According to a Runner's World magazine article titled "Facts on Foot Strike," every time you land, your foot impacts the ground with a certain amount of force, which is counteracted by an equal and opposite amount of force applied by the ground to your foot. This equal and opposite force is known as the ground reaction force (GRF). The GRF comes in a number of components, typically broken into anterior-posterior (along the direction you're traveling), horizontal (side to side), and vertical (straight up and down). Of these, the vertical GRF is the greatest in magnitude.
The amount of time each foot is in contact with the ground varies among people and at different running speeds, but about one-third of a second is a reasonable number for a typical recreational runner. Each foot comes into contact with the ground 80 to 100 times per minute on average, which translates to a stride rate or cadence of 160 to 200 steps per minute, with considerable variation from person to person and at different running speeds. The average runner will take 800 to 1,000 strides per mile. Over the course of a 5-mile run this means that each heel will strike the ground 5,000 times.
Studies have shown that each heel strike produces a force that is equal to 3 to 4 times your body weight. For a 150 pound runner, this means that each heel strike will generate up to 600 pounds of pressure. When I ran across America in 2006, I took about 6 million running strides from one ocean to another. I weighed approximately 145 pounds at the time. So, the total amount of impact force on my body to run 3,260 miles across the United States was at least 2½ BILLION POUNDS of pressure. Let's put that into perspective. The weight of the Empire State Building (a 102-story skyscraper in New York City standing 1,454 feet tall) is 730 million pounds. My coast-to-coast run applied impact force to my body which was over three times the weight of the Empire State Building. Yes, for that 108-day running adventure I experienced an average of 23 million pounds of impact force each day... while pushing an 80-pound support stroller of gear, food and water.
Impact force doesn’t just act at the foot. As the heel strikes the ground the impact force will then travel up the shin, through the knee, up through the thigh and hip, and into the pelvis and trunk. To ensure that the body is able to handle these forces, it is important to have proper mobility at the lower extremity joints as well as adequate strength, endurance, and balance of muscles that control the leg, pelvis, and trunk. As long as the muscles and joints are working properly the chance of injury is greatly reduced. However, because of the repetitive nature and high impact forces associated with running, even minor problems will increase the chances of pain and injury.
Keep Reaching For Life's Mileposts,