Reciprocal inhibition: the relaxation or inhibition of muscles on one side of a joint to accommodate contraction on the other side.
That’s some fancy PT/kinesiology speak that basically means you can bend your elbow with the biceps muscle (front of the arm) because triceps (back of the arm) will relax and allow the elbow to bend. If the triceps didn’t let go/relax, your biceps can’t shorten and bend your elbow—triceps-biceps would have a tug of war over your elbow resulting in no movement at all.
Muscle groups ideally work in balance around a joint. The agonist is prime mover and the opposing muscle is the antagonist. In this example the biceps is the agonist that bends the elbow and the triceps is the antagonist that lengthens to allow the elbow to bend. Other examples quads straighten the knee, hamstrings bend the knee. Psoas flexes the hip, glues/hamstrings extend the hip. Upper trap raises the shoulders, serrates anterior lowers the shoulders
Generally PTs/fitness peeps talk about or cue the agonist during an exercise, not really mentioning what happens on the other side, the side that has to let go. I’m going to ramp it up a notch. Instead of the antagonist (triceps) just “relaxing”, let’s give it something to do: lengthen it. Reach that elbow towards the floor to bring your hand to your shoulder. Think of a teeter totter, elbow goes down as the hand comes up. Try it next time you bring coffee cup to your mouth (or beer can, whatevs), elbow down hand comes up.
The downward motion with the elbow can keep those shoulders out of your ears and maybe down play the forward-backward hinge at the low back frequently seen when you add too much weight to a biceps curl.
Another image is this nasty hand towel thing that I thought was left in the 80s and
COVID had finally extinguished. I was wrong, spotted one in Utah in February of 2022. Thankfully there was a roll of paper towels right next to it, so I didn’t have to use it following a sweet snowmobiling trip with friends. If you pull on one side it comes down, the other side goes up. Blinds work in a similar way.
This can be applied to most joints in the body:
one side shortens + one side lengthens = movement occurs
Stay tuned for more, how to apply this idea to keep shoulders away from ears, how to decrease shoulder tension.