Measuring position sense

Position sense is arguably more important than any of the other proprioceptive senses, because it provides us with information about the position of our body and limbs in relationship to one another and to our surroundings; it has been considered to contribute to our self-awareness. There is currently no consensus over the best method of measuring position sense. We have recently measured position sense with three commonly used methods. These were two-arm matching, one-arm pointing and one-arm repositioning, all carried out by blindfolded subjects with their lightly loaded forearms moving in the sagittal plane. It is currently believed that muscle spindles are the principal position sensors. We posed the question, was there evidence for spindles participating in the generation of position sense with each method? The indicator of spindle activity we used was the presence of thixotropic errors in the position signal, in response to conditioning voluntary contractions of forearm muscles. Based on this criterion, there was evidence of spindles contributing to position sense with all three methods. It was concluded that the spindle contribution to the position signal and the extent to which this was processed centrally was different with each method. It is argued that a case could be made for the existence of more than one position sense. Differences between the methods have implications for their meaning in a clinical setting.