You don't want to put undue strain on your joints, but you don't want to underuse them either. Try to maintain a nice, even balance between motion and rest. Work too hard for too long without a break, and your joints are going to go on strike. They don't like constant use, overuse or abuse, and if you push those joints to their limits, you'll suddenly find it's a lot more difficult to get a strong grip on that bag of groceries.
But if you stay at rest too long, you may encounter the same problems because your joints have stiffened up. Before stiffness creeps in, remember to regularly "shake it out" throughout the day. When you're at rest (or working in a seated or still position), remember to move around now and then and keep those joints limber. Get up at least once every hour and put your body into motion for a few minutes.