MODERATE-INTENSITY RESISTANCE EXERCISE ALTERS SKELETAL MUSCLE MOLECULAR AND CELLULAR STRUCTURE AND FUNCTION IN INACTIVE, OLDER ADULTS WITH KNEE OSTEOARTHRITIS

High-intensity resistance exercise (REX) training increases physical capacity, in part, by improving muscle cell size and function. Moderate-intensity REX, which is more feasible for many older adults with disease and disability, also increases physical function, but the mechanisms underlying such improvements are not understood. Therefore, we measured skeletal muscle structure and function from the molecular to the tissue level in response to 14 weeks of moderate-intensity, REX in physically-inactive, older adults with knee osteoarthritis (n=17; 70 +/- 1 yrs). Although REX training increased quadriceps muscle cross-sectional area (CSA), average single fiber CSA was unchanged due to reciprocal changes in myosin heavy chain (MHC) I and IIA fibers. Intermyofibrillar mitochondrial content increased with training, due to increases in mitochondrial size in men, but not women, with no changes in sub-sarcolemmal mitochondria in either sex. REX increased whole muscle contractile performance similarly in men and women. In contrast, adaptations in single muscle fiber force production per CSA (ie, tension) and contractile velocity varied between men and women in a fiber type-dependent manner, with adaptations being explained at the molecular level by differential changes in myosin-actin cross-bridge kinetics and mechanics and single fiber MHC protein expression. Our results are notable compared to studies of high-intensity REX because they show that the effects of moderate-intensity REX in older adults on muscle fiber size/structure and myofilament function are absent or modest. Moreover, our data highlight unique sex-specific adaptations due to differential cellular and sub-cellular structural and functional changes.

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