Participants in this study were part of the ‘Exercise in prevention of Metabolic Syndrome (EX-MET)’ international multicentre project described previously [16]. This-sub-study investigated the change in FFI values in participants recruited from the trial site at Brisbane, Australia. Recruitment was conducted through several methods: i) a website was developed to serve as a recruitment link for social platforms and the University’s online magazine; ii) referrals from medical practitioners at the Princess Alexandra Hospital; and iii) advertising through posters, newspapers, television news and flyers disseminated across the university and local health care centres. Prospective participants were excluded if they presented with any of the following: recent myocardial infarction (last four weeks), unstable angina, uncompensated heart failure, severe valvular heart disease, uncontrolled hypertension, pulmonary disease, cardiomyopathy, and kidney failure. Written and oral consent were obtained from all participants prior to inclusion. Ninety-nine individuals diagnosed with MetS according to the International Diabetes Federation criteria [17] were included and randomized into the following exercise groups (stratified by age, sex, and centre): i) MICT (n = 34); ii) 4HIIT (n = 34); and iii) 1HIIT (n = 31) (Fig. 1). The randomization procedure was performed via a software employing random permuted blocks. De-identified details of participants eligible were entered into an online system to acquire group allocation.
Before and after the 16-week exercise interventions, participants underwent several tests at the university’s laboratory (Human Movement and Nutrition Sciences Building, St Lucia Campus, The University of Queensland, QLD, Australia) to assess the primary (FFI) and secondary outcome measures (MetS risk factors and body composition). Participants were instructed to refrain from strenuous activities for at least 48 h, and caffeine and alcohol for at least 24 h before each examination. All assessments were conducted at approximately the same time of the day (morning, ± 2 h). This study was approved by the Medical Research Ethics Committee, The University of Queensland (Brisbane, Australia).
Metabolic Syndrome
To determine the participants’ eligibility for the study, the following assessments were conducted after a 12-h fast: i) brachial systolic and diastolic blood pressure; ii) fasting lipid profile and glucose-level; and iii) anthropometric measures (height, waist circumference, weight, and hip circumference). Details of these assessments have been reported previously [18].
Fitness Fatness Index
The FFI was calculated as the ratio between cardiorespiratory fitness, expressed as the metabolic equivalent (MET), and WtHR. Waist circumference and height were measured according to the protocols presented in Coombes and Skinner [19]. Briefly, waist circumference was measured at least twice at the narrowest point between the lower costal (10th rib) border and the top of the iliac crest, perpendicular to the trunk’s long axis. The WtHR was calculated by dividing the waist circumference in cm by height in cm. Cardiorespiratory fitness depicted as the peak oxygen update (V̇O2peak, mL/kg/min) was assessed via indirect calorimetry using the Parvo Medics TrueOne 2400 and Metamax II system (Cortex, Leipzig, Germany) during a graded maximal exercise test. V̇O2peak was determined as the highest 15- second time averaged V̇O2, expressed relative to the participant’s mass in mL/kg/min. V̇O2peak in mL/kg/min was subsequently converted to METs by dividing it by 3.5 mL/kg/min. A cycle or treadmill ergometer was used during the test according to the participants’ preferred training method during the supervised exercise sessions or orthopedic limitations. In order to standardize nutrition for the test, participants were provided with a liquid nutritional supplement (Sustagen, 250 mL, Dutch Chocolate, Nestle, Gympie QLD, Australia) to consume two hours before the assessment. All tests were preceded with an 8-min warm-up which included 2 stages (stage 1 warm-up: 4 km/h at 0% incline or 50–60 revolutions per minute [rpm] at 0 W; stage 2 warm-up: 4 km/h at 4% incline or 50–60 rpm at 25 W). The speed (individualized: within 6–9 km/h) and load (2% incline or 50 W) were subsequently increased each minute until exhaustion. Standardized verbal cues were provided throughout the graded exercise test to motivate participants to reach maximal effort.
Body Composition
Dual-energy X-ray absorptiometry (DEXA; Hologic QDR 4500 version 12.6) was used to assess pre- and post-intervention measures of body fat indices (total body and regional [android and gynoid] fat distributions [%]) and lean mass. Participants were required to be in a 12-h overnight fasted state for this assessment. Total caloric intake was monitored from baseline to post-intervention using a 3-day food diary. A diet analysis software (FoodWorks 8 Professional; Xyris Software) was subsequently used to analyse the food diary data.
Moderate-to-Vigorous Physical Activity
Accelerometers (ActiGraph GT3X_, Pensacola, FL) were used to objectively assess average daily time spent in moderate-to vigorous physical activity (MVPA). The accelerometer was placed on the participants’ right hip during waking hours for 7 days at baseline and post-testing. The ActiLife 6.1 software was used to analyse the data in 60 s sampling frequency. Minimum wear time was determined as the accelerometer device worn: i) at least one day during a weekend day; and ii) 10 h per day for four of the 7 days. Non-wear time was determined as 60 min of consecutive zeros [20]. The time in MVPA was defined by a cut-off point of 2020 counts per minute [21].
Training Protocol
The MICT group completed five exercise sessions per week, whilst the HIIT group trained three times per week (at least a day between sessions). All participants were required to attend two supervised sessions per week at The University of Queensland, while the remaining session/s were performed unsupervised. The unsupervised exercise sessions consisted of participant-preferred outdoor or indoor pursuits involving large muscle groups such as walking, running, or cycling. Both exercise heart rate and rating of perceived exertion (RPE) were monitored and recorded throughout the exercise sessions using a heart rate monitor (Polar Electro, Kempele, Finland) and 6–20 Borg scale [22]. Participants recorded HR and RPE data during the unsupervised sessions in a training log. The MICT group trained continuously for 30 min at 60–70% peak heart rate (HRpeak)/RPE of 11–13 on the Borg Scale, whereas each 4HIIT and 1HIIT session began with a 10-min warm-up and concluded with a 3-min cool-down at 50–70% HRpeak. The 4HIIT intervention included four bouts of 4-min intervals performed at 85–95% HRpeak/RPE of 15–17 on the Borg scale, interspersed with 3-min of active recovery performed at 50–70% HRpeak, totaling 38 min per session. The 1HIIT intervention comprised of one 4-min bout of exercise performed at 85–95% HRpeak/RPE of 15–17 on the. Borg scale, totaling 17-min per session.
Statistical Analysis
Data were analysed using the SPSS version 25 package (IBM, New York, NY, USA). Chi-square tests were used to compare exercise adherence between exercise intervention groups. Analysis of covariance (ANCOVA) was used to determine the between-group difference in the change in continuous variables from pre- to post-intervention, with the change-value assigned as the dependent variables and the baseline value as the covariate. Eta squared (η2) group x time interaction effect sizes were calculated as between-group sum of squares divided by the total sum of squares and interpreted as follows: ‘small’ effect (0.01); ‘small-to-medium’ effect (0.01 to 0.10); ‘medium-to-large’ effect (0.10 to 0.25) [23]. Continuous variables are presented as mean ± standard deviation or median (range), whilst categorical variables are reported as frequencies.
To determine individual FFI training responsiveness, delta values (post-intervention value minus pre-intervention value) were calculated. A participant was considered a likely responder if the delta FFI value was ≥ 1 unit. Chi square tests were used to analyse the proportion of training response for FFI with subsequent Cramer’s V test to quantify effect size. Significance level was set at p < 0.05.