The basics are the same.  Our nervous system sends a signal. Groups of muscle fibers respond and contract.  Muscle groups work in concert with each other to create motion in our skeleton.  The action of this muscle contraction is across the hinge of any variety of joint.  Using this joint as a fulcrum, the two lengths opposite the joint either increase their angle or decrease their angle.

Got all that?  Ok good.  When our muscles contract and result in an increased or “opening” of this angle we call it extension. When we decrease or “close” this angle we call it flexion.  There are plenty of other actions that can occur. Rotation, supination, pronation… but we are going to focus on extension and flexion.

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