Measures of Human Position Sense Do Not Always Include Contributions From Peripheral Sensory Receptors

Our position sense helps to guide us as we move about in our environment. The position sensors are believed to be the muscle spindles. We have asked the question, are spindles always involved in generating position sense? We have identified three methods of measurement of position sense, applied to the passive forearm: two-arm matching, one-arm pointing and repositioning, carried out by blindfolded subjects in the sagittal plane. In matching, one arm is brought to a position where it is perceived to be aligned with the other. In pointing, the participant points with one arm to the perceived position of the other, hidden from view. In repositioning, the passive arm is moved to a test angle, and the participant is asked to remember that angle and, after a delay, to reposition the arm at the remembered position. We have used two ways to detect spindles contributing to position sense: using muscle history effects to alter spindle sensitivity and changes in gravity during parabolic flight to alter spindle responses. Both methods disturbed position sense in matching and pointing, but not in repositioning. It suggested that the repositioning mechanism did not directly involve peripheral receptors. Therefore, when interpreting measurements made with this method, it must be remembered that signals of spindles do not contribute to the position signal. We cite examples of studies where wrong conclusions have been drawn, based on assumptions about the peripheral origins of the position signal. This has led to wrong interpretations and confusion in the field.