May 2007 -- Question: In pictures concerning the modern game, I see players leaving the ground on the majority of shots. As a traditionalist in teaching, this concerns me a great deal. First, what am I to make of this "jumping" and, second, how should I relate it to my students?
Answer: This question has been examined and studied in biomechanics laboratories throughout the United States, in Europe, and in Australia. Through the medium of high-speed cinematography in the ’70s and ’80s to the use of incredible technology in high-speed video today, the strokes of the great players have been broken down frame-by-frame in an attempt to explain phenomena such as leaving the ground during a stroke.
First, I need to say that this is not new to the game. Players have left the ground on their serves and overheads for as long as they have tried to hit them with extreme force. It is true, however, that we see more players than ever leaving the ground on groundstrokes, and even on swinging volleys. The key is to understand what causes them to leave the ground and then to discuss how this action should be taught, or even if it should be taught. I say that, because knowing that you are a traditionalist in your teaching, you probably are asking that question.
The body, in virtually every stroke, acts as a kinetic chain (some call this a linked system). By that, I mean that many body parts contribute to the overall force generated by the body to strike a tennis ball. For the majority of tennis strokes, the sequence of force development takes place as follows. Initially, there is a ground reaction force as the feet push against the ground. That force is transferred up through the legs via knee extension. That force, in turn, is transmitted to the hips, which begin rotating. The hips add to that force and transfer the force upward to the trunk. The shoulders begin rotating and really take over at this point. You will usually see a large rotation of the shoulders where they, in turn, transfer that force to the upper arm and on to the hand-racquet movement. But, that, in and of itself, does not explain why a player would leave the ground.
To understand this, we need to examine exactly when the great players actually leave the ground. Studies show that these athletes never leave the ground early in a stroke’s development. Instead, they leave the ground later in the stroke, about the time when the summed forces are transmitted into the upper arm. Now, let’s briefly discuss Newton’s laws of motion and how we can better understand what is happening.
The law of inertia must first be dealt with, and motion must be initiated. The backswing begins and the knees flex. At an optimum knee angle for the player, he or she exerts force against the ground. This demonstrates the law of action and reaction. This law states that, with every action, there is an equal and opposite reaction. So, when you push against an object, the object pushes back against you. In the case of an athlete jumping, the body is relatively small compared to the earth. The athlete pushes against the earth and the earth pushes back but this enables the athlete to leave the ground (for as long as gravity will let him). In tennis, as the feet push against the ground, the ground pushes back with an equal and opposite force. This ground reaction force is one of the most important principles in hitting effective strokes. The force from the ground is then transferred upward and through the body as explained earlier. Now the critical question: When does the body leave the ground? Better yet, when can the body leave the ground and still hit a hard ball?
Bear in mind that, as force is transferred upward and throughout the stroke, it is a large body part (e.g., the hips) transferring to another large body part (e.g., the trunk), until the force is transferred to the upper arm. Now, you have a large body part (the trunk and shoulders) transmitting to a small body part (the upper arm). What if the player left the ground when the force was in the hips or just beginning in the trunk? My guess is that the player would lose a lot of force and could not transfer as much force upward toward the impact. And, this has been shown in high-speed film and video. I have high-speed films of Jimmy Connors hitting his famous forehand and leaving the ground, as well as numerous players from the ’70s, ’80s, and ’90s, leaving the ground on their serves. Every one of these players stays in contact with the ground until the force is out into the small body part (the upper arm). Thus, the action/reaction that takes place is a small body part (the arm) against a very large body part (the trunk), and very little force is lost in the stroke. Now, the argument could be that, leaving the ground at this point actually adds force to the stroke, enabling greater body rotation and increasing the leverage into the stroke. But, now, let’s ask the last question: Do the players actually jump and how do we teach this action to our players?
First, the players do not intentionally jump into the shot. The thought is that their bodies are pulled off the ground. That is a very important point to make to your players as you teach more forceful strokes, not to jump but understand that your body might get pulled off the ground. When players think of actually jumping into a shot, their intentional force development is not upward and into the stroke but into the jumping process and coordination of the linked system becomes very difficult.
Before discussing how to best teach this phenomenon, let’s be sure we clear up one area. In today’s game, where the players are hitting aggressive strokes off both sides, the most prevalent action comes from the angular or rotational momentum in the upper trunk and shoulders. If the player left the ground too soon, the high rotational velocity, with control, would be difficult to achieve.
So, how do we teach this or how do we teach around this? I think it will be a difficult road for an instructor to teach players to actually "jump" into a shot. The student becomes more focused on the "jump" and not striking the ball. Instead, the teaching professional must become innovative and develop "cues" that will help the player perform the desired movement. One such cue might be to "load" the back foot in preparing to hit the ball, then possibly advise the student to "drive up" toward the ball. A final thought might be to drive the stroke with the shoulders. There are obviously many ways to teach and develop cues such as these to help our students. I am sure that you have other ways to make this teaching effective. The discussion must continue to draw the best possible performance from both us and our students.
Send questions to jgroppel@LGEPerformance.com.
