It is a known fact that people use their shoes well after their use-by date. Shoebuddy provides the user with an estimation of the life of every shoe in the database. Using running and walking as an example sport, shoe-life determination is a two-step process:
1. The user on-boards into Shoe-buddy: body weight, sporting preference, frequency of activity and running speed.
2. Shoe-buddy determines the shoe protection classification, shoe weight and the material composition (what foams are used within the midsole. The outcome is an estimated shoe life.
Other than ensuring you have the correct shoes, the life of athletic footwear is an important criterion for performance and injury prevention.
While most of the visible wear to a shoe occurs on the upper fabric and the outsole of the shoe, the most important features of the shoe are those that provide cushioning, protection from running loads, comfort and performance. The midsole of the shoe, that componentry wedged between the upper and outsole is the mechanism that is mainly responsible of all the joy and comfort we really want from a shoe. Footwear companies devote a large proportion of R&D budgets investigating, testing and designing how midsoles can be lighter in weight, promote cushioning and protection. This aligns with consumer expectations that when asked in focus groups what they want in a running shoes, the most common answer is “.. a shoe that is light in weight and cushioned”.
Over the decades, shoe weight has become lighter and lighter. As this happens, the structural integrity of the shoe is compromised. Therefore, the 21st Century shoe, as it becomes increasingly more high tech, needs to be individually prescribed and usage monitored to determine when the shoe needs to be replaced (shoe-life).
The midsole of a running shoe is a layer of foams, gels, air and other (usually proprietary technology individualised to a shoe manufacturer). The midsole attenuates loads, cushions impact and depending upon the shoe type, adjusts foot mechanics. For example, some shoes are designed for unstable feet that roll inward, are flat or low arched. This midsole is known as “dual density midsole,” meaning it has a firmer density of foam placed on the inner side or medial side of the shoe;
under the arch. Running consists of repeated foot strikes that produce a ground reaction force (GRF) of 3-4 times an individual’s body weight providing forward propulsion (Keller 1996). The figure shows running ground reaction forces and it is this and body weight that will determine the life of a shoe.
Keller T.S., Weisberger A.M., Ray J.L., Hasan S.S., Shiavi R.G., Spengler D.M. (1996) Relationship between vertical ground reaction force and speed during walking, slow jogging, and running. Clinical Biomechanics 11(5), 253-259.
A comparison study (http://dx.doi.org/10.1136/bjsm.2008.047761) of three different types of midsole foams indicated that after 200 miles (360km) midsole degradation occurs. While there is no definitive answer as to how long a shoe should last, clinical data obtained from over 25 years of podiatric practice, along with runners feedback of shoe comfort, analysis of shoe material properties, industry data on midsole longevity and data from scientific literature suggests that running shoe life expectancy will be correlated to factors such as ground surface, body weight, shoe type, activity and shoe midsole foams. If you rely upon outsole wear, the shoe to be distorted or holes in the upper of the shoe as an indicator of when to replace the shoe, you have waited too long.
As shoe material engineering advances and shoes continue to be lighter in weight, the longevity of the shoe will decrease. As a general guideline replace shoes before 1000km but better still use a resource that calculates shoe life based upon body weight and shoe type. This will help desired running outcomes of comfort, performance and injury minimisation.