The heel drop of a shoe represents the difference in midsole cushioning height between the heel and toe of the shoe, measured in millimeters. A zero-drop shoe means there’s an equal amount of the midsole cushioning under the heels and toes.
The drop primarily affects how your foot strikes the ground. Topo offers 0MM, 3MM and 5MM drop shoes for a more natural fit. A lower drop will help promote a midfoot strike, which has a lower impact stride than a higher drop resulting in heel strike -- which is more optimal from a biomechanical standpoint. For those who are use to a traditional higher drop shoe (10mm+), wearing a lower-drop shoe may require an adjustment period. A gradual transition between lower drops is recommended.
Changing the drop of your shoes (or using multiple shoes which have varying drops in a rotation system) is not to be discouraged or feared, but be sure your body’s tissues can tolerate this, and are given the necessary time to adapt and attain the capacity if needed.
Based on a heel drop, shoes are split into 4 categories:
Zero drop (0mm),
Low drop (1-4mm),
Mid drop (5-8mm), and
High drop (8+ mm) shoes.
Heel drop and the total amount of cushioning (stack height) are independent of each other; it is possible to find ultra-cushioned shoes that still have a zero or a low drop.
The stack height of a shoe refers to how much cushioning material is between your foot and the ground. A running shoe with a higher stack height will have more cushioning, such as a maximalist running shoe so that your foot is elevated on more of a platform above the ground. A naturalist running shoe will have a low stack height, which means that there is very little cushioning material between your feet and the ground. Again independent of drop, it is possible to have a shoe with a large stack height with a low heel to toe drop.
Tips on trying on shoes:
Try on shoes at the end of the day: Your feet swell throughout the day and will be largest at the end of the day. This helps you avoid buying shoes that are too small.
Aim for a thumbnail's length of space at the toe: You should be able to wiggle your toes. The width should be comfortably snug but still allow a bit of room for your feet to move without rubbing. Laces should be tied snug but not tight.
Try on both shoes: Some people have one foot that is larger. Try on both the right and the left shoe and find the pair that fits your larger foot.