What Are You Trying to Measure?
By Kevin G. Keegan, DVM, MS, DACVS
I often get asked by colleagues and fellow equine lameness investigators to compare inertial sensors to other methods of objective lameness measurement. My question in return is, “What are you trying to measure?”.
In my honest opinion, if you are only interested in measuring lameness in horses, for whatever reason, then the only way to do it practically today is with body-mounted inertial sensors. If you are interested in measuring something else – for example, rider position on the horse – limb movement effects with shoeing, or if you are interested in developing a method of lameness evaluation that is not based upon the vertical movement of the torso, then other methods may be for you.
I will add, as words of caution, I doubt you will find a better method of lameness measurement based on anything other than vertical movement of the torso (I have tried). And be prepared to spend much effort – with some methods just trying to get enough measurements and with other methods searching for meaningful patterns in mountains of data.
I am taking for granted that you understand the benefit, and appreciate the necessity in many situations, of objective methods of lameness measurement and evaluation. If you do not understand and appreciate these, stop reading, the rest of this article will be a waste of time for you. Time is precious to all, and money to many.
The first order of business is to explain why inertial sensors are superior to the stationary force plate for studying and measuring lameness in horses. Many years ago, when I was a young veterinarian coming from private practice fresh into a tenure-track position, I was, in a way, forced to develop an investigative focus. This was how it was back then. I was not fully trained to develop an investigative focus. So, I had to turn to the experts.
I was interested in lameness because this is mostly what I had faced in private practice. I proceeded to read just about everything there was available at the time from experts in the field. Clayton, Schamhardt, van Weeren, van den Bogert, Roepstorff, Back, Leech, yes, even James Rooney (although his stuff was frequently way over my head) caught my attention. Many of these individuals one could say, served as mentors of mine; some I call friends today. But it was the late Dr. Henk Schamhardt, actually a physicist interested in the horse, who directed, no forced, me down a path away from the stationary force plate. “Kevin”, he said, and
I am paraphrasing here from memory, “if you want to study lameness in horses, you need to get data, lots and lots of data. Lameness is an elusive thing, with much variability, and lameness in one limb depends on what is going on in the other limbs, and this elusiveness is compounded in quadrupeds. The stationary force plate gives you data on one stride, in one leg. You need much more than this. Also, you will have to look at the horse always in the laboratory situation, under controlled environmental conditions of speed and surface”.
He made it clear to me that if you wanted to study lameness in horses you had to study, not force but instead, motion. I have since that time actually used the force plate, as a standard, to validate body-mounted inertial sensors, but the validation must be understood as limited, because use of the force plate requires limiting conditions. If someone could develop a set of force-measuring devices that could be easily applied to all four limbs in the horse; that was robust enough to stand up to the extreme vertical forces expected in even slowing moving horses; that was lightweight enough to not alter normal gait; that was inexpensive for normal, everyday use, then maybe measuring force directly would be more practical than measuring motion. This has not yet been accomplished.
Many have tried and are still trying. If you want to measure and study lameness in horses the easiest way to get enough meaningful data is with motion analysis.
The second order of business is to explain why inertial sensors are superior to line-of-site techniques using cameras with or without body markers, or the video-based, kinematic motion analysis systems. I have used two of these systems for many years and am very familiar with how they work, the data they generate, their accuracy and sensitivity (they are high), and their limitations and pitfalls. These systems have improved greatly over the last 25+ years.
Gone are the days of clicking on the video screen to select body points, or calibrating the field of view with video collection of stationary “frames”, or waving a wand with balls placed an accurately-measured distance apart, and having to download data to process before analysis of whatever movement parameter you decided to measure on that day, such that it was many hours or even days before you could determine how lame the horse was at that time. Not very practical.
Everything today is automatic, with automatic calibration, live marker tracking. Beautiful and impressive, it is, but still not very practical. There still is one major limitation, unlikely in the near future to change with line-of-site kinematic video systems. Your sensitivity is limited to the ratio of the size of your subject (the horse) and the area of the field of view. When the subject of measurement is small relative to the area of the field of view spatial resolution suffers. Also, things get in the way. Only highly thought-out (and expensive, like several cameras arranged side by side to cover large fields of view) or unnatural (treadmill, expensive too) measurement areas make it possible to get enough sensitive data to handle the many-times mild and frequently variable lameness seen clinically in horses.
Really, the only way to do it today is with body-mounted inertial sensors. I would be remiss if I did not point out the challenges of using body-mounted inertial sensors to measure lameness in horses. Motion analysis with inertial sensors seems a relatively straightforward thing to do these days, what with inertial sensors’ ubiquitous presence in just about every Fitbit-like step counter and Smart Phone around today. Inertial sensors are cheap. Inertial sensors are everywhere; however, for an inertial sensor device to be useful and effective, it must be designed from the ground up for the environment and use cases being measured. They need not measure with the frequency and precision required for a Mach 4 missile, but they do need to be designed with the appropriate size, weight, range, sampling frequency, battery life, durability, synchronization, and resistance to environmental elements. Also, the data that is generated needs to be collected, processed, analyzed, and presented in a meaningful way.
We are always working to make the evaluation more efficient for the veterinarian. We continue to examine current and potential use cases (multiple limb patterns, under saddle, flexion tests, wellness evaluation, neurologic lameness, neck and back pain), where sufficient data may have been amassed to provide additional evidence-based objective assessment. This is the beauty of body-mounted inertial sensors; it is the mechanism and technique for “big data” possibilities.
Inertial sensor-based lameness evaluation is still in its infancy. It took many years of perfecting the sensors, algorithms, and analysis method – and we will never stop improving them. There are many collaborators around the world collecting and sharing Q data. The future is bright!