I have a vivid memory of doing an exercise with a strong elastic rubber rope that had loops on each end. Note that one end of an elastic rope in the diagram is secured to the door knob of a closed door.
See if you can picture me doing this exercise.
1. Loop one end of the rope over a door knob (Round door knobs were the style.)
2. Sit on the floor near the door and place the free rope loop over one of your feet.
3. Lay down so that your leg is comfortably raised with the rope at full length but not stretched.
4. Raise your leg as far as possible. The objective is to stretch the rope as much as possible, and then slowly lower your leg. Repeat 5 times.
Science Application: Stretched Rope has elastic potential energy.
Now Make a Mental Picture of this:
I was partially sitting and raised my leg as instructed. Without warning the door knob was pulled out of the door and propelled forward. Due to the pull of gravity, my forehead was directly in the pathway of this projectile. It was truly a moment in which I saw stars twinkling. I can confirm that even after forty years the experience is as clear as when it happened. I can also confirm that the stored elastic energy of the rubber rope was transferred to kinetic energy and that a moving object really hurts when it hits you in the head. I decided that this exercise was not good for my health.