Locomotor muscle mass, total body mass, and physical function in older adults

Background Muscle mass is an important predictor of functional performance, and is dependent on several key factors, including sex, height, and activity level. Increasing evidence also suggests that muscle mass is dependent on total body mass, representing a chronic stress for adaptation during normal activities of daily living. Indeed, individuals with greater body mass often have greater muscle mass, regardless of sex, height, activity level, or disease status. Purpose To determine the best approach for representing muscle mass for a given individual. Methods To address this question, we leveraged a large multi-modal community cohort study, with comprehensive whole-body MRIs and functional performance measures. A total of 60 older adults (mean age: 68 ± 8 years; 68% women, see Table 1) were included in this preliminary analysis. Muscle mass was defined by the cross-sectional area (CSA) of a single mid-thigh axial T2-weighted image of the dominant leg. Each muscle was manually segmented by a single, experienced observer. Data are presented as absolute values (cm²), and relative values, normalized by femur length (cm), height (cm), total fat mass (kg; measured by dual-energy X-ray absorptiometry, DEXA), and total body mass (kg). Physical performance was assessed by 6–minute walk distance (6MWD). Correlation between CSA, physical features, and 6MWD were analyzed using Pearson or Spearman tests depending on the normality of the data, with significance set at p < 0.05. Results As illustrated in Figure 1, thigh CSA was weakly correlated to total fat mass and femur length, and moderately related to height and total body mass. To explore how each normalization metric relates to functional performance, CSA (absolute and normalized) was related to 6MWD. Absolute CSA and CSA normalized for height were poorly related to 6MWD. In contrast, CSA normalized for total body mass and total fat mass was strongly related to 6MWD. Conclusions Skeletal muscle mass is often reported in absolute terms or normalized to height or femur length. Here, we emphasize a crucial yet underappreciated factor—total body mass—which demonstrates a stronger association with both muscle mass and functional performance. We interpret this relationship to reflect a natural adaptation, whereby total body mass influences muscle development through routine activities of daily living, and is therefore the best metric to use when interpreting locomotor muscle mass.For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text.