Have you ever wondered how your leg reacts so fast when the doctor taps the tendon below your knee with a rubber mallet?
The tendon that is being tapped is called your patellar tendon and the muscles that react are the quadriceps. With most movements in the human body our mind has to process the information and send impulses to cause the chemical reactions that result in muscle contraction. When the patellar tendon is tapped the sensory receptors in the muscles send this information only to our spinal cord which in turn sends back impulses to contract. The reaction is so fast that the muscle contracts without our brain even knowing about it. And this is the secret behind plyometrics and jumping hihger. This reaction is called a stretch reflex. The tendon that was tapped was stretched and the body reacted by contracting. The same holds true when you lower your body rapidly to jump. Muscles in your legs are stretched rapidly and they respond by contracting which will result in jumping. Plyometrics has 2 main objectives. The first is to strengthen the contraction that results from the stretch and the second is to train all the fibers to contract at the exact same time. Because jumping is such a fast motion it makes sense to train the muscles involved in a very fast way. Weight training with the legs is necessary to have the strength required to go through the movement, but weight training alone really wont help you jump higher. That's why you don't see bodybuilders dunking basketballs. They have tons of leg strength but their legs have developed a lot of slow twitch muscles that don't react as well to fast movement.