Muscle Quality and Muscle Fat Infiltration in Relation to Incident Mobility Disability and Gait Speed Decline: the Age, Gene/Environment Susceptibility-Reykjavik Study.

Background. Aging is associated with increased risk of reduced mobility. However, data on muscle components in relation to subjective and objective indicators of disability is limited. Methods. Data were from 2,725 participants (43% men) aged 74.8 +/- 4.7 years from the AGES-Reykjavik Study. At baseline, maximal isometric thigh strength (dynamometer chair), and midthigh muscle area and muscle fat infiltration were assessed with computed tomography. Usual 6 m gait speed and mobility disability were assessed at baseline and after 5.2 +/- 0.3 years. Incident mobility disability was defined as having much difficulty or unable to walk 500 m or climb-up 10 steps. A decrease of >= 0.1 m/s in gait speed was considered clinically relevant. Results. Greater strength and area were protective for mobility disability risk and gait speed decline. After adjustment for other muscle components, greater strength was independently associated with lower mobility disability risk in women odds ratios (OR) 0.78 (95% CI 0.62, 0.99), and lower decline in gait speed risk among both men OR 0.64 (0.54, 0.76), and women OR 0.72 (0.62, 0.82). Larger muscle area was independently associated with lower mobility disability risk in women OR 0.67 (0.52, 0.87) and lower decline in gait speed risk in men OR 0.74 (0.61, 0.91). Conclusions. Greater muscle strength and area were independently associated with 15-30% decreased risk of mobility disability in women and gait speed decline in men. Among women, greater muscle strength was also associated with lower risk of gait speed decline. Interventions aimed at maintaining muscle strength and area in old age might delay functional decline.