Reduction in single muscle fiber rate of force development with aging is not attenuated in 1 world class older masters athletes

49 Normal adult aging is associated with impaired muscle contractile function, however to 50 what extent cross-bridge kinetics are altered in aging muscle is not clear. We used a slacken re51 stretch maneuver on single muscle fiber segments biopsied from the vastus lateralis of young 52 (~23y), older non-athlete (NA) adults (~80y) and age-matched world class masters athletes (MA; 53 ~80y) to assess the rate of force re-development (ktr) and cross-bridge kinetics. A post-hoc 54 analysis was performed, and only the mechanical properties of ‘slow type’ fibers based on 55 unloaded shortening velocity measurements are reported. The MA and NA were approximately 56 54% and 43% weaker, respectively, for specific force compared with young. Similarly, when 57 force was normalized to cross sectional area determined via the fiber shape angularity data, both 58 old groups did not differ, and the MA and NA were approximately 43% and 48% weaker, 59 respectively, compared with young (P<0.05). Unloaded shortening velocity (Vo) for both MA 60 and NA old groups were 62% and 46% slower, respectively, compared with young. Both MA 61 and NA adults had ~2 times slower values for ktr compared with young. The slower Vo in both 62 old groups relative to young, coupled with a similarly reduced ktr suggests impaired cross-bridge 63 kinetics are responsible for impaired single fiber contractile properties with aging. These results 64 challenge the widely accepted resilience of slow type fibers to cellular aging. 65 66