The protocol of this registered trial (ClinicalTrials.gov # NCT02164474) is published elsewhere . As such, an overview of the methods is presented. CONSORT checklist provided in Additional file 1.
Small Steps for Big Changes was a two-arm parallel group randomized trial that compared change in CRF and adherence to HIIT versus MICT 12 months following a two-week brief exercise counselling program in individuals who were low active and with overweight and obesity.
Eligible participants were between the ages of 30 and 65, were low-active (i.e., engaged in 2 or less bouts of moderate and/or vigorous aerobic exercise per week in the previous 6-months), had a body mass index (BMI) between 25 and 40 kg/m2, and were cleared to engage in vigorous exercise using the Physical Activity Readiness Questionnaire-Plus (PAR-Q+) . Participants were recruited through paper and online ad postings in the community (e.g., posters in community centres, coffee shops, online advertisements).
This study received clinical research ethics approval from the first author’s university research ethics board and met the ethical standards of the Declaration of Helsinki. Eligible participants provided written informed consent. An external statistician computer-generated (SAS PROC PLAN) random allocations to condition (1:1, HIIT or MICT) using permutated blocks of random size, stratified for sex; these were accessed by the project coordinator via password-protected website. Participants in both conditions completed 10 exercise sessions over a two-week period, seven of which were one-on-one supervised sessions conducted in the laboratory (exercise training plus counselling), while three were conducted at home to foster independence.
The exercise prescriptions for each condition were progressive and matched for estimated external work. HIIT involved sessions progressing from 4 to 10 × 1-min high-intensity intervals at ~ 80–90% VO2peak interspersed with 1-min rest periods at ~ 40% VO2peak and with 5 min of warm up and cool down. MICT involved sessions progressing from 20 to 50 min of continuous moderate-intensity exercise at ~ 45–55% VO2peak. All participants were exposed to a variety of exercise formats (e.g., stationary cycling, treadmill, elliptical, walking outside) and were able to self-select the exercise modality for four of the supervised sessions with the remaining three performed as stationary cycling to ensure accurate intensity based on the baseline VO2peak test. Participants wore a heart-rate monitor that provided them with feedback to understand the physiological exercise sensations (i.e., breathing, heart rate) associated with their prescribed exercise intensity zone. Following the two-week training program, participants were recommended to exercise three times a week performing either 10 × 1 min high intensity intervals or 50 min of continuous moderate intensity exercise. Participants could vary the number of intervals or duration to achieve of the prescribed total volume (i.e., 30 high intensity intervals or 150 moderate minutes).
Participants in both conditions received the same brief exercise counselling intervention delivered throughout the two-week supervised training program. Counselling was delivered in a one-on-one format at each of the seven supervised sessions (~ 10 min per session, 70 min total) and via take-home worksheets for the three home-based sessions. A detailed description of the behaviour change techniques used to promote exercise self-management are reported elsewhere . Briefly, task self-efficacy to perform HIIT or MICT was primarily bolstered through: providing instruction on how to perform the behaviour, behavioural practice, and helping participants identify physiological cues associated with the assigned exercise intensity. Self-regulatory efficacy was bolstered through: providing participants with opportunities to practice, with feedback, on self-monitoring, planning, and solving exercise barriers for independent exercise. Finally, salience of the positive psychological and physiological outcomes associated with exercise engagement (i.e., outcome expectations) was fostered through education and by bringing awareness to participants’ own subjective experiences of exercise and the experiences of similar individuals.
Participants were provided with a self-monitoring mobile application  to track their exercise during the 12-month trial and were sent monthly booster messages through this app to reinforce the psychological mechanisms addressed in counselling sessions. Exercise trainers monitored their participants through the app and contacted them when they failed to login for three consecutive days .
Age, sex, ethnicity, annual household income, marital status, and education level were collected at baseline.
was measured using peak oxygen uptake (absolute and relative VO2peak) and peak power output (Wpeak) at baseline, 6 and 12 months. VO2peak was assessed by a continuous incremental ramp (15 W/min) maximal exercise test on an electronically braked cycle ergometer (Lode Excalibur, The Netherlands) with expired gas collection (Parvomedics TrueOne 2400, Salt Lake City, Utah, USA). The metabolic cart was calibrated with a 3.0 L syringe and gases of known concentration before every test. VO2peak is defined as the highest 30-s average for VO2 (in l/min and ml/kg/min) and Wpeak the highest power achieved. Criteria for determining VO2 peak was indicated by a leveling (< 0.100 L・min-1) or decrease in VO2 with increasing workload; a plateau in heart rate (< 5 bpm) and (or) attainment of age predicted maximum heart rate; a respiratory exchange ratio > 1.1; and volitional fatigue.
Accelerometer-measured purposeful moderate-to-vigorous physical activity (MVPA)
adherence was assessed by accelerometry (Actigraph GT3X-BT, Actigraph, Pensacola, Florida, USA) at baseline and 3-, 6-, 9- and 12-month follow-up, using Freedson’s (1998) uniaxial cut-points . In line with suggestion by Migueles’ et al. (2017) review of accelerometry cut-points , the driving factors for selecting Freedson (1998) cut-points was comparability between studies, as it is the most commonly used cut-points with 5-s epochs summed as counts per minute. Participants were asked to wear the accelerometer on their right hip for seven consecutive days. A total of ≥10 h of valid wear time per day was required to be included in the analyses.
Due to the intermittent nature of HIIT and the lack of standardized methods to quantify HIIT based on accelerometry, purposeful physical activity was operationalized as minutes spent in MVPA in bouts of ≥10 min (MVPA10+) . MVPA10+ was also selected a priori as a measure of interest, as at the onset of the trial, bouts of 10 min or more of moderate and vigorous physical activity was considered the minimal amount required to elicit beneficial physiological adaptations . In line with Watson , scoring the MVPA10+ variable allows for drops of up to 1-min in intensity, which means the intermittent nature of HIIT would still be captured. “Rest” periods were performed at ~ 40% VO2peak, which would not result in a drop in accelerometer counts sufficient to cause a bout of HIIT to fail to be counted, as verified in our pilot research .
Total minutes of moderate and vigorous physical activity was also collected and analyzed ing Actilife v.6.11. Freedson cut points were used to identify time spent in each exercise intensity . We also examined the proportion of MVPA prescription achieved, by taking the total time spent in MVPA10+ and divided it by the number of minutes that was prescribed to participants in each condition as an indicant of meeting the prescribed exercise volume (HIIT was prescribed 75 min and MICT 150 min).
App-based self-monitored exercise
Participants used a mobile application  to self-monitor their exercise engagement. Participant could report the details of their exercise, including whether they did HIIT or MICT. Using this data as a measure of adherence to prescribed exercise, we extracted the total number of times that participants reported performing HIIT or MICT for those who self-monitored throughout the 12-month follow-up. We created weekly averages (# of bouts per week) for the first and second half of the 12-month follow-up.
Height and weight (SECA, 700 SECA, Hamburg, Germany), and waist circumference (WC, measured at the level of the umbilicus)  were taken by the same trained research assistant at each timepoint.
Dual energy X-ray absorptiometry (DXA; Hologic Discovery A) scans were used to assess total body fat percentage.
Based on Bandura’s (2006) methodological procedures , task self-efficacy assessed individuals’ confidence in their abilities to perform HIIT or MICT, dependent on assigned condition. Self-regulatory efficacy assessed individuals’ confidence in their abilities to manage their exercise behaviour (e.g., to schedule, plan). Responses were scored on a scale ranging from 0% (not at all confident) to 100% (extremely confident). Both task (4 items; α’s ≥ .86) and self-regulatory efficacy (14 items; α’s ≥ .90) had strong internal consistency. The overall scale mean was used for all analyses.
Sample size determination
Please refer to the published protocol for a thorough description of the sample size determination . Briefly, 15 participants per group were needed to detect a significant within-between interaction in cardiorespiratory fitness, our main outcome. This was based on a two-tailed alpha of 0.05 and 80% power, assuming a medium correlation amongst repeated measures of r = 0.5, a pooled mean of 20.8 (SD = 4.0) ml/kg/min , and a Cohen’s d = .48 difference between HIIT and MICT  (calculated using G*Power v3.1). However, our secondary outcome of MVPA typically produces greater measurement variability and we opted to determine the sample size based on the sample size requirements for detecting a clinically relevant within-group change in MVPA. To detect a difference of 10-min average MVPA per day within conditions, assuming a standard deviation of 16 , with 80% power at p < 0.05, 41 participants per group were required. A conservative ~ 25% loss to follow-up is anticipated and therefore the trial aimed to recruit 50 participants per group (i.e., 100 participants to be randomized).
Linear mixed effect regression was used to assess the change in outcomes at month 3, 6, 9, and 12 (relative to baseline) within and between each treatment group. The regression analysis considered the change in the measurements between the follow-up visits and baseline as the outcome. Time was considered as a categorical variable and an unstructured covariance matrix, which was allowed to differ by treatment group, was used to model the correlation over time. Other than data for self-efficacy and vigorous minutes, which are described below, our data met multivariate assumptions. For self-efficacy, the distribution was non-parametric, so analyses were based on quantile regression to compare the median change in the measurements. For vigorous minutes, due to the distribution of the data, Poisson mixed effects model was used. Analyses were performed using SAS 9.4 (SAS Institute, Cary NC).