Not doing too much exercise at one time, as others have said, is a good idea--but doing a little almost every day. A short walk is good, in the clean air, in addition to the walking around the house to do things. But a long walk will sometimes cause spasm (such as the one of the upper lip I was discussing yesterday, if it was a spasm, and I now think it was).

I concur with Goldie that magnesium will help. Many years ago, so long ago I forget the year, a neurologist at Scripps put me on magnesium for spasm, after Baclofen and Zanaflex made it impossible for me to function (unless walking into walls with Baclofen is considered OK). This helped and has helped all of the years since then. I still have some pretty bad spasm making my legs twist and move at night, but they are not especially painful, just very irritating. The only way (in addition to magnesium) that I can deal with them is stretching before bedtime (you said yoga helped) and rubbing out the knots in my legs. My arms have always been weak, but they are less weak since I started rubbing out the knots, at the suggestion of my pcp's assistant, who has a degree in movement therapy. It may take a good long time to conquer the knots, but it can be done usually, not always.
I take 1000 mg of magnesium a day and 1000 of calcium (cal at a different time than mag) and have upped my D3 lately to 1600 a day.