A VIRTUAL 5-LINK MODEL OF THE THUMB

Most researchers have modelled the thumb as three rigid links with con nections of two universal joints (carpo-metacarpal joint and metacarpo -phalangeal joint), an a hinge joint (interphalangeal joint). In this work, the thumb is modelled as a five-link manipulator with the virtua l links connected by hinge joints - one for each degree of freedom of the thumb. The axes of the hinges are not orthogonal to one another, t o the long axis of the bones or to the anatomic planes. Four static po sitions of hand function were analysed - key pinch, screwdriver hold, tip pinch, and wide grasp. The virtual five-link model of the thumb pr edicted similar muscle recruitment patterns to published EMG data. The force at the distal surface of the trapezium is between 6 and 24 time s the applied load depending on the posture.