Sketch study of the single leg squat. During running when your foot touches the ground during the plant phase of running you need strong hip stability, strong single hip stability.
The single leg squat differs from the single leg contralateral anterior reach in that it uses more knee flexion which results in more contraction from the quads.
Balance on the right leg while holding the left leg back for counterbalance. The hands can remain on the hips or reach out to the front for counterbalancing. Flex equally from the ankle, knee, and hip and drop the knee of the free leg as low as it will go while maintaining control.
From the front, the hip, knee, and foot will be aligned and the hips stable and parallel to the ground. Look for the 7-frame made possible by the stable hip and good knee alignment.