There are two supplements that may improve muscle metabolism, and they are used by athletes and body builders to some extent.

1) one is d-ribose powder. This provides ribose (which is not common in our diets) to the mitochondria (energy cells in our body's cells) to make adenosine. Adenosine is part of AMP, ADP, and ATP. Hence more energy may be had when using ribose.
We have posts about it here:
http://neurotalk.psychcentral.com/post708792-40.html

Ribose is available in powder form, and used by the scoop. 5 grams to start is a typical daily dose.

2) The other supplement is called creatine. Many athletes use creatine to build muscle mass and strength. This one you have to be more careful with. It may affect the kidneys in high doses, and you need to stay hydrated well while using it. Creatine is also a powder, and should only be used in moderation IMO.
It has been suggested also lately for people with neurological problems in the brain, like ADHD, Alzheimer's and Parkinson's.
Use only as directed on the label, and stay hydrated.
Start slowly if you decide to use this. There are many informational sites on the web you can consult for creatine.

The ribose really has no side effects, and it is also a slow working supplement. It may take a week or longer to manifest benefits. I find it very useful myself for weakness and stamina issues. I get my ribose from Puritan's Pride. But iherb.com also has it. I take it at night, as it helps me sleep.

When people engage in vigorous exercise often, sometimes they need some support. There are sites recommending ribose for fibromyalgia patients too.