Overcoming exercise barriers: home‐based HIT for reducing cardiovascular disease risk in obese individuals

Cardiovascular diseases (CVD) are the number one cause of death in the world and therefore represent a biomedical research priority. Physical activity is an important modifiable risk factor in the development of CVD, yet most adults do not meet recommended physical activity guidelines. The most prominently reported barrier to physical activity is lack of time; therefore, development of time-efficient modes of exercise that promote adherence are important for reducing CVD risk. High-intensity interval training (HIT) has emerged as a viable, time-saving alternative to moderate-intensity continuous aerobic exercise training (MICT), though there are still many research questions regarding HIT to answer. In particular, there is a need to assess adherence to HIT in 'real-world' settings, determine modes of HIT appropriate for specific populations, and establish the efficacy and mechanisms by which HIT may improve cardiovascular function in populations at risk for developing CVD (e.g. obese and sedentary) (Craighead et al. 2019). A study recently published in The Journal of Physiology compared 12 weeks of a home-based HIT (Home-HIT) programme to home-based MICT (Home-MICT) and a more traditional laboratory-based HIT programme (Lab-HIT) in obese sedentary adults with at least two additional CVD risk factors (Scott et al. 2019). The primary aim was to assess changes in muscle microvascular endothelial nitric oxide synthase (eNOS) to NADPH oxidase ratio, a measure of oxidative stress, and skeletal muscle capillary density before and after the three exercise training programmes. Additional measures included adherence (the percentage of prescribed exercise sessions completed), compliance (the percentage of exercise sessions performed at the correct heart rate), and other cardio-metabolic factors related to exercise adaptation (e.g. and whole-body insulin sensitivity). Importantly, two components of arterial function closely related to CVD risk, large-elastic artery stiffness and vascular endothelial function, also were assessed. Arterial stiffness was measured via aortic pulse wave velocity (PWV) and vascular endothelial function was measured via brachial artery flow-mediated dilatation (FMD). Both measures of arterial function were assessed at baseline and after 4 and 12 weeks of exercise training. Subjects self-allocated into one of the three intervention groups. The Home-HIT group (n = 9) performed self-selected body weight exercises in 1 min intervals at 80% of maximum heart rate followed by 1 min of rest. Participants were instructed to complete four intervals per session during weeks 1–4 and then progressively add one interval every 2 weeks up to a maximum of eight intervals. The Home-MICT group (n = 13) performed a self-selected mode of continuous aerobic exercise performed at ∼65% of maximum heart rate. They began by exercising for 30 min during weeks 1–4, then increased exercise duration by 5 min every 2 weeks up to a maximum of 50 min. Finally, the Lab-HIT group (n = 10) did supervised exercise on a cycle ergometer with the same interval protocol (i.e. bouts of 1 min intervals followed by 1 min of rest), target heart rate, and progression as the Home-HIT group. All three groups were instructed to exercise 3 days/week; however, adherence was only enforced for Lab-HIT. For home-based exercise, training intensity and adherence were assessed with a heart rate monitor synced to a smart phone application that provided real-time feedback to the subject and could be monitored by investigators. This design allowed for the efficacy of and adherence to Home-HIT to be compared to a training programme that met physical activity guidelines (Home-MICT) and to a well-controlled laboratory-based interval training programme (Lab-HIT). There were main effects for improvements in eNOS/NADPH oxidase ratio, skeletal muscle capillary density, , and insulin sensitivity with 12 weeks of exercise training, though there were no differences between groups. Adherence and compliance were high in all interventions, likely due in part to self-allocation to exercise groups. These results are encouraging as increasing adherence to physical activity guidelines is important for reducing CVD risk in sedentary, obese populations; however, more research with larger sample sizes is required to confirm whether Home-HIT specifically is a viable, time-saving alternative for promoting exercise adherence in 'real-world' settings. Although the emphasis was on investigating microvascular adaptations to Home-HIT exercise, the changes in large artery function were likely the most biomedically significant results. Aortic PWV, the gold standard in vivo measurement of large-elastic artery stiffness and an important risk factor for CVD, was unchanged after 4 weeks but decreased after 12 weeks of exercise with no differences between groups. This is notable because the subjects' baseline PWV were just over 6 m/s, representing only a very modest degree of physiological impairment. In general, it is difficult to elicit beneficial vascular adaptations with exercise when baseline values are in a healthy or near-healthy range. For comparison, 12 weeks (30–35 sessions total) of either MICT or sprint interval training failed to decrease PWV in sedentary but otherwise healthy young men with a mean PWV of 6.8 m/s at baseline (Shenouda et al. 2017). However, the varying results between studies may be attributed to differing subject characteristics, as the arterial stiffness of obese individuals with elevated CVD risk may be more responsive to changes with exercise compared to sedentary but otherwise healthy individuals. Additionally, differences in the exercise training programmes (e.g. interval duration and recovery, exercise frequency) may be involved. Therefore, the results of the present study are exciting and suggest the potential for HIT to improve cardiovascular function but need to be confirmed. Brachial artery FMD, a measure of vascular endothelial function, was unchanged after 4 weeks of exercise, but was significantly improved after 12 weeks of training. Again, the improvement in FMD was a main effect of exercise; no differences between groups or interaction were observed. Importantly, resting brachial artery diameter was not increased with any mode of exercise training. This is noteworthy because the pattern of adaptation in endothelial function to exercise in this study differs from what has been observed by other groups. For example, 8 weeks of HIT consisting of ten 1-min intervals at 90–95% of maximum heart rate performed on a cycle ergometer 3 days/week improved FMD in obese adults, whereas FMD was unchanged in obese adults cycling for 30 min/day, 3 days/week at 70–75% of maximum heart rate (MICT); this was attributed to a significant increase in baseline brachial artery diameter seen in the MICT group (Sawyer et al. 2016). Similarly, increased baseline brachial artery diameters have been observed in sedentary men who performed MICT for 45 min/day, ∼2–3 days/week at 70% of maximum heart rate. With this training programme, FMD increased after 6 weeks but returned to baseline values after 12 weeks, again attributed to increased resting brachial diameter (Shenouda et al. 2017). Indeed, with aerobic exercise training, an initial increase in FMD after only a few weeks of training is thought to supersede structural adaptations that occur over a longer period and return FMD towards pre-training values (Tinken et al. 2008). Therefore, it is interesting that in the present study, neither MICT nor HIT induced structural adaptations, and functional changes occurred later than expected. Reasons for these results are unclear, but the authors suggest the influence of elevated oxidative stress and inflammation on nitric oxide bioavailability in their obese subjects may have played a role in the time course of arterial adaptations. As acknowledged by the authors, this study was limited by a small sample size of only 32 subjects split between three exercise training interventions. Neither significant within-group improvements in arterial function with any of the exercise programmes nor significant between-group differences in adaptations to exercise could be detected. Future studies with larger sample sizes are needed to fully determine how the effectiveness of Home-HIT compares to traditional MICT at improving health outcomes; however, this initial investigation is promising. Despite the small sample size, the study addressed multiple research gaps related to interval training. By measuring both large artery and microvascular function, and interrogating underlying mechanisms, insight was gained for the efficacy of HIT to improve cardiovascular function. Additionally, using Home-HIT explored an effective and potentially more adherable exercise modality for people with obesity who may struggle with certain barriers to physical activity. Indeed, Home-HIT addresses many common barriers to exercise such as time constraints, limited access to facilities, and possible embarrassment and lack of self-efficacy when exercising in a public setting. Moreover, while mimicking a 'real-world' setting, Home-HIT produced high adherence, suggesting its efficacy as a promising method of exercise for people with obesity as well as other populations. Overall, this study highlighted the beneficial effects of multiple exercise modes for obese individuals with elevated CVD risk. The results suggest Home-HIT may be an effective method to overcome barriers to exercise and increase exercise adherence in this population, while also improving arterial function. However, more research with larger sample sizes is needed to confirm adherence and efficacy of Home-HIT for improving physiological function and vascular health in obese adults. None All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. This work was supported by an American Heart Association Postdoctoral Fellowship AHA 18POST33990034 (D.H.C.).