74 - The Power Wave in Tennis Stroke from Tennis Without Talent - https://www.tenniswithouttalent.com/PowerWave.html
The Power Wave
Theoretical Forehand Power Wave: Measured at the forearm. The shoulder and hips would see the same wave but would see it earlier. For example, the peak would come well before the load phase. The power is negative before and during the lock phase causing the hips to reverse-rotate creating the backswing proper. The wave also goes negative between the explode and follow-through phases as one puts the breaks on forward acceleration. This bi-directional behavior is typical of a wave and allows it to propagate through the body without resulting in excessive net twisting of the spine.
The Shape of the Wave
When we conceptualize a wave we like to take a tracing over time as in the diagram above. The horizontal axis represents time, while the vertical axis represents directional power. (Physicists will balk at this notion as their concept of power has no direction. Tennis players will understand this concept. Power in the positive direction is power directed towards the target. Negative power is in the opposite direction, i.e., in the direction of the backswing.) Imagine a sensor on the player's wrist that measures the forces trying to displace it. At first, the force is backward, dragging the wrist and racket away from the ball. This move is the lock, an essential component of every stroke from the drop shot to the serve and typical of wave motion which is always too and fro, forward and back or up and down. Suddenly the power changes direction towards the ball starting the acceleration forward.
At this point the racket flips, loading stretch shortening forces into the forearm which will ultimately be injected into the ball as impulse for spin and control. As the power wave continues to crest, the racket is dragged behind the wrist - accelerating and building racket head speed for pace - the lag phase. Suddenly, at the peak of the wave, the power reverses, and the shoulders start decelerating. The racket head's inertia allows it to catch up with the wrist, releasing the stored forces into the ball. At the moment of contact with the ball, all power neutralized, and there is no force on the racket coming from the shoulders. Eventually, the wave becomes completely negative, keeping the racket from wrapping totally around the player's neck. It is important to remember that the lower body generates the power wave - a conspiracy of the feet, legs, and hips. Although you can add power to the wave using your upper body, that can be a dangerous procedure. In particular, at the moment of contact power delivered through the shoulders should be zero, not positive, lest one misdirect the ball away from its target by injecting impulse in the wrong direction (see Push Syndrome).
Back and ForthEvery shot in tennis starts with the racket moving away from the oncoming ball. I call this move the lock or "backswing proper" to differentiate it from the unit turn - the larger and more ostentatious component of the backswing.
The lock, like every component of every stroke, requires a source of power. That power comes from an initial reverse-rotation of the hips as you transfer bodyweight from the front foot to the back foot.
Pushing Back Against the Push
How could Federer flub his forehand? Just dump an easy, short ball into the center of the net or hit it ten feet behind the baseline? What can we learn from his mistakes? That makes the most natural of all tennis strokes, the forehand, so glitchy, even for the best of the best? What is the universal forehand fail?
The culprit lurks in the second half of the power wave. Just after the power wave peaks, it takes a perilous nose-dive, indicating a sudden reversal of momentum from forward, in the direction of the oncoming ball, to back away from the ball. This reversal happens before the moment of contact. Ponder that for a second. Just before you lay into the ball you have to pull back on the reigns, stomp on the brakes, pull your punch. Could there be anything less natural than that? Does that even make sense?
In fact it makes perfect sense. To take control of the ball's flight path one must store and later deliver muscular forces to the ball. There are alternative ways of delivering spin and pace, but there is only one reliable source of control, and that is impulse. The forces one uses to gain hegemony over the ball are stored during the lock and load phases of the stroke, then held in place by inertia during the acceleration or lag phase. During the lag phase, forces are driving the racket towards the ball, but they are not appropriate control forces. Their direction is determined by the need to address the ball and the need to add spin to the ball. The acceleration forces are not pointing at the tiny window over the net through which one must direct the ball for it to find its target. If you continue to accelerate the racket through the moment of contact, those mighty acceleration forces will overwhelm any stored control or spin forces in the forearm and send your ball God knows where.
Thus, to deliver the stored control forces to the ball at the moment of contact, all acceleration forces must be gone. Imagine that for a second and be amazed. For a few tenths of a second, you are pouring as much power into the racket as you can muster, then 'poof' - power is gone, and the racket glides into the ball under its own steam. The timing of this loss of forward acceleration is pretty critical. A few milliseconds late and you drive through the ball and release the stored control and spin forces in the follow through with no effect on the ball. Generally, this fail results in a long ball. A few milliseconds too early and the stored forces will begin to contort the arm and racket into pretzel position before one makes contact with the ball. The pretzel position is best reserved for the follow through where it can't do any harm to the ball. Generally, when the forces are released early, you drive the ball into the center of the net, mishit it or flag it all together.
Forehand Topspin Power Wave: The feet generate the power wave in the forehand topspin. First, the front foot pushes the weight back to the back foot; powering the reverse-rotation of the backswing proper and loading the back foot. This reverse-rotation creates the initial 'dip' in the power wave. Next, the back foot pushes the weight to the front foot creating the peak of the power wave and driving the load and lag phases. Finally, the front foot pushes back neutralizing the forward acceleration, causing the power wave to plummet and stopping the shoulders so the arm, racket and stored control and spin forces can explode into the ball. This vital last weight transfer is frequently, fatally forgotten by pros and duffers alike.
The Power Wave ShuffleThe video above is a demonstration of how foot-to-foot weight transfers allow you to harvest directional momentum from the earth to feed the power wave for the topspin forehand. Remember that each weight transfers occurs hundreds of milliseconds before the portion of the wave that it creates reaches the racket. Waves must travel from the source to the sink, and that takes time. This delay between action and result messes with our heads as players. It syncopates the rhythms of the stroke and has made it hard in the past to understand the relationship, say, between "stepping into the ball" and stopping the forward swing. I always thought that stepping in somehow added power to the shot just by the translation velocity of my body. If you take the 3 miles per hour that I can move and add it directly to the pace of the ball, well, obviously that is not a valid reason for "stepping-in". Stepping-In has two functions; stepping-off of the back foot, which starts the upstroke of the power wave and motivates the forward portion of the swing, and stepping on to the front foot, which secures the front foot for pushing back against and reversing the direction of the power wave. So, just like a bullwhip, the footwork of the forehand groundstroke is back - forward - back because the stroke is back (backswing proper) - forward (load and lag) - back (explode). If you fail on the last portion, say because you don't plant your front foot sufficiently to push back against the stroke, you lose all of your control and most of your spin. You still have plenty of pace, though, to carry the ball into the back fence.
From the preceding discussion we can derive a fundamental rewrite of the old "Step into the ball!" tip. It is; "Step early into the ball!" The 'early' part is vital because stepping-off (of the back foot) has to finish well before the load phase begins, and stepping-on (the front foot) must finish well before the moment of contact. Only by completing both of these steps before the moment of contact can you use your front foot as an anchor to reverse-rotate your hips against the forward rotational inertia of the body. This action puts the brakes on the stroke and releases the stored forces.
Step into the ball early!
So what do you do if you can't get your front foot down? How do you stop the forward rotation of the shoulders? The pros instinctively counter-kick out forward with the front foot. That can provide just enough momentum to quell forward acceleration and allow control forces to be released.
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////Wave Theory and Tennis
The classic understanding of the tennis stroke is as a mechanical "rods-and-levers" contrivance vis: the legs turn the hips - the hips turn the shoulders - the shoulder turns the arm - the arm does some stuff to address the ball and get the racket head going in the right direction and presto! The ball is sent along to its target with unerring accuracy, irresistible power and screaming spin. That is the fantasy we carry around in our noggins, reinforced with terms such as the "kinetic chain" from the literature of tennis that reinforce such delusions. For some odd reason, this idea never seems to work out for us. Every time we adjust one cog in our corporal clockwork, it seems to throw our game into chaos. As an example, one might bend the elbow a bit to gain control over the position of the wrist to better guide the strings to the ball and avoid miss-hits. That makes sense - more control is good, right? The result is more mis-hits, a complete loss of pace, control, and spin, and a sore elbow. Is this beginning to sound familiar?. There are way too many objectives in tennis - adding spin, addressing the ball, creating pace and control, etc. to do it all with a series of tightly linked motions. Each of these objectives must be achieved by a system that is separate and distinct from the others in both time and space. Thus the muscles of the forearm are largely responsible for control while pace comes from the larger muscles of the shoulders, trunk, and legs. If you start tensing your forearm to get more pace, you lose control. If you fail to establish balance with your quadriceps muscles, your brain will recruit the muscles in your upper body and wreck the rest of your stroke. Each body unit in a stroke has its own set of responsibilities and is on its own schedule, and that schedule is not "first A completes then B starts, then B completes, then C starts ... " We achieve balance in the serve as the left hand is tossing the ball and the right arm is relaxed in preparation for the hitting sequence. They are independent of one another, and each has its own start and finish time. If each component has to wait for the other to get the energy and momentum it needs to complete its task then what you have is a house of cards, not a stroke.
Each of the functional units of a stroke requires power to operate. Power is the ability to make things move, stop moving or change direction. It is a combination of energy and momentum. Energy for tennis stroking comes mostly from the muscles and a bit from gravity. Energy is necessary to influence the flight of a tennis ball, but it is also insufficient.
PowerOne must also harvest momentum, mostly from the earth. If you have ever tried to hit a tennis ball while floating, weightless, in space ... sorry, bad example. You probably have not tried that yet. I have suffered from the delusion that I could stand flat-footed with no solid purchase on the court and hit a tennis ball 100mph with just the power of my muscles and my indomitable will.
Footwork is the primary source of momentum. Setting the back foot on a groundstroke, keeping the knees bent and the bodyweight bouncing from one foot to the next, orienting the feet so that the momentum generated is in a proper direction for the stroke are all essential for the production of power. Power must be created and distributed to each part of the body as that part needs it. Power is the raw material for pace, spin, and control. After all, what is control but the ability to direct a ball to the desired target; to change its momentum? Each component needs the power to reach it when the component need its and in the form that the component can use it. A mechanical linkage will not suffice. The power must be packaged and delivered in the form of a wave.
The Power Wave of the Topspin Forehand: Begins with a push-off against the earth and weight transfer from the front foot to the back foot (gold) during the unit turn which powers the lock (backswing proper). It propagates through the muscles as the first trough of the wave, rippling up the trunk, through the chest and shoulders and down the arm to the hitting hand where it is transferred to the racket, throwing the racket back away from the ball and helping set the tone of the forearm. Next one pushes off the back foot throwing the bodyweight to the front foot (green). This shift of weight produces the main peak of the power wave, and it accelerates the racket toward the ball to stretch-shorten the forearm muscles for directional control and spin and to develop racket head speed for pace. Finally, a weight transfer from the front foot to the back creates a final trough in the power wave which opposes the peak of the wave, terminating acceleration and thereby releasing the forces stored in the forearm. There is a delay between the production of power in the legs and the use of that power in the upper body, resulting in a troubling but unavoidable syncopation of the rhythm of the stroke.
Waves
Waves aren't passed from one part to the next. Instead, they propagate or flow through a medium. In tennis the medium is muscle. Bones and joints are not medium; they are a framework on which to hang the muscles and levers on which those muscles act. Muscles can pass the power along, use it to move things or store it for short periods for use after the power wave has passed.
The power wave typically starts as an oscillation in the feet. The bodyweight begins on the front or offsides foot - the foot opposite the hitting arm. As we transfer weight to the on-side foot, the hips reverse-rotate - they turn away from the oncoming ball. This is the backswing proper or lock. After the back foot accumulates bodyweight and is planted firmly onto the court, weight transfer is reversed and travels from the back foot to the front. This weight transfer is not an isolated movement but is part of a constant shifting of weight from one foot to the other to create dynamic balance. During the instant that bodyweight is moving from one foot to the other and is more or less evenly distributed between the feet, the feet push off against the court in opposite directions starting a powerful rotation in the legs and pelvis. There are always two rotations; one back to set the hitting foot and create the backswing proper, followed by one forward, generating the power used to load the muscles of the arm for control and spin and accelerate the racket head for raw, unadulterated pace. Finally, there is another reverse-rotation that puts the brakes on, triggering the explode phase and ultimately putting the brakes on the follow through. This back-then-forward scenario plays out throughout the game of tennis and typifies the wave nature of this and every sport. The crack of a whip is back-forward-back. That oscillation creates the wave that propagates the power of the swing along the whip culminating in a miniature supersonic boom at the tip.As the tennis power wave propagates through the body, power bleeds from the wave, either inadvertently wasted or harvested for creating spin, pace or control. Although a huge amount of energy and momentum pass through the body, the wave does not throw us off our feet. Waves leave the medium through which they propagate largely unchanged, like a cork on a stormy sea. If one is thrown off balance by the wave, it may indicate that one invested it with too much energy. I call this problem overmodulation.
Applying the Wave Concept
Let us take a concrete example to demonstrate the value of the wave concept. Why is the two-handed topspin backhand (2HBH) so consistent? Why is it that you can hit the shot as hard as you want and not wreck its control? Why is pace sometimes lacking? Of course, the answer to the first two questions must be "Because you have two hands on the racket ... duh!" I like a little more meat on my analysis if only because I am more interested in the third question - how do I hit the two-handed backhand with pace? The presence of two hands on the handle of the racket in the 2HBH presents both challenges and opportunities. There is an opportunity to store an enormous amount of control and spin force in the muscles of the two forearms. This surfeit of forearms is the source of the superior control and consistency of the 2HBH compared to all one-handed strokes. The immediate challenge presented by all this stored force is holding it in the muscles of the forearm until it is needed.
Short Stroke Power Wave: Compact and brief, the short stroke version of the power wave shows the absence of a lag phase and a hitch in the upstroke reflecting two sources of power; an early bump from the legs and hips followed by a late contribution from counter-rotation by the non-hitting hand , especially important in the two-handed backhand.
All strokes use the inertia of the racket head, being dragged behind the wrist during the acceleration phase of the stroke, to wind up the forearms,storing those forces and keep them bottled up. With two hands on the racket, we have twice as many "springs" to use for power storage. Keeping those springs stretched requires a power wave of twice the amplitude, meaning we need to maintain twice the acceleration going until we are ready to release the stored forces into the ball. We can't generate a power wave sufficient to create and hold that much acceleration for any length of time. The only solution is to shorten the stroke, eliminate the lag phase and dump the control and spin forces into the ball immediately after we store them. The amount of racket head speed that we can develop is dependent as much on the duration of the acceleration phase as it is on the peak acceleration. In short, the very brief acceleration phase stunts the pace of the shot, no matter how violently you lean into it.
The answer to the "pace problem" of the 2HBH is the additive nature of waves. Two waves that meet at a point in space and are "in sync" with each other, aka in phase, aka peaking at the same moment, will add together and combine forces. The additive quality of waves allows one to use the leverage provided by having both hands on the racket to enhance the power wave using energy from the shoulders and momentum generated by counter-rotation of the wrists. If one can manage to resist the temptation to enlist the forearm muscles in this process, the enhanced power wave will propagate through them without disturbing the precious control and spin forces stored there. In short, you can have it all; superior control, spin and pace. All you need is lots of fast twitch fibers and an exquisite sense of timing.Granted, the truly talented figured all this out a long time ago without resorting to wave theory. They did it by feel, sound, and superior muscle memory. What they could not do is convey to you the important and non-intuitive details such as counter-rotating viscously with your shoulders while keeping your forearm muscles completely relaxed, or synchronizing the shoulder's contribution to the arrival of the main power wave. For that, you have to know what you are up against. Then it is only a matter of summoning up the sources of power available to the two-handed backhand through proper stance, balance, weight transfer, counter-kicking, incorporating the deep shoulder muscles (by reaching back in the unit turn), counter-rotating the wrists and timing all of that so the power of each source reaches the racket at the same, very brief instant. Sounds impossible, I know, but it isn't.
From this source: https://www.tenniswithouttalent.com/WaveTheory.html
At this point the racket flips, loading stretch shortening forces into the forearm which will ultimately be injected into the ball as impulse for spin and control. As the power wave continues to crest, the racket is dragged behind the wrist - accelerating and building racket head speed for pace - the lag phase. Suddenly, at the peak of the wave, the power reverses, and the shoulders start decelerating. The racket head's inertia allows it to catch up with the wrist, releasing the stored forces into the ball. At the moment of contact with the ball, all power neutralized, and there is no force on the racket coming from the shoulders. Eventually, the wave becomes completely negative, keeping the racket from wrapping totally around the player's neck. It is important to remember that the lower body generates the power wave - a conspiracy of the feet, legs, and hips. Although you can add power to the wave using your upper body, that can be a dangerous procedure. In particular, at the moment of contact power delivered through the shoulders should be zero, not positive, lest one misdirect the ball away from its target by injecting impulse in the wrong direction (see Push Syndrome).
Back and Forth
Every shot in tennis starts with the racket moving away from the oncoming ball. I call this move the lock or "backswing proper" to differentiate it from the unit turn - the larger and more ostentatious component of the backswing.
The lock, like every component of every stroke, requires a source of power. That power comes from an initial reverse-rotation of the hips as you transfer bodyweight from the front foot to the back foot.
Pushing Back Against the Push
How could Federer flub his forehand? Just dump an easy, short ball into the center of the net or hit it ten feet behind the baseline? What can we learn from his mistakes? That makes the most natural of all tennis strokes, the forehand, so glitchy, even for the best of the best? What is the universal forehand fail?
The culprit lurks in the second half of the power wave. Just after the power wave peaks, it takes a perilous nose-dive, indicating a sudden reversal of momentum from forward, in the direction of the oncoming ball, to back away from the ball. This reversal happens before the moment of contact. Ponder that for a second. Just before you lay into the ball you have to pull back on the reigns, stomp on the brakes, pull your punch. Could there be anything less natural than that? Does that even make sense?
In fact it makes perfect sense. To take control of the ball's flight path one must store and later deliver muscular forces to the ball. There are alternative ways of delivering spin and pace, but there is only one reliable source of control, and that is impulse. The forces one uses to gain hegemony over the ball are stored during the lock and load phases of the stroke, then held in place by inertia during the acceleration or lag phase. During the lag phase, forces are driving the racket towards the ball, but they are not appropriate control forces. Their direction is determined by the need to address the ball and the need to add spin to the ball. The acceleration forces are not pointing at the tiny window over the net through which one must direct the ball for it to find its target. If you continue to accelerate the racket through the moment of contact, those mighty acceleration forces will overwhelm any stored control or spin forces in the forearm and send your ball God knows where.
Thus, to deliver the stored control forces to the ball at the moment of contact, all acceleration forces must be gone. Imagine that for a second and be amazed. For a few tenths of a second, you are pouring as much power into the racket as you can muster, then 'poof' - power is gone, and the racket glides into the ball under its own steam. The timing of this loss of forward acceleration is pretty critical. A few milliseconds late and you drive through the ball and release the stored control and spin forces in the follow through with no effect on the ball. Generally, this fail results in a long ball. A few milliseconds too early and the stored forces will begin to contort the arm and racket into pretzel position before one makes contact with the ball. The pretzel position is best reserved for the follow through where it can't do any harm to the ball. Generally, when the forces are released early, you drive the ball into the center of the net, mishit it or flag it all together.
Forehand Topspin Power Wave: The feet generate the power wave in the forehand topspin. First, the front foot pushes the weight back to the back foot; powering the reverse-rotation of the backswing proper and loading the back foot. This reverse-rotation creates the initial 'dip' in the power wave. Next, the back foot pushes the weight to the front foot creating the peak of the power wave and driving the load and lag phases. Finally, the front foot pushes back neutralizing the forward acceleration, causing the power wave to plummet and stopping the shoulders so the arm, racket and stored control and spin forces can explode into the ball. This vital last weight transfer is frequently, fatally forgotten by pros and duffers alike.
The Power Wave Shuffle
The video above is a demonstration of how foot-to-foot weight transfers allow you to harvest directional momentum from the earth to feed the power wave for the topspin forehand. Remember that each weight transfers occurs hundreds of milliseconds before the portion of the wave that it creates reaches the racket. Waves must travel from the source to the sink, and that takes time. This delay between action and result messes with our heads as players. It syncopates the rhythms of the stroke and has made it hard in the past to understand the relationship, say, between "stepping into the ball" and stopping the forward swing. I always thought that stepping in somehow added power to the shot just by the translation velocity of my body. If you take the 3 miles per hour that I can move and add it directly to the pace of the ball, well, obviously that is not a valid reason for "stepping-in". Stepping-In has two functions; stepping-off of the back foot, which starts the upstroke of the power wave and motivates the forward portion of the swing, and stepping on to the front foot, which secures the front foot for pushing back against and reversing the direction of the power wave. So, just like a bullwhip, the footwork of the forehand groundstroke is back - forward - back because the stroke is back (backswing proper) - forward (load and lag) - back (explode). If you fail on the last portion, say because you don't plant your front foot sufficiently to push back against the stroke, you lose all of your control and most of your spin. You still have plenty of pace, though, to carry the ball into the back fence.
From the preceding discussion we can derive a fundamental rewrite of the old "Step into the ball!" tip. It is; "Step early into the ball!" The 'early' part is vital because stepping-off (of the back foot) has to finish well before the load phase begins, and stepping-on (the front foot) must finish well before the moment of contact. Only by completing both of these steps before the moment of contact can you use your front foot as an anchor to reverse-rotate your hips against the forward rotational inertia of the body. This action puts the brakes on the stroke and releases the stored forces.
Step into the ball early!
So what do you do if you can't get your front foot down? How do you stop the forward rotation of the shoulders? The pros instinctively counter-kick out forward with the front foot. That can provide just enough momentum to quell forward acceleration and allow control forces to be released.
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
Wave Theory and Tennis
The classic understanding of the tennis stroke is as a mechanical "rods-and-levers" contrivance vis: the legs turn the hips - the hips turn the shoulders - the shoulder turns the arm - the arm does some stuff to address the ball and get the racket head going in the right direction and presto! The ball is sent along to its target with unerring accuracy, irresistible power and screaming spin. That is the fantasy we carry around in our noggins, reinforced with terms such as the "kinetic chain" from the literature of tennis that reinforce such delusions. For some odd reason, this idea never seems to work out for us. Every time we adjust one cog in our corporal clockwork, it seems to throw our game into chaos. As an example, one might bend the elbow a bit to gain control over the position of the wrist to better guide the strings to the ball and avoid miss-hits. That makes sense - more control is good, right? The result is more mis-hits, a complete loss of pace, control, and spin, and a sore elbow. Is this beginning to sound familiar?. There are way too many objectives in tennis - adding spin, addressing the ball, creating pace and control, etc. to do it all with a series of tightly linked motions. Each of these objectives must be achieved by a system that is separate and distinct from the others in both time and space. Thus the muscles of the forearm are largely responsible for control while pace comes from the larger muscles of the shoulders, trunk, and legs. If you start tensing your forearm to get more pace, you lose control. If you fail to establish balance with your quadriceps muscles, your brain will recruit the muscles in your upper body and wreck the rest of your stroke. Each body unit in a stroke has its own set of responsibilities and is on its own schedule, and that schedule is not "first A completes then B starts, then B completes, then C starts ... " We achieve balance in the serve as the left hand is tossing the ball and the right arm is relaxed in preparation for the hitting sequence. They are independent of one another, and each has its own start and finish time. If each component has to wait for the other to get the energy and momentum it needs to complete its task then what you have is a house of cards, not a stroke.
Each of the functional units of a stroke requires power to operate. Power is the ability to make things move, stop moving or change direction. It is a combination of energy and momentum. Energy for tennis stroking comes mostly from the muscles and a bit from gravity. Energy is necessary to influence the flight of a tennis ball, but it is also insufficient.
Power
One must also harvest momentum, mostly from the earth. If you have ever tried to hit a tennis ball while floating, weightless, in space ... sorry, bad example. You probably have not tried that yet. I have suffered from the delusion that I could stand flat-footed with no solid purchase on the court and hit a tennis ball 100mph with just the power of my muscles and my indomitable will.
Footwork is the primary source of momentum. Setting the back foot on a groundstroke, keeping the knees bent and the bodyweight bouncing from one foot to the next, orienting the feet so that the momentum generated is in a proper direction for the stroke are all essential for the production of power. Power must be created and distributed to each part of the body as that part needs it. Power is the raw material for pace, spin, and control. After all, what is control but the ability to direct a ball to the desired target; to change its momentum? Each component needs the power to reach it when the component need its and in the form that the component can use it. A mechanical linkage will not suffice. The power must be packaged and delivered in the form of a wave.
The Power Wave of the Topspin Forehand: Begins with a push-off against the earth and weight transfer from the front foot to the back foot (gold) during the unit turn which powers the lock (backswing proper). It propagates through the muscles as the first trough of the wave, rippling up the trunk, through the chest and shoulders and down the arm to the hitting hand where it is transferred to the racket, throwing the racket back away from the ball and helping set the tone of the forearm. Next one pushes off the back foot throwing the bodyweight to the front foot (green). This shift of weight produces the main peak of the power wave, and it accelerates the racket toward the ball to stretch-shorten the forearm muscles for directional control and spin and to develop racket head speed for pace. Finally, a weight transfer from the front foot to the back creates a final trough in the power wave which opposes the peak of the wave, terminating acceleration and thereby releasing the forces stored in the forearm. There is a delay between the production of power in the legs and the use of that power in the upper body, resulting in a troubling but unavoidable syncopation of the rhythm of the stroke.
Waves
Waves aren't passed from one part to the next. Instead, they propagate or flow through a medium. In tennis the medium is muscle. Bones and joints are not medium; they are a framework on which to hang the muscles and levers on which those muscles act. Muscles can pass the power along, use it to move things or store it for short periods for use after the power wave has passed.
The power wave typically starts as an oscillation in the feet. The bodyweight begins on the front or offsides foot - the foot opposite the hitting arm. As we transfer weight to the on-side foot, the hips reverse-rotate - they turn away from the oncoming ball. This is the backswing proper or lock. After the back foot accumulates bodyweight and is planted firmly onto the court, weight transfer is reversed and travels from the back foot to the front. This weight transfer is not an isolated movement but is part of a constant shifting of weight from one foot to the other to create dynamic balance. During the instant that bodyweight is moving from one foot to the other and is more or less evenly distributed between the feet, the feet push off against the court in opposite directions starting a powerful rotation in the legs and pelvis. There are always two rotations; one back to set the hitting foot and create the backswing proper, followed by one forward, generating the power used to load the muscles of the arm for control and spin and accelerate the racket head for raw, unadulterated pace. Finally, there is another reverse-rotation that puts the brakes on, triggering the explode phase and ultimately putting the brakes on the follow through. This back-then-forward scenario plays out throughout the game of tennis and typifies the wave nature of this and every sport. The crack of a whip is back-forward-back. That oscillation creates the wave that propagates the power of the swing along the whip culminating in a miniature supersonic boom at the tip.
As the tennis power wave propagates through the body, power bleeds from the wave, either inadvertently wasted or harvested for creating spin, pace or control. Although a huge amount of energy and momentum pass through the body, the wave does not throw us off our feet. Waves leave the medium through which they propagate largely unchanged, like a cork on a stormy sea. If one is thrown off balance by the wave, it may indicate that one invested it with too much energy. I call this problem overmodulation.
Applying the Wave Concept
Let us take a concrete example to demonstrate the value of the wave concept. Why is the two-handed topspin backhand (2HBH) so consistent? Why is it that you can hit the shot as hard as you want and not wreck its control? Why is pace sometimes lacking? Of course, the answer to the first two questions must be "Because you have two hands on the racket ... duh!" I like a little more meat on my analysis if only because I am more interested in the third question - how do I hit the two-handed backhand with pace? The presence of two hands on the handle of the racket in the 2HBH presents both challenges and opportunities. There is an opportunity to store an enormous amount of control and spin force in the muscles of the two forearms. This surfeit of forearms is the source of the superior control and consistency of the 2HBH compared to all one-handed strokes. The immediate challenge presented by all this stored force is holding it in the muscles of the forearm until it is needed.
All strokes use the inertia of the racket head, being dragged behind the wrist during the acceleration phase of the stroke, to wind up the forearms,storing those forces and keep them bottled up. With two hands on the racket, we have twice as many "springs" to use for power storage. Keeping those springs stretched requires a power wave of twice the amplitude, meaning we need to maintain twice the acceleration going until we are ready to release the stored forces into the ball. We can't generate a power wave sufficient to create and hold that much acceleration for any length of time. The only solution is to shorten the stroke, eliminate the lag phase and dump the control and spin forces into the ball immediately after we store them. The amount of racket head speed that we can develop is dependent as much on the duration of the acceleration phase as it is on the peak acceleration. In short, the very brief acceleration phase stunts the pace of the shot, no matter how violently you lean into it.
The answer to the "pace problem" of the 2HBH is the additive nature of waves. Two waves that meet at a point in space and are "in sync" with each other, aka in phase, aka peaking at the same moment, will add together and combine forces. The additive quality of waves allows one to use the leverage provided by having both hands on the racket to enhance the power wave using energy from the shoulders and momentum generated by counter-rotation of the wrists. If one can manage to resist the temptation to enlist the forearm muscles in this process, the enhanced power wave will propagate through them without disturbing the precious control and spin forces stored there. In short, you can have it all; superior control, spin and pace. All you need is lots of fast twitch fibers and an exquisite sense of timing.
Granted, the truly talented figured all this out a long time ago without resorting to wave theory. They did it by feel, sound, and superior muscle memory. What they could not do is convey to you the important and non-intuitive details such as counter-rotating viscously with your shoulders while keeping your forearm muscles completely relaxed, or synchronizing the shoulder's contribution to the arrival of the main power wave. For that, you have to know what you are up against. Then it is only a matter of summoning up the sources of power available to the two-handed backhand through proper stance, balance, weight transfer, counter-kicking, incorporating the deep shoulder muscles (by reaching back in the unit turn), counter-rotating the wrists and timing all of that so the power of each source reaches the racket at the same, very brief instant. Sounds impossible, I know, but it isn't.
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