Skeletal muscle mass and function assessment

Skeletal muscle mass was estimated according to the following validated formula: muscle mass (kg)=[(Height²/R x 0.401) + (gender x 3.825) + (age x − 0.071)] + 5.102, where height is in centimeters; R is resistance in ohms based on bioimpedance analysis (BIA); for gender men=1 and women=0; age is in years [52]. BIA was measured using Tanita Body Compo-sition Analyzer TBF-300A (Tokyo, Japan). Skeletal muscle index (SMI) was calculated as SMI=muscle mass (kg)/height (m)² and provided normalization of muscle mass for height [53]. SMI of ≤6.75 kg/m² and ≤ 10.75 kg/m² were defined as severe sarcopenia in women and men, respectively, based on associated high physical disability risk in older adults, as identified in the older population sample from the National Health and Nutrition Examination Survey III (NHANES III) [53, 54].

Assessments of muscle function were introduced into the study protocol at a later time (2007), therefore fewer subjects completed these tests. Grip strength was assessed with the use of dynamometer. The dynamo-meter was squeezed with the dominant hand and the maximum grip strength (kg) recorded out of three attempts was used for analysis. Weak grip strength was defined as either a reading that was 1 standard deviation (SD) below sex-specific means or inability to squeeze the dynamometer. Gait speed (m/sec) was calculated based on the time it took to walk a distance of 10 feet (3.048 m). Slow gait speed was defined as gait speed in the lowest quartile, stratified by sex and median height [55]. Subjects who were unable to complete the walking test due to physical limitations were also characterized as having slow gait. Individuals met criteria for frailty if they demonstrated three or more of the following deficits: 1. slow gait speed; 2. weak grip strength; 3. response of “No” to a question on the GDS that asks, “Do you feel full of energy?”; 4. report of less activity in the past 12 months; 5. body mass index (BMI), calculated as weight (kg)/height (m)², in the lowest quintile [56, 57]. BMI was calculated based on reported maximum adult height. Deficit in performing the chair rise test was defined as inability to rise from a chair 5 times without using one's arms for support [58].