Source: HEALTHandFITNESS.com
Summary:
By: Joann Bally CSCS
The Greek athlete of ancient times, Milo of Crotoa, was said to train by lifting a young calf every day. As the calf grew, Milo continued to lift, and eventually he was hauling around a full-grown cow. Milo, a wrestler, never lost a match.
This is considered to be the first example of progressive resistance exercise, getting stronger by increasing your lifting load by small increments over time. The basic philosophy hasn't changed, although now we might advise Milo to lift the cow 3 times every other day. (Or, according to some, he can lift it only once a week as long as he holds it until his muscles fail, hard as that might be on both Milo and the cow.)
It follows that if you want to maintain a physical skill, it may be a good idea to keep doing it. Let's take, for example, rising from a chair without using your arms for a boost. Lots of people as they get older (or maybe fatter) can't do that. If you are at the point where rising from a chair is hard for you, you have to practice doing it or you'll lose it. Don't use your arms because it's easier. Rise up from a chair and sit down without using your arms, then do it again . Do it every day.
Let's say you have trouble getting up off the floor. Practice doing it every day. Do some crunches while you're down there. How about walking? Forty percent of people over 70 have trouble walking a quarter mile. If a quarter mile is hard for you, walk it every day. Take your time, but do it. If a quarter mile is easy, walk a full mile every day, on general principles. You younger people find something hard for you--pushups, pullups, running 3 miles--and do it. You don't get off easy just because you've got youth on your side. (Of course, anybody with a sore back or a sore knee gets to take the day off; we want you to work through a little difficulty, not pain.)
Is this all you should do then, maintain what you've got? Well no, certainly not for performance purposes, that is, if you want to improve your golf game, your free-throw percentage, or your 10K time, you do have to do more than just stay where you are. Eventually, age will cause your skills to decline no matter what you do, but you can hold back the clock with consistent and appropriate exercise. For those of you who are just interested in maintaining your ability to function independently and keep your quality of life intact, it's still a good idea to get into a program to increase your strength and cardiovascular endurance to give yourself a margin in case you do get sick or injured. People with low fitness, even if they have no problems with everyday activities, may never be able to recover fully from a prolonged illness.
So get to the gym or the Y or your local community recreation program and find a knowledgable trainer or coach who can help you get started actually improving your fitness. And in the meantime, get away from the elevator and maintain your ability to climb stairs.
HEALTHandFITNESS.com 
