Older age was associated with 41% and 37% lower absolute force and power in fast muscle fibers, but intrinsic contractile function was preserved when normalized to 3D-derived cross-sectional area.
Observational (n=13)
3D imaging reveals that intrinsic contractile function of skeletal muscle fibers is preserved with aging, suggesting that previous reports of impairment may be due to methodological limitations in measuring cross-sectional area.
ABSTRACT Age-related reductions in muscle fiber size and contractile function, particularly in fibers expressing fast myosin heavy chains, contribute to declines in whole-muscle power. However, methodological limitations in estimating fiber size during contractile experiments have likely contributed to conflicting findings regarding whether reduced single-fiber force and power in older adults reflects their smaller size and/or impaired intrinsic contractile function. To address this, we coupled single-fiber contractile experiments with 3D-imaging in 7 young (19-40yrs) and 6 older (69-84yrs) males to assess intrinsic contractile function and compare agreement between 3D-derived cross-sectional area (CSA) and CSA estimates obtained either in air or solution. Fast fiber CSA from older males were ∼28–45% smaller across measurement conditions compared with young, whereas slow fiber CSA did not differ. Accordingly, absolute force and power of fast fibers were 41% and 37% lower. When normalized to CSA from measurements in air or 3D-imaging, size-specific force and power either did not differ or were greater in older adults, indicating preserved intrinsic contractile function in both fiber types. This was supported by no age-related differences in the rate of tension redevelopment (k tr ), a size-independent measure of intrinsic contractile function. In contrast, size-specific force and power calculated using solution-based CSA estimates were lower in older compared with young adults, and Bland-Altman analyses demonstrated the poorest agreement between solution-based and 3D CSA measurements. These findings indicate that intrinsic contractile function is preserved with aging and suggest that methodological differences in CSA measurement contributes to the disparate findings in the literature.
Zepeda et al. (Fri,) conducted a observational in Age-related reductions in muscle fiber size and contractile function (n=13). Older age vs. Young age was evaluated on Intrinsic contractile function (size-specific force and power). Older age was associated with 41% and 37% lower absolute force and power in fast muscle fibers, but intrinsic contractile function was preserved when normalized to 3D-derived cross-sectional area.