Wednesday, 4 April 2012

Perception of weight distribution in tai chi stances

Tai chi weight distribution scales
In tai chi we make a significant conscious effort to control the distribution of our weight in postures and stances. This led me to wondering, that although we talk about having 70% of our weight on one leg (for example), how accurate and exact is this 70% figure? I intend to carry out some tests myself, but in the meantime I did a quick search for scientific literature on the subject and discovered this particularly relevant journal paper entitled "Perception of unilateral lower extremity weightbearing during bilateral upright stance".

The abstract of this paper is below, but in summary it seems that humans can judge the weight distribution to within a 7% error for asymmetrical stances [i.e. if you feel like it's 70% of you weight, it is likely somewhere between 65% and 75% of your weight]. It seems reasonable to assume that with tai chi training, this error margin is reduced, as it is a conscious, learned and developed skill say to a 3% accuracy [i.e. if you think it's 70% of you weight it is likely somewhere between 68% and 72% of your weight] - which is pretty accurate.

What this means is that this 70% figure is not just an arbitrary feeling that our teachers are talking about (as it would be if the average percentage error was high implying humans were bad at judging weight distribution, like say >20%), but a real scientific quantification of what we should be doing. Hence when we talk about moving 70% of our weight onto one leg, not only should this feel like 70% of weight is on that leg, but practically it should also be 70%. Hence it makes sense to play around with bathroom scales at home to actually measure how good your stances are and improve your perception of weight distribution for better control.

Abstract of "Perception of unilateral lower extremity weightbearing during bilateral upright stance"
"The primary purpose of this study was to describe the error in 61 healthy subjects' perceptions of weight-bearing at three target levels during bilateral upright stance. The secondary purpose was to describe the effects of age, sex, lower extremity dominance and target weightbearing level on the error in perceptions of weightbearing. Weightbearing was determined while subjects stood on digital scales. They adjusted their weight in an attempt to bear 25, 50, and 75% of their weight through a designated lower extremity. Three trials were allowed at each weightbearing target, and the results were averaged. Each subject's error in perception of weightbearing at each target level was determined by taking the absolute value of the target percent weightbearing minus the mean actual percent weightbearing. The mean errors at the 25, 50, and 75% targets were 7.3, 3.3, and 7.7%, respectively. The magnitude of the error was unrelated to age. An analysis of variance showed that error was not dependent on sex or whether the dominant lower extremity was used for making judgements. The error did differ between target levels. Clinicians cannot assume, based on the findings of this study, that individuals can accurately judge the percent weightbearing they are placing through one of their lower extremities during bilateral upright stance."
Note I disagree with the final sentence of this abstract, as I believe that this weight distribution perception is the key skill that tai chi practitioners are developing, and hence their ability to judge it is significantly better than the normal population. You can hypothesise an extrapolation of this fact from the numerous scientific studies demonstrating that there are benefits to doing tai chi for improving balance. I would argue that improving your balance is primarily driven by improving the accuracy of your weight distribution perception.

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