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

Abstract

Objective.The purpose of this work was use a computer simulation of the action of extensor carpi radialis brevis during a typical backhand tennis stroke of novice and advance players to examine a potential mechanism of injury.

Design.This study uses established kinematic data in conjunction with a computer model to give a time varying description of muscle force and length changes.

Background. Lateral epicondylitis or tennis elbow has been attributed to over-exertion of extensor carpi radialis brevis with novice tennis players being particularly susceptible.

Methods. We used a simple Hill-type muscle model to predict muscle force and internal kinematics based on activation and joint angle changes as inputs. Magnetic resonance images were used to determine the morphometric dimensions of extensor carpi radialis brevis which were used to scale the mechanical properties determined from in vivo contractions of flexor pollicis longus.

Results. The simulation indicated that the novice group generated considerably less force and the muscle was subjected to a substantial eccentric contraction as a result of racquet-ball impact. This eccentric contraction occurred with the muscle at a very long length with diminishing tension capabilities.

Conclusion. The observed pattern of activation and joint kinematics of novice tennis players results in substantial eccentric contractions which are likely 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.

Relevance

Lateral epicondylitis can be extremely problematic because of its chronic nature and relatively high incidence. This study offers one aetiology of the condition that results from improper kinematics during the tennis backhand stroke.

Introduction

Lateral epicondylitis or tennis elbow is manifested by pain over the region of the lateral epicondyle of the humerus [1]. One aetiology of lateral epicondylitis (LE) is thought to be over exertion of the extensor carpi radialis brevis (ECRB) muscle resulting in microtrauma [2]. There is a risk of injury to persons performing repetitive wrist movements in sports such as tennis.

It appears that novice tennis players in particular are at risk of developing LE and often report pain on performing a backhand stroke. Lateral epicondylitis occurs in up to 50% of recreational tennis players, most of whom are over 30 years of age [2]. Examination of the kinematics of the backhand stroke initially led investigators to suggest that skilled players maintain a constant wrist position, whereas novice players are likely to use more wrist motion to produce the stroke [3]. While increased grip pressure has been suggested as an expert-novice difference that may contribute to over exertion of the ECRB muscle [3], measurements of grip pressure in expert and novice participants have revealed no differences when performing a backhand tennis stroke [5]. Recently, Ingelman [4] and Blackwell and Cole [5] investigated the kinematic and EMG profiles of expert and novice tennis players performing the backhand stroke and showed distinct differences between the two groups. The wrist joint angle patterns observed in novice players may indicate conditions that promote wrist extensor injury through eccentric contraction of the ECRB. Injury may result from at least two possible mechanisms. First, eccentric muscle contraction has been shown to result in muscle injury [6] characterised by muscle fibre degeneration. The second possible mechanism concerns the muscle's isometric force–length relationship which may move beyond its plateau, to a position that is less advantageous in counteracting the large loads experienced during the imposed stretch resulting from ball impact [5]. Until now, muscle forces and internal muscle kinematics could not be assessed in order to confirm or refute the speculative nature of the potential injury mechanisms.

The purpose of this paper is to use a computer simulation of the action of ECRB using established kinematic and EMG data describing the backhand tennis stroke [4]. It was expected that both qualitative and quantitative differences in the action of ECRB in advanced and novice players would be predicted and that these differences would help to explain why novice players are more at risk of developing lateral epicondylitis. The rationale for using a subject specific model was to examine the influence of wrist kinematics and activation pattern upon ECRB dynamics independent of the influences such as differences in experience and muscle morphology that may exist between these two groups.