I hear this term used all the time by sports announcers, internet posters, and people in general. It does not mean what any of them think it means. It does not mean something like, "I'm a runner who broke his hip and was out of commission for months. I finally started running again and it was surprisingly easy to find my stride again without thinking too much about it." That is not muscle memory. Muscle memory was a term coined by bodybuilders, and I think it may have originated with the High Intensity Training godfather, Arthur Jones. What it means, is that guys noticed back in the early 70s that when they went off-cycle in the offseason from bodybuilding competition and lost mass, they found that they regained their mass and muscularity much, much faster than bodybuilders trying to build up to an equal amount of mass and muscularity for the first time. As I recall, Arthur Jones' trainee, the great Casey Viator, injured himself and was off-cycle and not training for the better part of a year. He dropped like 30 pounds of mass. When he was cleared, he went back to training and went back on the juice and regained all that mass and then some very quickly. This same phenomenon happened with other bodybuilders. Once you attain a level of mass, your muscles "remember" how to regain that level of mass, and thus you can be at the level much quicker than someone who is trying to build to that level of mass the first time.
That is muscle memory. Sorry, but I get tired of people misunderstanding it.