## 4 Discussion
### Subjective Comfort Descriptor Ratings  
In order to evaluate the influence of the interface material stiffness on the human tactile perception, we firstly discuss the results of the subjective comfort descriptor ratings and compare all the product handles. All significance levels are \(p<0.05\), unless otherwise stated. The medium and hard deformable handles were rated the highest, with no statistically significant difference between both handles. However, a statistically significant difference was observed when comparing both aforementioned handles to the soft deformable and the original handle, which were rated significantly lower. The hard deformable handle was statistically significantly different from the original handle at \(p<0.001\), suggesting a strong difference in the perceived comfort rating predictor. The soft deformable handle was not statistically significantly different from the original handle and the optimally shaped hard plastic handle, which indicates that handle material has an influence on this comfort descriptor. No statistically significant difference was observed between the hard and medium deformable handles when compared to the optimally shaped hard plastic handle, suggesting that the shape of the handle had a major impact on the comfort descriptor, Fits the hand. This had been expected, since this comfort descriptor considers mostly the shape of the handle. Additionally, a statistically significant difference was also observed between the original manufacturer's handle and the optimally shaped hard plastic handle and both hard and medium deformable handles, which were rated higher, suggesting that the original manufacturer's handle was not optimized, either in terms of the shape and or in handle material. Based on the results, it is evident that both the shape of the handle, as well as the handle material in the case of deformable material, had an influence on the descriptor, Fits the hand. The importance of correct material parameter determination of deformable handles can be observed clearly when evaluating the results of the comfort descriptor, Has a good force and torque transmission. The soft deformable handle showed the lowest score with a value of 4.2, and was statistically significantly different to the optimally shaped hard plastic handle and the hard deformable handle. However, it was not statistically significantly different from the medium deformable handle and the original handle. This result suggests that forces and torques cannot be transferred effectively from the hand to the handle, and vice versa when too soft deformable handles are being used, due to the increased deformation of the handle material and the resulting movements and rotations of the handle and, hence, saw, in the hand. By contrast, no other statistically significant differences were observed, indicating that deformable handles can provide the same perceived force and torque transmission as hard plastic handles if the material properties of the deformable material are defined correctly. The comfort descriptor, Provides a nice feeling during grasping, clearly indicates potential for the improvement of tactile perception based on correct interface material mechanical parameter determination, since both hard and medium deformable handles were rated the highest, with values of 6.1 and 5.7, respectively. The comfort descriptor was rated significantly higher compared to stiff plastic handles and soft deformable handles. The hard deformable handle was statistically significantly different from the original handle at \(p<0.001\). The results suggest that stiff handles cannot provide the best tactile perception, and, hence, need to be deformable with exact material behavior as not to allow excessive deformation of the handle material during grasping.