by Dr. Phil Maffetone.

Walking is associated with first striking the heel, whereas a running gait involves landing farther forward on the foot—a midfoot strike in most cases with more forefoot landing as running speed increases.

Making contact with the ground imparts impact forces—the foot literally collides with the earth on each step. While impact is often seen as a negative aspect of running, equating to trauma and injury, a proper gait is potentially associated with better bone density and improved muscle and tendon function, better circulation, and otherhealthy benefits associated with exercise. With proper gait, colliding with the ground is well compensated for—humans have evolved an effective gait mechanism.

Impact forces during walking are relatively minor. But heel striking while running can be a significant loss of energy, a common example of an improper gait producing stress from impact. The overall mechanics of the foot, ankle and leg, and many body areas above are stressed with abnormal heel striking compared to the runner who lands farther forward. Mid- or forefoot running is associated with a more optimal gait that’s usually not impact impaired.

Let’s consider these two gaits.

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 the conversion of carbohydrates and fat.

Things are quite different with running. This action 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 counterproductive, 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 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 also slow you down, due to the higher cost of energy. This can be further compounded if you burn less fat for energy, thereby relying more on sugar that’s associated with the more rapid onset of fatigue. And the impact energy that’s not recycled often places a strain on muscles and tendons (and ultimately, ligaments and bones) and can contribute to an injury.

In addition, movements above the ankle, especially in the knees, hips, and low back can help—or hurt—the natural spring-ahead mechanism. Too much motion in these joints can reduce the body’s ability to recycle impact energy. By running more upright—you should be running tall—rather than adopting a lazy, slumped-over position, you’ll minimize knee, hip, and low-back movements, and thus helping to utilize the foot’s spring mechanism. This involves using muscles similar to when you have to stand up straight—they include the abdominals, gluteus maximus, and even the neck flexors that prevent the head from tilting back.

Other movements are different between walking and running. Most notably in the knee, which is locked during a walking gait but not while running. The slightly flexed knee is more active during running and requires much more effort by muscles to support the joint while the foot is on the ground. This is a key reason why many runners with improper gait have knee injuries.

Those who run slowly often wonder if it’s better to sometimes just walk fast as the pace can be the same. This is especially true on hills. Deciding on which option is best is the job of the brain, which will naturally tend to make the right decision about making the transition from walking to running.

The energy cost of walking and running not only varies with speed but with the type of ground surface and other environmental factors such as temperature, humidity, and wind. But when the gait is irregular, both walking and running share a common feature: Both movements will cost more in energy. The worse or more inefficient the gait, the greater will be the energy expenditure.

This essay was excerpted from The Big Book of Health and Fitness.