Friday, April 11, 2008
"Slo-Mo" Normal Video vs. "High Speed" Video: Why HS Counts in Gait Analysis
People love to take videos of their horses and then play them back in slow motion, but are ultimately disappointed when their clips don't have the detail that you see in a presentation by Drs Hilary Clayton or Alan Wilson, or in the toe-grab research footage shot by farriers Mitch Taylor and Scott Lampert. And why can't you see that twisting hock action in your videos that Haydn Price shows in his lectures? You know it must be there. Worse yet, speakers at conferences are horrified when their videos project as pixelated blurs in motion on the big screens. "It looked great on my laptop," they mumble.
What's the problem? With digital cameras, it's easy enough to understand the concept of "resolution", or pixels per inch. In an oversimplified nutshell: The higher the resolution, the more pixels per inch. The more pixels per inch, the sharper the image looks when printed on paper. Low-res images usually look ok on computer monitors because the monitors are low-res, too but they won't print in a magazine.
With video, if you want to slow something down, you need a lot of "frames" to see the phases of motion. High speed video delivers those frames, but it is very expensive, and also the very best way to look at a horse's foot in motion.
In the clip above, you are seeing the exact same footage, shown at three different speeds.
My favorite video clip, along with the Danish mousetrap, is one that Dr. Clayton likes to show when she speaks. She shows it first at normal speed: a horse trots across a force plate. There is a little glitch in the action, you see his head lower but you might not give it a second glance.
There's always some clever farrier in the audience who, as this juncture, blurts out: "He pulled a shoe". Most people don't realize it happened.
When she slows the high speed down, you see the hind foot reach forward and catch on the heel of the front shoe. For several tortured frames, front and hind are locked and the horse careens forward in a sliding motion--that would be when the horse's head goes down for a split second.
Then the shoe comes ripping off the foot and the entire lower limb is wrenched so that you would think the pastern would come unhinged. You swear the shoe will take half the hoof wall with it. It flies through the air, somersaulting in slow motion, much like the mousetrap in today's video.
Some barefoot advocates have to leave the room at this point.
And the horse keeps going, as if nothing happened.
High speed video has been a great gift to anyone interested in how horses move. The name is deceptive, since it is not speeding up at all. What it does is allow us to slow motion to a crawl and see every exaggerated detail. We see that little twisty rotational move before breakover. We see the slip in landing. We see that horses rarely, if ever, land symmetrically, even though it may look that way on normal video in slow motion.
Do you need a megabucks camera to record horses in motion? Certainly not. I know of people doing very clever things with decent cameras bought on sale day at Best Buy. Out of the box sports coaching programs like MotionCoach can get you started and if Nokia's cell phone Golf Edition will video-capture your golf swing and analyze it in the palm of your hand, can a stride-length cell phone or a railbird accelerometer be far behind?
At the Badminton Horse Trials in England next month, and at other advanced events there over the summer, a special fence will be in use. Designed by an engineering project called "Competitive Measure", the ascending spread obstacle will have an accelerometer attached to it. If the horse touches the fence, the accelerometer will measure how fast the horse was going when it hit. But that's only half of it: the fence is rigged with a built-in high-speed video camera that will record the horse's approach, the rider's position, the trajectory of takeoff and, should it go down, every detail of the fall.
There's no doubt that motion capture is looming in the future for riders and for their support teams. Just like the early days of the Internet, the best advice is to jump in and get your feet wet. Splash around at the shallow end of the pool and learn to use the cameras and editing software like iMovie, if you're lucky enough to have a Macintosh computer, or Final Cut.
By studying high speed clips from the pros, we can get inspired and we can relate to how difficult and time-consuming the work of true gait/motion analysis is, when proving or disproving lameness or a biomechanical syndrome is at stake. Video analysis will soon enhance the sales value of horses and track training progress. We'll take it for granted, but professional high-speed footage will still be the Holy Grail.
The way I describe it: It's like seeing a hoof--or a mousetrap--for the very first time.