Kinematics of Walking and Running

One important observation we can make from looking at the kinematics of the body is the amount of up-down and sideways motion. In actions such as walking and running, the body is attempting to move horizontally across the ground. However any other motion, especially vertical motion, does not help this objective, and uses upward precious energy.If the body had wheels it could avoid vertical motion altogether, but since we have legs, there must be some vertical motion. The reason for this is that at heel-strike and toe-off the two legs make up the sides of a triangle, while during mid-stance the stance leg is vertical. This has the effect of lowering the upper-body (often called the HAT segment for Head-Arms-Trunk) at heel-strike and toe-off (which together make up the phase known as double stance, when both feet are in contact with the ground), and raising it during mid-stance:

The body's center of mass (CoM), is located in the pelvis. In order for the CoM to rise up between heel-strike and mid-stance, energy must be expanded, the energy is not conserved when it drops at the toe but rather expended. This is an up and down motion of the CoM.