When the foot is on the ground, the glutes should be king. They control hip and pelvic stability, create rotation and extension, and they decelerate and accelerate the body and limb. If you are not powering each step with the glutes, you are sure to limit your speed, stride, power and acceleration.
Fix it: Do lunges with the back leg glute contracted through the entire lunge.
The Gait Guys are Drs. Shawn Allen and Ivo Waerlop (Thegaitguys.com).