Tennis, like golf, is a highly technical sport. The differences between taking a racquet back like this or like that have career-defining consequences. A player can be oozing with athleticism, good sense, and work ethic, but with the wrong technique her level has been capped at the start.
This is why it's difficult to teach yourself tennis well. Correct technique is surprisingly counter-intuitive and difficult to pick up, even from videos. For instance, beginners are often incredulous when they learn what the correct serve grip is. It's tough to know you should be swinging harder on your (more conservative) second serve. Even if you do figure these things out and more, you need a second pair of eyes making sure you implement the changes.
This does not mean there is a "one true way" to hit a tennis ball. Even at the highest levels, technique varies wildly. There are one-handed backhands, two handed backhands; eastern and western forehands; abbreviated and elongated service motions; masters of sublimity like Federer and daddy long-legs like Medvedev. Still, the best players share the same fundamentals.
Those fundamentals, according to Hugh Clarke, are degrading. Younger professionals have worse technique than their peers. Youth is on their side, but without technically sound strokes, they struggle with consistency. Nikolay Davydenko, former world number 3, said this recently:
In my opinion, tennis is not making much progress. The players who are at the top now – not Nadal and Djokovic, but the younger generation –are not that good technically. I got surprised by that. It’s more physical – big serves, hitting hard–, but we still see that Nadal and Djokovic can control all this power over the new generation. They are still winning Slams and beating guys who are ten years younger than them, which is amazing. Anyway, I do not feel that the new generation is playing on an unbelievable level.
According to Clarke, technical degradation especially pronounced on the forehand wing. This is important because forehands are usually the foundation of a player's game. Since forehands (usually) require only one hand on the racquet instead of two, players can get more extension and racquet-head speed on the stroke. In other words, they can be hit harder and with more control. Forehands are often a player's "weapon" or stroke they use to win points. Federer was so good because his forehand a whip. Opponents sweat when the ball goes to Rafa's forehand because he can crush it in any direction.
Forehands are also exploitable. Nearly every rally features one. If an opponent senses yours is weak, she can almost guarantee 90% of your shots will be forehands. Weak volleys and overheads can be mostly hidden. However, you cannot run from a bad forehand. Even decent players will make you pay.
What's so bad about the young-ins' forehands? Clarke does a fantastic job explaining why in his Death of a Forehand series. Those interested should read it for a detailed technical explanation of forehand degradation. I'll recapitulate Clarke's points in simpler terms here.
The margins in tennis are thin. On a service return, a pro is confronted with an 120mph+ little yellow ball hurtling towards them. Their job is to redirect that ball to a small area 72 feet away with spin and pace. To do this, they have a racquet with as little as 97 square inches of string surface area to work with. Realistically, only the central 50 inches are usable. Contact the ball with the other 47 square inches string and control suffers dramatically.
When things are this complex, you want a simple stroke. Wasted movements waste time. If you need to bring your racquet over your head, do a little twist, flare your pinky, and flex your wrist before a forehand, the ball may be gone before you hit it. It's best to take your racquet back simply. No bells and whistles.
To see what I mean, watch Juan Martin Del Potro's forehand.
It's simple. Del Potro brings the racquet up and back in a little loop and then crushes the ball. By pointing the racquet straight up, he can use gravity to bring the racquet down and pick up speed rather than use his muscles. Look how solid his wrist it. It doesn't move a muscle. There is very little excess movement in the stroke.
Now, look at Jack Sock's forehand.
First he brings that racquet to his head. Then he rotates it, so the tip of it is pointed towards the oncoming ball. He lowers the racquet from his head, but brings it back with the racquet parallel to the ground and the side of the stringbed he won't use facing the ball. When he's actually swinging to hit the ball, the racquet must rotate mid-swing to get the strings in the correct position.
Most younger players' forehands aren't as egregious as Sock's, but have similar elements. They might point the racquet tip towards the opponent, or flex their wrists on the takeback so the opposite side of the stringbed is facing the ball. Even something as simple as taking the racquet back parallel to the ground rather than pointing towards the sky is a technical trade off. Call this forehand style, with the flexed wrist and racquet facing parallel to the ground, the "nextgen" technique.
Nextgen forehands generate more power and spin. However, they come at the expense of consistency and control. A stroke with a lot of wrist movement is an unreliable one. Players can get away with it, but it hurts them in the long run.
This is concerning for younger players. Consistency and control are what separate the merely good from great. Many adult males can hit a tennis ball as hard as the pros. What makes tennis difficult is hitting the ball into the same spot, under pressure, for hours. Djokovic does not have the most firepower on tour. He wins in large part due to his command over where the ball goes. Even when he's on defense, Djokovic is able to put it exactly where he wants it. Other players can't do the same. Often, they look like they're struggling against their opponent's pace or to control their own power.
Nikolay Davydenko explicitly links Djokovic’s talent to his control.
We were in Rome and [Djokovic] was my partner for the warmup before the match. I was surprised with the way he controlled the ball – I was hitting really good balls and each one came back. He moved really well too.
He was 16-17 years old at the time, he was working with Riccardo Piatti, and he told me: ‘Novak will be the next top player’. And I answered: ‘It seems so’. In two years, he was already in the top 10. Amazing – boom. And he is still there.
Tennis technology does not explain technique degradation. Polyester strings were the latest technological advancement. They were introduced in the 90s, but Federer turned pro at 17 in 1998. If polyester strings were responsible for wristy forehands, Federer's generation would have them. Instead, Federer and his ilk are among the most technically sound.
Clarke claims degradation is instead due to the introduction of smaller and lighter kid's racquets and depressurized tennis balls. By shrinking the game to kid size, adolescents get a softer introduction to tennis. Rather than deal with heavy racquets and high-bouncing balls, kids can learn an easier version of the game graduate to full-sized gear and courts as they grow. The kid-sized equipment is supposed to aid development and prevent juniors from becoming discouraged while they learn.
While making the game easier, lighter racquets and depressurized balls negatively affect development. Skills developed in "mini tennis" aren't transferable to the real game. In the same way practicing ping-pong is poor training for badminton, learning to hit foam balls is inapplicable to tennis.
To try and corroborate Clarke's argument, ideally I would go back in time and weigh pros' childhood racquets and ask their parents and coaches if they used depressurized tennis balls. Djokovic should have been using "real" tennis balls and racquets at the start. Other players like Shapovalov and Kygrios should be using light racquets and/or the depressurized balls.
I can't time travel. The best I can do is look at youtube videos of pros practicing when when they were young and try to infer how heavy their racquet is. However, the way I'd do that is to look at how good their technique is. But this doesn't work. We're trying to see whether light racquets cause a certain kind of technique. If I try to infer a light racquet from poor technique, I'm assuming the relationship I'm looking for evidence of.
If we squint hard enough, perhaps we can see Djokovic struggling with a heavy racquet at age 6, and Shapovalov whipping around a light racquet at age 8, though I don’t trust my judgements here. Snippets of young Federer also don’t provide evidence of his racquet weight.
In light of this, the strongest point in Clarke's favor is this table. It records the weights of racquets used by players born before and after 1990.
Players born before 1990, at least in adulthood, use heavier racquets. Carlos Moya's racquet's swingweight (SW) was 408. The thing was basically a brick. Kyrgios' racquet has a swingweight of 323. This is surprising. For context, I'm barely 5'6 and my current racquet's swingweight is a little over 330. Kyrgios is 6'4, 187 lbs, and can definitely handle a heavier racquet. The light racquet explains how he can get away with a whippy forehand, as seen below.
By contrast, look at Moya's forehand. The racquet-head is pointed straight up, it drops, and he swings through the ball. The tip of the racquet doesn't start facing the opponent. The side of the racquet that hits the ball isn't facing straight down. It's a simple stroke that takes advantage of gravity to generate racquet-head speed and power.
I find Clarke's views on technique compelling. I used to play with a 295g Head Radical MP. I also had a "nextgen" style forehand where I took the racquet back with the wrist flexed and the strings facing the ground. Recently, I purchased a 330g Yonex VCore Pro 97 and put tungsten tape on the frame to make it even heavier. The difference is striking. I physically cannot use my old forehand technique with the heavier racquet. Taking the racquet back nextgen style risks injuring my wrist and my forearm. The only way to safely hit a forehand is to imitate Moya and begin with the tip facing straight up, let gravity do its work to bring the head down, and swing through. If I made solid contact, it was divine.
Solid contact was still far from guaranteed. A heavier racquet made everything, not just the forehand, more difficult. 30+ extra grams slows how quickly you can move the racquet into position. When you're playing a game of millimeters and and milliseconds, this makes a difference. I had to concentrate much harder hitting volleys, backhands, and slices than I would have otherwise.
The concentration took the form of better footwork and technique. Weird wristy 'feel shots' were off the table for me. As an example, sometimes when you're returning a fast groundstroke and getting into position would be tough, you'll keep a continental grip and just wrist the ball back with a bit of topspin. If you didn't understand the previous sentence, just know this is a technically offensive way to hit a tennis ball. With a heavier these shots are extremely difficult. I just couldn't get the racquet in position punctually without my engaging my entire body. To me, it's clear a heavier racquet introduces "desirable difficulties" that benefit the rest of your game.
Despite being convinced heavier racquets are better, technically and otherwise, I'm still puzzled. If they're so good, why did many of the best players of a generation use lighter racquets? Fame and fortune are at stake in professional tennis. If heavier racquets are superior, you'd think everyone would recognize this and adopt them. To go a step further, nobody even needs to realize heavy racquets make better players. The juniors who happened to use them should literally outcompete the others and come to dominate the sport. For some reason, this doesn't happen.
My pet explanation is light racquets are still pretty good. Heavier racquets give you a stable, consistent forehand, but light racquets give you power. As mentioned, Nick Kyrgios has one of the lightest racquets on tour. He has has one of its deadliest forehands. Players like Shapovalov, Alcaraz, and Khachonav also have light racquets, nextgen technique, and big forehands. Trading control for power can make sense in today's game, especially when courts are slow. Players have more time to get through a complex stroke.
Technique is also difficult to change. How a player hits the ball is burned into their nervous system via thousands, if not millions, of repetitions. Much of what players know about their technique is, then, subconscious. As Hugh Clarke has pointed out, Roger Federer doesn't know what his forehand grip is called and Nalbandian can barely explain how his own forehand works.. Dictates like "take the racquet back like X instead of Y" are useless when the player didn't know she takes it back like Y, or has a dim understanding of what X means. [1]
There are conscious factors inhibiting technique change as well. Things get worse before they get better. Your body will move in unnatural ways, and you'll have to think more about what you're doing than before. Both of these decrease performance. Even though you know technique change is the right thing to do, it's still psychologically difficult to commit to it for long periods of time. Professionals who have the most at stake may struggle with this more. If your pay is based on performance, it takes courage to jeopardize that in the short term for uncertain gains.
Footnotes
[1] Technique change is still possible, even for pros. Clarke mentions how Thiem was able to shorten his stroke to be competitive on hard courts.