Contact pressure distribution at hand-handle interface: role of hand forces and handle size

The distribution of localized pressure peaks and the resulting contact forces over the hand surface are investigated through measurements performed under applications of different combinations of hand grip and push forces in the 0-75 N range. Three different cylindrical handles of 30, 40 and 48 mm are used to measure the hand-induced forces and distributed pressures using a capacitive pressure-sensing mat wrapped around the handle. The data acquired with 10 adult male subjects are analyzed to derive the contact force distributions over the hand surface as functions of the grip and push forces, and the handle size. The peak pressures occurring in different regions of the hand surface are also derived and examined in view of the reported pressure-discomfort and pressure-pain threshold limits. For this purpose, the hand surface is split into five different zones. The results show that contact pressures of considerable magnitudes develop within the hand-handle interface, while the magnitudes of peak pressures strongly depend upon the handle size, grip and push forces. Application of high grip and push forces causes the peak pressures to exceed the discomfort threshold values, specifically for the thenar eminence. The peak pressures attained for the 48 mm handle under moderate levels of grip and push forces exceed the suggested sustained pressure values for the preservation of working efficiency. The results also suggest that the mean peak interface pressure for a given handle size can be expressed as a linear combination of grip and push forces. Furthermore, the proportions of hand-handle contact force developed within individual zones also vary linearly with grip and push forces, and strongly depend upon the handle size. The results show that the contact force developed in the vicinity of proximal phalanges of the digits and the palm is generally attributed to the push force, while forces at the fingers surface are caused by the gripping action. Owing to the difficulties associated with the measurement of interface pressures, the proposed relationships could be conveniently applied to obtain an estimate of the mean values of peak pressures on the basis of directly measurable grip and push forces.