This is an excerpt from Running Form by Owen Anderson.
This exercise enhances vertical propulsive force during ground contact, promotes stability during stance, and upgrades running economy and fatigue resistance.
Perform two sets of 12 reps on each leg, with a short break of about 10 seconds in between.
Maintain good posture as you squat. Don't lean forward with your upper body, rather let your torso descend during the squat until the hip of the squatting (support) leg is on a level with the knee. Then, straighten the leg and return to starting position. Balance the toes of the rear foot on a step or bench behind you, being careful not to bear any weight on the rear foot. During each squat, the knee of the non-support, rear leg should descend downward toward a line perpendicular to the support foot at the heel. Begin the one-leg squats with no added resistance. As strength and stability increase, gradually add resistance by holding steadily heavier dumbbells or by positioning a weighty bar on the shoulders (figure 14.2a and b).