Musculoskeletal demands placed upon the human body during tennis play have resulted in a characteristic pattern of upper extremity injuries in elite players (14,16,26). Injuries to the lower back can be included among the characteristic injuries in elite tennis players. Sward et al. (22) studied 30 elite players aged 17–25 and reported a 50% incidence of lower back pain, and a 47% incidence of radiographic abnormalities in the thoraco-lumbar spine. Hainline (11) reported a 2-yr musculoskeletal injury evaluation total from the U.S. Open Tennis Championships of 75 injuries, with 18 (24%) being low back complaints.
Rotation of the trunk during the tennis serve and groundstroke is an integral part of the development of power and transfer of energy up the kinetic chain from the lower to upper extremities (5,21). Trunk rotations during the forehand and backhand groundstroke have been reported between 110 and 120° at the completion of the backswing from parallel to the baseline of the court, with approximately 30° of trunk separation (angle of separation between shoulder rotation and the hips) (17). For a right-handed player, powerful concentric muscle contraction occurs during the serve and forehand, producing left trunk rotation. Conversely, the right-handed player rotates to the right during the backhand groundstroke. Additionally, eccentric muscular activation occurs during the deceleration of the torso following ball impact, along with cocontraction of the abdominal and lower back musculature to increase stability of the spinal column (15). The integral role trunk rotation plays in tennis performance has led to a greater awareness and utilization of conditioning programs to improve core stability to enhance performance and prevent injury. However, little research is available to guide clinicians in the proper design of these stabilization programs, and there is no established method to identify players who are deficient in core strength and require these programs.
Previous isokinetic strength profiling research on elite tennis players has identified selective unilateral strength development of the dominant arm internal rotators (1,3,4,8), shoulder extensors (3), elbow extensors (9), forearm pronators (3), and wrist flexors and extensors (3). Additional research has shown bilateral symmetry in the muscular strength of the quadriceps and hamstrings in the lower extremity (6), and in both the upper-extremity glenohumeral external rotators (1,3,4,7), elbow flexors (9), and forearm supinators.
Roetert et al. (18) used an isokinetic dynamometer to measure trunk flexion and extension strength in 60 elite junior tennis players. Results of this study showed that unlike testing in normal populations, where trunk extension strength exceeds trunk flexion strength, elite junior tennis players actually had greater trunk flexion strength. The importance of balancing muscular strength relationships for trunk flexion and extension and having players engage in a more complete trunk strengthening program, emphasizing both abdominal and lower back exercises, was recommended based on the results of this study.
No prior research has been published profiling trunk rotation strength in elite tennis players. Due to the repetitive use of trunk rotation in tennis-specific stroke performance, knowledge of any side to side differences in trunk rotation strength or muscular imbalances would be important to sports medicine professionals who design and implement both rehabilitation and training programs for athletes in this population.
The purpose of this study was to isokinetically measure bilateral trunk rotation strength in elite tennis players, and to determine whether differences exist in side to side rotational strength. Development of a descriptive isokinetic profile of trunk rotation strength data for male and female elite tennis players, and determination of the degree of correlation between isokinetically measured trunk rotation strength and a functional medicine ball rotation toss, were secondary purposes of this study.
One hundred nine elite tennis players were tested in this study. Fifty-four males (age 11–54 yr; mean 24.3 ± 10.5 yr) and 55 females (age 12–45 yr; mean 18.9 ± 6.7 yr) with an average competitive playing experience of 15 and 9 yr, respectively, participated in this study. All players were either ranked players and/or played competitive tournaments at the junior, collegiate or professional level. Players reported playing an average of 14.2 ± 9.7 tournaments per yr. Subjects were free from any back or pelvic injury and had not had any injuries in the past year before testing that had resulted in an inability to compete or practice.
Subjects read and completed an informed consent before data collection. The testing protocol was approved by an Institutional Review Board (Physiotherapy Associates, Memphis, TN).
Before isokinetic testing, a 5-min warm-up was performed on an Upper Body Ergometer (Cybex Inc., Ronkonkoma, NY) using a clockwise direction (right-hand view) at a resistance level of 90 kpm. Following the warm-up, subjects were placed in the Cybex Torso Rotation isokinetic dynamometer (Cybex Inc., Ronkonkoma, NY) in a seated position with 90° of hip and knee flexion. The subjects’ feet were secured in straps to a platform that could be raised or lowered to produce a consistent hip and knee position. Figure 1 shows the degree of stabilization afforded by the device with straps at the waist, mid-femur, and over the shoulders of the subject to secure the device to the upper torso. A 90° arc of total rotation was set using range of motion stops (45° of right and left rotation). Isokinetic testing speeds of 60 and 120°·s−1 were used for data collection, with four submaximal warm-up repetitions used before data generation at each speed. Five maximal repetitions at 60°·s−1 and 15 maximal repetitions at 120°·s−1 were used. A 30-s rest period was used between sets for all subjects. Consistent verbal commands were given, with no visual feedback allowed during the testing protocol. All subjects were tested by one examiner who was trained and experienced in the use of isokinetic testing devices (TE). The reliability of the Cybex isokinetic trunk system has been previously established and published (23).
Parameters measured by the dynamometer included peak torque relative to body weight, single repetition work relative to body weight, total work, and an endurance ratio consisting of the measurement of the work performed in the second half of the 15 testing repetitions compared to the work performed in the first half of the 15 testing repetitions.
A subset of 28 subjects volunteered to perform a functional medicine ball toss following performance of the isokinetic trunk rotation testing protocol. The subject was asked to maximally throw a 6-lb medicine ball, using both hands in a forehand (left rotation for right-handed players) and backhand movement pattern from a stationary start position (19). The best of three trials served as the representative value of functional trunk rotation performance.
Two repeated-measures ANOVA were used (one for males and one for females), and dependent t-tests were used where main effect differences were identified. Significance was based on the P (<0.01) level. For analysis, all trunk rotation data were entered relative to rotation during tennis groundstroke performance. Data from left-handed players were entered and analyzed in reverse, such that all players had forehand (left rotation for a right-handed player) and backhand (right rotation for a right-handed player) rotation measured in relative terms. A Pearson correlation coefficient was used to determine the relationship between the isokinetic strength parameters and the functional medicine ball performance measurement for the 28 subjects (males and females combined) who performed this portion of the testing protocol.
Table 1 shows the results of the repeated-measures ANOVA for the male and female subjects. No significant main effect differences were found in the male subjects between forehand and backhand rotation strength. Female subjects did show a significant (P < 0.001) main effect difference in the direction of rotational strength. Post hoc testing revealed that female subjects had statistically greater (P < 0.001) backhand rotation direction peak torque and single repetition work values at both 60 and 120°·s−1 as compared to forehand rotation. Backhand rotation endurance and total work at 120°·s−1 were also statistically greater (P < 0.001) than forehand rotation in females. Rotational strength of the trunk measured in the direction encountered during a backhand groundstroke did exceed the strength and endurance values measured in the direction of rotation during the serve and forehand in the female subjects in this investigation. Tables 2 and 3 list the descriptive isokinetic trunk rotation strength profiles from the male and female subjects, respectively.
Pearson correlations relating performance on the isokinetic strength test to performance on the medicine ball test are listed in Table 4 for male and female subjects. Significant (P < 0.01) correlations were found between the isokinetic trunk rotation parameters of peak torque and single repetition work and both directions of the 6-lb forehand and backhand functional medicine ball toss.
Unlike prior isokinetic muscular profiling studies of elite tennis players that identified significant bilateral differences in specific muscle groups of the upper extremities, this study shows essentially symmetrical strength patterning in trunk rotation strength in elite players. Despite statistically significant differences between forehand and backhand direction rotation strength in the female subjects, the actual percent differences between sides was small (4–8%). In fact, the mean forehand rotation/backhand rotation ratios formed by dividing one direction of rotation by the other ranged from 95 to 98% for males and from 92 to 96% for females. While this difference between backhand and forehand rotation in the female subjects was statistically significant, the actual clinical significance is small. This finding suggests that a large difference in trunk rotation strength does not appear to occur in elite players, and selective rotational training to obtain 4–8% greater backhand rotation in females would not be supported based on this research. Greater trunk rotation in the direction of the forehand and serve (left rotation in a right-handed player) was hypothesized since a greater number of overall repetitions in this direction ultimately are performed during actual tennis play and training for most players. This finding of symmetrical trunk rotation strength in elite players suggests that core stabilization programs focus on both directions of trunk rotation to promote muscular balance.
Further research is needed to determine trunk rotation patterns in injured tennis players based on the results of this study showing symmetrical strength in uninjured players. Comparison of the results of this study to other published research on trunk rotation strength in athletic populations is not currently possible, due to the limited data in the present literature. Research published using a large population of over 27,000 normal, uninjured subjects found left and right trunk rotation strength to be symmetrical in uninjured male and female subjects between the ages of 10 and 79 (24). No significant difference between left and right rotation was isokinetically measured in this large descriptive profile of normal, nonathletic subjects. Timm (24) concluded that symmetrical trunk rotation strength values should be expected in uninjured subjects with isokinetic trunk rotation testing.
The measurement of trunk rotation is of key interest in both athletes and normal nonathletic individuals. Research has shown a trend for larger incidences of injury in workers who sustain trunk rotation postures (12). Another parameter measured in this study was trunk rotation muscular endurance. No significant difference in the repeated function of the muscles producing forehand and backhand rotation was measured for males, and slightly more fatigue resistance in backhand rotation in the females was identified (95 vs 87%). Again, no comparison to other athletic populations is currently possible due to the limited isokinetic strength data available in the present literature.
Ng et al. (15) measured the isometric muscular endurance in left and right rotation using a standing testing protocol in normal healthy male subjects. No significant side to side differences in the amount of time subjects could maintain an 80% maximal force level were measured; however, EMG analysis of muscular function during rotational exertion did show side to side variability in the firing patterns of the external and internal obliques, latissimus dorsi, and illiocostalis lumborum muscles. Symmetrical muscular activation patterns and characteristics were reported in the left and right rectus abdominus and multifidus muscles. Premature fatigue of the muscles that stabilize the trunk has been shown to result in increases in neuromuscular control and coordinated spinal movement. Identification of elite tennis players with reduced muscular endurance capability in trunk rotation would allow clinicians to apply resistance training programs, with an overall goal of improving local muscular endurance of these target muscles (10). The use of training programs utilizing medicine balls and core stabilization exercises in various patterns of trunk and pelvic rotation are strongly recommended for tennis players (19,20).
The use of the isokinetic dynamometer to assess dynamic muscular function has distinct advantages over other methods (2,25). A widely used technique to assess “core stability” and abdominal strength is the bilateral leg-lowering technique described by Kendall (13). This technique, although widely used, has recently come under scrutiny following a biomechanical analysis showing early pelvic movement in nearly all subjects, regardless of abdominal bracing condition (27). The isolated testing position afforded by the isokinetic torso rotation device used in this study allows for measurement of trunk rotation at multiple speeds with accommodating resistance. Disadvantages include the lack of availability of isokinetic torso rotation testing equipment, and the inability of the device to be portable and accessible in all athletic training and testing facilities. The use of the functional forehand and backhand medicine ball toss in this study was meant to provide an alternative testing method that would be widely available and have virtually minimal testing limitations. Pearson correlations were statistically significant between isokinetically measured forehand and backhand rotation strength and the distance the medicine ball was tossed using a bilateral upper extremity movement pattern. This finding is consistent with research conducted by Roetert et al. (18), who found significant Pearson correlations ranging from 0.62 to 0.76 between isokinetic trunk extension flexion strength and a forehand and backhand medicine ball toss with a 6-lb medicine ball in elite junior tennis players. Further study with extended subject populations and regressional analysis must be performed before more definitive conclusions can be made on the exact relationship and degree of estimation that the functional medicine ball toss can provide clinicians, coaches and sport scientists.
Finally, one additional variable that initially was hypothesized to contribute to torso rotational strength was the two-handed backhand. Since no bilateral differences were measured in the male population, only the female population could be analyzed for study. However, of the 55 females tested in this study, fewer than 3% used a one-handed backhand. This left too few subjects utilizing a one-handed backhand to adequately perform the analysis. The widespread use of the two-handed backhand among female subjects may have been one determining factor explaining the slightly greater backhand-direction trunk rotation strength and muscular endurance in the female subjects tested in this investigation. A definitive statement regarding this theory cannot be made based on the methodology employed in this study.
An isokinetic testing profile of trunk rotation strength in elite players has identified symmetrical trunk rotation strength and muscular endurance parameters in males, and slightly greater trunk rotation strength and endurance in the direction of the backhand (right rotation) in females. Based on the results of this study, the use of a functional medicine ball toss did correlate significantly with isokinetic trunk rotation variables, and may be an acceptable alternative when testing with isokinetic dynamometry is not immediately available.
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