I started karate about three months ago and in that time my thighs have become remarkable in my usually overly critical opinion. The thing that did it, I believe, is the constant strain on my thighs when I do deep stances, front, back and horse stance. I usually am sweating in class from the muscle use in my legs as I try to do very deep poses, so I recommend doing an exercise of twenty minutes daily walking across the room and back taking deep stances.