Reciprocal inhibition

This describes muscles on one side of a joint relaxing to accommodate contraction on the other side of that joint.

The body handles this pretty well during activities like running, where muscles that oppose each other are engaged and disengaged sequentially to produce coordinated movement. This facilitates ease of movement and is a safeguard against injury. Sometimes, for example, a football running back can experience a "misfiring" of motor units and end up simultaneously contracting the quads and hamstrings during a hard sprint. If these muscles, which act opposite to each other are fired at the same time, at a high intensity, a tear can result. The stronger muscle, usually the quadriceps in this case, overpowers the hamstrings. This sometimes results in an injury known as a pulled hamstring.

To get an idea of reciprocal inhibition in action, hold a heavy book in one hand, palm up. With your other hand feel the front of your upper arm. It's contracted and hard. Then, feel the back of your upper arm. It's relaxed, and flaccid. Include this as a flexibility technique to improve your active and PNF stretching.