Biomechanics of the interaction of finger flexor tendons and pulleys in rock climbing

Abstract

In sport climbing, and particularly in rock climbing, the so-called crimp grip position is the finger position most often used to enable the athlete to hold the smallest ledges. The proximal interphalangeal (PIP) joints are thereby flexed at approximately 90 degrees, and the distal interphalangeal joints are hyperextended. In this position, the force transmission from the flexor tendon to the A2 and A4 pulley is at its maximum, and the load at the pulleys is approximately four times higher than at the fingertip. The use of the crimp grip may lead to tendosynovitis and partial or complete ruptures of the A2 and A4 pulley, particularly if warming up has not been conducted properly, which should involve at least 100 climbing moves. Under maximal load, the friction between the flexor tendons and the A2 pulley is responsible for up to 18 per cent of the grip force. Friction correlates with the degree of PIP joint flexion, which is maximal at approximately 90 degrees. Pulley injuries are mostly due to the bowstringing of the flexor tendon and peak forces at the edges of the A2 and A4 pulleys. However friction may also play an important role in the pathogenesis in a way that the tendon acts like a saw crossing the pulley fibers. These findings also explain why pulley injuries often occur during crimp grip and during a sudden and high eccentric load at the PIP joint. Friction between the tendons and pulleys may also physiologically act as a substantial part of the holding force, particularly in static or eccentric flexion of the PIP joint, and may increase the maximum muscular holding force. Friction therefore has to be taken into account as an important factor in the biomechanics of finger modeling in sport climbing and the explanation of the pathophysiology of pulley injuries.