손으로 내 뒷발 뒷꿈치를 터치
반드시 뒷꿈치를 들면서 터치
Touch the heel of my hind foot with your hand
Be sure to touch while lifting your heel
Biomechanics of the Lower Body
Although the major source of power on the serve comes from internal shoulder rotation, the base is the legs. The legs may not be the major source of power but think of them as the ignition key. Many younger players copy Andy Roddick's abbreviated backswing, but they can't get significant power until they physically mature.
Nick Saviano, the former Director of USTA's Player Development who now runs his own world-class academy in Florida, suggests that younger players have difficulty stabilizing their base, the legs. Failure to stabilized the feet undermines the kinetic chain. This chain is how power translates through the body. The first segment is the interaction of the legs and ground. Movement goes through the hips and core, then through the shoulders, and lastly, through the elbow, wrist, and hand.
Also, many club players mistime the knee bend which is critical to using the legs properly. That is, many juniors and club players will bend the knee before the toss or sway on the toss. Touring pros will toss and then sink the knees as Jim McLennan suggests (The Service Motion - Ground Reaction Forces). The pro then achieves what is known as the power position.
In the photo on the right, we see Donald Young show a terrific power position which is important for his recent serving successes. The body forms an L-shape with the bend primarily at the knees. The racquet has not fallen back yet as many club players allow it to do. Young uses a platform stance with the feet separate. Pros often call this stance the foot back position to differentiate between foot forward, feet together, or the pinpoint stance. In either case, the knees flex after the toss.