Relationship between quiet standing position and perceptibility of standing position in the anteroposterior direction.

We investigated the relationship between an individual's center of pressure in the anteroposterior direction in quiet standing (QS) and perceptibility of different standing positions. The position of the center of pressure in the anteroposterior direction (CoPy position) while standing was represented as the percentage distance (%FL) from the hindmost point of the heel in relation to foot length. CoPy position in QS was located from 31 to 58%FL. Perceptibility of standing position was evaluated by the difference between the reference position and the subject's attempt to reproduce that position. Subjects were tested for their ability to reproduce reference positions selected randomly from a total of 13 positions at 5%FL increments from 20 to 80%FL. Using an approximation formula curve, we identified the relationship between reference position and reproduction absolute error. The standing position range with reproduction error exceeding 90% of the difference between the maximum and minimum errors was defined as the low perceptibility range of standing position. The approximation curve had one peak near QS. CoPy positions in QS were located in the low perceptibility range, except for five subjects with a more posterior location. The correlation coefficient between CoPy positions in QS (x) and reference position (y) showing maximum error was 0.70 and the regression line was y=0.464x+28.2; the intersection point with y=x was 53%FL. Reproduction absolute errors in reference positions at 20-30%FL and 70-80%FL were significantly smaller than those at 40-60%FL (p<0.05). We concluded the following. (1) Standing positions showing the lowest perceptibility are located close to the QS position; however, in subjects whose QS position is located more posteriorly, the standing position showing maximum error is more anterior. (2) Perceptibility of extreme forward- and backward-leaning positions is very high and independent of individual QS position.