muscle memory

You have a long delay(lets say 3 months) you go back in the gym and you can only do 135 for 10 reps, but you could do 275*8. In a matter of a month, you should be back up to your previous weights. Since your muscle memory should kick in.
thats pretty much it
your muscle *remembers* where it once was so its easier to that get to that weight or condition(size) after a long lay-off in the gym
it all comes down to mature muscle mass. Kind of like a guy that has only been lifting will not have the dense full muscles of a guy that has been hitting it hard for 5.

In my opinion only the mature muscle has memory.
I think muscle memory can last for years. I started lifting again after 4 years off and I am really making good gains now. I started with 135 for around 10 reps on bench. I think that a lot of the initial strength gains made are from getting your CNS back in shape, because my lean bodymass is higher now than it was when I would use 225 four years ago. I can only guess that it had something to do with gaining 10% more bodyfat, combined with that muscle fullness development.
I believe muscle memory is also a mental aspect. Determination and aggressively pursuing to achieve the strength back will initiate more physical as well as mental strength. Of course our brain is the central control of our body, so it will send the message to the muscle receptors.
Muscle memory is the shit. A few years ago, I came back after missing a year and in a few months, I was back at full force. I went from benching like 165 lbs. for 8 reps to 275 lbs for 4 reps in 10 weeks.
I think that muscle memory is about gaining lost cordination, and very little to do with gaining muscle back that you once had.