Example of elbow bending, rotating, and moving in 
Example of elbow straightening, rotating and moving out
I always find the body and its intricacies fascinating. I smile when I think of the professor in PT school, that said your elbow only moves in one direction or plane as it straightens or bends. Truth be told it also rotates allowing you to turn your hand up or down. The elbow moves in a 3rd plane which can be seen as you straighten your arm with your palm facing forward. The hand moves farther out from your body. Compare this to bending your elbow to touch your hand to the same shoulder and you will see it move inward. So, the elbow straightens and bends, rotates and moves in and out.
Wow! Knowing all these intricate movements of the joint mechanics is what helps me restore lost mobility to a joint. If a joint can’t do one of those motions, you may be restricted or using another motion too much to gain the range of motion. This can result in pain or even a joint that is unstable. That is why it is important to find a therapist who understands manual therapy for rehab and does not use exercises only.
I think muscles are another part of the body that is viewed as only doing one thing. When you think of the biceps, do you picture biceps curls, bending the elbow? Or when you think of the quadriceps, the muscle on the front of the thigh, do you envision straightening your knee? And what about abdominals? Many think of an abdominal crunch. You may not realize it, but muscles shorten like mentioned above, but also are working when they lengthen. So, when you straighten your elbow, your biceps lengthen, when going downstairs or squatting, your quads lengthen and control your descent in synchronization with the hamstrings and when you do a plank or bridge your abdominals must lengthen.
Lengthening or eccentric movement is more forceful and generates higher forces on the muscle, than shortening or concentric strengthening, and is often the most important activity the muscle does. Training muscles to control lengthening and shortening is important to gain stability and improve muscle balance. Often knee pain is felt more going downstairs than up. This is because the quadriceps are weak lengthening. Or if the abdominals can only crunch and not lengthen our abdominal space is decreased, which makes less rooms for the abdominal organs, decreases rib expansion and diaphragmatic breathing and keeps you from standing upright. Strengthening should address both. That’s what allows you to transform your strength, flexibility and muscle balance.
A Jewel from Judy
When you are sitting and typing on a computer make sure your elbows are at your side and in line with your shoulders. This allows your forearm to face straight ahead. Often people sit with the elbows out and the forearms turning in. This often happens in office chairs when sitting with your elbows on the arm rests. Turn your elbows out and feel your shoulders move forward, your chest slump and your chin jut out. Now try staying in line and feel your shoulders relax back and your chin drops slightly in. You should also feel your chest lift and note less stress on the wrists.