Running is a form of human locomotion that involves cyclic movement of the legs and includes a period of time when neither foot is in contact with the ground. The gait cycle is used to describe this cyclic movement.
The gait cycle consists of two movement periods: swing and stance. The swing period is the movement of one leg while its foot is in the air. The stance period is the period of movement of the leg and foot while the foot is in contact with the ground. The stance period may be divided into the following three phases:
Impact (heel-strike) Phase:
The impact phase is initiated when the foot makes contact with the running surface. Forces of two to three times the runners body weight is imparted to the body during the impact phase.
Support (mid-stance) Phase:
The support phase consists of the foot supporting the body as the foot transitions from heel to toe. A controlled elongation of the arch of the foot maintains proper support and allows for neutral pronation. Too much elongation of the arch will tend to result in over-pronation while limited elongation of the arch will tend to result in supination.
Propulsion (toe-off) Phase:
Forces are distributed across the forefoot and the arch stiffens as the foot prepares to leave the ground.
A foot-strike characterizes the movement of the foot as it goes through the three phases of stance: impact, support and propulsion.
The human body is an amazing mechanical specimen when functioning correctly. Unfortunately weaknesses and imbalances within individuals often subject the running body to forces and motions that lead to injury. An imperfection of the body may be exhibited in the feet during running and walking. Ideally a foot should contact the ground and roll slightly outward before rolling slightly inward. Before the foot leaves the ground, it should roll slightly outward again. Collectively, these slight movements allow the foot to function correctly in absorbing or dissipating ground forces and are said to result in a neutral foot-strike. Too much or too little movement usually leads to problems. See below to learn about the three common foot-strikes.
Supination is the lack of an inward roll or a rolling out of the foot during its foot-strike. This motion inhibits the foots ability to absorb ground forces. Also referred to as under-pronation, a foot that supinates needs a soft ride and quick heel-to-toe transition. Also, excellent flexibility in a shoe is desirable. Minimum neutral shoes and moderate neutral shoes work well for supination.
Neutral pronation is the slight inward rolling movement of the foot during its foot-strike. Neutral pronation is considered to be the ideal motion of the foot during running and walking and greatly reduces the risk for injury. People with neutral pronation tend to find success in neutral shoes or mild support shoes.
Over-pronation is excessive inward rolling (pronation) of the foot during its foot-strike and often creates alignment problems within the legs. There is a wide range of degrees of over-pronation. Minimium support shoes, moderate support shoes and maximum support shoes are designed for feet, which need some guidance to steer the foot along a path of neutral pronation. Moderate motion control shoes and maximum motion control shoes are designed for the most excessive degrees of over-pronation and work to control the motion of the foot.
If you do not know what type of motion your feet have and do not have the opportunity to have your foot motion evaluated by another person, use the following information to estimate your foot motion.
The best indirect measure of foot motion is the wear of the midsole, which is indicated by creasing in the midsole EVA foam. Note that a shoe with a polyurethane midsole will not show creasing. Fortunately, most running shoes are made with EVA foam midsoles. As a midsole breaks down creases will develop. A greater number of stacked creases or deeper creases indicate greater wear. Old shoes will display creases without applying a load while shoes with less mileage will need to be compressed with your hands. A neutral foot-strike will create creasing in the lateral heel of the midsole and medial forefoot of the midsole. Excessive creasing of the midsole in the lateral midfoot suggests that supination is occurring. Excessive creasing in any part of the medial side of the midsole from the heel to just before the forefoot suggests that the foot is experiencing over-pronation.
A fair measure of foot motion is outsole wear. The pattern of wear on the outsole may represent how forces are distributed along the shoe during stance. Compare the wear pattern from your shoe with the images below.
A wet footprint test is a common method for determining foot motion. Get your feet wet and stand on a flat surface that will allow your footprint to be shown. A paper towel works well. Compare your footprint to the images below.
One problem with the footprint test is that it is a static measure and feet have dynamic movements during stance while running. To make the footprint test better, compare seated (non weight-bearing) footprints with standing (weight-bearing) footprints. If the standing footprint has a greater surface area than the seated footprint then you probably need some pronation control. Another problem with the footprint test is that some people have footprints that match the above images but their feet do not move in the indicated way.