Tensed Muscles

As shown in the picture, muscles when tensed form harder shapes under the skin versus relaxed muscles.

One way to emphasize an action in your drawings is to play up the contrast between tensed and relaxed muscles.  I recommend you act out a pose before drawing it to help you figure out which muscle groups are being used.  Sometimes your own body can be a better reference than the model posing in front of you.

from Human Anatomy for Artists by Eliot Goldfinger