Spring tendons enhance the foot’s efficiency: some of the energy spent to drop one’s weight down when taking a step is actually stored and then returned to the leg as it rebounds.

All other things being equal, when the ball receives a jolt of energy, it is more likely to drop down a step than to pop up one.

These products are drag and drop, and should be used as the last step before you upload your page (the files are harder to read after many of the returns are removed).