Measuring position sense

Abstract

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.

Keywords: antagonist muscle; muscle spindle; position sense; thixotropy; voluntary contraction.

FIGURE 1

Position sense, measured over the full working range of the human forearm, by two‐arm matching (a), by one‐arm pointing (b) and by repositioning (c). (a) One of the blindfolded subject's forearms, the reference, was conditioned with a half‐maximum voluntary contraction of elbow muscles at 125° (forearm flexed) and extended to a randomly selected test angle; the indicator was conditioned at 5° (forearm extended), then moved towards the reference to a matching position (red triangles). Then the sequence was reversed, with the reference conditioned at 5° and the indicator at 125° (blue triangles). Matching angles are shown as means (±SD) for three repetitions by each subject, pooled for 11 subjects, measured over the full range of elbow angles, 125°–5°. Dashed line, line of equality, the position of the indicator if it had accurately matched the reference. (b) Position sense by pointing. The position of the hidden forearm is indicated with a pointer moved by the other arm. The elbow muscles of the hidden arm were conditioned at 125° (red triangles) or 5° (blue circles), then the arm was moved to the test angle, where its position was indicated with the pointer. Values shown as means (±SD) for the two conditionings, for three repetitions, for 11 subjects, over the full range of elbow angles. Dashed line, zero error. (c) Position sense was measured by repositioning. From a starting position of 125° the forearm was moved into extension to one of four randomly presented test angles (95°, 65°, 35° and 5°), and the subject was asked to remember that angle. After returning to the starting position, they were asked to reproduce the remembered position. This sequence was repeated, but with a starting angle of 5°. Values are shown as means (±SD) for three repetitions for each of 11 subjects. Black open circles, arm muscles left unconditioned. Green open circles, arm muscles conditioned at 125° after the learning stage, before the reproduction stage. Red open circles, arm muscles conditioned before the both learning and reproduction stages. Dashed line, zero error. Figures redrawn from Roach et al. (2023).