We previously showed that 7 weeks of treadmill exercise training in late middle aged rats can reverse the modest reductions in skeletal muscle aerobic function and enzyme activity relative to values in young adult rats (Exp Physiol. 93.7: 863-871, 2008). The purpose of the current study was to determine if extending this training program into senescence would attenuate the accelerated decline in the muscle aerobic machinery normally seen at this advanced age. For this purpose, 29 mo old Fisher 344 Brown-Norway rats underwent 5 or 7 months of treadmill exercise training. Training resulted in greater exercise capacity during an incremental treadmill exercise test and reduced percent body fat in 34 mo old and 36 mo old rats, and improved survival. Despite these benefits at the whole body level, in situ muscle aerobic capacity and muscle mass were not greater in the trained groups at 34 mo or 36 mo of age. Similarly, the trained groups did not have higher activities of citrate synthase or complex IV in homogenates of either the plantaris (fast twitch) or the soleus (slow twitch) muscles at either age. Finally, protein expression of citrate synthase (a marker of mitochondrial content) and PGC-1 (relating to the drive on mitochondrial biogenesis) were not higher in the trained groups. Therefore, although treadmill training from late middle age into senescence had significant benefits on running capacity, survival and body fat, it did not prevent the declines in muscle mass, muscle aerobic capacity or mitochondrial enzyme activities normally seen across this age, revealing a markedly diminished plasticity of the aerobic machinery in response to endurance exercise at advanced age.