The purpose of this study is to understand how to make exercise more attractive to people.

This investigation will address increased physical activity and will yield empirical evidence to inform developing tools to help individuals make healthy choices regarding physical activity (PA) and maintenance of healthy body weight. Behavioral Choice Theory provides a framework for understanding choices people make and how to shift an individual's choice toward healthier alternatives. The theory holds that choice is based on the relative motivating values of alternatives. One way to shift choice toward a healthier alternative is to increase the relative reinforcing value (RRV), or motivating value, of that alternative. Thus, increasing the RRV of PA behaviors could shift choice towards PA and away from less healthy, sedentary alternatives.

Increasing the RRV of PA may allow PA to compete with more reinforcing, sedentary behaviors, resulting in a shift in behavioral choice. This process is termed "incentive sensitization", which was originally proposed to explain drug addiction. Incentive Sensitization theory posits that the RRV of a behavior is increased through repeated exposures, which produce neuroadaptations that increase craving of the behavior - after repeated exposures to a stimulus, a 'sensitization' or hypersensitivity to the incentive motivational effects of the stimulus follows. Another factor that influences motivation for a behavior is the development of tolerance to unpleasant aspects of that behavior. Opponent-Process Theory would account for the acquisition of motives where the initial reinforcer may be negative - the affect experienced after repeated exposures is opposite to that experienced during the first few presentations. For exercise, this would include tolerance to any discomfort, pain, fatigue and displeasure and a greater positive post-exercise affective response: the affect experienced during and after exercise has a greater motivational significance in PA participation than knowledge and beliefs regarding health benefits. Therefore, the investigators propose that repeated exposures to PA will produce tolerance to unpleasant aspects while increasing the post-exercise affect. Sensitizing PA reinforcement and increasing discomfort tolerance may be associated with increasing the duration and intensity of daily activity. There are likely to be individual differences in sensitization of PA reinforcement and development of discomfort tolerance. Single-nucleotide polymorphisms (SNPs) that alter dopamine tone in the central dopaminergic reward system and SNPs associated with altered pain perception may contribute to such individual differences. The investigators anticipate that a moderate PA dose (300 kilocalorie [kcal] expenditure) will produce greater incentive sensitization and tolerance than a low dose (150 kcal) and that SNPs related to central dopamine tone and pain perception may contribute to individual differences in incentive sensitization and changes in physical activity after repeated exposures to physical activity.

Inclusion Criteria:

• BMI within 19-35 kg/m2
• Healthy enough to exercise
• Sedentary (not regularly engaging in exercise more than once per week)

Exclusion Criteria:

• Taking any medications that affect energy expenditure or eating
• Have gained or lost more than 10 pounds over the past 3 months
• Use tobacco
• Pregnant or lactating or plan to become pregnant in the next 6 months
• Have any medical conditions that prevent the individual from safely joining in physical activity
• Have high work-related activity such as construction and farm work