Mitochondria
Mark Rippetoe recently published an article in which he advocates maintenance instead of training for some older people. We all get to a point where injury risk and lack of recovery make weight training a challenge.
A recent study regarding glutathione precursers glycine and nac is promising. The problem for older people is that the mitochondria don't make ATP as well, but we need ATP to lift heavy. This is not as much of an issue for cycling where we are burning glycogen primarily. If you try to rest for 7 minutes or more between heavy sets you lift with cold muscles and risk injury.
Supplementation with NAC and glycine was shown to improve grip strength in older men by making glutathione available to the mitochondria. I have not tried this yet. I have been reluctant to try testosterone replacement due to possible side effect risks.