Urinary incontinence (UI) in older adults is common and associated with mobility limitations.[1] Lower extremity muscle health is critical for maintaining mobility in older age. Therefore, age-related thigh muscle changes are potential, yet unexplored, shared mechanisms of both UI and mobility limitations.

This analysis includes 227 women and 231 men enrolled in the Baltimore Longitudinal Study of Aging. We restricted the analysis to community-dwelling older adults age ≥60 years with no history of stroke or Parkinson’s disease and no missing UI or muscle measures seen between February 2011 and September 2015. UI severity and type were assessed by asking “During the past week (7 days), how many times did you leak urine under the following conditions?” and “In the past 12 months, how often have you leaked urine?” Stress UI was defined by “With an activity like coughing, lifting, or exercise”, urgency UI was defined by “When you had a sense of urgency and could not get to a toilet fast enough”, other UI was defined by “Unrelated to an activity or urge to urinate”, and mixed UI as the presence of both urgency and stress UI. We excluded participants who reported at least weekly UI at analytic baseline. Given the limited number of events for individual UI subtypes, our primary outcome was new at least weekly UI of any subtype. Thigh muscle strength (Nm) was defined as maximum concentric 30°/s knee extensor torque. Thigh muscle area (cm2) was estimated using mid-femur cross-sectional 10-mm CT images. Thigh muscle specific force (Nm/cm2) was defined as strength divided by area. We used multivariable interval censored cox regression to model associations between tertiles of thigh muscle measures (strength, area, specific power) and incident at least weekly UI. Associations were stratified by sex and adjusted for age, race, height, weight, height*weight, and physical activity. We conducted sensitivity analyses further adjusting for number of live births or hormone replacement therapy among women.

Baseline characteristics of the analytic study population are reported in Table 1. Mean follow-up time was 3.2 years. Overall, the incidence of at least weekly UI was 9.1/100 person-years for women and 7.8/100 person-years for men. Among the 75 women with incident UI, 53% developed urgency UI, 31% stress UI, and 16% other/mixed UI. Among the 82 men with incident UI, 50% developed urgency UI, 19% stress UI, and 31% other/mixed UI. Compared to women in the lowest tertile of thigh muscle area, the hazard ratio for incident UI was 49% lower among women in the middle tertile (hazard ratio=0.51, 95% CI 0.27, 0.97) and 54% lower among women in the highest tertile (hazard ratio=0.46, 95% CI 0.20, 1.07; Figure 1). Conversely, after multivariable adjustment thigh muscle area was not significantly associated with incident UI in men and thigh muscle strength or specific force were not significantly associated with incident UI in women or men (95% CI included 1.0 for all). Further adjusting for number of live births or hormone replacement therapy among women did not meaningfully impact the results.

In this prospective cohort study of community-dwelling older adults, we observed an association between greater thigh muscle area and lower risk of incident UI in older women but not older men. Thigh muscle strength and specific force were not associated with incident UI in either sex. This study extends existing cross-sectional data suggesting that older women with UI have poorer lower extremity strength and function.[2] Although we are not aware of other longitudinal studies that have examined this relationship, our findings are also consistent with prior literature demonstrating that older women with incident UI have increased risk of lower extremity functional decline.[3] Although prospective studies with larger sample sizes are needed to confirm our findings, low thigh muscle area may reflect emerging sarcopenia, an unexplored mechanism of UI that could partially explain why older women with UI have increased risk of mobility limitations.

This novel study provides preliminary evidence for a relationship between thigh muscle area and risk of UI in older women. While we did not observe statistically significant associations between thigh muscle measures and incident UI in older men, our power may have been insufficient to detect modest but clinically meaningful effects. These relationships should be evaluated in larger prospective cohort studies of older adults with detailed assessments of UI subtypes.

Sanses, T.V., B. Kudish, and J.M. Guralnik, The Relationship Between Urinary Incontinence, Mobility Limitations, and Disability in Older Women. Curr Geriatr Rep, 2017. 6(2): p. 74-80.
Le Berre, M., et al., Characteristics of Lower Limb Muscle Strength, Balance, Mobility, and Function in Older Women with Urge and Mixed Urinary Incontinence: An Observational Pilot Study. Physiother Can, 2019. 71(3): p. 250-260.
Parker-Autry, C., et al., Characterizing the Functional Decline of Older Women With Incident Urinary Incontinence. Obstet Gynecol, 2017. 130(5): p. 1025-1032.

Continence 2S2 (2022) 100275
DOI: 10.1016/j.cont.2022.100275