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The aim was to investigate injury risk factors in junior tennis players. Fifty-five players, 35 boys and 20 girls, answered a questionnaire about training habits, time of exposure, previous injuries and equipment factors. A battery of clinical tests and functional performance tests were also carried out. All tennis-related injuries that occurred during a 2-year period were identified and recorded. An injury was defined as an injury if it was impossible to participate in regular tennis training or playing matches during at least one occasion, a time loss injury. Potential injury risk factors were tested in a forward stepwise logistic regression model for injury. Thirty-nine players sustained totally 100 new and recurrent injuries. Injuries to the lower extremity were the most common ones (51%) followed by the upper extremity (24%) and the trunk (24%). Injured players performed more singles per week (P<0.0001) and played more tennis hours per year (P=0.016) than the uninjured players. Playing tennis more than 6 h/week was found to be a risk factor for back pain. A previous injury regardless of location was identified as an injury risk factor, and a previous injury to the back was a risk factor for back pain.
The overall injury rate in tennis varies greatly from 0.04 injuries/1000 h to 21.5 injuries/1000 h of playing tennis depending on injury definition (Pluim et al., 2006). In the majority of investigations at the junior level, elite junior players have been the population studied, mostly during tournaments (Kibler & Safran, 2005). The more an athlete is exposed for tennis and other sports, the higher the frequency of injuries according to Kibler and Safran (2005). Recreational junior players experience relatively few injuries (Hutchinson et al., 1995; Kibler & Safran, 2005). Pluim et al. (2006) found only three investigations on injury risk factors in tennis and these studies have a number of limitations. Many potential injury risk factors have been discussed in the literature. The majority of these have not been verified to be risk factors, although.
Muscle imbalance in terms of changes in strength and flexibility, can alter joint biomechanics and lead to decreased maximal force output (Kibler & Safran, 2000). Tennis players often present with a decreased internal rotation of the shoulder joint and an increased external rotation of their dominant side, which may result in clinical implications in terms of injuries (Chinn et al., 1974; Chandler et al., 1990; Ellenbecker et al., 1996, 2002; Kibler et al., 1996; Vad et al., 2003; Schmidt-Wiethoff et al., 2004). Elite junior tennis players with symptoms from their dominant shoulder joint showed a decreased internal shoulder rotation compared with asymptomatic players of the same age and level (Vad et al., 2003). Players with back pain from the lumbar spine showed a decreased internal rotation of the leading hip joint as well as deficits in extension of the lumber spine (Vad et al., 2003).
Oversized head, heavier, stiffer, more tightly strung rackets, incorrect grip size and increased vibration have been associated with problems like “tennis elbow” (Kamien, 1989; Pluim et al., 2006). However, this is a fairly rare syndrome in young tennis players (Pluim et al., 2006).
A significantly lower injury rate has been reported by tennis players when playing on clay or synthetic sand when compared with playing on hard court (Nigg & Segesser, 1998).
The purpose of the present study was to evaluate potential intrinsic as well as extrinsic injury risk factors in junior tennis players.
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The main finding of the present investigation was that a previous injury regardless of body location was found to be an injury risk factor. In addition, we also found that a previous injury to the back and playing tennis more than 6 h per week were identified as risk factors for back pain.
Earlier studies on other sports have shown strong evidence that a previous injury increases the risk of a recurrent injury or a new injury (Van Mechelen, 1992; Van Mechelen et al., 1993; Murphy et al., 2003). This finding has led to a general suggestion, although not evidence based, that the injury risk increases with inadequate rehabilitation. In our opinion, a young player may benefit by seeing a professional therapist with good knowledge of the biomechanics and profile of demands in tennis before considering a return to tennis after an injury. It is of utmost importance to include sufficient time for recovery before a gradual return to playing tennis is allowed. Furthermore, functional performance tests tailored for tennis players may to some extent predict a “safe” return to sport. However, hitherto there is no consensus on what specific tests that should be used for evaluating tennis players. To our knowledge, no studies have been performed about this.
Pluim et al. (2006) reported exposure to tennis to be a risk factor for injuries in general. In our study, we found that exposure to tennis was a risk factor for sustaining an injury to the back. In addition, the injured players in the present study played more than double the amount of single sessions a week and played almost the double amount of hours per year than the uninjured group. Furthermore, the lesser number of years of playing tennis the higher the risk of injury to the upper extremity. The reason for this finding although is difficult to explain. One explanation could be that these junior players have not yet learnt a good stroke technique. This may be true for the service motion, the most difficult and powerful stroke in tennis, which may result in a disturbance in the kinetic link (Kibler, 1995). If there are any deficits in strength anywhere in the kinetic chain, there will be an increased load of other joints and muscles, which may lead to increased injury risk. The kinetic chain is crucial for developing power in the strokes (Kibler, 1995). Fifty-four percent of the total force is developed in the leg/hip/trunk link and 21% in the shoulder. Many young tennis players present with weak lower extremity muscles, and therefore do not have sufficient strength to hit the ball the way the older players do (Kibler, 1995). They must instead rely on biomechanical efficiency to hit effective shots. However, we did not find any differences in leg strength between injured and uninjured players when measured with the one-leg hop test for distance and the vertical hop test.
Although not significant, players with lower extremity injuries tossed 1.0–1.6 m shorter in all the medicine ball tests. This may stress the importance of a good balance in terms of muscle strength between the lower and upper body (Chandler et al., 1998). Muscle weakness of the trunk and upper extremity may put high demands on the lower extremity and thereby an increased injury risk. Roetert et al. (1996) found significant correlations between medicine ball tosses (reverse, overhead, backhand and forehand) and isokinetic muscle torques of the trunk muscles during flexion and extension. Thus, this may indicate that medicine ball tosses are likely to put stress on the strength of the trunk. Stockbrugger and Haennel (2001) however, showed that the reverse medicine ball toss was correlated to the vertical hop test, and therefore suggested that this test may reflect the movement pattern of the total body.
Several authors have discussed tennis-generated musculo-skeletal maladaptations such as differences in a range of motion and muscle strength between the dominant and non-dominant side as being potential injury risk factors (Knapik et al., 1991; Young et al., 1996; Chandler et al., 1998; Kibler & Safran, 2000). Renkawitz et al. (2006) found distinct neuromuscular imbalances between the right and left erector spinae at the second and fourth vertebrae of the lumbar spine during maximum voluntary trunk extension among tennis players with low back pain. Players without low back pain did not show any such imbalances. Interestingly, they also found that the direction of electromyographic (EMG) imbalance was amazingly closely related to handedness. Nearly all right-handed players showed significant lower integrated EMG (IEMG) measures on the left side of erector spinae, and left-handed players showed lower IEMG on the right side. The authors state that these results suggest that asymmetric trunk loading as it is in tennis and other racquet sports induce neuromuscular imbalances Renkawitz et al., 2006). However, the question whether these changes is a cause or a result of low back pain remains unanswered. In the present study, players with a side-to-side difference in total shoulder rotation, and a muscle strength difference of more than 10% between forehand and backhand medicine ball tosses showed a tendency to back pain. Players with a side-to-side difference in scapular winging showed a tendency to upper extremity injuries.
A decrease in internal rotation of the shoulder joint has been reported to be associated with both age and number of years in tennis and may further be a risk factor for injury (Ellenbecker et al., 1996; Kibler et al., 1996; Vad et al., 2003). In our study, we did not find any differences in the range of motion of the shoulder joint between injured and uninjured players. One reason may be the young age of our players and that they sustained relatively few shoulder injuries. However, we found that players who sustained injuries to the lower back presented with a decreased range of motion in several joints including the shoulder joints, compared with uninjured players, although only the dominant and non-dominant lateral flexion of the neck were found to be significant.
Chandler et al. (1990) found a decrease in the sit and reach test in tennis players compared with athletes in other sports. In the present investigation, we did not find any difference in sit and reach test between injured and uninjured players. However, we found a decrease in the non-dominant lateral flexion of the neck in players with back pain. This may force the player to increase the extension of the back, in particular lumbar lordosis, at early cocking and cocking phase in the service motion leading to an increased load of joints, ligaments and muscles of the back and thereby a risk of being injured. As the movements of the cervical spine in the early cocking and cocking phase in the service are a combination of lateral flexion of the neck at the non-dominant side, rotation and extension of the dominant side, we do not know whether this range of motion was even more decreased in players with back injuries, as combined movements were not tested. Young et al. (1996) hypothesized that the cervical spine may be involved in force generation during the throwing or tennis serving motions through the mechanism of torque–counter torque as well as of the lower back. However, this is an area that needs to be investigated more thoroughly.
Imbalance between muscle strength and flexibility may be risk factors for injury as specific strength and flexibility imbalances have been reported to be associated with lower extremity injuries in female collegiate athletes, tennis included (Knapik et al., 1991). In the present investigation, we did not, however, find any such imbalances in the players with lower extremity injuries. We found that the players who regularly performed stretching exercises “in general” had sustained a lower extremity injury more often than players who did not stretch. This is in agreement with a study by van Mechelen et al. (1993). In accordance, a systematic review about the efficacy of static stretching as part of a warm-up for the prevention of exercise-related injuries, showed moderate to strong evidence that static stretching does not reduce overall injury rates. However, the same review reports that there is preliminary evidence that static stretching may reduce musculo-tendinous injuries (Small et al., 2008).
Some limitations of the present study need to be addressed. The sample size was somewhat small because we only included junior players from one typical tennis club and the participation was voluntary. Furthermore, some members did not fulfil the inclusion criteria by being healthy and uninjured at the start of the study. We had 10 players (15%) that dropped out. However, these 10 players did not differ from the other 55 players that completed the investigation in any parameters that we have studied. According to Bahr and Holme (2003) when identifying injury risk factors in sports, 20–50 participants are needed for a moderate to strong association and 200 for a small to moderate association.
Based on the literature when it comes to power calculation, we performed an 80% power analysis for the medicine ball toss performance and found quite large differences (1.6−1.0 m) between players who sustained an injury to the lower extremity and those who did not sustain any lower extremity injury. For this variable/factor, the power analysis showed a need for 279 players. On the other hand, we also performed a power analysis using an identified significant injury risk factor, earlier injuries for injuries in general, which showed a power of over 0.99. These examples really show how difficult it is to make a proper power analysis in this type of research.
Those players that reported a previous injury regardless of injury location showed a higher number of injuries during the 2-year registration period. Injury to the back was the only previous injury location that was identified as a risk factor for a new or recurrent injury at the same location. The young players had, however, some problems to describe their previous injuries in detail in the open questions, which is a study limitation. Therefore, a well-structured interview may probably have been better in order to obtain a more complete picture of their previous injuries.
Recall bias has been reported to be a major problem when using a self reporting study design (Junge & Dvorak, 2000). We were in contact with our players every third month in order to make sure that no injuries were forgotten to be reported. The PI, who worked at a physical therapy clinic in connection with the tennis arena, had a close collaboration with both the tennis coaches and the players throughout the entire study period (2 years). This may at least to some extent prevent recall bias.
In light of this investigation, we conclude that a previous injury regardless of body location was found to be an injury risk factor. Furthermore, a previous injury to the back and playing tennis more than 6 h per week were identified as risk factors for back pain.