A key difference between walking and proper (mid- and forefoot) running is how the foot muscles work, and, in particular, the energy used for propulsion. The walking body acts more like an inverted pendulum, swinging along step-by-step, literally vaulting over stiff legs with locked knees. Muscles use the body’s metabolic energy created by conversion of carbohydrates and fat.

Things are quite different with running. This action or “elastic recoil” is sometimes referred to as an “impulsive” and “springy” gait, rebounding along on compliant legs and unlocked knees. Instead of using all the body’s energy, the leg and foot have a built-in “return energy” system for a significant amount of energy. This relies on the Achilles and other tendons to recycle impact energy. (Don’t confuse this with claims made that some running shoes have a “return energy” system, they don’t—it’s simply marketing hype.)

In running, the body has an effective muscle work-minimizing strategy—many of the foot muscles don’t technically push you off the ground like during walking. Instead, the muscles provide an isometric-type tension to stabilize the tendons and help in the function of the unique mechanism that takes impact energy, sometimes referred to as “elastic energy” associated with gravity and impact, and uses it for propelling the body forward. In particular, the large springy Achilles tendon on the back of the heel that runs up the leg and attaches into the large calf muscles (the gastrocnemius and soleus) plays a key role in recycling energy for propulsion. This tendon must function with sufficient tension to help in the return energy process, and the muscles it attaches to, also important postural supports, require a certain level of tautness, even at rest. (Trying to “loosen” these muscles and tendons through stretching, aggressive massage or other therapy may be counter-productive, impairing the natural springy gait. Excessive tightness of the Achilles certainly can induce poor function as well—think balance.)

Those with shorter, more compact Achilles tendons, unlike taller runners who also have longer heel bones attached to the Achilles, generally have a more efficient spring mechanism—one reason why shorter runners typically can run faster, especially in sprinting, although there are exceptions. Carl Lewis and Usain Bolt, past and present Olympic champions, respectively, are taller than average. Bolt’s height advantage worked against him in the start, but then he would later cover more ground using fewer strides than his competitors.

Here’s how the body’s natural gait uses recycled energy or elastic recoil for propulsion. As a runner’s foot hits the ground, impact energy is stored in the muscles and tendons, and 95 percent of this energy is then used to spring the body forward like a pogo stick. This mechanism provides about 50 percent of the leg and foot energy for propulsion (the other 50 percent comes from muscle contraction). If this process isn’t working well, such as if you land on your heels, are wearing bad shoes, or have muscle imbalance, the impact energy is dissipated or lost, and you must make up for the problem by contracting more muscles for propulsion which requires the use of more energy. Not only is this mechanically inefficient but it will slow you down, due to the higher cost of energy. –by Dr. Phil Maffetone