Participants (n = 23) were recruited from Kansas State University and surrounding community to participate in a free exercise program which would consist of standard aerobic and resistance training or a CrossFit™ program. There was no racial or gender bias for participant selection. Participants were stratified on median age (< or ≥ 28) and BMI (<or ≥30.5) and randomized to eight weeks/24 sessions on non-consecutive days (Monday, Wednesday, and Friday) of HIFT or ART. Participants completed written informed consent and procedures were approved by the Kansas State University Institutional Review Board. The intervention was conducted in 2012 with analysis in 2013.
Setting and participants
Participant characteristics included age (M = 26.8 ± 5.9 years), weight (M = 89.5 ± 14.2 kg), BMI (M = 31.1 ± 3.5 kg/m2), and body fat percentage (M = 42.0 ± 7.4). Race/ethnicity included white (n = 16), Hispanic (n = 3), Asian (n = 2), black (n = 1), and not specified (n = 1). Over half had a bachelor’s degree (60.9%), were female (56.5%) and were married/engaged (52.2%).
The ART intervention was designed to meet current physical activity recommendations and all sessions were supervised by an American Council on Exercise certified personal trainer . Participants completed three exercise sessions per week on Mondays, Wednesdays, and Fridays. Participants first warmed up using aerobic exercise machines at their own discretion (time not recorded). They completed a total of 50 minutes of aerobic exercise on the machines (minimum of 10 minutes per machine) each session at 40-50% heart rate reserve (HRR) for weeks 1–4 and 50-60% HRR for weeks 5–8. Full-body resistance exercises were completed in about 20 minutes during the sessions on Mondays (i.e., bicep curls, military presses, lat pulldowns, and leg extensions) and Wednesdays (i.e., tricep pulldowns, bench presses, reverse leg curls, and seated leg presses). After establishing 1 repetition maximums (1RM) in week 1, participants completed three sets per lift for weeks 2–8 (i.e., weeks 2–3 = 50% 1RM, 15 reps; weeks 4–5 = 60% 1RM, 12 reps; weeks 6–7 = 70% 1RM, 10 reps; week 8 = 75% 1RM, 8 reps). Three sets of 15 crunches were performed both days, and participants rested one minute between each set and exercise.
The HIFT group utilized CrossFit™ training for 60-minute sessions led by CrossFit™ certified trainers. Nine movements (i.e., air squat, front squat, overhead squat, press, push press push jerk, deadlift, sumo deadlift high pull, and medicine ball clean) were introduced in sessions 1–2. Remaining sessions included warm-up and stretching (10–15 min), instruction and technique practice (10–20 min), workout (5–30 min), and cool-down and stretching (5 min). Workouts utilized aerobic (e.g., rowing), bodyweight (e.g., pushups), and weightlifting (e.g., deadlifts) exercises in singular or multiple combinations that were completed for time, repetitions, or weight (e.g., see Table 1). Workouts were completed at relative (self-selected) high-intensity for each participant, with movements and weights individually scaled.
Main outcome measures
At baseline, participants were asked to indicate why they were interested in participating in the study (exercise initiation; qualitative). At baseline and posttest, participants were asked to complete a single-item to rate their exercise enjoyment from 1-strongly disagree to 5-strongly agree for the statement, “I enjoy doing exercise” . This measure has been found to have fair test-retest reliability and strong construct validity and to significantly correlate with objective and self-reported physical activity . At posttest, participants were asked if they planned to continue exercising and what type of exercise they planned to do (exercise intentions; qualitative). Adherence was defined as completing 90% of exercise sessions.
In light workout clothing without shoes at baseline and posttest, participants’ height to the nearest 0.5-cm was measured with a SECA® 214 portable stadiometer (Chino, CA) and their weight to the nearest 0.5-pound was measured with a Detecto scale (Webb City, MO). These measurements were used to calculate BMI, after conversion of pounds to kg. Body composition was measured by Lunar Prodigy dual-energy X-ray absorptiometry (DXA) scan (Madison, WI). Times to complete daily workouts (excluding time for warm-up, stretching, skill work, and cool-down) were recorded.
Qualitative data were analyzed using NVivo (version 10). Reasons for exercise initiation were coded as either intrinsic (i.e., focused on the process of completing the behavior such as pleasure, satisfaction, and skill development) or extrinsic (i.e., focused on completing the behavior to gain benefits, improve image, or avoid negative consequences) [19–21]. Categories of qualitative answers were counted and entered into SPSS (version 20) for statistical analysis.
Quantitative data were summarized with descriptive statistics. To examine between-group differences at baseline, independent samples t-tests were conducted for age, BMI, body composition, and exercise enjoyment; chi-square analysis was conducted for gender, marital status, race/ethnicity, education, and income. Paired samples t-tests were used to compare dropouts to adherers. To examine differences from baseline to posttest in exercise enjoyment, BMI, and body composition, 2 (group) × 2 (time) repeated measures ANOVAs (between-groups) were conducted. Chi-square analysis was conducted to compare exercise intentions between groups. Independent samples t-tests were conducted to compare workout times. Statistical significance was set at p < .05.