Muscles move us. If you want to raise your hand, take a step, or pick up a bowl of ice cream, your muscles do the moving. They do this by contracting, changing their shape to be shorter and thicker. That contraction causes the muscle to pull.

Often, muscles are not attached directly to your bones. Instead, the muscle is attached to tendons, which attach to the bones. A good example of that can be seen in your hands. Open and close your hand. Where are the muscles that open and close your fingers? Place your left hand around your right arm, just below the elbow. Open and close your right hand. You should be able to feel the muscles in your arm contracting to move your hand. Those muscles are attached to long tendons that connect to the bones in your fingers. The muscles contract, pulling the tendons, which pull the bones to move your fingers.

Often those tendons are compared to the strings that move a puppet, but there is much more to a tendon that just acting as a string. Tendons are stretchy, and as they stretch, they store energy. Instead of strings, imagine your tendons as rubber bands. You can store energy by stretching a rubber band, and then get that energy back by releasing the rubber band to fly across the room. We often do the same thing with our tendons, storing energy to get a sudden burst of power.

To examine that, you will need:

• several tennis balls or other brightly colored objects of similar size and weight.
• a place where you can throw the objects a long distance without hitting windows, flowers, little brothers, or other things that might be damaged.

Pick up one of the tennis balls, and get ready to throw it as far as you can. As you move your arm into position, pay close attention to what your muscles and tendons are doing. Why do you tighten your muscles? A muscle that is already contracted does not help you move. Throw the ball as far as you can.

Now pick up another tennis ball. Once again, get ready to throw it as far as you can, but just as you get ready to throw, stop. Without moving, relax the muscles of your arm. Take a few seconds to get the muscles in your arm, shoulder and back to relax. Now, without retightening those muscles, throw the ball as far as you can.

The ball probably did not go as far this time. Why? The first time, as you drew your arm back, you tightened your muscles to stretch the tendons in your arm and shoulder. Stretching those tendons stored energy. When you threw the ball, muscles in your arm and shoulder contracted to move your arm. At the same time, your tendons released their energy, just like releasing a stretched rubber band. That extra energy helped the ball go farther.

With the second throw, relaxing your muscles released the stored energy in your tendons. The contracting muscles in your arm and shoulder gave the ball just as much energy as they did for the first throw, but the ball did not get the extra push from stored energy in your tendons.

Now, lets try it one more time. Get another tennis ball, and very slowly move your arm into position for throwing. This slow movement simulates the cat slowly stalking its prey. Once your arm is in position, notice the sensation of your muscles and tendons. There is some “stretch” to the tendons, but probably not as much as for your first throw. Also notice that by moving very slowly, those tightened muscles are getting a bit tired, probably not giving their best stretch.

Instead of throwing, put your arm back down by your side. Then very slowly move it into position again, but this time keep your muscles fairly relaxed. Once your arm is in position, then begin to tighten your muscles. As you do that, move your arm slightly from side to side, just as the cat does with its back legs. That slight movement helps your muscles stretch the tendons, storing extra energy. Once you feel that you are ready, throw the ball. It should go at least as far as it did the first time.

Your cat keeps its muscles fairly relaxed as it stalks. That lets it move very slowly, without tiring its muscles by trying to keep the tendons stretched. Once it is in position, it tightens the muscles, and uses that slight wiggle to help stretch the tendons a bit more, storing energy for the final pounce.