JOHN VARUN MAGUIRE
When an individual slips or rapidly changes position, one or more muscles quickly stretch while their antagonists are shortened. The spindle cell mechanism of the overstretched muscle sends a message to the brain: “Help! I’m being stretched too far and if you don’t do something fast, I’m going to turn into silly putty!” The brain responds with a protective reaction and says to the muscle, “Chill out, Dude! Contract your fibers and you won’t get bent out of shape.” In the meantime, the spindle cells of the hypershortened, antagonist muscle are silent, so there is no communication between the brain and the muscle. Before this shortened muscle knows what’s happening it is whipped into a stretch by the protective reaction that the brain ordered to the initially stretched muscle. The shortened muscle’s spindle cells fail to reset properly in this rapid change of position. They continue to tell the muscle to maintain a shortened position which keeps the muscle in a contracted or hypertonic state.