Skeletal Muscle Fiber Adaptations Are Distinct In Different Exercise Training Programs In Healthy, Older Adults

Older adults are susceptible to age-related skeletal muscle dysfunction, which increases physical disability and limits functional capacity. Specific exercise interventions, such as high-load, low-velocity resistance training or low-load, high-velocity power training, that optimally improve skeletal muscle function in older adults has yet to be defined. PURPOSE: To determine the effects of resistance and power training on skeletal muscle fiber contractile function in healthy, older adults. METHODS: Older males (n = 10) and females (n = 18) underwent 16 weeks of exercise training (leg press, extension and flexion), where one leg performed resistance training (80% one-repetition maximum or 1-RM, 3 sets of 8 repetitions) and the contralateral leg performed power training (40% 1-RM, 3 sets of 16 repetitions). Using vastus lateralis biopsies, single fiber force generation, myofilament mechanical properties and myosin-actin cross-bridge kinetics were measured pre- and post-training under maximal Ca2+-activated conditions in myosin heavy chain (MHC) I (slow-contracting) and MHC IIA (fast-contracting) fibers. RESULTS: Both sexes showed variable cellular and molecular responses to exercise training, such as specific tension (force/cross-sectional area) increasing, decreasing or remaining unchanged within individuals. However, significant physiological relationships were observed. With power training, alterations in specific tension were most highly correlated with changes in myofilament stiffness in both fiber types (MHC I: r2 = 0.70, p < 0.001; MHC IIA: r2 = 0.53, p < 0.001). With resistance training, alterations in specific tension in MHC I fibers were most highly correlated to changes in the number of strongly-bound myosin-actin cross-bridges (MHC I: r2 = 0.60, p < 0.001), due to alterations in myosin attachment time (r2 = 0.29, p < 0.05). MHC IIA fibers showed specific tension with resistance training was altered primarily using myofilament stiffness (r2 = 0.66, p < 0.001), although number of cross-bridges also played a role (r2 = 0.45, p < 0.001). CONCLUSION: Our results suggest that the molecular responses to exercise training may be intervention-specific and depend upon the fiber’s MHC isoform expression. R01 AG-047245