adrenergic sensitivity, while decreasing antilypolytic alpha receptors.
An increase in growth hormone in response to sprint interval training is another proposed mechanism underlying greater lipid utilization. Growth hormone has substantial lipolytic effects and elevations are linked to power output and released to a greater degree in response to a 30-second maximal effort treadmill sprint in sprint-trained athletes than endurance-trained athletes (21). None of the studies demonstrating body fat loss in women in response to SIT have measured growth hormone or catecholamine levels so it is unclear how much of an impact hormone elevations play.
A third possible mechanism contributing to the significant reduction in body fat in response to SIT is the elevation in metabolism that occurs after the exercise session during the recovery period. Known as excess postexercise oxygen consumption (EPOC), there is a demonstrated increase in calories burned by the body during the extended recovery period (reaching 24–48 hours) that is associated with the removal of lactate and hydrogen, increased pulmonary and cardiac function, elevated body temperature, catecholamine effects, and glycogen resynthesis (4). EPOC is more influenced by exercise intensity than duration and is correlated with elevations in lactate produced during exercise (2,19).
In considering the possible role of EPOC on body fat changes in response to SIT in women, it should be noted that the limited number of studies testing EPOC have been done on male subjects with very few done on women. A study by Townsend et al. (30) addressed this, comparing EPOC in men and women after a running SIT session, a cycling SIT session, and a control group. Results showed no significant difference in total EPOC based on sex, suggesting that men and women have a similar metabolic response during recovery from intense exercise and that studies testing EPOC using male subjects are relevant to women. Naturally, female-specific studies should be conducted, and this is a key point for future research.
The impact of EPOC on body composition changes with exercise is controversial because although studies show an increase in postexercise metabolism over resting levels (7), some studies show only small elevations in postexercise energy expenditure (17,35). Williams et al. (35) found that EPOC in the 2-hour period after exercise was not different after interval training compared with endurance exercise and the overall magnitude of EPOC was small. Compared with a control group, energy expenditure increased during the recovery period by 33.5 ± 16.3 kcal after a sprint interval protocol and by 41.5 ± 13.8 kcal after endurance exercise. Energy expenditure during exercise was significantly larger in the endurance training group with a total energy expenditure (EPOC + exercise) of 560 kcal compared with a total energy expenditure of only 85 kcal in the SIT group.
This study only tested EPOC in the 2 hours after exercise and it is possible that there is a sustained elevation in metabolic rate lasting 24–48 hours as the body restores metabolic processes to baseline (13,26). For example, a trial that compared four 30-second maximal effort sprints with 30 minutes of MCT at 70% of V̇o2 max found that when an 8-hour period that included the workouts was compared, oxygen consumption was 17% greater in the MCT group than the SIT group because of the greater energy cost of the exercise trial (13). However, over a full 24-hour span, the sprint protocol resulted in oxygen consumption that was equal to that of the MCT program (SIT group: 498 L O2; MCT group: 500.2 L O2). The elevated metabolism during the 24 hours after SIT resulted in a similar 24-hour energy cost as that observed in the MCT condition (18).
To date, none of these explanations fully explain the superior effect of sprint interval training for body fat reduction in women. Further research should explore the effect of SIT over the long-term on appetite and energy intake. EPOC and resting energy expenditure should also be further investigated, particularly in leaner female populations who have not demonstrated as much of a reduction in body fat as overweight and obese subjects in response to SIT.
One important issue regarding interval training is whether it is well tolerated in obese and overweight populations. There is concern that the high level of perceived exertion that is typical with SIT is associated with a reduction in both affect and enjoyment (5,22), which could impair adherence. Affect has been identified as a predictor of engagement in long-term exercise behavior for up to 12 months (34), suggesting that the more difficult a training protocol is perceived, the less likely it is to be adhered to. Interestingly, there is some evidence that sprint training does not fit into this paradigm.
A study that compared fatigue, mood state, and enjoyment in response to SIT and MCT found that although the SIT workout produced greater perceived exertion than MCT, enjoyment was not significantly different (22). A second trial found that high-intensity training (6 × 3 minutes at 90% of V̇o2 max interspersed with 3 minutes active recovery) was rated as more enjoyable than 50 minutes of moderate-intensity continuous running at 70% of V̇o2 max despite higher ratings of perceived exertion with the HIT protocol (3). Another trial compared enjoyment and affect responses to an interval protocol (ten 60-second intervals with 60-second active rest) at a high (100% of peak work rate) or moderate (70% peak work rate) intensity (5). Affect decreased more after the high-intensity protocol; however, participants rated both the high- and moderate-intensity workouts as equally enjoyable. Furthermore, participants in both groups expressed a high degree of confidence in their ability to successfully complete their prescribed interval protocol and schedule it into their weekly routine for the future. Finally, a study that measured exercise enjoyment in overweight men and women who performed 3 weeks of high-intensity training showed that enjoyment levels were relatively high for the first session (4.2 ± 1.0 of a 7-point scale) (27). Enjoyment ratings improved significantly over the course of the study despite higher training heart rates and RPEs as the workouts progressed. Enjoyment ratings were similar between men and women, suggesting that studies testing enjoyment of exercise in male subjects are relevant to women. Together these studies support reports of enjoyment of high-intensity interval exercise. Future research should confirm these findings and explore adherence rates over the long-term. How intensity affects enjoyment, rating of perceived enjoyment, and adherence also needs to be explored. None of the cited studies used the maximal training intensity that is typical of sprint protocols.
With regard to drop out rates in controlled SIT studies, the literature suggests properly designed protocols are as well tolerated as moderate-intensity aerobic training and the greater efficacy for reducing body fat may make them preferable. In a study conducted by Tremblay et al. (33), the dropout rate was approximately the same in the SIT and MCT groups and corresponded to 25–30% of the initial sample. In a study conducted by Trapp et al. (32), 4 subjects dropped out of the SIT program and 7 from the MCT group. When moderate-intensity interval training (5 × 3 minutes at 85% V̇o2 max) was compared with low-intensity aerobic exercise (40 minutes at 50% V̇o2 max), 3 women dropped out of the interval program and 4 from the aerobic program, suggesting that adherence rates are comparable at submaximal training intensities as they are at high intensities (25).
RECOMMENDATIONS FOR FUTURE RESEARCH
Interval training at a range of high intensities is an effective time efficient method for both normal-weight and obese women in the general population to reduce body fat and improve body composition. Future research should attempt to identify the specific training parameters (mode, intensity, duration) that encourage adherence as well as produce the greatest improvement in body composition in women. To date, a number of different protocols have shown beneficial effects in women and future research should identify the optimal SIT protocol, particularly for special populations that struggle with obesity. The most commonly used protocol is the Wingate test (30 seconds of maximal effort sprinting) repeated 4 to 7 times with 4 minutes rest. This protocol is physically and mentally challenging, and it will be interesting to see if the results of more moderate protocols can be replicated.
Identifying the minimal dose for a maximal response is important. Both longer duration (12 or 15 weeks) and shorter duration studies (6 weeks) have demonstrated significant reductions in body fat and increases in lean mass (12,15,20,32,33). It should also be determined whether SIT is appropriate as a long-term weight management tool or if it should be strictly used as a time-limited intervention to reduce body fat, after which, trainees can transition to other forms of exercise. Future research should also explore how completing a SIT or MCT intervention affects the regain of body fat over the long-term.
Determining if there are any differences in outcomes that are linked to modality is necessary. It is unclear whether rowing, walking, swimming, or stair climbing would yield similar benefits in body composition. Most studies that have demonstrated body composition changes have used a stationary cycle ergometer, but at least 2 studies have used a self-propelled treadmill. It should be noted that both of these modes provided an external resistance that subjects had to overcome, which may have contributed to increases in lean mass. Both Macpherson et al. (20) and Hazell e al. (12) recorded significant elevations in lean mass and used running sprints that used body weight as resistance on a self-propelled treadmill, which likely generated enough overload to stimulate this increase. Over the ground running or other modes with less resistance may not produce comparable increases in lean mass.
Determining individual fat-loss responses is an important issue for future investigations. In Trapp et al. (32), there was a wide range of individual responses to SIT with fat loss ranging from 8 kg to a gain of 0.10 kg and a mean decrease of 2.5 kg. If fat-loss responders alone were examined in the study (women who lost rather than gained fat), then average fat loss was 3.94 kg (4). It has been suggested that leaner women are lower responders and do not lose as much body fat in response to the same protocol as those with higher body fat. This should be further tested, as should the impact of sex on fat loss. Furthermore, it is important to explore the efficacy of sprint interval training in trained female athletes who carry excess body fat. Future studies should explore if sprint training could be a useful tool for improving body composition without compromising recovery or negatively impacting sports practice.
It is likely that other program design factors could impact reductions in body fat in women. For example, when sedentary women with metabolic syndrome were instructed to eat a Mediterranean diet, take 3.3 grams of fish oil, and do SIT, body fat mass decreased by 2.6 kg, and body fat by 6.2% as measured by DEXA (10). The role of diet should be further investigated, as should the potential impact of sprint interval training on total energy intake and appetite.
Strength and conditioning coaches and personal trainers working with women can be confident that interval training is an effective and efficient method for reducing body fat. When appropriate, interval training can be used as an alternative to continuous aerobic exercise for improving body composition. The following factors should be considered when designing protocols:
- Sprint training can be effective for reducing body fat without a reduced calorie diet; however, coaches should advise trainees to be cognizant of portions and food intake to avoid “compensation” whereby food intake is increased as an unconscious “reward,” thereby negating the energy deficit achieved through training (12).
- Maximal intensity sprints can be used by healthy, obese, overweight, and normal weight women. However, moderate-intensity protocols, such as ten 60-second intervals at 85–90% of heart rate reserve, can also be used to provide mental relief and increase tolerability (12,26,28).
- Sprinting against resistance, either with a resisted stationary bike, a self-propelled treadmill, or weighted sled may be favorable to nonresisted modes (such as over the ground running or cycling) to provide sufficient overload to trigger an increase in lean mass for more favorable body composition (12).
- For athletes who require body fat reduction, training should generally be mode-specific to the athlete's sport. Protocols should be designed to closely mimic intensity and work-to-relief ratios that are used during sport competition. In monitoring intensity, it should be noted that at supramaximal intensities, heart rate is a poor indicator of exercise intensity. Therefore, providing a goal pace can ensure the appropriate effort is exerted (23).
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