An investigation of elbow loading in one-handed tennis backhand groundstrokes using computer simulation

A 3D subject-specific computer simulation model of a ball-racket system linked to an upper-limb and torso was developed to investigate factors which may result in adverse loading at the elbow during one-handed backhand groundstrokes. Rigid hand, forearm, upper-arm and torso segments were driven by joint angle time histories obtained from backhand performances. Wobbling mass segments were incorporated to represent soft tissue motion. The upper-limb model was attached to a forward dynamics model of the racket-ball system using spring-dampers at the thenar and hypothenar eminences of the hand. The racket frame was represented using two rigid bodies with two torsional spring-dampers to allow motion in and out of the racket plane. The stringbed was represented by nine point masses connected using elastic springs. A point mass representation of the tennis ball allowed normal and oblique impacts at the nine locations on the stringbed. Inertia parameters for the elite tennis player and the rackets and visco-elastic parameters for the rackets and ball were determined from independent experimental tests. Visco-elastic parameters for the hand and wobbling masses were determined within the matching process of six backhand trials. Excellent agreement between performance and matching simulations was obtained with a mean RMS difference of 1.3% based on racket kinematics, outbound ball velocity and time of ball contact. Simulation results suggest that the inertia and stiffness parameters of the racket frame and the stringbed tension have a relatively small influence on elbow loading within current design ranges. In contrast, the off-centre ball impact simulations resulted in an 11% increase in peak internal elbow joint force, a 22% increase in peak pronation-supination net torque and a 19% increase in peak elbow-flexion extension net torque around the elbow joint. This research suggests that racket frame vibration is an unlikely mechanism for tennis elbow and that an accumulation of peak loads from off-centre hits is a more likely cause.