Number of sampling movements and manual stiffness judgments

Objective: It has been suggested that when using posteroanterior pressure to assess the stiffness of the spine, no more than two or three oscillations should be used. This study sought to examine this clinical impression by investigating the effect of the number of sampling movements on stiffness perception. Design: Perceptual study using a mechanical device to provide stiffness stimuli, with university staff and students as judges. Setting: University psychophysics laboratory. Interventions: In part 1, subjects were directed to sample the stimuli a prescribed number of times (either 1, 2, 3, 4 or 5 pushes), whereas in part 2, subjects were free to choose the number of sampling movements. Main Outcome Measure: Measures of interstimulus discriminability and bias. Results: An inverted-U relationship between the number of directed sampling movements and stiffness discriminability was found, with three cycles providing best discriminability. When subjects were given a choice, most chose to use three cycles. The number of sampling movements had no effect on bias. Conclusion: This study confirms the hypothesis under investigation and suggests that therapists will be maximally sensitive when using three testing cycles of posteroanterior pressure to assess stiffness.