Motivation to Exercise

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