If you've visited a running store recently, you've certainly heard the term "drop." Here's what it means: Drop is the height differential between where the heel and forefoot sit. For many years, shoes had drops between 12 and 15mm, with the idea that thicker cushioning and a higher heel would reduce impact. But recent studies have suggested that a large drop is one of the culprits in common running injuries because it encourages a heavier heel-striking gait. During the minimalist-running-shoe revolution of the past few years, many new training shoes have been designed with 4 to 10mm heel-toe drops, and a new category, "zero drop" shoes, has emerged at 0 to 4mm. Gait analyst Jay Dicharry, director of the Speed Clinic at the University of Virginia's Center for Endurance Sport, says running in shoes with a low drop is ideal because they put the body in an optimal running posture and increase stability. "Not every runner needs a zero-drop shoe," he says, "but if joint health and better form are the goals, running in a shoe with a minimal drop of about 3 to 5mm is clearly better for most."