A PRELIMINARY KINEMATIC ANALYSIS OF THE UPPER LIMB DURING THE WHEELCHAIR TENNIS SERVE

INTRODUCTION Up to 78% of wheelchair users, athletes and non athletes, report shoulder pain [1]. The weight-bearing role of the shoulder is thought to be one cause as this joint was designed for mobility rather than for stability [2]. Wheelchair athletes, in addition, put increased load and repetitive stress through their shoulders during sporting activities. An accurate and quantitative biomechanical analysis of the athletes during the execution of the specific sport exercises is the only way to understand the causes of this overuse injury. This type of analysis has been performed extensively on able-bodied tennis players (APs) and weakly on wheelchair tennis players (WPs). Only a recent study examined the specific kinematic and kinetic analysis of wheelchair tennis serve [3]. However, the protocol adopted did not consider all the degrees of freedom of the shoulder joint. CLINICAL SIGNIFICANCE The aim of the present study was to better understand the causes of the shoulder joint overuse injury in WPs, using a validated specific protocol of the upper limbs [4]. METHODS Fig.1 Subject on wheelchair with marker-set. Two caucasian Italian right-handed athletes were acquired using a stereo-photogrammetric system (Vicon 612, 9 cameras, 100Hz). The first subject (SubW) was a WP (31st in the world ranking) and suffered from a complete L1 spinal cord injury, with no pathologies at the shoulder. The second subject (SubC) was an AP, middle national level. Two repetitions of the 9 combinations among 3 serve types were acquired: first and second, lateral and central, and from the deuce and the advanced service box. A validated specific protocol of the upper limbs was used [4]. From the biomechanical viewpoint, the racket arm was modelled as an open kinematic chain formed by 4 segments (thorax, shoulder girdle, humerus and forearm), with 7 degrees of freedom: 2 describing the mobility of the shoulder girdle [5], 3 of the glenohumeral joint, and 2 of the elbow. The following phases were identified: cocking (from the backswing racket’s highest point (BHP) to maximum external rotation of the glenohumeral joint (MER)) and forward-swing (from MER to the racket ball pre-impact (IMP))[3]. RESULTS 316 No differences of girdle, glenohumeral and elbow kinematics were found among serve types in both athletes. As expected, the trunk range of motion of SubW was limited with respect to SubC. The mean and standard deviation values of range of motion in the sagittal, the transverse, and the frontal plane were 29.2±5.9°, 36.5±5.2°, 45.7±5.0°, for SubW and 80.1±5.8°, 67.8±7.6°, 114.0±19.4°, for SubC. The girdle joint of SubW showed clearly a different pattern of movement in both degrees of freedom (fig 2). At the BHP the SubW girdle was more retracted (-32.6±5.1°) than the SubC girdle (-9.5±1.4°). This difference, during the cocking phase, increased and only at the IMP the subjects exhibited more similar values. Furthermore, the girdle of SubW was more depressed during the cocking phase reaching a minimum of -11.1±4.2°. As for the glenohumeral (fig 3) and the elbow joint, different pattern of motion in the cocking phase were found. On the contrary, during the forward swing phase, the players exhibited more similar pattern of motion (fig 3). Although the MER showed similar values (100.5±5.2° and 103.5±3.9°), it had a different timing (79% and 93% of the serve for SubW and SubC, respectively). DISCUSSION The comparison between AP and WP showed similar patterns for the glenohumeral and the elbow joints only during the forward-swing, while patterns of the shoulder girdle were different during the entire serve. The differences in the cocking phase are justified by the limited trunk range of motion and the constrained position on the wheelchair. The typical WPs shoulder pain could be related to these kinematics differences. However, to support these hypotheses a larger number of WPs and APs will be acquired and analyzed.