Muscle Perfusion and Protein Metabolism in Elderly

Inclusion Criteria: All inclusion criteria must be fulfilled throughout the study. At enrollment, subjects must fit the following criteria: 1. Age: young 18-40 yrs; elderly 60-85 yrs. 2. Availability of transportation (i.e., subjects must be able to provide their own transportation to UTMB). 3. Ability to sign informed consent (score \>24 on 30 item mini-mental status exam and no errors on assessment of judgment). Exclusion Criteria: None of the following exclusion criteria should be present at enrollment or throughout the study: 1. Patients with limiting or unstable angina or who demonstrate cardiac abnormalities such as \> 0.2 mV horizontal or downsloping ST segment depression, frequent arrhythmias (\>10 PVC/min), or valvular disease. 2. Any patient with atrial fibrillation, history of syncope, angina, or congestive heart failure. 3. Patients with vascular disease, as determined by the presence of risk factors of peripheral atherosclerosis (i.e., hypertension, obesity, uncontrolled diabetes, and evidence of venous or arterial insufficiency upon palpation of femoral, popliteal, and pedal arteries). 4. Peripheral vascular disease as determined by history or abnormal ankle-brachial index by Doppler (\< 1.0). 5. Any subject with blood pressure on three consecutive measurements taken at rest on separate occasions that has a systolic pressure \>170 mm Hg or a diastolic blood pressure \>100 mm Hg will be excluded. Subjects will not be included if they are taking blood pressure medication and have a blood pressure above these criteria. 6. Any person HIV-seropositive, with active hepatitis, or any other immunosuppressive or autoimmune disease. 7. Any patient taking beta blockers, vasodilators, angiotensin-converting enzyme inhibitors, calcium channel blockers, or alpha blockers. 8. Any patient with uncontrolled metabolic diseases including any patient with liver or renal disease. 9. Glucose intolerance: fasting plasma glucose concentration 110-126 mg/dL (6.1-7 mmol/L) and/or 2-h plasma glucose 140-200 mg/dL (7.8-11.1 mmol/L) during oral glucose tolerance test (OGTT). 10. Currently in muscle strengthening program. 11. Total knee replacement or moderate to severe degenerative joint disease of knees. 12. Anemia (hemoglobin \<13 g/dL in males or \<12 g/dL in females). 13. Any history of hypo- or hyper-coagulation disorders, including patients taking Coumadin or with a history of deep venous thrombosis (DVT) or pulmonary embolism (PE) at any point in their lifetimes. 14. Currently taking aspirin and cannot stop for 7 days (i.e., medical indication for continued aspirin such as transient ischemic attacks). 15. Presence of acute illness or metabolically unstable chronic illness. 16. Heart disease requiring therapy or recent myocardial infarction (less than 1 year) 17. Currently on weight-loss diet. 18. Active cancer or in short-term remission (less than 3 years). 19. Alcohol or drug abuse. 20. History of \>20 packs per year cigarette smoking. 21. Recent (within 6 months) ingestion or injection or transdermal administration of anabolic steroids, corticosteroids, estrogens, or progestagens. 22. History of falls (1 fall in the past year which required medical attention, or more than 2 falls/year regardless of severity). 23. Allergy to iodides, penicillin or shellfish. 24. Any subject with a know hypersensitivity to octafluoropropane. 25. Any subject with a cardiac shunt. 26. Any subject with chronic obstructive pulmonary disease (COPD).