Muscles in Focus: Quadriceps for Stronger Sexier Legs
Straight leg lifts:
Sit on an exercise ball with your hands by your sides or stand still. Lift one leg while keeping the other one just short of hyperextension. Stop lifting your leg at the point where your knee begins to bend. This is an excellent and elegantly simple exercise which uses both your hamstrings and your quads, as well as your lower abdominal muscles.