Bone responds according to the load placed on it. In this study, investigators want to know if exercise that loads the body in exercises, like walking or jogging (Ground Reaction Forces: GRF), affects bone differently than exercises that load the joints, like weight lifting (Joint Reaction Forces: JRF). Participants will exercise at least 3 times a week for 6 months. Measures of bone strength and mass, muscular strength, and aerobic fitness will be taken at the beginning and the end of the 6 month program. Period blood samples will be taken for future analysis of blood markers.

Inclusion Criteria:

• non-smoker
• in generally good health
• not physically active (less than 60 minutes of purposeful exercise/week)
• able to travel to the exercise facility a minimum of 3 times/week

Exclusion Criteria:

• use of glucocorticoids
• abnormal resting EKG
• angina and/or evidence of acute myocardial ischemia during a treadmill test
• resting blood pressure above 150 mmHg systolic of 90 mmHg diastolic
• anti-resorptive drug use in the last 2 years
• initiation of treatments that could impact bone metabolism (diuretics, hormones, bine anabolic agents) in the prior 6 months
• dual-energy x-ray absorptiometry (DXA) t-score <+ -2.5 at the femoral neck and/or lumbar spine
• serum vitamin D concentration <20 ng/mL
• uncontrolled thyroid disease
• any contraindications to regular exercise