The effect of low volume sprint interval training on cardiorespiratory fitness: study protocol for a definitive randomized controlled trial

Abstract Background Prior studies show significant increases in cardiorespiratory fitness (V̇O2max) and other health-related outcomes in response to reduced exertion high-intensity interval training (REHIT), yet these studies are characterized by small sample sizes which casts doubt on the true efficacy of this form of physical activity for enhancing fitness and health. We propose a definitive randomized controlled exercise training study in a large sample of inactive adults to examine the effects of REHIT on outcomes related to cardiometabolic health in comparison to a non-exercise control group. Methods After baseline testing, 120 inactive men and women ages 18–64 years will be randomized to a non-exercising control group or undergo 12 weeks of REHIT. Participants will be healthy, weight stable, and have BMI < 35 kg/m2 and be required to maintain their habitual physical activity and dietary patterns during the study. Training will be supervised and performed 2 days per week. Changes in V̇O2max, body composition, and substrate metabolism will be monitored to assess adaptations in cardiometabolic health in response to REHIT. To assess the feasibility of REHIT, various perceptual measures including RPE, affective valence, and enjoyment will be recorded, and quality of life will be measured. Discussion Data from the current study will advance physical activity research by bolstering the implementation of REHIT, which will contribute to reduced risk factors of cardiovascular disease and, in turn, improved public health. Trial registration OSF (osf.io/wvcz4). Registered on November 3, 2025. ClinicalTrials.gov NCT07328568. Registered on January 8, 2026.