A vital concern of tennis performance is the ability to repeat intermittently muscular force at high speed (14). Nevertheless, the ultimate functional performance of any complex chain of torque transfers (i.e., serve or ground strokes in tennis) depends on several factors including technique, flexibility, muscle strength, speed, and power (6). Standardized testing is commonly used to provide a useful supplement to subjective coaching appraisals in an attempt to assess strengths and weaknesses of a given player. Therefore, research has been conducted with athletes of various backgrounds (e.g., age, performance level) in order to identify the most influencing factors of significance in successful tournament play (3,17,22,23).
In a 5-year study of 500 preadolescent athletes, a number of track (e.g., running, jumping) parameters were found to be weak predictors of overall tennis performance (3). This is not surprising considering the relative lack of physical development in young prepubescent (8-12 years old) males and females at this stage of maturation (20). Rather, it appears that success in this group of young tennis players can be primarily attributed to their ability to generate consistent, accurate, and powerful shots (22).
Few studies have addressed the performance needs of competitive teenage (12-16 years old) tennis players (23). Dramatic changes in physiological attributes typically occur at the age of about 12-15 years, but there is a large intersubject variability in the timing of maturation. For example, the age of peak height velocity is known to influence to a great extent the physical factors. However, it is still unclear how the stage of maturation influences the acquisition of the specific/technical skills (18). Therefore, the influence of physical abilities on tennis play may be more apparent during this time of development due to dramatic increase in strength, size, and endurance (20). Knowledge of the relationship between various physical attributes (e.g., speed, explosive power, leg stiffness, muscular strength of upper and lower limbs) and the ranking of adolescent tennis players could assist in determining the relative importance of such measures and providing optimal training programs. In addition, little is known regarding the nature of the relationships between such physical variables in this age group of tennis players.
Therefore, the aims of this study were (a) to examine the relationship between several physical attributes (e.g., speed, explosive power, leg stiffness, muscular strength of upper and lower limbs); and (b) to determine to which extent they relate to tournament play in a group of competitive teenage (14 year old) tennis players.
Experimental Approach to the Problem
Subjects were tested in early July during a summer training camp. After a standardized warm-up lasting approximately 15 minutes (e.g., submaximal run, accelerations and knee extensions), each subject repeated a series of physical tests three times to assess their speed, explosive power, and muscular strength on two separate occasions (day 1: sprinting and muscular strength; day 2: jumping and hopping). These dependent variables were used to describe the link between physical attributes in competitive teenage tennis players.
Based on the ranking established by their national tennis federation, calculated from the results of the preceding season achieved in official tournaments, the subjects were ranked 1 to 12. Within 2 weeks, they competed in a tournament to adjust the ranking of homogenous players (e.g., performance level, tennis stroke ratings) based on their final results. This procedure was used to determine to what extent physical abilities relate to competitive ranking in this age group of tennis players. All testing sessions were conducted on an outdoor Greenset tennis court.
A total of 12 male junior tennis players (mean age 13.6 ± 1.4 years; height 163.4 ± 11.1 cm; weight 50.5 ± 11.7 kg) with an International Tennis Federation (ITF) number ranging from 3 to 6 volunteered to participate in this study. Regarding tennis experience, each athlete had played competitive tennis for a minimum of 4 years prior to the start of the study. All subjects were members of various tennis academies and trained 7.2 ± 1.8 h·wk−1 in the 6 months preceding the experiment.
The program focused on development of on-court technical/tactical player's behavior, as well as on tennis-specific aerobic and anaerobic capabilities enhancement, which included on court-aerobic exercises, agility exercises, and repeated sprints. Additionally, the players performed a variety of plyometric (e.g., medicine ball, hopping) and resistance training modalities (e.g., rubber tubing) in order to build a good strength base but without using free weights. Their maturational status was estimated at pubertal stage III (n = 6) or IV (n = 6) according to Tanner classification via direct visual observation of primary and secondary sexual characteristics (e.g., abdominal, shoulder, chest and facial hair; Adam's apple; voice) (25). Subject and parental written informed consent was given prior to participation in this investigation. The human subjects committee of the local university approved the study.
Physical tests were performed for sprinting, as well as for jumping and hopping. For sprinting, subjects were asked to run a 5-m, 10-m, and 20-m sprint as fast as possible. In both tests, subjects had to complete the distance in a straight line alongside the tennis court as fast as possible. The feet were placed side to side in the tennis-ready position behind the starting line. For each distance, the time trials were measured using two photocells connected to an electronic timer (Globus, Treviso, Italy).
For jumping and hopping, athletes performed the following vertical jumps/hopping tests with hands kept on the hips to minimize the contribution of the upper limbs: (a) squat jump (SJ) starting from a static semisquatting position (∼90° of flexion) maintained for ∼1 second and without any preliminary movement; (b) countermovement jump (CMJ) starting from a standing position, squatting down and then extending the knee in one continuous movement; (c) drop jump (DJ) starting from a standing position on a 30-cm height, dipping, and then extending the knee in one continuous movement; (d) multi-rebound jumps (MRJ) rebounding to the highest possible point 6 times. For MRJ, subjects were asked to keep their knees as stiff as possible and to have as brief a contact time as possible.
An electronic timer was connected to an optical acquisition system (Ergotest, Azur Systèmes, Grasse St-Jean, France) for measuring ground contact (MRJ, DJ) and flight times (SJ, CMJ, DJ, and MRJ). From the flight times, the peak power (W·kg−1) in the SJ, CMJ, and DJ tests was calculated (15). From the flight and contact times, leg stiffness (K in N·m−1·kg−1) and reactive power (W·kg−1) in the MRJ test were estimated (7).
In order to assess the muscular strength of upper and lower extremities, two isometric tests were performed: (a) grip strength of the dominant and nondominant arm was measured using a handgrip dynamometer (Captels, St. Mathieu de Treviers, France); (b) maximum voluntary contraction torque of the dominant and nondominant plantar flexors was recorded by a dynamometric pedal (Captels, St. Mathieu de Treviers, France). The subject's position was standardized with pelvis, knee and ankle angulations of 90°, the foot securely strapped on the pedal.
Before data collection, the subjects had to perform each test twice. Whatever the testing modality, subjects were then requested to exert a maximal effort three times, with a 1-minute rest between trials to avoid fatigue effects. The mean performance was retained and used in subsequent statistical analysis. During the tests, the subjects were verbally encouraged to produce maximal efforts.
Intrasession variability was assessed by determining the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) for each dependent variable. Both CV (absolute reliability) and ICC (relative reliability) refer to intrasubject variation between measurements (1). A coefficient of variation (CV = [SD / mean] × 100) was calculated for each variable. ICC analysis was calculated using a downloadable spreadsheet (16).
Pearson correlation coefficients were used to examine the relations between study variables. Anthropometric characteristics and physical performance scores were correlated with the ranking of the players using Spearman rank order correlation. Grip strength and plantar flexor torque measured at the dominant and nondominant limbs were compared using a student t-test. Values of p ≤ 0.05 were considered to be statistically significant.
Table 1 displays the results of various athletic performance variables. Intrasession variability (intertrials reliability) was high, with CV less than 10% and ICC greater than 0.80 for each dependent variable. The relationships between the competitive ranking order and the physical performance scores are shown in Table 2. The correlational matrix for the relationships between anthropometric characteristics and physical performance variables are shown in Table 3.
Competitive teenage tennis players tested in the present study possess physical attributes values similar to those recorded in players of similar age (14), higher than prepubescent competitors (22) but obviously lower than adults (13). Of importance is the high intraplayer's consistency, with ICC > 0.80 and CV < 10%.
In this study, sprint, vertical power abilities, and maximal strength in the dominant limbs (both handgrip and plantar flexor) were found to be good predictors of tennis performance. These results are not in good agreement with previous findings on advanced prepubescent male tennis players (8-12 years old), which indicated that physical performance tests in this group of young athletes do not predict their ability to play tennis at a competitive level (3,20). For example, Birrer et al. (3) found, in a 5-year study of over 500 preadolescent athletes, that athletic ability performance parameters were poor predictors of rankings. More recently, Roetert et al. (22) concurred with the findings of Birrer et al. (3). They indicated that agility was the only physical performance variable used to predict competitive rankings in 83 prepubescent tennis players.
Investigating relationships among an array of performance and clinical variables (e.g., dynamic, isometric, and isokinetic strength; joint laxity and flexibility; speed; agility; peak oxygen consumption; ball velocities of the serve, forehand and backhand) in female collegiate (∼20 years old) tennis players, Kraemer et al. (17) observed that no single variable strongly explained tennis performance. Taken together, these findings also reinforce that performance in tennis is multifactorial; it depends on mental, tactical, and technical factors, as tennis stroke ratings appeared to be strong predictors of tournament play in preadolescent tennis players (3).
Nevertheless, the difference regarding the nature of the link between physical attributes and player's ranking between previous studies focusing on young tennis players (8-12 years old) and the present investigation on teenage players might be due to progressive increases in muscle strength and therefore speed and power during puberty (18,20). The chronological age of the present players (13.6 ± 1.4 years) is very close to the age of peak height velocity (13.8 ± 0.8 years) described in similar male European adolescents (19). In addition, the estimation of Tanner stages confirms that the subjects were teenage players.
The present study was also done to identify the nature of the link between several physical attributes in adolescent tennis players. Although sprint times, muscle power, leg stiffness, and muscular strength are implicit determinants of tennis performance (22), the significant correlations observed between sprint times, vertical jumps and muscular strength of the dominant limbs underline the importance of muscle strength and power in the lower extremities to produce rapid on court movements in teenage tennis players. Positive correlations between speed (e.g., 10-m and 20-m sprints) and vertical jump ability (e.g., power during CMJ and DJ) have already been observed in women players (17). It is also interesting to note that leg stiffness is a quality independent of leg strength/power, since K was related neither to maximal voluntary contraction torque in plantar flexors nor to vertical jump performance (e.g., SJ, CMJ, DJ). Functional links between sprinting performance and muscle power, which are needed to produce initial acceleration, have been also observed in a group of adolescent handball players (5).
Leg stiffness has been reported to be an influential factor of sprinting performance, at least after the acceleration phase for maintaining maximal velocity (5). This is confirmed by our results because K was not correlated to sprint performance over 10 or 20 m corresponding to the acceleration phase in (5) but interestingly correlated to 5-m time. However, the 5-m performance is strongly dependant on factors other than leg power, as the starting position or the reaction time. Another interesting result is the correlation observed between K and PF-D. It is known that leg stiffness while hopping depends on the ability to control the pronation (11). To our knowledge, the relationship between this ability and the ankle strength (e.g., in the present case, PF strength) is unanswered.
Although the standard muscle strength (e.g., MVC torque, grip strength) and power (e.g., vertical jumps, hopping) tests used in the present study could be valid for general assessments of neuromuscular function, their external validity in terms of their relationships with performance in various functional movements tests (i.e., serve or ground strokes in tennis) was shown to be moderate, if not insignificant (26). In addition, it is unclear if vertical jump performance or MVC torque measured isometrically could reflect lower-limb activity during tennis strokes production or on court movements. In this view, previous studies have demonstrated that only a few vertical jumps or sprints (3,22) or isometric/dynamic knee or shoulder strength (17) values correlated with tennis performance (e.g., ball velocities, tennis stroke ratings).
Of interest is that the isometric strength was greater for both the dominant handgrip (17%) and plantar flexion (12%). Large side-to-side differences in grip strength have been reported [e.g., ∼20% in (17)] between the dominant and nondominant upper limbs, with larger differences recorded in competitive teenage tennis players than in nontennis players [e.g., swimmers, sedentary subjects (2)] or in tennis players with a lower ranking (21). Handgrip has been used previously in tennis as a static measurement (24). It is known that the dominant arm is stronger in elite players, even in junior categories, such as the internal rotators (4) or the elbow extensors (9). The greater handgrip force observed at the dominant side may be attributed to the unilateral loading (e.g., racket control) involved during tennis playing. To our knowledge, this was the first time that such asymmetry is reported for the lower limbs in adolescent players.
Contradictory to the present study, bilaterally symmetric knee extension and flexion strength have been reported in elite junior (8,10) and competitive women (21) tennis players. However, due to differences in the anthropometric/training characteristics (e.g., training volume, sex, age) of the players and in the nature of the tests performed (e.g., strength indicators used, muscular groups tested, and/or type of contraction involved) between the aforementioned studies, direct comparison is irrelevant. Interestingly, Ellenbecker et al. (10) observed that increase in normalized knee extension strength occurred until the age of 14 years in male tennis players, but did not change after this chronological age. It is known that the lower limbs muscles are initiating the kinetic chain and that the player' performance level significantly influences their contribution (12). In our study, the higher PF torque at the dominant extremity might be partly due to a higher muscular activity, implying that more force could be translated to the racket movement (kinetic chain). This also underlined the specific role of each particular leg during on-court movements and stroke production (12,13). It is likely that this asymmetry can be attenuated by appropriate conditioning training.
In conclusion, this study demonstrated functional links between sprinting and vertical jumps abilities as well as muscular strength in dominant lower and upper extremities in a group of competitive teenage tennis players. An asymmetry in both upper and lower limb strength was observed. Results of this study also indicate that these physical variables were statistically linked to specific performance level identified via their tennis ranking in a tournament. However, the leg stiffness calculated during hopping was not correlated to the player's ranking. In attempting to shed light on the contribution of physical attributes used to predict competitive rankings in junior tennis players, the possibility of testing the relationships between these measures and performance during on court movements and strokes production may warrant further investigation.
Junior elite tennis physical characteristics have been described (3,4,8,17,20,22,23). It is also known that physical performance is related to biological maturation during male adolescence (18,19). However, there are very few accessible tools for the coach to evaluate the cross-effect of maturation and training on physical features of teenage players. By the means of simple field tests, we showed that there is an asymmetry between dominant and nondominant limbs that was correlated to tennis performance. In addition, explosive power and speed were shown to be influential. By regularly monitoring these factors (we suggest twice a year) during puberty, the conditioning coach can modify a program in order to compensate the imbalances. This would minimize the risks of injuries during this critical period.
The authors thank Jean-Paul Micallef, Laurent Court-Fortune, and Julien Roussillon for their technical assistance during the study.
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