Spring is here, and that means its running season. Most races, from 5Ks to ultramarathons, occur in summer. Therefore, training is starting to kick into high gear. And as the miles accumulate, our running shoes will take a beating.

To ensure that your running shoe remains structurally sound and able to handle loads, it should be replaced before breakdown occurs. While different sources have varying opinions on when that should happen an interesting study in the Journal of Biomechanics shed light on the distance most running shoes can adequately handle.

Before we delve into the study, let’s focus on the midsole of the shoe, the part that plays a pivotal role in determining when it should be replaced. The midsole is the most functional part of a running shoe as it provides torsional rigidity and helps the shoe adapt to the loads from the runner and the ground. Midsoles are primarily made up of ethylene-vinyl acetate, a lightweight multi-density material that can be manipulated to provide varying amounts of support. EVA is known to wear down over time, and the amount of time it remains functional is the critical factor in determining how long a running shoe can be safely used.

In the study, “Heel-shoe interactions and the durability of EVA foam running-shoe midsoles,” researchers examined the structural integrity of EVA in new shoes and shoes that had endured the stresses of 500 km and 750 km of running. Researchers took the shoes, cut the EVA in half, dipped it in 24-carat gold, and studied the material under an electron microscope.

In new shoes, researchers noted EVA had nice, well-defined, round pores. The structural integrity was intact. In shoes with 500 kilometers of running, researchers noted that the EVA still had nice round pores, but there was a slight breakdown. In shoes with 750 km of running, researchers noted that the pores were frayed, and holes had also appeared in the structure of the EVA.

When the midsole of the shoe breaks down it is less capable of acting as a shock absorber between the ground and the runner. Therefore, to ensure that a running shoe is structurally sound and able to handle the loads applied to it, it should be replaced. Since that happens at 500 km, roughly 300 miles, it should be replaced just before that. For some runners, that can be as often as every few months.

Dr. Jordan Duncan is from Kitsap County and writes a monthly health column for the Kitsap News Group. He is the owner of Silverdale Sport & Spine.