Sure, you could have your clients run drills at the park using trees and dirt patches as landmarks, but luckily, there is a better way.
Field marker discs such as the 50-Pack Kevenz 2″ High Multicolor Soccer Disc Cones Markers
is the preferred way of marking a designated course for speed and agility training. This 50 pack of bright colors is particularly useful in outdoor settings so you can highlight the route for your clients, allowing them to focus on the exercises rather than finding the markers.
