Low levels of physical activity (PA) are related to poor health and greater risk of premature death in adults. Lack of time is cited as a primary barrier to partaking in PA. In the last 20 years, a lot of attention has been directed towards the efficacy of high intensity interval training (HIIT), which consists of brief, intense bursts of PA separated by recovery. One primary benefit of HIIT is a sizable increase in cardiorespiratory fitness (CRF), which enables adults to exercise better and reduce health risks due to the strong link between health status and CRF.

Completion of cardiovascular exercise is typically recommended for all adults to increase CRF, yet it takes a lot of time and may be perceived as boring. Alternatively, HIIT requires less time and tends to cause greater feelings of enjoyment in many adults. Hundreds of studies report an increase in CRF in response to HIIT in various groups of adults ranging from athletes and those with obesity, diabetes, cancer, stroke, and even spinal cord injury, which emphasizes the potency of this vigorous form of PA. Yet, many studies are weakened by a small sample size which questions the feasibility of these findings.

This randomized controlled trial will test the efficacy and feasibility of a very small amount of HIIT, referred to as reduced exertion high intensity interval training (REHIT), in inactive adults. In the proposed study, REHIT will consist of 2 days per week of 1 to two 10 - 20 second sprints on a stationary bike. The proposed sample will include 60 adults who complete 18 sessions of REHIT over a 9 week period, and their responses will be compared to a non exercising control group. During the study, changes in CRF, fuel metabolism, and psychological responses will be monitored, with the latter outcome shedding light on the overall feasibility of HIIT in inactive adults.

Overall, this novel study has profound public health applications as it will assess fitness and health related changes to a small dose of PA in the largest sample to date. If substantial changes in CRF are shown, these data have the potential to modify public health guidelines for implementing PA in inactive adults.

Low levels of physical activity (PA) and cardiorespiratory fitness (CRF) are significant issues for societal health. Public health guidelines for adults recommend > 150 min of moderate-intensity continuous training (MICT) or 75 min of vigorous exercise per week, and strength training on > 2 days per week. However, in 2020, 75% of adult Americans did not meet these PA recommendations. Physical inactivity is the fourth leading cause of death worldwide, as it increases the risk of 40 chronic conditions. Low maximal oxygen uptake (V̇O2max; the gold standard index of CRF) is strongly associated with increased risk of future morbidity and all-cause mortality. In fact, many large-scale studies have reported V̇O2max to be a more powerful predictor of risk for future morbidity and all-cause mortality than more traditional risk factors such as smoking, hypertension, obesity, high cholesterol, and insulin resistance. For population-level health, the largest benefits from improved PA and CRF are gained by the least fit individuals, with even small increases in fitness shown to improve health. Despite exercise being the only viable means to improve V̇O2max, it remains underutilized for primary disease prevention. Nevertheless, adults do not engage in PA due to a perceived lack of time. High intensity interval training (HIIT) is a time-efficient form of PA requiring brief intense bursts of activity separated by rest periods. Empirical data show that HIIT elicits similar and in some cases superior benefits versus MICT, often with less time. A special type of HIIT is reduced exertion high intensity interval training (REHIT) requiring only two 20 second sprints within a 10 minute session. A few studies show that REHIT increases CRF and other health-related outcomes, yet these studies have been criticized due to their small sample size, which serves to question the broad application of REHIT in adults. The proposed randomized controlled trial will recruit 60 inactive adults to complete 24 sessions of REHIT and compare resultant changes in CRF, substrate metabolism, and psychological responses to a non-exercising control group. This sample size is threefold higher than used in prior work and provides adequate statistical power to detect meaningful increases in V̇O2max. The proposed study will advance further research in the area of HIIT and if successful, may promote inclusion of REHIT in PA recommendations as a feasible alternative or adjunct to moderate or vigorous-intensity exercise as promoted in current PA recommendations.

Inclusion Criteria:

• Healthy
• BMI < 35 kg/m2
• younger than 65 yr old
• no joint issues

Exclusion Criteria:

• Those adults who are currently active and not weight stable
• Adults with BMI > 35 kg/m2
• Use of medications which may alter study outcomes

All participant data will be reported in articles published related to this project.

Investigators are masked to participant randomization which is performed by an individual who has no interaction with participants.