Subjects will participate in high-intensity interval training (HIIT) only.

After a brief warm-up, participants will pedal on an ergometer 75-85 revolutions per minute (RPMs) at low resistance of approximately 2 metabolic equivalent of task (METs). The resistance will increase by 15% per minute until respiratory exchange ratio (RER) exceeds 1.0, maximum oxygen consumption (V02 max) is reached, or participant can no longer maintain pedaling speed.

Following a brief warm-up, participants will be instructed to pedal on an ergometer as hard and as fast as they can for 30 seconds at a resistance of 1 watt·kilogram-1, followed by a cool-down period. Changes in anaerobic power, will be as assessed by as changes in power outputs in watts (rpm·braking force).

Participants will be instructed to lie supine with hands by their sides on a GE Lunar iDXA machine for a full-body x-ray scan which will measure grams and percentage changed in fat mass.

Participants will be instructed to lie supine with hands by their sides on a GE Lunar iDXA machine for a full-body x-ray scan which will measure grams and percentage changed in fat-free lean mass.

Changes in preference for tolerance for physical pain and discomfort will be assessed by self-report questionnaire responses to the Discomfort Intolerance Scale (DIS). This scale measures individual differences in ability to tolerate uncomfortable sensations. There are 7 items total. Although there are two subscales (measure ability to tolerate discomfort / pain and avoidance of physical discomfort) only total scores will be used. There are 7 items total. Items are scored 0 - 6 (total range 0 - 42), with higher scores indicating greater ability to tolerate physical discomfort. Average score on the total scale will be used.

Changes perceived support for physical activity from friends and family members will be assessed by self-report questionnaire responses to the Social Support and Exercise Survey. This questionnaire measures 13 items are scored on a scale of 1 - 5, with higher scores indicating greater perceived support. Total scores will be used, with ranges from 13 - 65. Higher scores indicate greater perceived support. Average score for the total scale will be used. Items may be answered twice, once for family members and once for friends.

Changes in perceptions of how much exercise satisfies autonomy, relatedness, and competence needs will be assessed by self-report questionnaire responses to the Psychological Need Satisfaction in Exercise Scale. This scale measures how much the psychological needs of autonomy, relatedness, and competence are satisfied during exercise. There are three subscales: autonomy, relatedness, and competence. Each subscale has 6 items rated on a 6-point scale. Higher scores indicate greater amounts of the construct. Total subscale scores may range from 6 to 36. Subscale averages will be used in analyses.

Changes in intrinsic, external, interrogated, identified, introjected and amotivation for physical activity will be assessed by self-report questionnaire responses to the Behavioral Regulation in Exercise, 3rd Edition (BREQ-3).This scale measures how much exercise behavior is driven by intrinsic motivation, external motivation, interrogated motivation, identified motivation, introjected motivation, and amotivation (lack of motivation). There are 6 subscales with 4 items each: intrinsic, external, interrogated, identified, introjected, and amotivation. Items are measured on a 5-point scale, where higher scores indicated greater amounts of the construct. Subscale scores may range from 4 to 20. Average subscale scores will be used in analyses.

Changes in how enjoyable physical activity is perceived to be will be assessed by self-report questionnaire responses to the Physical Activity Enjoyment scale (PACES). This scale measures how enjoyable or unenjoyable physical activity is perceived to be. The measure consists of 18 questions and no subscales. Each question is measured on a 7-point scale, where higher scores indicate greater enjoyment. Total scores may range from 18 to 126. Average score will be used.

Changes in perceived effort expended during exercise will be assessed by self-report questionnaire responses to the Borg Rating of Perceived Exertion Scale. This scale measures perceived physical exertion during exercise. There is only 1 item and no subscales. Scores may range from 0 to 10, with higher scores indicating greater perceived exertion. The total score will be used in analyses.

Changes in perceived pain/discomfort during exercise will be assessed by self-report questionnaire responses to the Numeric Ratings Pain Scale. This single item scale measures muscle pain/discomfort associated with physical activity. There are no subscales. Scores may range from 0 to 11, with higher scores indicating greater pain/discomfort. The total score will be used in analyses.

Changes in perceived ability to continue exercise long-term will be assessed by self-report questionnaire responses to the Barriers Self-Efficacy Scale (BARSE). This scale measures perceived ability to continue meeting exercise guidelines over the next 1 to 8 weeks. There are no subscales. 8 items are measured on an 11-point scale where higher scores indicate greater certainty in ability to complete exercise. Average score will be used in analyses.

Changes in perceived ability to stick to an exercise routine will be assessed by self-report questionnaire responses to the Exercise Confidence Survey. This scale measures perceived confidence in ability to make time for exercise and persist in an exercise routine. There are 2 subscales: sticking to it (8 items) and making time for exercise (4 items). Items are measured on a 5-point scale where higher scores indicate greater confidence. Total scores may range from 8-40 (sticking to it) or 4-20 (making time). Average subscale scores will be used in analyses.