Mode of Exercise and Bone Biomarkers in Older Veterans

Inclusion Criteria: * Healthy older (60+ y) Veteran women and men in the Denver Metro Area * Normally active (e.g., recreational cycling or walking exercise) Exclusion Criteria: * Impaired renal function, defined as an eGRF of \<60 mL/min/1.73m2 * Hepatobiliary disease, defined as liver function tests (AST, ALT) \>1.5 times the upper limit of normal * Thyroid dysfunction, defined as an ultrasensitive thyroid stimulating hormone (TSH) \<0.5 or \>5.0 mU/L * Serum Ca \<8.5 or \>10.3 mg/dL * Serum 25(OH)D \<20 ng/mL * Uncontrolled hypertension, defined as resting systolic blood pressure (BP) \>150 mmHg or diastolic BP \>90 mmHg; * History of type 1 or type 2 diabetes * Cardiovascular disease, defined as subjective or objective indicators of ischemic heart disease (e.g., angina, ST segment depression) or serious arrhythmias at rest or during the graded exercise test (GXT). Volunteers who have a positive GXT can be re-considered after follow-up evaluation, which must include diagnostic testing (e.g., stress echocardiogram or thallium stress test) with interpretation by a cardiologist * Anemia, defined as a serum hemoglobin \<12.1 g/dL for women and \<14.3 g/dL for men * Fracture in the past 6 months * Current diagnosis or symptoms of COVID-19 In the event of abnormal BP, live function, TSH, 25(OH)D, or hemoglobin values, volunteers can be reassessed, including after appropriate follow-up evaluation and treatment by a primary care provider. Those who have experienced symptoms of COVID-19 or have been formally diagnosed will be allowed to participate once symptoms have resolved and they are approved to return to exercise by their primary care provider.