Athletic shoes protect your knees and other joints by providing support and cushioning during high-impact activities. While athletic shoes should be replaced approximately every 350-500 miles or every 3-6 months, your shoe quality is also influenced by factors like body weight, intensity, and exercise surface. When you lose track of the exercise distance or time you’ve spent in your current pair, pay attention to these signs that you need new athletic shoes.

Your knees start hurting.

If you start to develop knee pain, it could be because your shoes are past their prime. The loss of support can lead to iliotibial band syndrome, which is inflammation of the iliotibial band that runs from your hip to below your knee. The inflammation can cause pain on the outside of your knee joint.

You have pain in your shins.

Shin splints result from an imbalance in the muscles controlling the movement of the foot. Old shoes that lack support can contribute to shin splints.

There is a noticeable difference when you try a different pair.

It’s a good idea to buy new shoes before your current shoes are completely worn out. This allows you to transition into the new shoes by alternating between the current and new pair every few workouts. If the lack of support and cushioning is noticeable when returning to the old pair, it’s time to retire those shoes and move on to the new pair for good.

The tread is visibly worn.

Check the bottoms of your shoes. While a little wearing is normal, if you notice spots where the tread is worn a lot more than other areas, or where it is smooth and no longer grips the exercise surface, it’s time for a new pair of shoes.

The shoes have changed shape.

Over time, your shoes will conform to the shape of your foot and reflect your walking pattern. If you notice that the heels are unevenly worn causing your foot to turn, or if there is little stability on the sides of the shoe causing your ankle to shift towards the center, move on to a new pair.

Lori Rice, M.S., is a nutritional scientist and author with a passion for healthy cooking, exercise physiology, and food photography.