Background: Training manual therapists in the judgment of spinal stiffness traditionally requires the spines of human volunteers. Presenting the range of stiffness values representative of spines requires both time and resources. When investigating the ability of therapists to judge spinal stiffness, several studies have therefore utilized spine analogues, nonbiological devices designed to present either purely elastic stimuli or viscoelastic stimuli with unknown relative contributions of elastic and viscous components. Previous research has suggested that the viscous component of stiffness is more difficult to judge.
Objective: This study determined for each subject the just-noticeable difference, or discrimination threshold, for manual judgment of viscous stiffness against a constant elastic and friction background.
Design: This psychophysical study utilized the Weber fraction as a measure of the ability of subjects to discriminate between viscoelastic stiffness stimuli.
Methods: Twenty-five subjects with both physiotherapy and lay backgrounds volunteered to participate in the study. Stiffness stimuli were generated by a device incorporating a fluid-filled plunger and a spring, with only the amount of viscous stiffness being manipulated by the experimenter. The method of constant stimuli was used to estimate the just-discriminable change in viscous stiffness, and results were expressed as a percentage of the base stiffness or Weber fraction.
Results: The mean Weber fraction for manual judgments of viscous stiffness was 14.7%. For 13 of the subjects who had previously participated in elastic stiffness discrimination studies, the Weber fraction for viscous stimuli was significantly greater than that for elastic stimuli.
Conclusion: The significantly higher Weber fraction for viscous stiffness perception compared with that for pure elastic stiffness suggests that the poor reliability of manual judgments of spinal stiffness may be due to the difficulty in judging the viscous stiffness component.