Individuals randomized to HIIT will be asked to engage in only HIIT 5 times/week for 4 weeks. HIIT consists of 5 repeated vigorous intervals of 1-min duration at 80-90% of heart rate max interspersed with 1-min recovery periods. A 3-min warm-up and 2-min cool-down are also included, for a total session duration of 15 minutes. This matches the guidelines of 75 min of vigorous exercise per week. During the supervised exercise sessions, intensity will be based on HR monitors and psychophysiological responses of perceived exertion. The exercise training is paired with 30-50 minutes of behavioral counselling that is identical for all three interventions.

Participants will first be familiarized with HIIT and MICT in a randomized, counter-balanced order during supervised sessions 1 and 2, and will subsequently self-select the exercise type for the supervised and unsupervised exercise sessions. The HIIT and MICT protocols will match to the parallel imposed conditions. During the supervised exercise, intensity will also be based on HR monitors and psychophysiological responses of perceived exertion. The exercise training is paired with 30-50 minutes of behavioral counselling that is identical for all three interventions.

Change in physical activity behavior from baseline to 6 months later will be measured using the Godin Leisure-Time Exercise Questionnaire (GLTEQ). The GLTEQ requires individuals to report their frequency of engaging in strenuous, moderate, and mild exercise during a typical week in bouts of 15 minutes or longer. Scoring of the GLTEQ consists of first multiplying reported frequencies by their related metabolic equivalent (MET) values (strenuous: 9METs, moderate: 5METs, mild: 3METs), and then summing the products for a total score. Interpretation of the scores follows the categories proposed by Godin, where a score of 24 units or higher indicates an individual is active, a score between 14 and 23 indicates an individual is moderately active, and a score below 14 indicates an individual is insufficiently active. The minimum score is 0 and there is no maximum score. Higher scores indicate higher levels of physical activity behavior.

Change in physical activity behavior from baseline to 6 months later will also be measured using 7-day triaxial accelerometry data. Participants wear an Actigraph GT3X-BT for seven consecutive days at the top of their right hip at a sampling rate of 100Hz. Accelerometry data is considered valid if the participant wears the accelerometer for a minimum of 10 hours/day and at least 4 of the 7 days of the week. Non-wear time is defined as any period of 60 minutes or longer with no recorded activity counts. Physical activity is measured by summing activity counts in 60-second epochs, and cut-off activity count thresholds for defining intensity follows Troiano and colleagues' convention. Free-living physical activity behavior is operationalized as average daily minutes of MVPA and is calculated by summing both moderate and vigorous activity count minutes and subsequently dividing by the number of valid wear days.

A wearable activity tracker (Fitbit Luxe™) is also provided to each participant to track change in physical activity behavior from baseline to 6 months later in free-living conditions. We define a valid wear day as any day with a minimum of 10 hours of wear time determined by continuous minute-by-minute heart rate recordings. Non-wear time is defined as a period of 60 continuous minutes with no heart rate recordings. A minimum of 4 out of 7 consecutive days of the week are required for the week to be considered valid. Free-living physical activity behavior is operationalized as average daily minutes of MVPA and is calculated by summing the daily minutes spent in moderate and intense activities per day and subsequently dividing by the number of valid wear days. Activity intensity levels are determined by using Fitbit-derived algorithms, and number of minutes per day at each intensity level are imported into Fitabase.

Participants' change in cardiorespiratory fitness (CRF) from baseline to 6 months later is calculated based on the minutes and seconds participants are able to achieve in submaximal 12-lead ECG stress tests performed at a cardiac rehabilitation clinic. Calculation of CRF is conducted according to Bruce and colleagues' equation for estimated maximal volume of oxygen intake (VO2max) in mL/(kg*min). Calculations are stratified by biological sex, where w is the weighting factor for sex (1=men; 2=women) and t is the duration of the protocol in seconds:

VO2max=6.70-2.82w+0.056t

Five repeated vigorous intervals of 1-min duration at 80-90% of heart rate max interspersed with 1-min recovery periods; 3-min warm-up and 2-min cool-down, making the total session duration 15 minutes for five times/week, equated to match the guidelines of 75 min of vigorous exercise per week.

HIIT: Individuals randomized to HIIT will be asked to engage in only HIIT 5 times/week for 4 weeks. HIIT consists of 5 repeated vigorous intervals of 1-min duration at 80-90% of heart rate max interspersed with 1-min recovery periods. A 3-min warm-up and 2-min cool-down are also included, for a total session duration of 15 minutes. This matches the guidelines of 75 min of vigorous exercise per week. During the supervised exercise sessions, intensity will be based on HR monitors and psychophysiological responses of perceived exertion. The exercise training is paired with 30-50 minutes of behavioral counselling that is identical for all three interventions.

Participants will be introduced to HIIT and MICT during sessions 1 and 2 of the 4-week intervention in a counterbalanced randomized order, and will thereafter self-select one of the two exercise types for remaining sessions. Exercise will be matched to the parallel imposed conditions.

CHOICE: Participants will first be familiarized with HIIT and MICT in a randomized, counter-balanced order during supervised sessions 1 and 2, and will subsequently self-select the exercise type for the supervised and unsupervised exercise sessions. The HIIT and MICT protocols will match to the parallel imposed conditions. During the supervised exercise, intensity will also be based on HR monitors and psychophysiological responses of perceived exertion. The exercise training is paired with 30-50 minutes of behavioral counselling that is identical for all three interventions.

Total of all reporting groups

Race and Ethnicity were not collected from any participant.

Five repeated vigorous intervals of 1-min duration at 80-90% of heart rate max interspersed with 1-min recovery periods; 3-min warm-up and 2-min cool-down, making the total session duration 15 minutes for five times/week, equated to match the guidelines of 75 min of vigorous exercise per week.

HIIT: Individuals randomized to HIIT will be asked to engage in only HIIT 5 times/week for 4 weeks. HIIT consists of 5 repeated vigorous intervals of 1-min duration at 80-90% of heart rate max interspersed with 1-min recovery periods. A 3-min warm-up and 2-min cool-down are also included, for a total session duration of 15 minutes. This matches the guidelines of 75 min of vigorous exercise per week. During the supervised exercise sessions, intensity will be based on HR monitors and psychophysiological responses of perceived exertion. The exercise training is paired with 30-50 minutes of behavioral counselling that is identical for all three interventions.

Participants will be introduced to HIIT and MICT during sessions 1 and 2 of the 4-week intervention in a counterbalanced randomized order, and will thereafter self-select one of the two exercise types for remaining sessions. Exercise will be matched to the parallel imposed conditions.

CHOICE: Participants will first be familiarized with HIIT and MICT in a randomized, counter-balanced order during supervised sessions 1 and 2, and will subsequently self-select the exercise type for the supervised and unsupervised exercise sessions. The HIIT and MICT protocols will match to the parallel imposed conditions. During the supervised exercise, intensity will also be based on HR monitors and psychophysiological responses of perceived exertion. The exercise training is paired with 30-50 minutes of behavioral counselling that is identical for all three interventions.

Participants' change in motivation regulation from baseline to post-intervention and subsequently 6 months later is measured using the Behavioral Regulation in Exercise Questionnaire-2 (BREQ-2). The BREQ-2 is a 19-item self-report questionnaire that measures one's motivation to exercise. The questionnaire uses a 5-point Likert scale ranging from 0 "not true for me" to 4 "very true for me". The relative autonomy index (RAI) is used to provide an index of the degree to which participants feel self-determined ranging from a minimum score of -24 to a maximum score of +20, with higher positive scores indicating more autonomous forms of motivation regulation. The RAI is calculated by first averaging each subscale, then multiplying each average by a subscale weighting, and subsequently summing the weighted subscale scores. Weighting for each subscale are as follows: amotivation (-3), external regulation (-2), introjected regulation (-1), identified regulation (+2), intrinsic regulation (+3).

Change in physical activity behavior from baseline to 6 months later will be measured using the Godin Leisure-Time Exercise Questionnaire (GLTEQ). The GLTEQ requires individuals to report their frequency of engaging in strenuous, moderate, and mild exercise during a typical week in bouts of 15 minutes or longer. Scoring of the GLTEQ consists of first multiplying reported frequencies by their related metabolic equivalent (MET) values (strenuous: 9METs, moderate: 5METs, mild: 3METs), and then summing the products for a total score. Interpretation of the scores follows the categories proposed by Godin, where a score of 24 units or higher indicates an individual is active, a score between 14 and 23 indicates an individual is moderately active, and a score below 14 indicates an individual is insufficiently active. The minimum score is 0 and there is no maximum score. Higher scores indicate higher levels of physical activity behavior.

Change in physical activity behavior from baseline to 6 months later will also be measured using 7-day triaxial accelerometry data. Participants wear an Actigraph GT3X-BT for seven consecutive days at the top of their right hip at a sampling rate of 100Hz. Accelerometry data is considered valid if the participant wears the accelerometer for a minimum of 10 hours/day and at least 4 of the 7 days of the week. Non-wear time is defined as any period of 60 minutes or longer with no recorded activity counts. Physical activity is measured by summing activity counts in 60-second epochs, and cut-off activity count thresholds for defining intensity follows Troiano and colleagues' convention. Free-living physical activity behavior is operationalized as average daily minutes of MVPA and is calculated by summing both moderate and vigorous activity count minutes and subsequently dividing by the number of valid wear days.

A wearable activity tracker (Fitbit Luxe™) is also provided to each participant to track change in physical activity behavior from baseline to 6 months later in free-living conditions. We define a valid wear day as any day with a minimum of 10 hours of wear time determined by continuous minute-by-minute heart rate recordings. Non-wear time is defined as a period of 60 continuous minutes with no heart rate recordings. A minimum of 4 out of 7 consecutive days of the week are required for the week to be considered valid. Free-living physical activity behavior is operationalized as average daily minutes of MVPA and is calculated by summing the daily minutes spent in moderate and intense activities per day and subsequently dividing by the number of valid wear days. Activity intensity levels are determined by using Fitbit-derived algorithms, and number of minutes per day at each intensity level are imported into Fitabase.

Participants' change in cardiorespiratory fitness (CRF) from baseline to 6 months later is calculated based on the minutes and seconds participants are able to achieve in submaximal 12-lead ECG stress tests performed at a cardiac rehabilitation clinic. Calculation of CRF is conducted according to Bruce and colleagues' equation for estimated maximal volume of oxygen intake (VO2max) in mL/(kg*min). Calculations are stratified by biological sex, where w is the weighting factor for sex (1=men; 2=women) and t is the duration of the protocol in seconds:

VO2max=6.70-2.82w+0.056t

Five repeated vigorous intervals of 1-min duration at 80-90% of heart rate max interspersed with 1-min recovery periods; 3-min warm-up and 2-min cool-down, making the total session duration 15 minutes for five times/week, equated to match the guidelines of 75 min of vigorous exercise per week.

HIIT: Individuals randomized to HIIT will be asked to engage in only HIIT 5 times/week for 4 weeks. HIIT consists of 5 repeated vigorous intervals of 1-min duration at 80-90% of heart rate max interspersed with 1-min recovery periods. A 3-min warm-up and 2-min cool-down are also included, for a total session duration of 15 minutes. This matches the guidelines of 75 min of vigorous exercise per week. During the supervised exercise sessions, intensity will be based on HR monitors and psychophysiological responses of perceived exertion. The exercise training is paired with 30-50 minutes of behavioral counselling that is identical for all three interventions.

Participants will be introduced to HIIT and MICT during sessions 1 and 2 of the 4-week intervention in a counterbalanced randomized order, and will thereafter self-select one of the two exercise types for remaining sessions. Exercise will be matched to the parallel imposed conditions.

CHOICE: Participants will first be familiarized with HIIT and MICT in a randomized, counter-balanced order during supervised sessions 1 and 2, and will subsequently self-select the exercise type for the supervised and unsupervised exercise sessions. The HIIT and MICT protocols will match to the parallel imposed conditions. During the supervised exercise, intensity will also be based on HR monitors and psychophysiological responses of perceived exertion. The exercise training is paired with 30-50 minutes of behavioral counselling that is identical for all three interventions.

Diagnosis was not related to the current study.