IMPROVE MY GAME
Articles
Measuring the Timing of the Golf Swing from Video
Mon Mar 11, 2013 by Phil Cheetham
What you will learn in this article:
- The difference between camera frame rate and shutter speed.
- How to calculate timing of backswing, downswing and follow through from high-speed video.
- The TPI 3D Touring Pro Ranges for these swing phases.
- The 3:1 ratio of backswing to downswing.
- A fast swing is 750ms back and 250ms down; a slow swing is 900ms back and 300ms down; (where ms means milliseconds).
If you have a high frame rate video camera such as the Casio FH100, and video analysis software such as the free software from Kinovea, it becomes very easy to make direct measurements from video. In this article I will show you how to measure the timing of the backswing, downswing and follow through; plus use this information to calculate backswing to downswing tempo. If you don’t have the Kinovea software download it now from www.kinovea.org, and follow along with this tutorial. You will also need a Windows laptop.
I will use video that I filmed at 120 frames per second (fps). I would suggest this as a minimum; you could also use 240 fps. The extra frames you get from these high speeds provide you with a higher timing resolution and hence help you to be much more accurate than standard 30 fps video in your timing calculations. For example, in a 100 mph drive the club head is moving at about 150 feet per second. So if you are filming at 30 frames per second, the club moves about 5 feet (i.e. 150/30) between every picture. So you don’t get much information around the impact zone! If you are filming at 240 fps; you have eight times more pictures to look at. At this frame rate the club head will have only moved about 7.5 inches between pictures; (150/240 feet). This gives you much more information around the impact zone!
Calculating timing from high-speed video is simply counting the number of frames between events and then doing the appropriate math to turn frames into seconds. The Kinovea software has this conversion built in so it makes it really easy. Here’s how to do it.
Firstly you need to have a good quality clear video of the swing. Film at either 120 or 240 fps and make sure you set the shutter speed to at least 1/1000 second. Just a quick note to explain the difference between shutter speed and frame rate; shutter speed is the time that the camera shutter is open between each picture that is taken. Frame rate is how many pictures (or frames) the camera is taking every second; they are two different things. If you film at 120 fps and don’t set the shutter then your effective shutter speed is the same as the frame rate, i.e. 1/120 second. At this rate the club head will still appear to be blurry and hard to see. We can separately set the shutter speed to say 1/1000 sec while still filming at 120 fps. What this means is that the camera aperture will stay open for 1/1000 sec at the beginning of every picture and the camera will then wait the rest of the 1/120 second to take the next picture. Let’s think of it another way; 1/1000 second is one millisecond and 1/120-second is 8.3 milliseconds. So for the first millisecond the aperture is open, then for the next 7.3 milliseconds it is shut and then it opens again. This way you get a sharp picture of the club at high speed even though you are only filming at 120 frames per second. Notice in the picture of impact above, I didn’t have the shutter set to 1/100th and the club head is blurry.
On the Casio FH100 you set the frame rate by first getting into the video recording mode, then press the menu button at the bottom right of the camera; choose “Quality” and scroll down to “HS Speed” by using the down control (the bottom part of the wheel on the back right). To choose the speed, use the right part of the same wheel and scroll through the selections. Choose the speed you want and press the center button of the wheel. That sets frame rate. To set the shutter speed first set the wheel on the top of the camera to the S position (S for shutter). Then press the bottom of the wheel on the back and then press the top to move to the S selection. Here you can choose the shutter speed for either snap shots or video. Set it to 1/1000 for video. Now put your camera on a tripod and get a square on view of the front of the golfer during the swing.
Once you have taken the video, transfer the memory card to your computer and copy the video to a folder on the computer. Run Kinovea and open the video you have just transferred. Right click on the screen, select “High Speed Camera” and enter “120” (or whatever your frame rate was) and click “Apply”. Now the software will make the correct timing calculations automatically. Advance the video to the first movement of the club head, step back one frame from that point and set the Stopwatch. Do this by selecting the Stopwatch tool from the tool bar at the bottom left of the video image, then click on the screen where you want the timer to appear. To reset and start the Stopwatch you must right click on it and choose “Start the Stopwatch”. Play the video until the golfer’s club changes direction from the backswing to the downswing. In the video I am looking at for example, I get 0.966 seconds (or 966 milliseconds), just less than a second. So this is the time it took for the backswing to occur.
This gives us an indication of the tempo or speed of the backswing. A high number means a slow backswing and a low number means a fast backswing. How do we know what is slow and what is fast? TPI 3D has a database of many tour swings captured in 3D. From this database we see that the average time of backswing for PGA tour golfers, with a driver, is 0.847 seconds (or 847 milliseconds) with a standard deviation of 111 milliseconds. So from basic statistics, if the backswing takes less than 736 milliseconds (average minus one standard deviation) then the golfer is faster that about 8 out of 10 of the golfers in the database (84% to be exact). If the backswing takes more than 958 milliseconds (average plus on standard deviation) then the golfer has a slower backswing than most of the golfers in the database. So from my analysis, my golfer has a backswing tempo of 966 milliseconds and therefore has a comparatively slow backswing.
Let me continue and measure both the downswing and follow through times. Get back to the Kinovea analysis screen and reset the Stopwatch. Do this by right clicking on it and choose “Start Stopwatch”. This resets it and gets it ready to measure the next time interval. Now advance the video until you find the club at the closest point to impact. This will either be a fraction before or after the ball is hit. It will be very unusual to get it dead on. The ball is on the club face for about a half a millisecond, so to get exact impact every time you would need to be filming at a minimum of 2000 frames per second. For my golfer it is a fraction after ball contact and his time reading is 291 milliseconds. The TPI 3D ranges for this are 233 ms to 296 ms. So this golfer has a “normal” speed downswing (but slightly on the slow side of the average). As a rule of thumb; a fast swing is 750ms back and 250ms down; a slow swing is 900ms back and 300ms down.
Zero out the Stopwatch again and measure the time of follow through up to the finish point. This is a little subjective, as golfers tend to finish in many styles. I look for the point where the club has stopped for an instance and bounce back. For my golfer I get 433 milliseconds, this is a fast finish as the ranges for the tour pros are 525 milliseconds to 825 milliseconds.
There is one more value we can calculate and that is backswing/downswing ratio. The Novosels at Tour Tempo have done some of their own research by measuring the swings of many tour pros and the have found that this range is extremely consistent for most pros. They see a ratio of 3:1. If we do this calculation for the golf pros in the TPI 3D database we get 3.2 as the ratio. So we are very closely in agreement with their numbers. My golfer has a ratio of 966/291, or about 3.3, not bad but as we said earlier his back swing is maybe a little slow; or maybe a little too long. I have seen swings where the ration is close to 4:1 with a very slow backswing of more than one second.
So in summary I have explained how to measure the timing of the backswing, downswing and follow through and how to determine if they are slow or fast compared to the TPI 3D tour pro database. I also showed you that the typical ratio of the backswing to downswing for the tour pros using the driver is 3 to 1. And all this can be done quickly and easily with your own gear.
© Phil Cheetham