While running, your sneaker loses its Durability, which can be found at the bottom of the screen when tapping on an item in your 'Inventory' tab.
Each sneaker has its own durability level which depends on its rarity.
It is crucial to keep your sneaker repaired as it affects your earnings. The more damaged your gear is, the less KCAL you earn.
Switch to the 'Inventory' tab,
Tap on a sneaker you would like to repair,
Press the 'Repair' button at the bottom of the screen,
Use the slider to adjust the amount of Durability you would like to restore.