Failure to accelerate the foot backward before foot-strike presents an impossible task for the foot. At push-off, the foot must be moving backward, relative to the ground in order to generate propulsion.
A runner moving eight miles per hour, waiting until foot-strike to begin propulsive acceleration, demands that the leg, while weight bearing, perform a 16 mph acceleration (from minus-eight to plus-eight) instantaneously. No runner can accomplish this.
This movement requires correct biomechanics and precise timing. Most runners swing the foot too far forward in leg recovery. As you swing the leg forward, initiate the foot drag movement, from the hip, as soon as the foot gets directly underneath the knee.
Set a treadmill to LT speed. Keeping your weight on one leg on the side of the treadmill, practice your stride cycle with one leg, touching the tread without fully weighting the leg.
The force of the tread pushing your foot backward at impact demonstrates how much you brake. Practice initiating the foot-drag movement as soon as the foot gets underneath the knee, pulling the foot backward into the tread right next to the supporting leg.
The foot-drag movement right reduces braking and impact stress while increasing propulsion without increasing energy cost. Get this right and you will run faster.
Ken Mierke is head coach of Fitness Concepts, author of The Triathlete's Guide to Run Training and developer of Evolution Running. Ken coaches several of the fastest runners in triathlon. His book and video are both available at www.fitness-concepts.com.