BACKGROUND: Functional training is commonly conducted using high-intensity interval training (HIIT) where heart rate exceeds 85% of age-predicted maximum. Traditional forms of HIIT have primarily been studied using cycling or running, and more work is needed to define the effects of functional HIIT and to compare it to combined training that incorporates an equal mix of traditional and functional work. The purpose of the present study was to compare 4 weeks of functional HIIT, endurance, and combined HIIT and endurance training on markers of health-related fitness. METHODS: Fifteen recreationally active participants (age= 21.8 + 3.1 years, BMI = 24.5 + 3.9 kg/m2, VO2max = 33.6 + 8.7 ml/kg/min, percent body fat = 33.2 + 8.9 %) were divided into 3 groups: HIIT, endurance, or combined training. HIIT workouts consisted of 3 rounds of 8 bodyweight exercises with a 2:1 work-rest ratio, and participants were coached to exercise maximally or near-maximally during each block with heart rate recorded continuously throughout each session. Endurance exercise was performed on a treadmill at an intensity of 60-70% heart rate reserve (HRR) for 30 minutes. All participants completed 3 exercise sessions per week over 4 weeks, with the combined group completing 6 sessions of HIIT and 6 sessions of endurance exercise. VO2max, body composition, and muscular endurance (1-minute push-ups, curl-ups, and weighted goblet box squats) were measured pre- and post-training. Separate factorial ANOVAs were used to assess changes in each dependent variable with Bonferroni post-hoc testing. RESULTS: Significant main effects for time were found for VO2max (p=0.01) with improvements in the HIIT group post-training (p=0.014). Significant interaction (p=0.013) and main effects for time (p=0.013) were observed for lean body mass with significant increases in the HIIT group (p<0.001). Push-ups and curl-ups displayed significant main effects for time (p<0.001 and p=0.001, respectively), with improvements in the HIIT(push-ups: p=0.005; curl-ups: p=0.016) and combined (push-ups: p=0.007; curl-ups:p=0.001) groups. CONCLUSIONS: Engaging in functional HIIT 3 days per week for 4 weeks improves VO2max and lean body mass. HIIT and combined training are effective for improving muscular endurance, but more than 6 sessions of HIIT are necessary to induce changes in aerobic fitness or body composition.

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