EXAMINING THE EFFECT OF MUSCLE ASYMMETRY ON PHYSICAL FUNCTION IN MIDDLE-AGED WOMEN
Importance and Objective: The prevalence of physical function limitations in middle-aged women (40-64 years of age) has increased over the past several years. While factors such as age, level of physical activity, body composition, muscle strength, and muscle quality have been associated with declines in physical function, the influence of muscular strength asymmetry has not been well characterized. Additionally, the prevalence of muscular strength asymmetry among middle-aged women is also unknown. The primary aim of this study was to examine the prevalence of lower limb muscular asymmetry in middle-aged women and assess the independent contribution of asymmetry of the lower limb musculature on physical function performance. With the hypothesis that middle-aged women with greater muscle asymmetry would exhibit poorer performance on physical function assessments compared to their more muscularly symmetrical counterparts.
Methods: In this secondary data analysis, 116 middle-aged women (52.93 ± 6.10 years; BMI =26.38±5.10) were assessed for lower-extremity asymmetry using the Limb Symmetry Index-2 equation and a cut point of ≥ 10 %. Five objective measures (transfer task, 8-foot-up-and-go, 30-second chair stand test, 6-minute-walk-test, lift-and-carry) and a composite measure (lower-extremity physical function composite score) were used to assess physical function performance. An accelerometer and physical activity logs were used to measure physical activity and dual energy X-ray absorptiometry was used to measure body composition (whole body lean mass and fat mass). Isometric knee flexion and extension at 60 degrees and isokinetic knee flexion and extension at 60 degrees per second and 180 degrees per second were measured using isokinetic dynamometry. Finally, muscle quality was calculated as muscle strength normalized for thigh lean mass. Bivariate correlation analysis was utilized to assess the strength of the association between muscle asymmetry and physical function measures and a series of independent samples t-test were used to assess differences among asymmetry groups. Significance was set at a p-value < 0.05.
Discussion: Lower limb muscular asymmetry was not significantly associated with objectively measured physical function or with the physical function composite score (all p < 0.05) and there were no significant differences among muscular asymmetry groups for any physical function measures (all p < 0.05). Muscle quality was significantly associated with the majority of the objective physical function measures.
Conclusion: The lack of an association between asymmetry and physical function task performance refutes our hypothesis and indicates that lower limb muscular asymmetry may not negatively impact middle-aged women's physical function performance. The present study confirmed that physical function performance and muscle quality are significantly associated, which warrants the measurement of muscular strength and body composition in future studies. Interventions aimed at maintaining or improving physical function in middle-aged women may not need to target muscular asymmetry, but focus on minimizing fat mass, maximizing lean mass, and improving muscular strength.