Discussion in 'Tennis Tips/Instruction' started by Greg G, Jul 1, 2012.
Those look great!
There is always a lot of interest in measuring the serve speed. It is probably the easiest and most accurate velocity to get because you can place the camera so that the ball's trajectory is pretty parallel to the plane of the sensor. Viewing perpendicular to the trajectory gets that. If the alignment is off by a few degrees (<10°) it probably does not cause much error in serve speed.
Did you use your racket length as a scale? Did you use all Kinovea tracking or do some manually? When I tried tracking it worked very well until an object approached a white line on the court. It would break track and I'd have to help the tracking get going again after the line. It tracked 3 or 4 things at once - ball, racket, black tape marker on my arm, etc..
I've always thought that this was one of the better displays of the serve. I believe that there are timing ticks along the path to indicate racket position for each frame, but I'm not sure. If they are timing ticks maybe they were placed in manually.
Have you seen the displays with color coded lines - I guess manually applied - that show racket head position, for example, before and after impact?
Thanks! Kinovea was pretty decent at tracking until you come to some sort of junction where there's a change of background- a white line, my body, the ballboy, etc. So I had to manually edit the track, maybe about 50%, especially the tip of the racquet, where there wasn't much of a marker to track. The wristband was easy for the software to track, same with the ball except after impact, where I had to help it out, probably because of the sudden big change in ball position.
Now to analyze what it means...
The good thing I see is that the racquet head movement is constant (no hitch), with good acceleration until contact. The difference between hand speed was also interesting to note...a factor of 2x at PTD, all the way to ~4.5x at contact. Whether or not that is ideal, I have no idea. Maybe I'll try measuring some pro footage to compare.
Could I get more RHS? Probably...the trick is figuring out where I'm losing it.
Can we measure the amount of energy put into the ball? I'll leave that to the physics professors... :shock:
Here, data overload!
This ping pong ball marker worked well against a dark tree background. Still, the white line on the court made it lose track. Tie on a piece of bright yellow police tape.
Yeah, I might set aside one racquet and set it up for "crazy time"
OK here's Dimitrov. I guessed on the frame rate, but it looks correct, ~90mph forehand. Not sure of the absolute values, just note the acceleration.
Interesting the swing path on the racquet head! Mine is more circular because of the little WTAish wrist roll...
Sure we can. Kinetic energy equals one half of the mass going into the shot times the racket head speed squared. K=(1/2)m*v^2. Unfortunately, some of that energy will be lost due to the inherent inelasticity of the collision. In order to calculate the coefficient of elasticity, we must use the formula e=(v2'-v1')/(v1-v2). The ' symbol designates outgoing velocity whereas the vs without the ' refer to the original velocities of the ball (v1) and racket head (v2) respectively. The key to the whole thing is that momentum is always conserved (m1v1+m2v2=m1v1'+m2v2'), so knowing the incoming and outgoing masses and velocities, you can figure out how much energy was lost in the collision.
Unfortunately, I did a really bad job of explaining the whole thing, but yes, it in fact is possible and of no help whatsoever to your tennis game. :shock:
Overlaying the 2 swing paths to see the difference, particularly in the arm position at the backswing. (hey my cap matches the outfit LOL)
This looks like the money shot for my particular issues. It reminds me of a serve where I don't get into the correct trophy pose. Which begs the question: is it right to think of the forehand like a serve (in a way)?
Hips are also more open. Cheetah, chime in anytime here
That really makes things stand out that otherwise would be missed. Didn't you have some 'pat the dog' issues? Compare that part of his stroke to yours. Your path is a circular path and Dimitrov's has a delay, with no forward motion, and then straighter forward. That part of Federer's forehand, in particular, shows a very distinct stretching of muscles by the body turn, relaxed, not guided. Maybe an upper arm downward acceleration stretches the arm also?
Take care to follow sound techniques with that forward high acceleration part of the stroke. I read that golfer's elbow affects elite players more often than amateur players. ? Forehand, serve and overhead being the most likely strokes.
To separate players see how the display looks with - all your lines blue, all his red, etc. Different weights of the same color for wrist & racket. Ball impact is always an informative frame to show. Find videos with small motion blur so that the exact angle of the racket, when viewed top-on, can be shown at impact.
Yup, that darn PTD....it's come a long way though...sometimes I go back to the first and second videos on page one for a shock. Good motivation to keep at it
OK today's session's goal...address the PTD yet again! I just did a straight back takeback at the start, to eliminate the chance of the 'presupination'/wrist roll.
So did I get it? I think there is some improvement, I did get a bit more pop and spin, with less effort. The kinetic chain isn't all there though.
Ended the session with alternating forehands and backhands. The forehand is inconsistent because I don't have my balance, compared to the backhand. Really really need to fix that.
You appear to have an aversion to lowering your stance.
It's okay, I can't tell you how many times I've seen even pros hitting a shot with a stance like they're standing at a bar, ordering a drink.
Try getting off your a**, looking as though you're riding one, and get up on a horse, in a saddle-position.
Another way to think about it is; look like you're going to hit something.
Most of the time, you're hitting your strokes looking like you're ordering a drink and riding an a**.
Lower your stance, move into the ball, hit it early as possible, and you'll ride that stallion of good form to great strokes.
RHS speed analysis:
I seem to be picking up RHS much closer to contact and with more acceleration, which suggests a better utilization of the stretch-shortening cycle, compared to the last RHS analysis I did.
The path of the racquet head in the backswing is less curved now, and has a bit of an elbow. The forearm is supinated, but only after the forward move. An improvement.
Here I think the upper arm is a bit decoupled from the shoulder (should be more forward/less lagged).
Forget the swing, listen to Tennissean.
Bend ze knees, keep the torso upright.
Overlay showing the result of a better PTD:
I'm on it Lee! Just having fun playing with Kinovea
The Dimitrov forehand video that you used was the subject of a thread last week because he might have a different body & arm turn than most other pros.
"Torso rotation does NOT precede the swing."
Yours has a more downward and then upward pattern than Dimitrov's. His is more back, sort of stop and then more directly forward. I don't know how the ball heights compare or, for now, other forehand details. Is Dimitrov a good model for the forehand?
I like the discussion in the Elliott, Reid, Crespo book, Technique Development in Tennis Stroke Production(2009). Purchase only from the ITF I believe, $20.
Elliott discusses the difference between the angle that the hips turn back and the angle that the upper body/shoulders turn back. A little extra for the shoulders - body twist - is used for the SSC. Dojkovic often uses more of this hip-shoulder angle difference that others, in my opinion. Stop action on Dojkovic on TV. He also has had a few back tweeks in the past, maybe 2 years ago, cause unknown. Maybe we should be very careful with adding body twisting.
You need a model. Are you looking at the Tennisoxygen stroke 'comparisons', Youtubes, on the forehand? Some videos are very detailed on each phase of the forehand. One of these videos, for example -
Or FYB or Tennisplayer.net?
The Macci video - "Secrets of the Forehand" ?
I hope that you will post some of your stroke tracks and overlay comparisons on Kinovea, it's a very informative way to display the strokes.
I'd like to see some kinovea analysis on this swing:
Sure, but...is there a point you're trying to drive home? I might be missing it....
No, don't do any analysis. Just thought you'd like to see a different kind of.. strange, swing.
Looks painful! I might do Gulbis for kicks
Or this guy.
His name is Marsel Ihan.
That Jack Sock forehand looks as if it could be stressful to the area injured in Golfer's Elbow. Probably not for Jack, but someone experimenting with a new stroke should be especially careful. ??
I read that golfer's elbow is an injury that affects elite level tennis players at a relatively higher rate than rec players and that it is believed to be associated with forehands, serves and overheads. Anyone with information on this issue?
OK added Tommy Haas to the mix!
The heights of the balls are different. Dimitrov's ball is the highest and Haas's is the lowest. Haas's stroke looks closer to yours. You should account somehow for ball height in comparing the stroke tracks.
Kinovea also has side-by-side video comparisons, between you and a pro. Step through the strokes together frame by frame, etc. . You might use that for quicker comparisons to more pro strokes, selecting model forehands, etc..
Yes Chas, I forgot to mention that Haas was hitting a low ball and skipped off the ground a bit, explaining the steepness of the path after contact. Plus a bug in Kinovea nuked the overlaid videos I made :evil:
That's right! (not that an obsessive-compulsive like you would have a clue)
You're talking to LeeD right? Cos I don't think I'd classify him as O-C
Some Information on Golfer's Elbow & Tennis
Roddick forehand video, looks stressful -
Reference for Golfer's Elbow -
Biomechanics of the elbow joint in tennis players and relation to pathology.
"Flexor-pronator tendinosis or rupture
Unlike to the common “tennis elbow”, or lateral epicondylitis, this tendinosis is more common in high-level tennis players than it is in recreational players. The pronator teres and flexor carpi radialis have been identified as the most common sites of pathologic changes.17 18 Athletes complain about tenderness distal and lateral to the medial epicondyle; resisted wrist flexion and forearm pronation exacerbate pain.
Treatment is in general a non-operative program for at least 6 months; persistent symptoms after 6 months can be an indication for surgical treatment after exclusion of any other pathologic causes, especially UCL insufficiency. Medial epicondylitis represents an “absolute overload” of normal anatomy and physiology due to supra normal forces; possible related factors are an excessive wrist snap, “open stance hitting”, opening too soon on serve and short arming of the strokes.5 More research needs to be performed to clarify the relation of biomechanics in tennis and flexor-pronator tendinosis."
That has got to be one of the ugliest pro fhs I've ever seen. I hope it's effective, because it's never going to get any style points.
Trying out an overhead camera position:
Looks good for evaluating weight transfer/rotation/footwork....all of which need work
Some set play footage, to evaluate footwork. Been playing too much doubles lately, getting rusty.
Live ball hitting. Swing feels nice and loose, contact out in front, need to watch the ball better. I think it's time to switch focus from swing path to footwork...more gains to be had there.
nice swing path there. much better than the old scandinavian ballroom dance move days you used to put on display haha.
I'd say a little less arm usage. some of them a touch too army. some looked pretty sweet tho.
But yea... time for footwork now. base is still shaky.
I'm almost ready to say you have a good stroke. ...After you fix the footwork that is.
Some of those shots looked real nice. Almost like real tennis now.
OMG I got half a compliment from you! Scandinavian dance move, that's new though
That was in the back of my head though, whether or not I could use less arm...I think it gets arm-y when I don't have my feet underneath me, and I end up late/rushing. Hopefully fixing the feet gets rid of that issue too.
This return game is a perfect example of how my forehand is driving me nuts....after 30 minutes of hitting only forehands, and less than 10 backhands...
FH winner (yay)
Here's my one bit of feedback on your game. I haven't read this thread much, so I don't know everything that's going on other than that Cheetah's been giving you a lot of help. From watching your latest video, I noticed that the number one reason you make so many errors (footwork is number two) is that you hit the ball too low over the net. You seem to be giving yourself a foot or so of margin, which is quite low, even for a professional level player. The average pro gives himself an average of 2.5-3 feet of net clearance (5.5-6 feet in the air) on his rally ball, possibly even more since average also takes into account approach shots and slices that cross lower over the net. By playing the ball with such a flat trajectory, you can't help but miss every five or so shots, which is a very bad number, especially considering that you and your hitting partner aren't even running each other around. Aim for about three feet of net clearance, which is slightly on the high end, but then again, you don't have the racket control of a professional.
Thanks TS! Will keep that in mind! I'm still working on the swing path mainly, so I was more process oriented, making sure the path is correct and the kinetic chain is in place. The forehand was never a kill shot for me, and I'm trying to find that.
Will work on a higher, consistent rally ball when I go to the next step.
p.s. you should watch the videos on the first page to see where it all started. I can't even watch anymore
All right, keep up the good work! So I looked at your first video, and you're way ahead of where you were then, so it seems like it's all paid off. BTW, take a look at your avatar picture. You've got great extension, but it's all flat out to the target, which is indicative of the way you're hitting. I'm sure you're familiar with the term windshield wiper forehand. Well, your swing would shatter the windshield! Low to high, sit and lift, brush up the back of the ball with a vertical racket face, whatever it takes to get a more consistent rally ball! And work on the footwork too. Happy hitting!
Not the best video quality/angle, but I tried mapping the swing path/RHS.
There is no level path and then come at the ball from below for topspin. That does not look typical of your forehand comparisons with Dimitrov.
This is a running forehand on a low ball but it has some detail on the racket path and lag from a good angle.
^ That is one funky picture
Regarding mine... It's the front quarter angle, we're looking at the "windshield wiper" arc from a point almost perpendicular to it. it's not comparable to the arcs I posted earlier taken from side view.
If the ball was struck where the yellow dotted lines meet it looks as if the racket might not meet the ball based on the track. There is a lot of motion blur so check what the tracker did in comparison to the original video.
I manually checked every point. The red line marks the tip of the racquet head. There's also pronation which affects the path of the racquet head. Hmm...gets more interesting the more we look at it!
Anyway, here's the frame just before contact (didn't get actual contact):
What was the frame rate?
Maybe you had a very planar swing path and just happened to view that plane edge on?
Repeat in direct sunlight at a high frame rate.
100 MPH = 1760"/sec
1760"/ 240 fps = 7.3" movement between frames
1760"/ 120 fps = 15"
1760"/ 60 fps = 30"
1760" / 30 fps = 59"
Since it was too dark indoors to get any good high speed video, I set the FC150 to auto, so it's probably 30 fps.
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