I started working out regularly (every other day, mainly cardio work) several months ago, mainly for more energy and endurance; weight loss isn’t really important. I then tried to take an introductory yoga class and was quickly reminded that my upper body is much weaker than my lower. (The chauturangas (sp?) absolutely killed me. I was sore for days.) So, I decided to hire one of the club’s personal trainers to design an upper-body workout for me. Her routine seems great; it’s challenging but I can really feel the work during the exercise, and afterwards I feel like I’m glowing. 
Anyway, I’ve been doing this routine for a few weeks now (and I can see some definition on my poor little deltoids!) but I’m having surgery next week. It will be at least one week of couch time.
My question is, how long does your body ‘hold on’ to your improved strength? If I don’t do cardio for a week, does my body ‘lose’ part of the increased endurance? What about muscles? If I gain strength, does it go away without regular maintanance? Are there muscles that ‘lose’ their development at different rates?
Your muscles (including your heart) have grown to accomodate your increased activity level. If you stop working out, those muscles can be expected to get weaker as your body adapts to its new, lower activity level. It takes energy to maintain big, strong muscles. If you don’t use them, your body will consider them a frivolous expense and will reduce them.
How fast will this happen? In my experience (and I’ve a lot of experience of working out for months, then stopping cold-turkey for months, repeat) you’ll lose muscle at about the same rate that you gained it. Being laid up for a week or two shouldn’t be a problem. You won’t lose much.
Just be sure to warm up carefully and take it a bit easy when you do start up after taking time off. You can easily injure yourself if you jump right where you left off in after not exercising for a while.
Here’s a similar thread I started on this topic a while back. Hope you recover quickly from surgery.
It’s been my experience that I lose cardiovascular endurance much more quickly than I lose muscle. Also, the amount of muscle I lose varies per group–my hamstrings seem to go the quickest.
It really depends on your activity level. Doing modest stretching routines during your convalescence should make mass loss minimal. Lying flat on your back for six weeks after back surgery can cause up to fifty percent loss of small but important back stablization muscles.
Message? Don’t use, you lose. But if you even have everyday activity, should be able to stop any significant loss in two weeks just by stretching and simple stuff.
I’ve read – and don’t ask for a cite because I don’t have one – that all you have to do is work the same muscles once a week in order not to lose any strength.
I’ve heard the same thing as barbitu8, so take that for what it’s worth.