To comprehensively assess the influence of aerobic training on muscle size and function, we examined seven older women (71±2yr) before and after 12-wk of cycle ergometer training. The training program increased (P<0.05) aerobic capacity by 30±6%. Quadriceps muscle volume, determined by MRI, was 12±2% greater (P<0.05) after training and knee extensor power increased 55±7% (P<0.05). Muscle biopsies were obtained from the vastus lateralis to determine size and contractile properties of individual slow (MHC I) and fast (MHC IIa) myofibers, myosin light chain (MLC) composition, and muscle protein concentration. Aerobic training increased (P<0.05) MHC I fiber size 16±5% while MHC IIa fiber size was unchanged. MHC I peak power was elevated 21±8% (P<0.05) after training while MHC IIa peak power was unaltered. Peak force (Po) was unchanged in both fiber types while normalized force (Po/CSA) was 10% lower (P<0.05) for both MHC I and IIa fibers after training. The decrease in normalized force was likely related to a reduction (P<0.05) in myofibrillar protein concentration after training. In the absence of an increase in Po, the increase in MHC I peak power was mediated through an increased (P<0.05) maximum contraction velocity (Vo) of MHC I fibers only. The relative proportion of MLC_1s (Pre: 0.62±0.01; Post: 0.58±0.01) was lower (P<0.05) in MHC I myofibers after training while no differences were present for MLC_2s and MLC_3f isoforms. These data indicate that aerobic exercise training improves muscle function through remodeling the contractile properties at the myofiber level in addition to pronounced muscle hypertrophy. Progressive aerobic exercise training should be considered a viable exercise modality to combat sarcopenia in the elderly population.