BACKGROUND: Understanding exercise participation for overweight and obese adults is critical for preventing comorbid conditions. Group-based high-intensity functional training (HIFT) provides time-efficient aerobic and resistance exercise at self-selected intensity levels which can increase adherence; behavioral responses to HIFT are unknown. This study examined effects of HIFT as compared to moderate-intensity aerobic and resistance training (ART) on exercise initiation, enjoyment, adherence, and intentions. METHODS: A stratified, randomized two-group pre-test posttest intervention was conducted for eight weeks in 2012 with analysis in 2013. Participants (n = 23) were stratified by median age (< or >= 28) and body mass index (BMI; < or >= 30.5). Participants were physically inactive with an average BMI of 31.1 +/- 3.5 kg/m2, body fat percentage of 42.0 +/- 7.4%, weight of 89.5 +/- 14.2 kg, and ages 26.8 +/- 5.9 years. Most participants were white, college educated, female, and married/engaged. Both groups completed 3 training sessions per week. The ART group completed 50 minutes of moderate aerobic exercise each session and full-body resistance training on two sessions per week. The HIFT group completed 60-minute sessions of CrossFit with actual workouts ranging from 5 to 30 minutes. Participants completed baseline and posttest questionnaires indicating reasons for exercise initiation (baseline), exercise enjoyment, and exercise intentions (posttest). Adherence was defined as completing 90% of exercise sessions. Daily workout times were recorded. RESULTS: Participants provided mostly intrinsic reasons for exercise initiation. Eighteen participants adhered (ART = 9, 81.8%; HIFT = 9, 75%). HIFT dropouts (p = 0.012) and ART participants (p = 0.009) reported lower baseline exercise enjoyment than HIFT participants, although ART participants improved enjoyment at posttest (p = 0.005). More HIFT participants planned to continue the same exercise than ART participants (p = 0.002). No significant changes in BMI or body composition were found. Workouts were shorter for HIFT than ART (p < 0.001). CONCLUSIONS: HIFT participants spent significantly less time exercising per week, yet were able to maintain exercise enjoyment and were more likely to intend to continue. High-intensity exercise options should be included in public health interventions. TRIAL REGISTRATION: ClinicalTrials.gov identifier NCT02185872. Registered 9 July 2014.

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