Athletic performance,training characteristics,and orthopedic indications in junior tennis Davis Cup players

Subjects

Twelve nationally ranked male junior DC and two samples of junior squad players of the regional federations (RS1, n = 60 males, n = 47 females; RS2, n = 59 males) participated in this study. Anthropometric data are shown in Table 1. All players were familiar with the test procedures and the testing surroundings. Written informed consent was obtained before participation. The study was approved by the local ethics committee and conformed to the recommendations of the Declaration of Helsinki.

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Table 1.

Individual characteristics of the players and classification to the experimental set up. Data are presented as mean ± SD.

					Participation in
	N	Age (year)	Weight (kg)	Height (cm)	Fitness test	Interview (training)	Orthopedic questionnaire
Davis Cup players (m)	12	15.6 ± 1.1	69.8 ± 11.7	180.7 ± 9.6	x	x	x
RS2 (m)	59	15.2 ± 0.6	62.4 ± 8.7	176.3 ± 7.7	x	x
RS1
Male	60	14.9 ± 2.5	58.6 ± 15.4	171.2 ± 13.9			x
Female	47	14.6 ± 2.1	54.1 ± 10.6	166.1 ± 10.9			x

Experimental set-up

This study examined athletic performance, training workloads, and orthopedic injuries of junior DC in comparison with their regional counterparts. The experimental design was divided into different parts (Table 1):

Athletic performance measurements: For assessment of athletic performance, subjects (DC, RS2) performed the German Physical Condition Tennis Test. ¹⁰ Data of DC were obtained during a training workshop in laboratories at the Department of Training and Exercise Science at Ruhr University Bochum, while data for regional squads were gathered at the players’ respective federation base over a three-week period of time.

Athletic performance and anthropometric measurements

Physical performance was assessed using the standardized fitness test battery of the German Tennis Federation. ¹⁰ Using the German Physical Condition Tennis Test Protocol, an assessment of athletic performance can be conducted by norm values and percentiles as described by Ulbricht et al. ¹⁰ and Fernandez-Fernandez et al. ¹⁵ The percentiles can be used to obtain norm-referenced interpretations of the performed fitness score within a distribution consisting of scores from a comparable section. ¹⁶ By means of this fitness-testing concept, individual profiles of the players with their respective strengths and weaknesses in comparison with their peers could be obtained. ¹⁰ Physical performance tests were conducted in an indoor tennis court (Rebound Ace surface) and began after a 15-minute individual warm-up, consisting of low-intensity forward, sideways, and backward running, acceleration runs, hopping exercises, and jumps of increasing intensity. ¹⁰

Training documentation

Data collection on training characteristics for each participant was undertaken during a standardized interview, the contents of which were determined by several experts in the field of tennis coaching and science. The isolated workloads of tennis training (TT), physical training, and of additional sport activities performed were retained for a normal training week (out of competition phase) and recorded in hours per week. Total training volume and the ratio of TT to physical conditioning (PT) was calculated. In addition, training history (years of practice, age of start playing tennis (years)), the organization form of tennis training (relation player: coach; 1:1, 2:1, 3:1, and 4:1 in percentage), and the distribution of training contents (warm-up, small-sided practice, technique, match training, and on-court physical training in percentage) throughout TT were obtained.

Orthopedic injury prevalence

Prevalence of musculoskeletal symptoms (MS) was recorded by a questionnaire developed in accordance with the standardized and validated Nordic Questionnaire. ²⁵ The applied questionnaire includes (1) questions of limitations in the region of neck, shoulder, elbow, hand/wrist, thoracic spine, lumbar spine, hip, knee, and ankle/feet throughout the whole tennis career; (2) questions of limitations in upper extremities (shoulder, elbow, and hand/wrist) during the previous 12 months and in accordance with defined stroke performance (forehand, backhand, and serve); and (3) a pain score for the region of shoulder, elbow, and hand/wrist while performing the tennis serve. For pain severity measurement, the Numeric Pain Rating Scale was utilized, using ratings ranging from “0” indicating “no pain” up to “10” indicating the “worst possible pain.” ²⁶ Non-standard questions were thoroughly pilot tested. Due to the fact that injury patterns are more likely to be sport than gender specific, it was decided to forego dividing the sample into males and females.

Statistics

All data are presented as mean values and standard deviations (±SD). Independent sample t tests were used to determine mean differences between junior DC and regional squads in anthropometric and physical performance as well as training characteristics. Further, the standardized difference or ES of changes in each parameter between the two groups was calculated using the pooled standard deviation. Threshold values for Cohen ES statistic were <0.2 (small), 0.5 (moderate), and >0.8 (large). ²⁷ Injury prevalence between groups was compared using chi-square statistics. For analyzing the correlation between training volume and age, as well as athletic performance, the Pearson product–moment correlation coefficient (r) was used. Spearman’s rank correlation coefficient was calculated between training volume and tennis performance (i.e., national youth ranking). The magnitude of correlation was classified as trivial (0–0.1), small (0.1–0.3), moderate (0.3–0.5), large (0.5–0.7), very large (0.7–0.9), nearly perfect (0.9), and perfect (1.0). The p ≤ 0.05 criterion was used to constitute statistical significance. Data analyses were performed using the SPSS statistical software package (version 18, SPSS Inc., Chicago, IL, USA) and Open Office (version 3.2.1; Oracle).

Training characteristics

The data obtained reflect that players devote a great amount of time to improving their tennis performance throughout training interventions. Junior DC start playing tennis at a young age (6.4 ± 1.0 years) and spend 22.7 ( ± 6.8) h/wk for total training workout (tennis training and physical training). Because of the predominance of techniques and tactics in tennis, ⁴ training priority is focused on the improvement of these skills with an average of 15.1 h/wk, while 7.6 h/wk are spent for improvements of athletic performance. These data differ significantly in comparison to the training volume of the RS (Table 3), thus supporting the idea that high training volumes are necessary to compete at a high performance level. This fact is also reflected by the high correlation (r = −0.78, p < 0.05) observed between the total training volume and player’s ranking position (Table 4). However, comparing these findings to the previous research is difficult since data on training characteristics in tennis do not receive much attention. Although the importance of intervention programs has not been questioned, there is less information about training characteristics compared to other sports, i.e., endurance-based sports.^28,29 However, a few studies reported total training volumes, not evaluating a separate physical training. In adult elite tennis players (age 21.4 ± 2.6 years), Hornery et al. ³⁰ found a total training workload of 32 h/wk during out of competition phases. In the investigation by Sanchez-Munoz et al., ³¹ 23.2 h/week of training volume were recorded in elite male junior players (age 16.2 ± 0.4 years); the sample and results are in agreement with our findings in DC. Furthermore, training data of DC are in line with the recommendation of the International Tennis Federation (ITF) to spend 15–20 h/wk on technical and tactical skills. ⁹ Consequently, due to the fact that high training volumes seem to be indispensable for competing on an elite level even at a young age, training interventions must be organized as efficiently as possible to achieve an optimum cost–benefit ratio.^10,15 Taking into account the abovementioned distribution of limiting performance factors (i.e., predominance of techniques and tactics), ⁴ the application of specific diagnostics, notably of physical parameters, helps to design individualized training programs according to the respective needs, thus leading to a minimized training volume focused on fitness training thereby saving time for tennis-specific training.^10,15

Correlation analysis between training volume and the players’ age refers to an increased importance of physical training in older players (Table 4). While the volume of tennis training (i.e., techniques and tactics) is at a high level also in young players, physical training volume is enhanced in older ones. Consequently, and not surprisingly, there is also a positive moderate correlation between total training volume and age, as well as a high correlation between the ratio of tennis training and physical training and the age of players (Table 4). This might be due to fundamental changes in the biological system (i.e., hormone status) during and after maturity where some physical parameters (i.e., strength) are more likely to be enhanced. ³² For example, with increasing age (11–15 with girls and 13–17 with boys), the influence of the strength region is increased through hormone distribution (testosterone and estrogen) and resultant muscular growth. ³² Nevertheless, interindividual variation has to be considered since the physical development of athletes in particular differs during the process of maturation (i.e., retardation and acceleration), independent of age.^17,33 We suggest that the interindividual stage of maturation, which is reflected by our results of estimated age from/to peak height velocity ranging from −0.67 to + 3.9 years, could also be one reason impeding general training guidelines and recommendations for physical training volume.

Athletic performance

In spite of the limited training time for physical training, data obtained for physical fitness indicate a moderate to high athletic performance level in junior DC according to norm values established by Fernandez-Fernandez et al. ¹⁵ and Ulbricht et al. ¹⁰ The qualities of the upper body strength and power-related characteristics (i.e., MBT and serve velocity) especially have to be highlighted with percentile values up to 85%; this data differed significantly when compared to RS (Table 2). In this regard, results are in line with previous research reporting differences in physical performance measures between athletes with different performance levels in soccer players.^34,35 In agreement with Ulbricht et al., ⁸ analyzing the effect of performance level in tennis players, results reinforce that players require well-developed physical fitness for competing on elite levels, and it is reasonable to assume that the qualities of upper body power and strength are one of the determining factors in tennis performance as supported by previous research.^8,36,37 Therefore, greater upper body power allows for the chance to improve tennis performance with higher stroke velocities. Since the modern game becomes faster, with the tennis serve being the most powerful and potentially dominant shot that influences the match results in the game of tennis, the implementation of high-serve velocities where 210 km/h serves are common, has led to an advantage in winning the rally either through an ace or indirectly due to a defense situation of the opponent because of the given time pressure.^1,2,38 In conclusion, the development and evaluation of upper body intervention programs for improving tennis performance is of great interest and utility. ⁸

Medical orthopedic status

Results of medical orthopedic assessments refer to moderate and high injury prevalence in young athletes with no differences between performance levels, but with a tendency toward higher rates in DC (Table 5). Notwithstanding, comparisons are difficult since we are not aware of similar studies evaluating the effect of performance level on medical orthopedic status in elite junior tennis players. Concerning the effect of performance level, there is no consensus in the literature whether there is an increased or decreased risk of injury with increasing skill level. Baxter-Jones et al., ³⁹ contrary to widely held opinion found, reported that while elite youth athletes do not appear to be at increased risk of injury, there is an increased risk of injury with increasing skill level in wrestling, as reported in a review of Emery. ⁴⁰ Further, Jayanthi et al. ⁴¹ reported no differences in prevalence rates across different skill levels in recreational competition adult tennis players. We suppose that the tendency of higher injury rates in DC could occur due to the evidently higher demands in DC, particular in training volume and match participation (Table 3). This assumption is supported by previous research. In this regard, DiFori and colleagues ⁴² summarized that the volume and intensity of training is strongly related to overuse injury risk.

With respect to an entire tennis career, the highest frequencies of MS are observed in the upper (i.e., shoulder with 67% in DC and 50% in RS) and lower extremities (i.e., knee with 79% in DC and 54% in RS), and also in the lower back (Table 5). These findings are in line with previous research.^43–46 Epidemiologic studies evaluating tennis injuries reported that the lower and upper extremities are the body areas where injuries occur most frequently, with the shoulder, elbow, and knee being the most widely involved joints. ⁴⁴ In agreement with this, Priest and Nagel ⁴⁷ recorded that over 50% of the world’s best tennis players had experienced shoulder symptoms during their career, mostly secondary to overuse injuries with muscle imbalances being a risk factor for developing disorders.^14,48 For prevention strategies and performance enhancement, it is of great importance to identify and monitor these predisposed body areas so that prescribed limitations can be considered by coaches, conditioning professionals, and physiotherapists.^44,46

Concerning the 12-month prevalence of MS of the upper extremities, results indicate that nearly 55% of athletes had at least one disability in this body region during the last year. Shoulder injuries (with values up to 44%) were the most common, followed by injuries to the hand/wrist (33%) and elbow (19%) (Table 6). The assumption that shoulder pain and injuries are often related to the serve is reported by many authors^13,14,49 and confirmed by our results, which indicate that 87% of the players with shoulder disabilities throughout the last 12 months have discomforts notably during serve performance and less during forehand (50%) and backhand strokes (11%). Moreover, the highest pain scores for pain severity during the tennis serve is related to the shoulder (4.00) followed by the elbow (3.95) and wrist (2.50) (Table 6). The actual high injury rate of the upper extremities, especially of the shoulder and in relation to the serve motion, could be explained by the strenuous repetitive forces during stroke production. ¹³ Serve performance in particular generates a high load on tennis players’ shoulders, ¹⁴ and in addition to the shoulders’ natural hypermobility and the excessive laxity of the glenohumeral joint, ⁴⁴ make the joints prone to injury. Johnson and McHugh ¹³ emphasized that the shoulder plays an essential role in the kinetic chain to transfer the forces to the hand and the racket. The serve inherently requires repetitive powerful concentric internal shoulder rotation for power generation followed by eccentric forces to the external rotators to decelerate the overhead arm motion. ⁴⁴ Rotation velocities of 2420°/s (internal rotation) ⁵⁰ and large range of motion demands, especially in external rotation during the cocking phase, intensify the high stress on the serving shoulder. ¹³ Taking the high repetitive nature of this stroke into consideration, only an adequate service technique and optimal strength and conditioning status for stabilizing the shoulder joint may prevent injuries, since improper serve techniques could result in increased loading of the upper limb joints.^14,44,51