# Upper Extremity Kinematics and Electromyographic Activity in Uninjured Tennis Players
## 4 Discussion
### Two-Handed Backhand  
The normative data for the two-handed backhand groundstroke have applications in injury prevention. At peak backswing, the elbow is flexed bilaterally, and the wrist unlarly deviates bilaterally, with the non-dominant wrist positioned in a slightly increased position of unlar deviation, compared with the dominant wrist. The non-dominant side is positioned with a flexed wrist and pronated forearm, while the dominant side presents an extended wrist and supinated forearm. Moving from peak backswing to ball impact, the elbow maintains flexion, and increases in wrist ulnar deviation are seen bilaterally. Forearm and wrist sagittal plane positions are maintained bilaterally. Peak ulnar deviation does not occur at ball impact bilaterally, but at 0.10 ms and 0.12 ms after ball impact, on dominant and non-dominant sides, respectively. The kinematics described in this study agree with previous reports of the two-handed backhand [17]. EMG activation occurred by peak backswing for all muscles and was maintained throughout the swing. Non-dominant FCU onset occurred the closest to peak backswing, and the ECR experienced the most activation among subjects throughout the entirety of the swing bilaterally. The EMG  
Figure 5: SPM analysis of the forehand and two-handed backhand groundstrokes in the groupings of age, sex, and USTA ranking. The graph represents 500 ms before and 300 ms after ball impact, with t(0) = ball impact. The y-axis represents degrees with zero representing neutral. The dotted vertical line represents the end of backswing \(\pm\) 1 standard deviation. The solid vertical line represents ball impact. Areas of significant difference are indicated by black horizontal bar. Five areas of significance were found in the two-handed backhand: (**a**) younger subjects (ages 12–13, blue solid line) maintained a more pronated wrist immediately after ball impact than older subjects (ages, 15–17, red solid line). The most differences were found between sex (male—blue dotted line, female—red solid line). Female subjects maintained significantly greater elbow flexion in both the dominant (**b**) and non-dominant (**c**) elbow during racket acceleration. Female subjects maintained significantly greater wrist flexion angular velocity (**d**) during backswing; (**e**) subjects with a USTA ranking <500 (blue dotted line) reached greater wrist extension angular velocity right before ball impact than those with a ranking >500 (red solid line). One area of significance was found in the forehand: (**f**) male subjects (blue dotted line) maintained significantly increased supination angular velocity at the end of swing.  
activation onset and offset times reported are like those observed in previous studies [27]. With the dominant wrist moving towards extension during the racket acceleration phase, it allows for a concentric coactivation of extensor muscles during ball impact, which has been shown to be advantageous [28]. Current knowledge of EMG studies suggests that novice players impact the ball with a dominant flexing wrist, causing repetitive eccentric contraction and lengthening of the wrist extensor muscles [29], which may cause injury due to the extensor activation during eccentric contraction where higher forces can be generated due to the muscle force-velocity relationship.