The relevant part of the article:

> When I pressed Kram and Swinnen for their preferred explanations, they pointed out that efficient runners use their stretchy tendons and ligaments to store elastic energy to be “recycled” from stride to stride. The push and pull between tendon and muscle is so finely tuned that your muscles stay roughly the same length throughout the stride instead of shortening and lengthening with each contraction. Optimizing this aspect of running is invisible to the naked eye and beyond conscious control, but it may be one of the crucial skills that improve with experience.