An article comments on the way that leggings might cause our muscles to relax too much, because of the support. In other words, muscles will not have to work as hard, so they begin to grow weak.
I think that that is bunk. I think that there is not enough support in normal leggings and hosiery to make our muscles weaken. Most of the exercise that we get will be from day to day movement. If we walk to the bus stop daily, and eat right, then we will probably get enough exercise to deal with that problem.
That article has lots of discussion. I recommend checking it out, if you have time.