A metric for assessing acuity in positioning joints and limbs

In this paper, we present a method for assessing the exactness of sensing and setting the positions of joints and limbs, using a measure we call target resolution. Target resolution, derived from information theory but ultimately based on variance, estimates the fewest number of discrete, equally spaced targets required within a range to provide the maximum possible information transfer from any target set. We argue that target resolution provides better insight into the exactness of position sense than does the usual measure of accuracy based on mean or constant error. Studies have shown that measures of mean error in setting or indicating positions of joints or limbs exhibit lability; they drift and show considerable sensitivity to factors such as previous positions of the limb and learning. We derive the equation for calculating target resolution and give example resolutions for several joints we have tested. Target resolution often gives a quite different impression of proprioceptive exactness than do measures of accuracy based on mean error.