10. Have you ever experienced quiver in your thighs after having sex? And not in a good way? Thighs have large muscles that are used during sex. These muscles can be fatigued quickly if you dont do your exercises. The right thing to do is: seated leg extensions! Just find a stable chair and sit up straight, with your knees bent at 90 degree angles. Lift one foot to straighten your leg slowly and bring it to the floor. Do three sets of ten repetitions for each leg.