I think we have enough torso studies now. It's time for some legs.
The thigh is build very similar to the arm. On the front side is a triceps (tri = three), consisting of a bundle of three muscles, to straighten the leg. The triceps on the back of your arm does exactly the same.
Of course the counterpart is the biceps (bi = two). Logically it is placed on the opposite side of the leg / arm to pull on the bones so the joint forms an angle. It's really simple.
About the illustration: 1: back view of the thigh 2: outer side view 3: front view 4: inner side view
The part that takes practise (as always) is to understand the forms in a threedimensional way.