The serve is an important stroke in high level tennis. A well-mastered serve is a substantial advantage for players (Girard et al., 2005; Johnson et al., 2006). However, the serve is extremely complex and requires a wide range of technical and physical skills (Elliott, 2006; Girard et al., 2005; Kovacs and Ellenbecker, 2011). This stroke is learned and improved upon throughout the entire player career development process, from beginner to professional level (Whiteside et al., 2013). Because of its repeatability and its intensity, this stroke is potentially deleterious (Kibler and Safran, 2005; Martin et al., 2013a; Renstrom and Johnson, 1985). It could lead to various muscular and articular pathologies of the upper and lower limbs (Campbell et al., 2014; Kibler and Safran, 2000; Perkins and Davis, 2006; van der Hoeven and Kibler, 2006) but also of the trunk (Maquirriain et al., 2007). The trunk is at the center of energy flow (Martin et al., 2014) observed during the proximo-distal sequence (Kovacs and Ellenbecker, 2011; Kibler and Van Der Meer, 2001; Liu et al., 2010). Previous studies show that abdominal muscle disorder could be a source of potential risk for local injury in tennis (Natsis et al., 2012; Sanchis-Moysi et al., 2010), however, it is not yet demonstrated that a specific serve kinematic could cause abdominal disorder during this energy transfer (Bahamonde, 2000; Girard et al., 2005; 2007b).
The two-dimensional method has been used for a long time to analyze tennis serving (Bahamonde, 2000; Sprigings et al., 1994). However, 3D methods enable more objective quantification of this stroke. Indeed, 3D methods precisely measure the kinematic of the body segments (Elliott et al., 2003; Tanabe and Ito, 2007). Authors collect high accuracy and high frequency 3D data in all three planes of space. In addition to 2D or 3D, researchers utilize force plates, radar and isokinetic dynamometer to evaluate performance (Antunez et al., 2012; Croisier et al., 2008; Elliott et al., 1986; Forthomme et al., 2013; Girard et al., 2007b; Julienne et al., 2012; Silva et al., 2006). The combination of all these techniques in a kinematic and kinetic analysis could be an original way to better understand the tennis serve mechanism and so optimize performance and prevent injury (Abrams et al., 2011; Elliott and Reid, 2008; Kovacs and Ellenbecker, 2011; Knudson, 2007).
Biomechanics play an important role in comprehension, prevention and management of injuries caused by sport practice (Abrams et al., 2011; Chan et al., 2008). The literature describes generalities of the tennis serve movement (Kovacs and Ellenbecker, 2011) but the throwing gesture, and particularly the service action itself is unique and specific for each individual player. It is therefore interesting to provide an individualized analysis of the player kinematic. In this case report, we performed a kinematic analysis of a high level tennis player with a previous history of abdominal injury. The injury originally appeared during a tennis service movement. We discuss retrospectively his kinematic during his serve. We expect that a combination of medical examination and kinematic analysis can help us to better understand the injury mechanisms. In order to have a reference, this study compares the previously injured player with a non-injured reference group composed of five international Professional Tennis Association (ATP) ranked players. The aim of our study is to provide a hypothesis of the injury mechanism based on a biomechanical evaluation.
The injured athlete was a 22 year-old international tennis player (height: 1.80 m and weight: 69.8 kg). He is right-handed and was ranked in the top 50 of the ATP in 2014.
History: The player suffered from a medical tear on the left rectus abdominis muscle. According to the player, the pain "appeared in the beginning of the trunk flexion when the trunk was in extension and starting the flexion". At that moment of the stroke, abdominis muscles would have been at the end of eccentric contraction and at the beginning of concentric contraction.
A 12 mm tear located on third bottom of left rectus abdominis was objectified by clinic and para-clinic examinations. MRI (Magnetic Resonance Imaging) showed a hypertrophy of rectus abdominal muscle and was confirmed by ultrasound diagnosis. This hypertrophy had already been demonstrated for other professional players (Sanchis-Moysi et al., 2010) as a specific localized site of injuries caused by the tennis serve (Maquirriain et al., 2007, Natsis et al., 2012, Chow et al., 2009, Balius et al., 2012).
Treatment and back assessment: Following the diagnostic, the player performed 18 sessions of physiotherapy treatments. Thereafter, an experienced physiotherapist performed an isometric evaluation of the player trunk muscles (flexors, extensors, lateral-flexors and rotators) using specific trunk dynamometers (the David 110, 120, 130 and 150) and in accordance with the manufacturer's instructions regarding placement (David Back[TM], David Health Solutions Ltd, Helsinki, Finland) (Grosdent et al., 2014). Results showed a weakness of the right lateralflexors (2.67 N.m.[Kg.sup.-1]) in comparison with the left lateral-flexor muscles (3.32 N.m.[Kg.sup.-1]). In addition, we observed that the agonist/antagonist ratio (flexors/extensors) for this player is 0.77 which is higher compared to the classical value seen in professional tennis players (0.57), highlighting dominance of flexors muscles of the player (Grosdent et al., 2014).
After treatment, and with the aim of better understanding the abdominal injury, the player carried out a 3D kinematic evaluation of his serve as well as functional evaluations: passive joint mobility and isokinetic force. Afterward, we compared the results of the player with the reference population who had performed the same assessments in standardized conditions.
Follow up: A few weeks after these evaluations, the player presented a new injury, a tear on the distal insertion of the right psoas muscle. This injury caused a temporary cessation of competition.
The study protocol reported is approved by the Medical Ethics Committee of the University of Liege. The established protocol provides reproducible results when analyzing the tennis serve.
Reference population: We compared the results of the injured player with those of five professional players among the top 600 ATP rankings. All the players are right-handed, 22 years old ([+ or -] 3), 75 kg ([+ or -] 4) and 1.81m ([+ or -] 0.02). At the time of testing, all players were considered as being fit for competitive practice. Except for our case study subject, no other player reported abdominal tear history. No players reported significant joint injury, history of pain or surgery on the dominant arm or their legs. They performed all the evaluations (a 3D evaluation, a passive joint mobility and an isokinetic force assessment) within a one to three week period.
3D kinematic and kinetic evaluation: In the laboratory, we reproduced one half of a tennis court (Figure 1). The width of our court was smaller (5.8 m) than the normal size (8.23 m) in order to fit into the laboratory. Players served from two force plates located behind the baseline. We placed the net at a regulatory distance and height (International Tennis Federation, Roehampton, England) from the baseline and ground.
Before the tests, the players performed a general cardio-vascular warm-up with lower limb, (skipping rope, running and/or ergometric bicycle) and upper limb (rubber band) exercises. Afterward, they undertook a general short stretching routine for legs and arms. Finally, players engaged in a specific warm-up procedure for tennis serves, first without markers and then with markers placed on the skin. This specific warm-up allowed players to get familiar with the laboratory context (field, target and markers on the skin). Each player decided the number of serves necessary for warming-up and for familiarization with a maximum of 30 serves allowed in order to avoid fatigue.
After the general and specific warm-up, the test began and the players served 25 times each, with 30 seconds between each serve. The instructions were to serve in the target ("T" area) with the highest ball speed possible and minimal ball rotation (flat serve). Afterward, the three best serves were kept for analysis (Reid et al., 2015, Whiteside et al., 2014) in order to consider the derivation of accurate and representative movement kinematics (Mullineaux et al., 2001). The selection criteria were precision (serve performed successfully in the 1 [m.sup.2] area or "T" zone of the deuce square (Gillet et al., 2009)) and highest forward velocity of the racket at impact (Reid et al., 2014, Whiteside et al., 2014).
We used a three-dimensional optoelectronic system (Codamotion[TM], Charnwood Dynamics, Rothley, UK) to measure the movements. We tracked the 3D positions of the player's racket, dominant arm and forearm, trunk, pelvis and legs with 28 markers and four Codamotion CX1 units. The acquisition rate was equal to 200 Hz.
We placed three markers on the trunk, three markers on the pelvis, four markers on both legs, four markers on the dominant arm, four makers on the dominant forearm and three on the dominant hand in accordance with the recommendations of the International Biomechanical Society (ISB) (Wu et al., 2002; 2005) (Figure 2A). We also placed three markers on the racket: one on each side and one on the top (Martin et al., 2014, Martin et al., 2012) (Figure 2B). We identified additional anatomical points by reference to the placed markers: T8, left and right posterior-superior iliac spine, dominant side lateral epicondyle, dominant side medial epicondyle and center of dominant side glenohumeral joint (Figure 2A).
The marker placement allowed us to measure the ankle, knee, pelvis and shoulder joints and segments' amplitude ([degrees])...