PRORehab Library

Newton's Third Law: A Runner's Perspective

“For every action there is an equal and opposite reaction.” – Sir Isaac Newton

Picture Sir Isaac sitting under an apple tree after completing his first marathon, his long hair tied back in a ponytail, his arches aching and his shins throbbing, contemplating the forces involved during each heel strike of his run. Okay, so maybe he was actually thinking of the motions of stars and planets around the sun when that apple hit him on the head, but his laws of motion define physical interaction, including those encountered during running.

It’s Newton’s third law that comes to mind every time a runner presents with an overuse injury. Unlike acute injuries that involve an identifiable cause and effect, overuse injuries are often insidious and multifactorial, but ultimately can be attributed to the physical interaction of the foot with the ground. Foot structure, flexibility, running surface and body mass are just some of the common factors that determine how the foot reacts to the ground during running. Peak vertical ground reaction forces (VGRF) during recreational jogging are estimated to be 2 to 3 times body weight. So what can you do to help reduce the chance of becoming injured?

Selecting appropriate footwear can help minimize ground reaction forces and offer control of normal or excessive motions of the foot during contact with the ground. Running shoes combine different materials, designs and technology to offer you a number of shoe types. Identifying what shoe is right for you will help to absorb GRF and offer motion control features that may be the key in preventing injury. Common shoe types include: cushioned shoes for rigid foot types (moderate to high arches) designed to emphasize shock attenuation, motion control shoes designed to inhibit excessive rearfoot motion (flat feet, heavy runners), and stability shoes designed to control pronation ( the normal flattening of the foot from heel strike to mid-stance).

Explanation of a few terms and materials may help you determine what shoe is best for you. Outsoles (bottom of running shoes) are composed of blown rubber (expanded or mixed with air to make it lighter and offer more cushion) and/or carbon rubber (mixed with carbon additive to increase durability). Ethylene Vinyl Acetate (EVA) is the most common midsole foam (layer between outsole and upper) used in running shoes, while a polyurethane midsole is denser and heavier. The heel counter surrounds the heel and controls rearfoot motion, and a medial post helps control overpronation by adding material to the inner side of the shoe. The shoe last (curved, semi-curved and straight) is what the shoe is built on and determines the shape of the shoe, and the toe box is the front of the shoe and should allow adequate room for your toes.

Follow these tips when selecting your shoes:
· Find a running store with a knowledgeable staff of running experts (New Balance and Fleet Feet are stores I recommend)
· Research the latest models and shoe types, and identify features that you feel are important for your needs (Runner’s World and Running Times are excellent resources)
· Wear your running socks when trying on shoes
· Fit your shoes at the end of the day when your feet are the largest
· Take your old shoes with you to show the salesperson
· Take any orthoses or inserts you plan to wear during running

Finally, alternate between a second pair of running shoes and limit the life of your shoes to 500 miles.

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