The Study: Relation between postural sway magnitude and metabolic energy cost during upright standing on a compliant surface
a. Postural control can be described in terms of postural sway.
b. In general the lower the sway the better for the patient.
c. The authors wished to look at the effort expended (metabolic cost) during postural sway.
d. The study looked at 19 subjects and examined the “center of pressure (CoP) excursion, and muscle activation”
e. They found “that people naturally select a magnitude of sway that resides at the bottom of the observed energy-sway relationship.”
f. “These results are in line with the notion that metabolic cost could be an optimization criterion used for selecting motor control strategies in postural control
g. The underlying pathways of how this is accomplished is not yet understood.
The sway that the patient has tends to be at the lower end of what it costs in terms of energy expenditure.
It takes energy to stand upright and in general, as sway increases, it takes more energy to stand upright. Although the authors found in the study that the patient could make an effort to reduce the sway, and could in fact reduce the sway, but that making this extra effort increased energy costs. So the amount of sway that the patient naturally had without expending extra effort to reduce sway was generally at the lower end of needed energy expenditure. We don’t know the pathways for this mechanism and it is interesting but the reason that I reported on this is to once again show that researchers are interested in how structure affects function. In this case the sway away from the midline increases the effort needed for the patient just to stand upright.
Reviewer: Roger Coleman DC
Editor’s Comments: I remember that Don Harrison used to frequently describe the patient’s habitual posture as their “minimal energy” state. In other words, patients tend to habitually stand in the posture that requires the least amount of energy to be expended. That was a pretty prescient observation considering that it was thirty years ago. Note that this says nothing as to whether or not their resting posture is particularly healthy or functional, only that the body tends to gravitate towards those static postures (or as in this study, amounts of postural sway) which require the least effort to maintain.
Editor: Mark R. Payne DC
Reference: Houdijk H, Brown SE, vabDieen JH. Relation between postural sway magnitude and metabolic energy cost during upright standing on a compliant surface. J Appl Physiol. 2015;119:696-703.
Link to Abstract: https://www.ncbi.nlm.nih.gov/pubmed/26159762