Add this new paper in Nature Cell Biology (subscription required) to the long list of reasons that getting healthy isn’t as easy as staying healthy:

Here, we describe a new subpopulation of fibro/adipogenic progenitors (FAPs) resident in muscle tissue but arising from a distinct developmental lineage. Transplantation of purified FAPs results in the generation of ectopic white fat when delivered subcutaneously or intramuscularly in a model of fatty infiltration, but not in healthy muscle, suggesting that the environment controls their engraftment.
There’s lots of jargon in there, but I think the concept is fairly straightforward: there are cells (FRP’s) that can become either fibroblasts (which in this case direct production of muscle cells) or adipocytes (cells that store fats), and these cells decide which fate to choose based on their environment.
Muscle tissue gets damaged all the time as a result of normal activity. Normally, this tissue is repaired by the generation of new muscle cells from precursor stem cells that are hanging out in the tissue (not the multipotent stem cells you normally hear about in the news, these guys can only become one or a few different cell types). In fact, the concept behind weight-training is to tear down the muscle so that it grows back stronger.

This new generation of muscle can be directed by fibroblasts, which in turn come from FRP’s. According to this paper however, certain conditions can cause the balance of FRP’s can be directed towards forming adipocytes which store fat. This in turn makes it more difficult to re-build muscle after injury. Previously, it was known that
The ectopic development of adipocytes and increased scar tissue formation (fibrosis) in skeletal muscle observed in several conditions (obesity, sedentary lifestyle and ageing) can perturb muscle regeneration and result in decreased muscle function.
This research gives a good indication as to why that might be. It’s important to realize that this research is in a model system, but understanding how these processes are regulated may lead to therapies that can help people that normally have difficulty recovering from muscle injury.