I wanted to touch on the basic functions of most muscles that are drawn on the human figure in action. When it comes to the arms and legs, and even the major torso movements, we're talking about Flexion and Extension. The biceps and hamstrings (and I would say the abs, too), are flexing muscles. Biceps pull your forearm up. Quads pull the lower leg back. Abs pull the ribcage and pelvis together. On the opposite side, the triceps, quadriceps lower back muscles extend or straighten the arms, legs and spine. There are lots of other flexor/extensor pairs throughout the body. For example, the outer forearm muscles extend the fingers, and the inner forearm muscles flex to close the hand for squeezing, etc... In the sketches I've drawn here, I also made a note of the forearm bones. When drawing the arms and hands, the position of these bones dictates EVERYTHING. The ulna is fixed to your elbow and forms the outer edge of your forearm. The other bone is the radius, and it actually pivots around the ulna. The various forearm muscles are responsible for pulling the radius back and forth, which allows us to turn our wrist and hand 180 degrees. At every point in that twist, the arm muscles take on a different look. By understanding how they work, you'll be able to envision and draw them properly.