Effects of Barefoot Running vs. in Shoes on Physiology and Mood

Inclusion Criteria: * Male/Female subjects between the ages of 18 and 45 at study entry (per a self report- females must be in the follicular phase of their menstrual cycle, within \~14 days of the end of their last period) * Fully ambulatory and in good medical health (see exclusion criteria below for specifics) * Ability to read/understand English * Consistent practice of running at least three times a week for a total of at least 30 minutes at a time. Exclusion Criteria: * Potential subjects will be excluded for a diagnosis of any serious ongoing medical or psychiatric condition that might unduly influence results or increase risk of study participation, including but not limited to: * Malignancy * Auto-immune disorder * Cardiovascular disease (except treated hypertension) * Neurologic disorder * Endocrinopathy (other than treated hypothyroidism) * Chronic infection (i.e. human immunodeficiency virus, hepatitis B or C) * Any renal, hepatic or hematological abnormality (other than history of mild anemia) * Current major depression or major depression requiring hospitalization or resulting in suicide attempt in past year. * Subjects will also be excluded for use of medications that might impact ANS functioning, including but not limited to beta-blockers, calcium channel blockers and alpha-blockers, and any medication with anticholinergic properties (e.g., many antihistamines).