Muscle Mitochondrial Respiration and Cardiorespiratory Fitness Contribute to Slower Walking Speed of Older Individuals Who Identify as Black

In the United States, older adults who self-identify as Black have a disproportionately higher incidence of mobility disability compared to those who are White. Whether older adults who are Black also have lower fitness and mitochondrial energetics has not been adequately investigated. The study of muscle, mobility and aging (SOMMA) examined 879 participants aged ≥ 70 years old, including 116 who self-identified as Black. Mitochondrial respiration (Max OXPHOS) was measured in permeabilized fibers from muscle biopsies. Cardiorespiratory fitness (VO₂ peak) was determined by a cardiopulmonary exercise test. Education, income, financial resources, race, sex, and age were determined by self-report. We used propensity score matching to match Blacks with Whites with a 1:1 ratio. Black (n = 90) and White (n = 90) groups were matched for age, sex, SOMMA multimorbidity index, BMI, muscle mass, physical activity, marital status, educational achievement, and whether financial needs were met (all p > 0.05). Despite being well matched for these variables, those who identified as Black had a slower 400-m walking speed (0.97 vs. 1.03 m/s, p = 0.014), lower Max OXPHOS (50.8 vs. 60.9 (pmol/(s*mg)), p = 0.0002), and lower cardiorespiratory fitness (1391 vs. 1566 mL/min, p = 0.007) when compared to those who identified as White. Multivariate regression showed that VO₂ peak and Max OXPHOS, but not socioeconomic factors, attenuated the race difference in 400-m walking speed. In conclusion, while the etiology of race differences in mobility is multifactorial, our data indicate that muscle mitochondrial respiration and cardiorespiratory fitness may contribute to the slower walking speed of individuals who identify as Black compared to White.