An experimental study of human hand pressures during suspension and implications for fossil hominin locomotion

Studies of the human hand can be useful for furthering our understanding of how fossil hominins interacted with their environments, particularly in relation to hypotheses concerning the significance of arboreal behaviors. Using a custom-built apparatus, we measured hand pressure experienced during dynamic and static suspension in six human participants to determine where load is experienced by the hand during suspension. Participants grasped 45mm, 80mm, and 105mm support diameters and three activity categories were investigated: static suspension with thumb adducted and abducted, and dynamic suspension using participants preferred hand grip. Qualisys® motion capture and Novel Pliance® pressure systems were synchronized to measure 3D hand posture data and the location of peak pressures, normalized by body mass, across the human hand. Peak pressure was typically experienced by the second through fourth digits across all support diameters and activities. Significant differences in the pressure value were found between the 45mm and 105mm diameters for the thumb adducted and abducted categories. In each activity, the 45mm diameter had the greatest normalized peak pressure. The location of peak pressure was observed to move to more distal and proximal areas of the hand as the diameter of the support increased. The distal and proximal locations of peak pressure on the largest diameter are concordant with previous human ergonomic research and captive bonobo hand pressures experienced during arboreal suspension. These results suggest that if fossil hominins still engaged in arboreal suspension, that morphological signals of this behavior may be found in the second through fourth digits.