Screening Posture or Leg Length with a Dual Scales Device
Recently I have been asked by a few people what my opinion was about the use of dual scales for an assessment of posture. If you have never seen one of these before, it involves a portable platform with two identical scales. The person being assessed then stands with one foot placed precisely on each scale to measure the weight distribution of each foot when standing. Usually there is a rigid frame which has a kind of plumb line and adjustable wires that run horizontally. These horizontal wires are placed at landmarks on the body (eg. the shoulders) as way of evaluating posture by comparing the heights of the landmarks from one side to another.
This kind of device is not something I use in my office. To my knowledge, is not a tool taught in chiropractic colleges, at least not where I went to school in California, nor where I taught in New York. My colleagues who studied Chiropractic in Canada did not use it in school either. However, being curious to know more I searched the Pubmed database of scientific articles. I used search terms such as posture, posture assessment, dual scales, double scales, bilateral scales, weight distribution, and even the brand name of a device I have seen, but I was unable to find any recent research.
A search of a manufacturer's website did find some interesting results. They have a monograph about the use of their dual scale equipment for assessing leg length and how that fits into their system of detecting and treating postural problems. This work is based on publications mainly on leg length inequalities (not on posture or the use of dual scales). An anatomical leg length inequality is where one leg is physically shorter than the other (for a better understanding of leg length inequalities and the different types (anatomical, functional, environmental) – see the facts page coming soon). In the monograph the author(s) point out that measuring leg length with a tape measure has a lot of error (which is common opinion). However their system to detect an anatomical short leg is better determined by:
- one side of the pelvis being higher (on the longer leg side)
- and if the body is shifted to one side
They propose that once you determine there is a short leg, then the information gained by the two scales can help determine if the difference in leg length is greater than or less than 6mm. They explain that a person with a leg length difference of 6mm or less tends to have more weight on the short leg, but someone with a leg length difference greater than 6mm tends to have more weight on the longer leg. They base this premise on three research studies:
- One study does not involve leg length inequality at all so its findings cannot be used to support their claim. However one thing observed in the study is the normal random postural swaying of body weight from one side to the other. This shifting of body weight actually refutes the use of dual scales as it demonstrates a person's weight continuously changes side moment to moment.
- A second article investigated the immediate effect of standing with one foot raised by a heel lift. The authors of this study clearly warn that their results would not specifically apply to someone with an anatomically short leg. So this does not support the use of dual scales either.
- The third article noted that people with a shorter leg by 1-4 mm tend to put more weight on the shorter leg and those with a leg length difference of 6 mm or more tend to put more weight on the longer leg. However the data in the study indicate that this premise would probably be incorrect in about 1-in-3 people.
The manufacturer's monograph does not describe their treatment program, for that you would have to buy their equipment, but they do imply treatment would include exercise and/or foot orthotics with possibly a lift to raise the shorter side. But using dual scales would not determine exactly how thick a heel lift you would need. To find that I assume you would need then you would then have to either measure each leg with a tape measure or prop the short leg up by a known amount to try to make the pelvis level which makes the use of the scales redundant.
My opinion is that the research for the dual scales is weak at best. The idea that a person with a short leg may put more weight one side does seem plausible, but there are too many other factors and no good evidence to rely on a dual scale device. Furthermore, if an anatomical leg length was suspected you would have measure the difference somehow anyway otherwise you would not know how much you would have to correct. Based on what I can tell the two scales do not add any clinical information which is probably why it is not endorsed by chiropractic colleges.
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