Changing muscle function throughout joint ROM

To continue my discussion on human tissue let's now look at a unique type, and that is muscle. A muscle has the unique ability to perform many select tasks during one motion of a human joint. It can be the prime mover, provide the opposing force, work in synergy with other muscle groups, stabilize the articulating joint surfaces, or neutralize unwanted motions. These functions all come into play as the joint moves through its range of motion and the orientation of force changes. 

All joint motion is in the form of angular rotation and the articulating bones aren't fixed, but free to move in all directions. If there is to much excess play or pressure on these surfaces; injury can result. This is why it is important to maintain good strength and flexibility in all these muscles. I have included 2 photos below to better illustrate these functions. 

I ask you to first look at the image of the posterior shoulder and notice all the muscles attaching to, or crossing the scapula (shoulder blade). Now view the gymnast and visualize the different actions of these muscles as he performs his technique. You should realize that different muscles are constantly changing their main action as the joint travels through it's full range of motion.