Figure 1. The relationship between potential and kinetic energy in walking and running. The relationship between potential and kinetic energy is one of the crucial differences between walking and running. In walking, they are out of phase. In running, they are in phase.
The relationship between potential and kinetic energy is critically different between walking and running activities (Fig. 2). In walking, the two are out of phase. When potential energy is high, kinetic energy is low, and vice versa. Walking has been referred to as controlled falling (from the zenith of the center of mass in mid-stance to its nadir during double support) and is similar to a swinging pendulum. In running on the other hand, the two are in phase.
Because of this difference, the body completely alters the methods it uses to maintain energy efficiency. Large fluctuations in total energy going into and out of the system would be disadvantageous regardless of the pace of movement. Efficiency in walking is maintained by the effective interchange between potential and kinetic energy. They are out of phase. In running, because the two are in phase, this is not possible. Instead, efficiency is primarily maintained in two ways.
1. The storage and later return of elastic potential energy by the stretch of elastic structures (especially tendons);
2. The transfer of energy from one body segment to another by two joint muscles such as the rectus femoris and the hamstrings.
These two concepts will be addressed separately in the next two sections. These mechanisms do not occur without some cost of their own. It is the repetitive cycling of tendon stretch and recoil that is responsible for many of the chronic overuse syndromes in runners.