The thumb during the crimp grip.

During rock-climbing, fingers grasp holds of various shapes with high force intensities. To ideally place the fingertips on the holds, the thumb is sometimes positioned on the nail of the index finger. This allows using the thumb as an additional actuator by exerting a supplementary force in the same direction as the index, middle, ring and little fingers. This study analysed how the forces exerted by the fingers are modified by the additional action of the thumb. The results showed that the thumb increases the resultant forces exerted on the hold. It was shown that the pathology risks of the middle, ring and little fingers were not modified in this condition. The finger force sharing was totally re-organized due to the support of the thumb. This led to the conclusion that the central nervous system organised the association of the 5 fingers. The results were discussed in regard to the established theories of the virtual fingers and the neutral line of the hand.