"Make sure you understand that muscles are in reciprocal relationships with one another, that is, when one is flexed, the opposing muscles is extended." Not quite. Flexing, movement toward the front of the figure, and extending, movement away from the front of the figure, are both types of work which muscles perform. When a muscle is working, whether flexing or extending, or rotating, its opposition muscles will be at rest. The forms of working versus resting opposing muscles are the crucial things to look for in achieving an "organic" look. For example, the biceps flexes the fore-arm with respect to the upper arm, the opposing triceps muscle rests. The triceps extends the fore-arm with respect to the upper arm, and the biceps rests. A muscle performs work like flexing or extending by pulling its insertion to its origin, insertion and origin being the names we give to where muscles attach to the skeleton. This pulling action causes a muscle to become shorter, fuller, and rounder than the muscle would be at rest. Thus when flexing the fore-arm with respect to the upper arm look for the working biceps, dominating the front portion of the upper arm to be short, full, and round, while its resting opposition muscle, triceps, which covers the rear portion of the upper arm, will be comparatively longer and flatter. With extension of the fore-arm biceps becomes the longer, flatter muscle, being at rest, while working triceps becomes shorter and fuller, just not to the same extent because extension of the fore-arm is checked by the locking of the elbow. This occurs throughout the figure. The left-hand external oblique muscle rotates the rib-cage to the left with respect to the pelvis, becoming shorter and fuller than the external oblique muscle resting on the right side, which becomes longer and flatter as a result of the pulling of the working left side oblique muscle.
Oh God. I think I just found an exception to the "muscle pulls insertion to origin rule". The external oblique originates from the lower portion of the rib-cage and inserts into the pelvic crest. This would mean in rotating the rib-cage with respect to the pelvis the origin is pulled toward the insertion! Man, wish that'd occurred to me when my sculpture prof first proposed that rule 30 years ago...ah, it's still a good rule of thumb. One thing about rotating muscles for sure; in form they spiral from origin to insertion, like the external oblique.
To make a long story short, in drawing the figure you can play the shortness and fullness of working muscles against their comparatively flatter and longer resting opposition muscles to achieve a sense of rhythm and flow and so avoid stiffness.
"Three's so little room for error."--Elwell