If I flex my arm, where is the "equal and opposite" reaction? In my sight, nothing happens at all. Is the opposite reaction pressure applied to my bones? It certainly seems so; however, since I flex my arms in a curve, shouldn't the opposite reaction direction also be in a curve as well?
 A: If you look at the instantaneous motion of your arm at any moment while it is flexing it will have a single direction; your arm goes up and then toward your center. During this time the opposite reaction is on the rest of your body, your torso is pulled slightly downward while you lift your arm and then slightly outward as you pull your arm toward your center. It's easy to miss this because your arm is much lighter than your body, but if you move quickly the effect may become noticeable.
A: Imagine sitting on a swivel chair; hold your arm in front of you with your elbow to the side, then flex it quickly. Your chair will start to rotate (I know, I am sitting on a swivel chair as I write this and just did the experiment).
The question you can ask yourself in a situation like you describe is this: if I am floating in space, would something move? Obviously, conservation of linear and angular momentum must hold; so your bent arm moves the center of mass (and your body will be pulled towards the arm), and if the motion causes part of the body to rotate with respect to the body, then the rest of the body must rotate in the opposite direction in order to maintain the angular momentum of the system constant.
You may not "see" or "feel" these forces - but they are there. Muscle pulls on bone, and bone pulls back on muscle. And as you bend joints, there will be torques on them.
