A psychophysical evaluation of manual stiffness discrimination.

One possible explanation for the poor reliability of clinical judgments of spinal stiffness is that humans do not possess a good ability to discriminate stiffness stimuli. This study sought to investigate this hypothesis by conducting stiffness discrimination tests on 72 subjects, using a mechanical device to provide the stiffness stimuli. The relative increment in stiffness needed so that two stimuli could be differentiated 75 per cent of the time was found to be 11 per cent when the pisiform grip was used to assess stiffness of stimuli in the range from 6 to 11 N/mm. Thus, by inference, other factors are more likely to be the cause of the poor reliability of clinical judgments of spinal stiffness.