Did you ever wonder why your knee jerks when it is struck by the doctor’s hammer? Or why your head snaps up when you fall asleep sitting up? How about why your leg shakes when you try that hamstring stretch?
It’s all due to special “organs” in your muscle fibers and tendons that are protecting you from injury.
Say you are watching tv at 1am and that infomercial on Swiffer just isn’t doing it for you anymore. Your head starts to roll to the side and next thing you know you jerk yourself awake and realize your neck is stiff. This is called the Myotatic Stretch Reflex. A specialized nerve in your muscles detects that the force of your head falling is too great and would cause damage to the body so it sends a message to your brain telling it to contract the muscle that is being stretched too far. Snap! You are suddenly awake and realizing maybe QVC doesn’t need your business after all.
When the doctor hits your knee with the hammer it is a similar reflex, only this one involves the Golgi Tendon Organ. This does the same thing as the Myotatic Reflex but for your tendons. The blow to your patellar tendon causes a forceful stretch that the Golgi reflex shuts down by contracting your quadriceps.
And the hamstring shake? This one happens to me all the time. When you are laying on your back and pulling your leg higher into the air your Golgi and Myotatic reflexes engage which stops your stretch, so you pull harder, the reflexes continue to engage, etc. You are playing tug-o-war with yourself! It may not feel as effective, but it’s time to back off the stretch and hold it for a longer amount of time at the point where shaking ceases.
This concludes your spin-off of G&D Discuss Science. Class dismissed!