While running provides many health benefits, runners must be mindful that it is a weight-bearing activity that involves substantial loading and twisting of the feet and ankles. Heel impact is typically measured at 3½ to 5 times body weight, toe-off loads regularly vary from 4 to 6 times body weight, and double-digit heel rotation angles are common. Multiply those numbers by the 1500 strides that most runners take every mile and it is apparent why 97% of running injuries are overload injuries. Too often we simply ask our bodies to deal with too much loading and/or twisting.
The primary purpose of running shoes is to lessen the runner’s risk of injury by reducing loading and excessive twisting. These goals are accomplished with the use of high-tech cushioning and stabilizing components. But just like any mechanical piece exposed to continuous loads and torques, a running shoe’s protective parts do gradually lose their functionality.
So, how do you know when your shoes are no longer providing adequate protection against overload injury? Here are four ways to tell.
When the forefoot has lost its cush.
Many people mistakenly assume that running shoes first wear out in the rearfoot. (“These shoes are done. I’ve worn the outersole rubber completely off the heel!”) In reality, a running shoe’s forefoot cushioning is almost always its weakest link. The reason is twofold: First, the forefoot can’t be so thick or stiff as to feel restrictive during toe-off, when the toes bend “up” approximately 30 degrees relative to their flat-footed position. Second, during propulsion the forefoot is subjected to loads equal to 4 to 6 times the runner’s body weight. These situations combine to result in a relatively thin, flexible material being regularly subjected to massive loads.
If you wait until you can feel the gravel underneath the balls of your feet to replace your shoes, you’re waiting too long. A better forefoot cushioning check involves sticking your hand inside your shoe and feeling the area where the ball of your foot sits. When your shoe was new, the forefoot felt flat – or even a bit convex. Any depression in the midsole you notice is the result of the breakdown of cushioning materials. Not only is your body now absorbing additional loads, but the resulting forefoot concavity also causes your toes to slide together at push-off, which leads to an inefficient, unstable toe-off and increasing your likelihood of pinching a nerve. Through experience you will learn how much of a concavity you can deal with before foot discomfort begins.
When heel lean becomes extreme.
Research conducted by Dr. Reed Ferber of the University of Calgary and the Running Injury Clinic indicates that injury risk increases dramatically when a runner’s heel rotates more than 10 degrees “inward” relative to vertical. Runners are encouraged to select a shoe style that has the appropriate level of stability control to keep them below this risk point. As is the case with cushioning componentry, however, stability parts (heel counters, midfoot shanks, medial posts, etc.) lose the functionality over time. If you put your shoe on a table and its heel is tilted inward at an angle of 10 degrees or greater, your shoes need replacement. If your shoes didn’t last as long as you expected, consider replacing them with a more stable shoe.
When your body tells you so.
If your training has remained the same but it is leading to more aches and pains, the protection offered by your shoes has likely fallen below the level you currently need to run comfortably. Your needed protection level changes with training intensity, frequency, and mileage - as well as age, weight, injury history and other factors. A low-protection shoe may have served you well as a young runner or occasional 5k participant, but will no longer serve you well as an older runner or regular marathoner.
If you have a weak link, use it to gage when you need new shoes. For example, when my shoes wear down, my lower back begins to hurt. I know to take a good look at my shoes when my lower back starts hurting.
When the calendar says so.
Runners who self-identify as “mid-mileage runners” typically replace their shoes every six months. Self-reported “low-mileage” runners update their shoes once a year. And “high-mileage” runners freshen their shoes every three or four months. It is assumed that these average timeframes are the result of each group (as a whole) determining through trial and error the replacement schedule that keeps them healthy.
David Spetnagel and his wife, Debby, opened Fleet Feet Sports in St. Louis in April 1993. There are currently six St. Louis locations. Originally an engineer at NASA and McDonnell Douglas, David has since served as a Sr. Writer at Running Times Magazine and a track and cross country coach at both the high school and collegiate levels. In early 2015 he completed a streak of 1000 straight days of 5Ks (or more). (2015-05-05)