Chapter 9: Experiential Learning in Biomechanics
9.2 Segmental motion analysis
Alix Bellemare and Rob Pryce
Introduction
The act of recording an activity to be able to review it, whether to assess technique, to compare to another activity or person, or to assess progress, is becoming more popular as the ability and access to cameras at our fingertips at practically any time has skyrocketed in recent years. This is much more convenient to do versus bringing an individual or team into a research lab, setting them up in a passive or active marker set, and recording the desired task in that lab, and then subsequently having to process and filter that data before being able to review it.
With the increased time of set up, and cost to set up, there is almost certainly more accuracy and reliability with these research lab set-ups, though. However, is the difference in quality enough to offset the cost? How do we, or can we, know this? What are things that need to be considered?
Firstly, the question of how accurate does the data need to be, needs to be asked. If the goal is simply qualitatively comparing two movements, or two individuals, then the simpler set up is probably all that is needed? But what if we need a bit more than that; or how do we make sure that even while using the simpler set up, we are getting the most accurate data as possible? There are many factors that can contribute to good quality video recording, and subsequent processing/analyzing.
Factors Contributing to Good Quality Video Recording
There are things that must be done to be able to produce accurate data, while there are other things that, while helpful, will not disallow data to be gathered completely.
Necessary Factors
Calibration Factor
When taking a video recording of a movement, and subsequent measurements are needed to be taken from the recording, how is one to know how long distances are, such as stride length, leg length, distance covered in a jump or stride? There must be some sort of calibration factor included in the video. This can be a physical object in the frame, such as a meter stick or scaling image, as seen below. This can also be simply knowing the length of something in the frame, such as the length of the bat being swung, the diameter of the ball being kicked, or the length of the brachium of the subject being recorded. An advantage to using something inherently in the frame is that it is certainly in the right frame of reference. Because it is part of the recorded movement, we know that it is not behind or in front of the movement. However, this measurement can be less reliable or accurate compared to a meter stick or scaling image, as we cannot be sure that it is 100% perpendicular to the video recording device, whereas we can with an external calibration factor.
Figure 1: Measurement Scale with black marks. Retrieved from Adobe Stock
Figure 2: Calibration Checkerboard. Retrieved from Calib.io
Perpendicular arrangement of recording
For the most accurate