A simulation of muscle force and internal kinematics of extensor carpi radialis brevis during backhand tennis stroke: implications for injury

Objective. The purpose of this work was use a computer simulation of the ac tion of extensor carpi radialis brevis during a typical backhand tennis str oke of novice and advance players to examine a potential mechanism of injur y. Design. This study uses established kinematic data in conjunction with a co mputer model to give a time varying description of muscle force and length changes. Background. Lateral epicondylitis or tennis elbow has been attributed to ov er-exertion of extensor carpi radialis brevis with novice tennis players be ing particularly susceptible. Methods. We used a simple Hill-type muscle model to predict muscle force an d internal kinematics based on activation and joint angle changes as inputs . Magnetic resonance images were used to determine the morphometric dimensi ons of extensor carpi radialis brevis which were used to scale the mechanic al properties determined from in vivo contractions of flexor pollicis longu s. Results. The simulation indicated that the novice group generated considera bly less force and the muscle was subjected to a substantial eccentric cont raction as a result of racquet-ball impact. This eccentric contraction occu rred with the muscle at a very long length with diminishing tension capabil ities. Conclusion. The observed pattern of activation and joint kinematics of novi ce tennis players results in substantial eccentric contractions which are l ikely the cause of repetitive microtrauma leading to tennis elbow injuries. Adopting the technique seen in advanced players would limit the eccentric contractions and reduce the likelihood of injury.