Current public health guidelines recommend at least 30 min of daily exercise of moderate intensity (e.g., walking, cycling, and gardening). However, studies consistently show poor adherence to guidelines (1) with lack of time often cited as a major barrier (2). An alternative approach is high-intensity interval training (HIIT), defined as “brief, intermittent bursts of vigorous activity, interspersed by periods of rest or low-intensity exercise” (3). A proposed major advantage of HIIT is that it takes considerably less time than conventional moderate-intensity exercise options to obtain comparable health benefits (4). Even very small amounts of intense exercise (as little as 5–10 min·d−1 or 30 min once per week) improve health outcomes (5) and reduce mortality risk (6,7). Several meta-analyses have demonstrated that laboratory-based HIIT is at least as effective as standard continuous training for improving aerobic fitness (V˙O2max) (8–11) and important clinical indicators such as blood pressure, glucose handling, and visceral fat (3,12,13).
However, whether HIIT works in the real world is currently unknown (14,15) as virtually all trials to date have been conducted in laboratory settings over less than 6 months (9–11). Although concerns have been raised regarding the safety of unsupervised HIIT in terms of cardiovascular events and injury risk (16), others argue that such concerns are unfounded, indicating that HIIT can be more enjoyable than conventional exercise (17), promote greater adherence (18), and be safe even in clinical populations (19). Certainly, longer-term outcomes from unsupervised HIIT have been evaluated in cardiac patients, who show improvements in aerobic fitness for periods up to 12 months (20,21). However, given their motivation to adhere should theoretically be higher than the general population, it remains important to examine the feasibility of unsupervised HIIT in nonclinical groups (22). If proven to work, such evidence will contribute to public health recommendations, indicating that HIIT is a viable alternative to conventional moderate-intensity exercise. As recently highlighted (22), several issues remain to bridge the translational gap from the laboratory to public health policy. Real-world effectiveness studies are required that trial low-cost, accessible HIIT protocols over the long term (22). Therefore, the aims of this study were to determine whether (i) overweight individuals would choose to participate in unsupervised HIIT on a regular basis over a 12-month period and (ii) an unsupervised HIIT program influences body composition, physical activity, and health more than conventional moderate-intensity exercise recommendations.
The Support Strategies for Whole-Food Diets, Intermittent Fasting and Training (SWIFT) study was a 12-month randomized controlled trial investigating whether the addition of monitoring strategies (face to face contact, self-weighing, dietary monitoring, or monitoring of hunger) to dietary and exercise advice increased weight loss over 12 months compared with the provision of advice alone (23). SWIFT is registered with the Australian New Zealand Clinical Trials Registry (ACTRN12615000010594), and ethical approval was obtained from the University of Otago Human Ethics Committee (H14/024). All participants provided written informed consent. As a protocol paper (24) and main outcome findings (23) have been published, only relevant details are provided here.
Participants were recruited by advertisement and screened using an online questionnaire followed by an initial face-to-face screening visit (Fig. 1). Eligible participants were 18+ yr of age, had a body mass index (BMI) of ≥27 kg·m−2, and no history or symptoms of cardiovascular disease or ongoing serious medical issues. Exclusion criteria were type 1 or 2 diabetes, inflammatory disorders, or any respiratory or musculoskeletal condition impeding ability to exercise. Women who were pregnant or breastfeeding were also excluded. Physical screening then applied further exclusions to those with previously unrecognized hyperglycemia (≥7 mmol·L−1) or hypertension greater than stage 1 (Fig. 1). In addition to preparticipation screening, further medical assessment was provided to HIIT participants as per the American College of Sports Medicine/American Heart Association guidelines in 2014 (25). This was to improve safety by identifying any participants with occult heart disease or other serious disorder who could be at a higher risk of a cardiac event during exercise.
After baseline assessments, eligible participants were able to choose whether to follow current exercise recommendations or HIIT. Both were unsupervised exercise programs after a single intervention/training session. Those selecting the current New Zealand (NZ) guidelines were instructed to perform “at least 30 min of moderate-intensity physical activity on most if not all days of the week.” Trained researchers (medical doctor and dietitian) spent approximately 10–20 min discussing with each individual how they could best meet this goal, typically using options such as brisk walking, cycling, and exercise classes. A widely available brochure produced by the NZ Ministry of Health was provided, and high-intensity exercise was not emphasized.
HIIT participants underwent a 60-min preparatory session, performed on a cycle ergometer to experience near-maximal intervals, by undertaking three intervals of up to 30 s in duration, and were encouraged to attain an RPE of 8 or greater (10-point scale). HR monitoring (Polar RC3/RCX GPS) was used to ensure participants achieved at least 80%–90% of their estimated maximum HR (using the formula 220 − age) (25). Although most participants attempted maximal intervals, fitness levels varied greatly, so modification of this protocol to use submaximal but still high-intensity intervals was occasionally necessary for tolerability and safety reasons for participants who were severely deconditioned. HIIT participants were provided with verbal and written instructions (see Document, Supplemental Digital Content 1, Supplementary methods detailing these instructions, https://links.lww.com/MSS/B282) outlining evidence-based HIIT options for ongoing unsupervised training (4,8,26–29).
All outcome measurements were obtained at 0, 6, and 12 months (unless noted differently) by trained assessors blinded to support and exercise groups. Height, weight (primary outcome), waist circumference, and blood pressure were obtained in duplicate using standard protocols (24). Body composition was measured by dual-energy x-ray aborptiometry at 0 and 12 months (GE Lunar Prodigy; GE Healthcare, Madison, WI). Visceral fat volume was estimated using the Lunar Encore software CoreScan (Version 16, GE Healthcare). A fasted venous blood sample was collected by a registered nurse; plasma total cholesterol, HDL cholesterol, and triglyceride levels were measured by enzymatic methods using a Cobas Mira Plus Analyser, and LDL cholesterol concentrations were derived using the Frielwald formula (30). High-sensitivity CRP was measured using a CRP Unimate kit (Roche Diagnostics, Indianapolis, IN) and glycated hemoglobin (HbA1c) by enzymatic methods on a Cobas Mira Plus Analyzer (Roche Diagnostics). Active ghrelin levels were assessed via immunoassay (Human Gut Hormone Panel LINCOplex Kit; LINCO Research, St Charles, MO). Total activity (counts per minute) and time spent performing moderate to vigorous physical activity (MVPA) were measured with all participants wearing an ActiGraph accelerometer (GT3X; ActiGraph, Pensacola, FL) for 7 d. Data were analyzed using both 15-s epoch (to capture shorter bouts of intense exercise) and 60-s epoch (to enable comparison with the international literature). Data were analyzed using an automated program developed in MATLAB (MathWorks, Natick, MA), which “removes” all sleep data, specific to each day and each individual (31). Physical fitness was ascertained using a modified YMCA submaximal cycle ergometer (Monark ergometer, model 828E, Sweden) to estimate V˙O2peak (32). A submaximal test was used for this overweight and largely unfit population, as maximal testing in a trial of this size was not pragmatic, nor likely to result in true peak values (25,33). Information on demographics (age, sex, ethnicity, education, and employment status) and personality (10-item personality inventory) (34) was obtained at baseline. At each time point, participants completed questionnaires assessing perceived self-efficacy and enjoyment of physical activity (35).
Additional measures of adherence to HIIT were recorded using Polar RC3/RCX GPS HR monitors worn during all unsupervised HIIT sessions over 1 wk at 0, 3, 6, 9, and 12 months. Within each HIIT session, HR was recorded continuously, providing measures of maximum HR, and the duration of time exceeding predefined thresholds of 80% and 90% of the predicted maximum HR (HRmax, calculated from 220 − age). HR data were subsequently uploaded and analyzed using Polar online software (http://polarpersonaltrainer.com).
Mixed models, with a random effect for participant, adjusting for age, sex, the monitoring group to which the participants were randomized, and the relevant baseline variables were used to analyze the data. No adjustments were made for multiple comparisons. For those outcomes that were not collected at 6 months, linear regression was used, adjusting for age, sex, and baseline to estimate differences between the exercise groups. The results are presented as differences (95% confidence intervals) between groups.
A subsequent analysis, limited to those choosing HIIT, used similar methods to compare participants with different levels of adherence to HIIT. Full adherence was defined a priori as providing at least two recordings of adequate HIIT sessions at three or more of the three monthly reviews. Partial adherence was defined as providing only one recording of an adequate HIIT session at three or more of the review periods, and nonadherence was defined as less HIIT data than these amounts. All analyses were performed using Stata 13.1 or a later version (StataCorp, College Station, TX).
Table 1 shows that the 104 participants (41.6%) that chose to try HIIT did not differ at baseline from those choosing standard exercise recommendations in terms of demographics, BMI, body composition, aerobic fitness, or physical activity levels. The psychological measures were also similar, with small significant differences observed in just two personality indices; the HIIT cohort showing higher scores for agreeableness (P = 0.02) and extraversion (P = 0.03) than the current recommendations group.
The initial supervised HIIT training session was completed by 102 participants. Sufficiently intense intervals (HR exceeding 80% HRmax) were achieved by 88 participants (86.2%) with 41 (40.2%) exceeding 90% HRmax. Twelve participants were unable to attain adequate intensity, primarily limited by leg fatigue specific to the cycling modality.
Table 2 illustrates the differences in outcomes according to exercise program for all the participants who remained in the trial, regardless of adherence to either exercise protocol. Retention was 70% in the HIIT cohort and 67% in the current recommendations group at 12 months. No significant group differences were observed in weight, body fat, or blood pressure at 6 or 12 months. Despite recommendations for duration and intensity of exercise being markedly different between the groups, estimated V˙O2peak and physical activity were similar at 12 months. Both groups achieved 32–52 min of MVPA per day, and self-efficacy for exercise remained unchanged throughout the trial in both groups. However, at both 6 and 12 months, enjoyment of physical activity was approximately 1 SD higher in HIIT compared with the current recommendations group (difference in scores = 2.5; 95% confidence interval [CI] = 0.6–4.3).
To gauge adherence to HIIT, the number of unsupervised HIIT sessions recorded by HR monitoring at 0, 3, 6, 9, and 12 months is shown in Table 3. The proportion of participants who did not provide data increased from 18 (17.6%) at baseline to 73 (71.6%) at 12 months. Those adhering to HIIT completed 1–3 sessions per week, with up to nine participants completing four or more sessions a week at each time point.
HIIT participants spent 21–24 min·wk−1 in total exceeding 80% HRmax, of which approximately 9 min was above the 90% HRmax threshold (see Table, Supplemental Digital Content 2, Detailing the number of participants who achieved set exercise intensities at each time point, https://links.lww.com/MSS/B283). Participants recorded a variety of HIIT modalities, including using hills, stairs, running, exercise equipment such as bikes, elliptical trainers, and rowing machines, as well as home-based circuit-type exercises such as burpees and star jumps. A few participants recorded taking part in commercial gym, online or app-based HIIT workouts, and some high-intensity sport-based training such as futsal or squash, and all of these activities produced exercise of adequate (≥80% HRmax) intensity (data not shown).
Given the apparently poor adherence to HIIT long term, a further analysis was undertaken to ascertain how adherence affected outcomes. On the basis of the a priori adherence categories, 24 participants (23.1%) were considered fully adherent, and 17 (16.3%) were partially adherent, with the majority (n = 63, 60.6%) not meeting adherence criteria. It was apparent that fully adherent participants were more likely to be male (P = 0.03) and leaner (P = 0.03), but they did not differ in terms of age, BMI, physical activity, or aerobic fitness at baseline from nonadherent participants (see Table, Supplemental Digital Content 3, Comparisons of baseline characteristics between adherent and nonadherent participants, https://links.lww.com/MSS/B284). However, differences in outcomes were apparent (Table 4). At 12 months, weight (−2.7 kg; 95% CI = −5.2 to −0.2), waist circumference (−2.4 cm; −4.7 to −0.2), visceral fat volume (−292 cm3; −483 to −101), and HbA1c (−0.9 mmol·mol−1; −1.7 to 0.0) were significantly lower in fully adherent compared with nonadherent participants. Interestingly, although self-efficacy for physical activity was also higher (2.5; 0.7 to 4.3), enjoyment of physical activity was lower in fully adherent participants (−2.2; −4.4 to 0.0). By contrast, adherence was not associated with differences in blood pressure, total body fat, or aerobic fitness, although partially adherent participants were less physically active than nonadherent participants (Table 4).
Our study illustrates that HIIT was chosen by a large number of overweight individuals, who were able to initially perform it effectively and independently using a variety of modalities. However, opting for three weekly HIIT as an alternative to 30 min of daily moderate-intensity exercise did not result in meaningful differences in any health outcomes at 12 months, most likely because of poor adherence in the real-world setting by the majority of participants. Without supervision, adherence to HIIT declined rapidly over 12 months, although participants maintained similar levels of physical activity overall compared with those following standard exercise guidelines. Significant decreases in weight and visceral fat were achieved by a relatively small group of participants who reported adherence to the HIIT protocol.
Although it has been argued that overweight nonathletic individuals would be unlikely to try HIIT (1,15), 42% of participants opted to do so as an alternative to daily moderate-intensity exercise. It has been contended that overweight or obese people would be unable to successfully perform high-intensity exercise without support (1). The results of this study suggest this may be partially true. Our findings demonstrate that the majority of participants proved able to perform HIIT at an adequate intensity (greater than 80% HRmax) under supervision, but much smaller numbers met our participation targets of at least twice per week, particularly over the longer term. Thus, it appears that HIIT can be effectively undertaken by most overweight and obese people with minimal training, but that long-term adherence remains a significant challenge.
Several existing trials have concluded that supervised HIIT can lead to improvements in fitness, body composition, and blood indices (8–10,12). By contrast, this study demonstrates that a single HIIT intervention without ongoing support did not lead to clinically significant changes in these outcomes over 12 months, most likely due to inadequate adherence. Only those who were adherent to the protocol (23%) showed the meaningful improvements in weight and visceral fat that have typically been shown in the laboratory setting (13,36).
In this study, the adherence of the participants choosing to undertake daily moderate-intensity continuous exercise could not be directly compared with the adherence of the HIIT group, as exercise performance was measured using different methods. Adequate execution of HIIT was best established using HR monitoring to ascertain that sufficient intensity was achieved because accelerometry may not effectively capture nonambulatory intense activity such as stationary cycling. By contrast, for participants opting for usual forms of exercise, accelerometry provided the best means of evaluating physical activity levels. However, it is well established that adherence to any structured exercise program is poor, especially for those with obesity (1,37,38). This study has demonstrated that if HIIT is advised but not supported, the majority of overweight participants will not obtain additional health benefits from this form of exercise by 12 months. Although previous studies have suggested that HIIT is considered enjoyable and even preferred in supervised environments (17), our findings support other work, indicating that in free-living environments, poor adherence leads to more modest health outcomes (39). It is important to note that our HIIT participants did not become less physically active than those choosing to follow more traditional exercise regimes.
The strengths of our study include its real-world design, in a large number of participants, and over a longer time frame than has typically occurred in HIIT research. We also used dual-energy x-ray aborptiometry to measure body composition and collected habitual physical activity data by accelerometry, with objective assessment of HIIT participation by HR monitoring. Our study also has some limitations. To best represent free-living conditions, participants were allowed to choose their exercise program. Although we observed very few differences in our exercise groups at baseline, it is possible that differences existed in variables we did not measure. Attrition was relatively high at 31.6% (23), although this is not unusual for lifestyle interventions. Small changes in aerobic fitness may not have been detected because of the use of submaximal estimation of V˙O2peak, which was the safest and most pragmatic test for this population, but inherently less accurate than maximal oxygen uptake testing (32,40). It is possible that adherence to HIIT was overestimated by our use of 1 wk of HIIT recordings every 3 months, as some participants may have undertaken a HIIT session only when provided with a HR monitor. However, alternative options such as daily records over the full 12 months in a sample of this size were not feasible. Allowing our participants to undertake a wide variety of HIIT protocols is important for real-world use, but it does not allow any mechanistic interpretations or delineation of more “successful” protocols.
In conclusion, our study indicates that HIIT may be a tolerable alternative to daily moderate-intensity exercise for people who are overweight or obese, although it is unlikely to lead to differences in important health outcomes for all who attempt it. However, those participants who do adhere to a HIIT regime experience important health benefits, indicating that HIIT can be included as a suitable exercise option. Further work to determine how best to improve long-term adherence in the real world may lead to HIIT being a viable public health strategy to improve health outcomes.
The SWIFT Study was funded by the University of Otago. R. W. T. is partially funded by a Fellowship from the Karitane Products Society Limited. The funders had no role in the design of the study; the collection, analysis, and interpretation of the data; the writing of the manuscript; or the decision to submit the article for publication. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The authors declare that they have no competing interests. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
M. R. was a PhD student on the SWIFT study and designed and undertook the HIIT intervention. She saw all participants, collected and analyzed all HR monitoring data, and wrote the first and subsequent drafts of the paper. R. C. B. was a coinvestigator, assisted with study design, and commented on the manuscript. S. M. W. was a coinvestigator, assisted with study design, was responsible for all statistical analyses, and commented on the manuscript. K. M.-J. was a coinvestigator, assisted with study design, completed the accelerometry analyses, and commented on the manuscript. H. O. was a coinvestigator, assisted with study design, oversaw the medical aspects of the study, and commented on the manuscript. M. J. was a coinvestigator, assisted with study design, and commented on the manuscript. R. W. T. conceived the idea for the study, was the principal investigator of the project, and was responsible for overall study design and monitoring of data collection. All authors read and approved the final manuscript.
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