This is an excerpt from Running Form by Owen Anderson.
Six key elements of form assessment represent the biggest influencers of performance, running economy, and risk of injury. Therefore, these six should be measured carefully via video analysis:
- Determination of how far ahead of the body the leg moves during the swing phase of gait and thus calculation of maximum shank angle (MSA)
- Measurement of reversal of swing (ROS), or how far back the shank and foot move from MSA before hitting the ground
- Determination of the ROS-to-MSA ratio (ROS/MSA)
- Measurement of the shank angle at initial impact with the ground (SAT, or shank angle at touchdown)
- Determination of the foot angle at touchdown (FAT)
- Confirmation of posture
A smartphone with video capabilities or a video camera are necessary to carry out the video analysis. The brand of the device does not matter, but the video instrument must be capable of recording at a rate of no less than 240 fps (frames per second).
Bear in mind that it is critical to determine MSA, ROS, SAT, and FAT with great precision. If you cannot determine exactly when a runner’s foot strikes the ground, for example, you will not be able to assess SAT and FAT accurately.
Take the case of a runner moving along with a cadence of 180 steps per minute (three steps per second). As she runs, the goal is to “see” the exact moment when each foot makes contact with the ground—the precise instant when the stance phase of gait begins and SAT and FAT can be measured. Imagine what might happen if the video device were taking images at only 30 frames per second: That would mean an image would be captured every 1000/30 = 33.33 milliseconds.
That may seem like plenty of footage. But when a video device captures only 30 frames per second, it will be impossible to tell if the first frame showing the foot touching the ground represents initial impact, or if it shows the foot after it has been on the ground for a number of milliseconds. In the intervening milliseconds after the true initial impact, the runner’s body may have moved significantly ahead over the foot, and thus the shank angle might have changed significantly. In other words, the measured SAT would be wrong.
Now, imagine this same scenario with a video device recording at 240 Hz. Instead of 33 milliseconds between images, there are now 1000/240 = approximately 4 milliseconds between images. On average, when estimating SAT, the video analysis will be off by about two milliseconds per frame, compared with about 16 milliseconds per frame for a 30 Hz recording. The degree of accuracy at 240 fps is simply much greater.