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Comment by jonnypotty | Hacker News Reader

World record tennis serve is 144 miles an hour and a human can't really move across a court and return a ball moving at this speed. If they're lucky they can reach it and react in time to hit it. I'm a bit confused by an article that claims tennis players can react to and return serves up to 160 miles an hour. I think evidence suggests that returning balls anywhere near this fast is dependent on analysing factors before the ball starts moving, the other players body position, racket position etc. Players have an intuition about where the ball is going to go without having to look at and analyse the flight of the ball.

Just did some very basic checking. Tennis court 23m 260mph = 72m/s Ball takes approx. 0.3ms to travel length of court. human reaction time to visual simulous 0.25ms So the idea is they move and hit the ball in the remaining 0.05ms? Hmmmmm.

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Serves in tennis don't go directly down the line, they go cross court. Returning players will often be standing behind the baseline. Additionally balls start ~2.5-3m above the ground, bounce and then come up again. The total distance traveled is probably closer to ~27m.

The air resistance slowing the ball down is significant - combined with the energy the ball loses bouncing, the ball has lost more than half of its initial velocity by the time it gets to the returning player.

I found these speed guns stats on a tennis forum for the ball speed at different points:

Speed after being hit: 126mph

Speed before hitting court: 89mph

Speed after hitting court: 67mph

Speed at returner's baseline: 58mph

Even after doing the calculations correctly, there still isn't a lot of time for reactions, but it is more plausible than your initial analysis suggests. (Your units also seem off - should be seconds not milli-seconds).

Thanks for this. Much better than what I did although I don't think 58 is a third of 126, more like a half.

'Players have an intuition about where the ball is going to go without having to look at and analyse the flight of the ball.'

This is pretty easily observable with baseball players as well. After playing thousands of games while standing in the same (relative) place on the field I/they can anticipate where the ball is going to go based on a variety of variables in real-time... instantly.

50% of the time it works 100% of the time

Almost all of the "amazing" things the brain does are basically continuously refined predictive branch execution.

Which is why practice is important. You're essentially strengthening certain neural pathways with continuous exposure to certain inputs.

But this strength also makes us susceptible to misdirection and slight of hand.

In the book “Thinking fast and Slow” the Baseball example is given for a learned intuition. The ball is too fast for batters to react so they learn to anticipate the trajectory of the ball from the way the pitcher throws. When they let professionals players play against a female softball player with lower throwing speed their intuition was off and they missed the ball more often than against a male professional player.

There was a festival of jugglers in my city and they taught me to juggle in like 15 minutes, I was amazed it's so easy (the basic 3-ball juggling, and just for a monute or two, the more difficult juggling is HARD and I had to train later to be able to keep juggling forever).

There is a very easy trick - you look forward in the distance keeping the balls in peripheral vision and there's 2 automatic reactions you have to develop:

1. when the ball going up is at the top of the curve - throw another ball up

2. when a falling ball goes out of your peripheral vision - do the "oh shit something's falling let's catch it" routine with the hand that has less balls in it.

Hands learn very quickly how to move to catch the balls that leave the peripheral vision "by itself" basing on the trajectory you've seen.

It's actually harder to juggle when you look at the balls directly, and it's impossible when you think about it and try to do the moves consciously because you're too slow.

It was mindblowing to me that it's easier to catch a ball when you don't look at it.

You're right that it's not possible to react if all the information you have is the trajectory of the ball after it leaves the racquet. Good players will subconciously be predetermining the path of the ball by looking at how the opponent is striking the ball.

It doesn't specify who serves it at that speed either, could be some kind of "serving machine".

I guess so. But I'd say humans would have no chance at this speed without another player to analyse visually.

The article’s articulation of what goes into returning a serve is a bit simplistic, but the underlying idea is not crazy.

* When you return a serve in tennis, you are doing so from only one side of the court. The opponent’s serve can only land in a service box that provides 13 feet of lateral space.

* Practically, there are relatively few spots in the service box that can be reached by a serve. Because of human physiology (the length of our arms, joints in the arms etc.), it would be extremely painful to try to hit a fast serve to certain parts of the service box. Either that, or the server would have to stand in atypical positions on the service line (i.e. not at the center tick) that would be a dead giveaway of where the server was trying to hit to.

* So, in simplistic terms, most tennis players are choosing between more-or-less staying in place (to return a body serve), or leaping to their left or right. The serve must bounce before you hit it, and it will be bouncing “towards you” vertically. The returner thus is very rarely going to move vertically. This usually only happens when you are moving in to pummel a slow and short serve.

* At the highest levels of tennis, the vast majority (60%+) of serves are going out wide, or down the middle (https://www.atptour.com/en/news/berrettini-infosys-serve-loc...). Mind you, these are also the same player who would have the physical conditioning and athleticism to actually be able to hit these blazing fast serves.

* Additional information for the returner is conveyed by the serve toss. Almost all players are giving away tells here. For example, if I’m a right hander serving from the deuce court, and I toss my ball to the left (the “11 o’clock position”), it’s high unlikely that I’m hitting the ball down the middle. Doing so would require one of those aforementioned contortions in my arms and legs, and I would then be unlikely to generate the power needed to strike the ball in a way that leads to a super fast serve.

* So in reality, by the time that the server is making contact between their racket and the ball, the returner will have a general idea of the direction that the ball is going in.

* The article does conflate getting your racquet on the ball, and making a successful return. Just as with any other tennis shot, there is not guarantee that your return does not go into the net, or go flying out. I think it’s a far more plausible claim that professional tennis players can get their ball on the racquet vs claiming that they can cleanly/successfully return these super fast serves.

Some other thoughts:

* Placement is just as important as speed in determining how returnable a serve is.

* For example, there are plenty of examples of top tennis players returning extremely fast serves. Federer against Isner (140 mph): https://youtu.be/5gcvLbtaNxM, Murray against Raonic (147 mph): https://youtu.be/8GYX4ZIPJsg

* The commonality between these successful returns is that the serves themselves were fast, but poorly placed. By serving right down the middle, the servers allowed Federer and Murray to take one small step, and then make good contact with the serves for an “easy return”.

* One small quibble with the “world record tennis serve” you cite. It’s not 144 mph, but rather 157.2 mph (hit by John Isner). If anything though, this is helps your argument.

* The unofficial record is 160+ MPH (hit by Sam Groth), but this was at a second tier tournament with a questionable radar gun (https://youtu.be/uKeL-W7xft0). Notice how even with this serve, the returner correctly guesses where the serve is headed, and even looks to have gotten a racquet on it.

* It’s a bit of a chicken and an egg problem as well. There is a very tiny sliver of people in the world who are physically fit enough and who possess the natural physical traits (like height and broad shoulders) necessary to hit serves in the 140+ MPH range. These people are likely playing on the ATP against the players in the world best equipped (mentally and physically) to return their serves.

* So all this is to say, returning serves in that 140-160 MPH range is a low probability proposition. Heck, a perfectly placed and well disguised serve even in the 110 MPH range can be unreturnable (as seen in two decades of Federer highlights). But, humans are indeed “capable” of returning serves in that speed range.

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