The American Academy of Orthopaedic Surgeons provides guidlines for buying athletic footwear:

• Because feet swell with activity, always try on athletic shoes later in the day or after a workout.
• Try on shoes with the same type of socks that you use to work out.
• Make sure you can easily wiggle your toes when the shoes are laced up, and that your heel doesn't slip as you walk.
• Try walking or running a few steps in the store to be sure the shoes are comfortable. You should not have to "break in" athletic shoes.
• Always fully lace shoes when trying them on, as you would to work out.
• If you work out frequently, buy a shoe specific to the sport or exercise that you perform.

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