Sarcopenia, the age-related decline in muscle mass and function (widely recognized as "frailty"), is increasingly being appreciated, primarily in the research environment. Interventions to prevent or treat sarcopenia can be anticipated to reduce falls, fractures and thereby to facilitate independence and improve quality of life for older adults. Unfortunately, there is no current consensus definition of sarcopenia, thereby impeding clinical recognition and treatment. It has been advocated that low appendicular (arm and leg) lean mass, as measured by DXA, be utilized as a clinical diagnostic tool to define sarcopenia. While such an approach is possible, however, muscle strength loss is more rapid than mass loss, indicating deterioration of muscle "quality." Muscle quality may be affected by changes at the neuromuscular, cellular or subcellular levels; parameters not detected by measuring mass alone. Clearly, tools evaluating muscle performance, not simply mass, are needed to optimally identify, and subsequently monitor, treatment of older adults with sarcopenia. While current tests of muscle power/function (e.g., chair-rising, self-selected gait velocity, etc.) do correlate with functional limitation in older adults, these existing tests have limitations in that they cannot be performed in all people, may have "yes/no" results rather than a continuous scale and may not be highly precise. Thus, improved muscle function assessment tools are needed, both clinically and in research venues. Jumping mechanography is very likely one such methodology.

Inclusion Criteria:

• Ambulatory, community dwelling men and women age ≥ 70 years
• Able and willing to sign informed consent
• Able to stand without assistance

Exclusion Criteria:

• Abnormalities on screening laboratory assessment deemed to be clinically significant by the study investigators
• History of myocardial infarction within the prior six months or ongoing angina
• History of injury or surgery within the prior six months which limits the ability to ambulate
• History of severe end-organ disease, e.g., cardiovascular, hepatic, hematologic, pulmonary, etc., which might limit the ability to complete this study
• History of malignancy with metastasis to the musculoskeletal system
• Neuromuscular disease impairing balance to the degree of not being able to stand without assistance
• BMD T-score of less than -3.5 at any measured site and a prior hip or vertebral fracture

Ambulatory community dwelling adults who are able to stand without assistance. Both men and women age ≥ 70 years from the Madison Wisconsin area. Specifically, participants will be enrolled using the following strata in each gender group: low vitamin D/low functional status (12 men and 12 women), normal vitamin D/low functional status (12 men and 12 women), low vitamin D/high functional status (12 men and 12 women), and normal vitamin D/high functional status (12 men and 12 women). Low vitamin D will be defined as 25(OH)D concentrations < 25 ng/ml, normal vitamin D status will be defined as 25(OH)D concentration of 30 ng/ml or greater. Functional status will be based on screening short physical performance battery (SPPB) score dichotomized at <9 vs. 9 and above.