Archive for August, 2007

h1

Are Figure Skaters Projectiles?

August 28, 2007

The title above is eye-catching on a number of levels.  Have you ever been on those high sessions with 25 skaters?

Actually, this post is a follow-up to my discussion of slo-motion versus computer analysis.  In that post I made the bold claim that the minimum height for a double axel is 60% higher than the minimum height for a double lutz.  In this post, I’ll show how I got those numbers.

Warning:  This post is extremely technical so if you just want the final answer, see the height and flight time tables below.

The work of figure skating video analysis experts has demonstrated that there is a minimum flight time requirement for each of the jumps.  For example, a double lutz needs to be in the air for 0.367 seconds or longer while a double axel needs to be in the air for 0.467 seconds or longer.  These values have been determined by analyzing hundreds of jumps with various video analysis programs such as Dartfish or Pro-Trainer.

Using the video analysis software, a coach can count the number of frames a skater is in the air.  You do this by advancing the jump entry and take-off frame-by-frame until the skate blade just leaves the ice.  You mark that as your starting point and advance the jump frame-by-frame through the flight until the skate blade just touches the ice.  The number of frames from the start to the end point give the flight time, as video cameras shoot frames at fixed time intervals.

In North America, the video standard is 29.97 frames per second but the magic of modern video analysis software such as Dartfish and Pro-Trainer allows those programs to double the number of pictures for certain consumer video cameras.  (In a future post I’ll discuss the technical details of interlace vs. progressive scan.)  That means we can resolve the jump flight time to 1/60th of a second (actually 1/59.94 of a second).

So here’s an example.  I’ve never analyzed a double axel with less than 28 frames of flight time at 60 frames per second.

28/60 = 0.467 seconds.  Dartfish and Pro-Trainer have timers so you don’t actually have to count the frames.

Now to calculate an estimate of how high the jump was, I apply the laws of physics.  In basic physics courses, there is almost always a part of the course devoted to “projectile motion” and the associated equations.  We’re going to consider our in-flight skaters as projectiles!  For our situation, the equation of interest is the simplified time-acceleration-distance equation. 

This equation states that the distance an object travels under constant acceleration from rest is one half the acceleration rate times the square of the time.  The equation looks like:  Distance = 1/2 x Acceleration x Time x Time.  For our skating jump analysis, we need to use half the total flight time as the Time in the equation as the skater at the peak of the jump has no vertical speed (“from rest” at the top of the jump to full speed at landing… to use the simplified equation).

The acceleration of a projectile is simply the pull of gravity.  And gravity has a constant acceleration of 386.088 inches/sec2.  Using the double axel minimum flight time of 0.467,  the decent-only time is 0.467/2 = .2335 seconds.  Plugging all of this into the equation yields:

Distance = 0.5 x 386.088 x .2335 x .2335 = 10.53 inches

Just for theoretical completeness, the actual jump height is slightly lower.  The reason is that a skater always points his or her toe at take-off but usually flexes the toe in the air and upon landing.  This makes the number of frames method a tiny bit inaccurate.  But the result is close to the theoretical.  A double axel must be 10.5 inches high or you can forget it.

(Some skaters land with their landing leg slightly bent.  Their jumps are actually slightly smaller than that indicated by the table below.)

Here’s the whole table:

Frames in Air

Flight Time in Seconds

Height in Inches

10

0.1667

1.3

11

0.1833

1.6

12

0.2000

1.9

13

0.2167

2.3

14

0.2333

2.6

15

0.2500

3.0

16

0.2667

3.4

17

0.2833

3.9

18

0.3000

4.3

19

0.3167

4.8

20

0.3333

5.4

21

0.3500

5.9

22

0.3667

6.5

23

0.3833

7.1

24

0.4000

7.7

25

0.4167

8.4

26

0.4333

9.1

27

0.4500

9.8

28

0.4667

10.5

29

0.4833

11.3

30

0.5000

12.1

31

0.5167

12.9

32

0.5333

13.7

33

0.5500

14.6

34

0.5667

15.5

35

0.5833

16.4

36

0.6000

17.4

37

0.6167

18.4

38

0.6333

19.4

39

0.6500

20.4

40

0.6667

21.4

It’s pretty fascinating to understand the ramifications of this table.  For example

  • A double lutz needs to be have a miminum flight time of about 0.36 seconds, so it will be 6.5 inches high. 
  • If a triple lutz needs to have a minimum flight time of 0.58 seconds (estimated), it will be 16.4 inches high.

OK…has that sunk in yet???  The triple lutz needs to be 250% higher than the double!!  WOW!  Do you see why so few skaters actually get all those triples?

I hope this was interesting and useful.  If you are enjoying this blog, please pass the URL along to your friends that may be interested.  Also, please leave a comment as I would love to hear from you.

Trevor

Advertisements
h1

Why Computer Analysis And Not Just Slo-Motion?

August 27, 2007

One 0f the main uses of video analysis software with figure skaters is determining flight times.  Why?  Because flight time or jump height largely determines jump success.  For example, based on all the video that I have analyzed, I have never seen a double axel that had less than 0.467 seconds of flight time.  And other video analysis experts have confirmed this value.

Dartfish experts Chris Conte and Audrey Weisiger use a value of 0.500 seconds for double axel.  But I have personally measured a handful of double axels at 0.467 and in fact, one of the skaters I work with regularly in Madison lands it consistently with that flight time.  But she is very slender and has an awesome rotation rate.  So most skaters that are not as slender will probably have to meet Chris and Audrey’s 0.500 second minimum.

If a skater is ready to work on double axel but cannot perform a single axel with at least 0.467 seconds of flight time, she should spend nearly 100% of her effort making her jump bigger.  The chances are simply not good that she will be the first skater ever to land a double axel with less than 0.467 seconds of flight time.

This feature of video analysis software truly sets computer jump analysis apart from basic slow-motion video analysis.  Of course, basic slo-motion has been used by coaches for years and is very common in figure skating.  But computer analysis provides flight time and takes coaching to another level.  Knowing the flight time of the jump is critical for understanding what is possible.

All the time, I see skaters that are attempting double axels and triples that simply do not have the height necessary to land those jumps.  And unfortunately, they continue to practice them over and over, teaching themselves to fall or teaching themselves to land cheated jumps.  Since their coaches don’t realize they will never land it without more height, they keep doing it, and ingraining an incorrect movement that can be nearly impossible to change.

And that largely explains why double axel separates the really good skaters from the rest.  At the recent Frank Carroll  workshop in Milwaukee, Mr. Carroll described skaters having jumps and jump combinations through double lutz as being very good… but skaters with a double axel are in another class.

And that’s also evident in some surveys I’ve done.  Many coaches rarely get the chance to work on double axel.  And when they do, they are almost never successful.  The percentage of ladies that actually learn to do it consistently is very small indeed.  And no wonder… just getting any female athlete to jump 10.5 inches off one leg is quite a feat! (10.5 inches represents 0.467 seconds of flight time based on projectile motion calculations… more about that in a future post!)

And here’s some more astonishing but useful numbers.  Minimum flight time for double lutz is about 0.366 seconds.  That makes the difference in flight time between double axel and double lutz just 0.1 seconds.  But guess what?  Those 0.1 seconds represents a whole 4 inches!!!  So you can do a double lutz with just 6.5 inches of height but you have to jump over 60% higher to land a double axel!  No wonder so many skaters never get one…

If you have skaters working on double axel, do yourself and them a favor by getting a computer analysis done.  Find someone to help you if you don’t have the tools yourself.  The bottom line is – computer video analysis is truly a major leap forward in figure skating coaching for advanced skaters. 

Trevor

h1

Great Dartfish Substitute for Figure Skating Video Analysis

August 27, 2007

I have a couple of video analyses ready to put online but I’ll need a day or so to convert them to streaming format.  So check back soon for an analysis of a double flip and a double toe loop.

People always ask me if I use Dartfish for my video analysis work.  Dartfish has become the standard software package for analyzing figure skating.  You even see Dartfish Stro-Motion images being used on television broadcasts of major skating events.  And rumor has it that the PSA has been talking to Dartfish about becoming “strategic partners.”

But I don’t use Dartfish.  Dartfish is very expensive and the extensive feature set is not required for day-to-day analysis work.  Instead I use another program called Pro-Trainer.  Pro-Trainer can be purchased from Sports Motion.  This software sells for $169 at the time of this post.  Considering that the least expensive Dartfish version is nearly $1000, Pro-Trainer is a great substitute.

Pro-Trainer has been used extensively in minor league baseball and is relatively stable.  I have found a number of bugs but the program does a nice job considering the price.  It does provide 60 frame per second capture if you have a mini-DV camera.  (If you’re looking to buy a video camera for video analysis work, go with a mini-DV camera instead of a hard disk or DVD version.  It’s all about video compression and I’ll explain in a later post.)

Pro-Trainer is a bit more difficult to operate than Dartfish but what do you expect for $169!  Once you get used to the series of actions required, it’s very simple.  And Pro-Trainer also has a recording capability…the video analyses that I post on this blog were all created with it.

When you install Pro-Trainer for the first time, it defaults to golf analysis.  But you can customize it for figure skating.  I’ll create a screen capture of how to do this in the near future.

Want to get into computer video analysis but can’t afford Dartfish?  Check out Pro-Trainer…it works for me.

Trevor

h1

Frank Carroll Workshop In Milwaukee, WI

August 16, 2007

Today I attended my first Frank Carroll workshop.  It was very interesting to hear Frank’s views on technique, performance and the new judging system.

As with nearly every elite coach I’ve met, Frank emphasized lifting up into a jump before rotating.  He also emphasized the basics of checking turns, strong landing positions, and a neutral head position.  When I have more time, I’ll comment further on much of this including specific recommendations for each of the jumps.

I thought one part of our conversation was worth further discussion now.  Frank says he regularly uses video in his coaching.  He doesn’t use Dartfish or any similar software but uses regular old slow motion video playback.  He said that focusing too much on static positions within Dartfish can confuse a skater.  He thinks it is important to observe the positions dynamically so that skaters don’t try to replicate positions they see in freeze frame in Dartfish.

I thought his comments were very interesting and I’d like to know what you think.  Please leave a comment below.

Trevor

h1

Example Dartfish Analysis of Figure Skating Jumps

August 14, 2007

I did a Google search today for examples of figure skating elements that were analyzed using Dartfish.  Dartfish is a video analysis program that has been around for many years but is expensive and therefore beyond the reach of most skating coaches.  Still, I know of at least 22 figure skating coaches currently using Dartfish on a regular basis.

So why aren’t there any example analyses available online?

Well, now there is.  To see an example of a Dartfish analysis performed by Chris Conte (of a double axel), check out the Skating Coach Quiz.  This is a classic example of how Dartfish and other video analysis programs can be used to speed the training of skaters.

Tomorrow I’m attending a Frank Carroll workshop at the Petit Center near Milwaukee, WI.  I’m looking forward to learning a few things.

Trevor

h1

Inconsistent Axels May Be Lacking Airtime

August 13, 2007

Today was a routine day at the rink.  No breakthroughs.  Some skaters are stressing about the upcoming DuPage Open competition this weekend.  It’s also time to decide exactly who is ready to test at the upcoming test session here in the Madison area.

One of the skaters I work with regularly was upset today because she wasn’t landing her double toe and the technical changes I suggested made it “feel” worse.  This is so common in coaching that I felt it deserved a comment here.  I find myself constantly repeating to my skaters that to learn a new skill, there’s a good chance they’ll have to get out of their “comfort zone.”  As obvious as this sounds to many of us, it is not obvious to our skaters.  Especially if they have landed the jump in question before.  But we know that this is true with anything…without stretching ourselves, we wouldn’t grow.  And there’s discomfort in stretching ourselves.

Anyway, today’s video analysis is another axel.  Click the link below to see one common problem that reduces axel flight time and the effect it can have.

 Common Axel Flight Time Problem

If you find these videos helpful, please post a comment below.  Thanks.

Trevor

h1

Figure Skating Video Analysis Example

August 13, 2007

Today I spent a few minutes creating a short video analysis of an axel attempt.  If you look closely, you’ll notice there is no Dartfish symbol on the final video.  The reason is that I do not use Dartfish for my video analysis.

 Of course Dartfish is the industry standard at this time, but considering the cost (>$1500), it just isn’t practical for the majority of figure skating coaches.  Ialready owned a medium cost laptop produced in early 2006 and an inexpensive digital video camera, and created a full video analysis system for less than $300.  I’ll reveal how I did it in a future entry.

 To see today’s analysis, click the link below:

Axel Two Foot Landing Video Analysis

 The video shows clearly one of the major errors that many skaters have when learning the axel.  The error is usually caused by fear as described in the video. 

Please comment on the video and let me know what you think of it.

Trevor

P.S. Don’t forget to check out Skating Coach Quiz!  There’s a great Dartfish analysis performed by Chris Conte you should really see. 

Figure Skating Video Analysis Rocks!