Hand contact area, force applied and early non-linear stiffness (toe) in a manual stiffness discrimination task

Despite research that demonstrates that humans possess a good ability to discriminate linear stiffness stimuli, which are non-biological (that is produced by a mechanical device), this is not the case when discriminating spinal stiffness in vivo, where there is poor reliability. One of the most obvious differences between device-generated stiffness stimuli and human spines has been the absence of the early non-linear elastic toe in the device tests. This study set out to investigate whether the addition of an early non-linear stiffness (or toe) to otherwise linear elastic stimuli, affects discrimination. The presence of padding, which was used to mimic a toe, did not affect the accuracy of judgements of linear stiffness. The study also investigated the relationship between a measure of discrimination, force used and contact area of the hand during stiffness sampling. A relationship was found between ability to discriminate stiffness, force used and area of hand contact with the stimulus. It is argued that this result has clinical implications for the standardized use of force and contact area during stiffness palpation.