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Age specific isokinetic glenohumeral internal and external rotation strength in elite junior tennis players
Age specific isokinetic glenohumeral internal and external rotation strength in elite junior tennis players T Ellenbecker I & EP Roetert 2 1physiotherapy Associates Scottsdale Sports Clinic, Scottsdale, Arizona, USA. 2USA Tennis High Performance Program, Key Biscayne, Florida, USA.
Ellenbecker, T, & Roetert, EP (2003). Age specific isokinetic glenohumeral internal and external rotation strength in elite junior tennis players. Journal of Science and Medicine in Sport 6 (1): 6370. The high incidence of overuse shoulder injuries in elite junior tennis players is often attributed to both the high repetitive stresses inherent in the game, as well as muscular imbalances in the humeral rotators. The purpose of this study was to isokinetically measure concentric glenohumeral joint internal and external rotation strength, to develop a bilateral descriptive profile in elite junior tennis players across two age ranges. One hundred forty seven elite junior players between the ages of 12 and 21 were bilaterally tested on a Cybex isokinetic dynamometer with 90 ° of abduction. Players were analysed in two groups (12-17 and 18-21 years of age), using isokinetic variables of peak torque and work relative to body weight. Data analysis consisted of ANOVAs, with dependent t-tests used for post-hoc testing where main effect differences were identified. Results indicated no significant main effect difference in the isokinetic relative parameters between the two age groups (p>.01). Significantly greater (p<.001) dominant arm internal rotation was measured in both groups for both males and females, with no significant difference in external rotation strength measured between extremities in either age group. These data show specific adaptations in shoulder strength and identify a relative muscular imbalance between the internal and external rotators on the dominant arm of elite junior tennis players. Normative data presented in this study can assist clinicians and researchers in interpreting isokinetic test results in elite junior tennis players.
Introduction Repetitive muscular exertion in the upper extremity, required during p e r f o r m a n c e of t e n n i s specific m o v e m e n t p a t t e r n s , l e a d s to t h e d e v e l o p m e n t of s p o r t specific m u s c u l a r a d a p t a t i o n s i n elite level t e n n i s p l a y e r s (1-6). M u s c u l a r i m b a l a n c e s in t h e r o t a t o r c u f f a n d s c a p u l a r m u s c u l a t u r e , c o u p l e d w i t h inadequate muscular endurance and improper stroke biomechanics, can lead to o v e r u s e i n j u r y in t h e g l e n o h u m e r a l j o i n t of elite level t e n n i s p l a y e r s (7,8} P h y s i o l o g i c a l profiles of elite level t e n n i s p l a y e r s h a v e b e e n p r e v i o u s l y p u b l i s h e d . M u s c u l o s k e l e t a l profiles d o c u m e n t i n g r a n g e of m o t i o n a n d m u s c u l a r s t r e n g t h h a v e i d e n t i f i e d u n i l a t e r a l d o m i n a n c e in s e l e c t m u s c l e s in t h e u p p e r e x t r e m i t y of elite j u n i o r (2.3) a n d collegiate p l a y e r s (4,6}, a s well a s in adults{ 11. I n c r e a s e d d o m i n a n t a r m i n t e r n a l r o t a t i o n s t r e n g t h , w i t h o u t a c o n c o m i t a n t i n c r e a s e in g l e n o h u m e r a l j o i n t e x t e r n a l r o t a t i o n , h a s b e e n a c o n s i s t e n t f i n d i n g in t h e s e s t u d i e s . T h e s e p r o f i l e s h a v e p r o v i d e d d e s c r i p t i v e profiles u s i n g s m a l l e r s a m p l e sizes t h a t c a n l i m i t effective a p p l i c a t i o n i n clinical rehabilitation or physiological performance enhancement evaluations(9). T h e p r i m a r y p u r p o s e of t h i s s t u d y w a s to p r e s e n t a d e s c r i p t i v e a g e
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Age specific isokinetic glenohumeral...
specific profile of concentric isokinetic glenohumeral joint internal and external rotation strength in a larger sample of elite junior tennis players.
Methods and Procedures Subjects Male and female elite junior tennis players were isokinetically tested in this study. Subjects were divided into 2 testing groups; 12-17 and 18-21 years of age. Total subject population included (n=50) 12-17 year old males, (n=31) 1821 year old males, (n=31) 12-17 year old females, and (n=35) 18-21 year old females. All subjects were free from a glenohumeral joint injury and did not have an upper extremity surgical history. Additionally, subjects were excluded from the study if they had experienced glenohumeral joint pain that limited tennis play or practice for three consecutive days in the year prior to testing.
Instrumentation A calibrated Cybex 6000 isokinetic dynamometer (Cybex Inc., Ronkonkoma, NY) and upper body testing table (UBXT) were used for testing all 147 subjects in this study. The mechanical and physiological reliability of the Cybex isokinetic dynamometer's concentric mode has been previously established and published (10,11).
Isokinetic testing protocol An informed consent was completed by all subjects prior to the performance of isokinetic testing. A five minute warm-up on an upper body ergometer (UBE) (Cybex Inc., Ronkonkoma NY) was performed at 90 kpm in the clockwise direction (right hand view) prior to isokinetic strength testing. Extremities were measured in a random stratified sequence to minimise the effects of a learning bias. Testing was performed at 210 and 300 degrees per second, in that order. Randomisation of testing velocity was not followed to enhance the reliability of data acquisition (12). Subjects performed 4 submaximal warm-up repetitions at
Figure 1: Isokinetic set up for glenohumeral joint internal/external rotation with 900 of abduction
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Age specific isokinetic glenohumeral.., /
each testing speed, followed by five maximal repetitions for data generation. A thirty second rest between testing speeds was followed, based on tennis specific physiologic stress patterns. Subjects were placed on the UBXT in a supine position, with stabilisation straps applied at the pelvis and mid-thoracic region to minimise substitution (figure 1). The glenohumeral joint was abducted 90 ° in the coronal plane, with the d y n a m o m e t e r input axis aligned with the centre of rotation of the glenohumeral joint. The range of motion used for testing included (0-90 °) of external rotation and (0-65 °) of internal rotation. Range of motion stops were used to e n s u r e that all subjects performed identical ranges of motion during testing. Testing was initiated with the shoulder in a position of 90 ° of external rotation. Standardised visual and auditory feedback was given by the test administrator during all tests (9,13). Data analysis
The p a r a m e t e r s of peak torque and single repetition work relative to body weight were analysed for the male and female subjects in both age groups. Unilateral strength ratios (external/internal rotation ratios) were computed by the Cybex 6000 software and were not subjected to further statistical analysis. SPSS for Windows (SPSS Inc., Chicago, IL) statistical software was used to perform two repeated measures ANOVAs; one for males and one for females. A 2 (age) X 2 (dominance: dominant, nondominant) X 2 (rotation: external, internal) X 2 (parameter: peak torque, single repetition work) ANOVA was applied with statistical significance set at the (p<.01) level. Post-hoc testing was performed using dependent t-tests in areas where main effect differences were initially identified. A Boneferroni a d j u s t m e n t was utilised to minimise the chance of type II error with the application of multiple t-tests. Statistical significance was set at the (p<.001) level for the post-hoc tests.
Results Table 1 lists the results of the two ANOVAs performed on the male and female isokinetic data. No significant difference was found for the between-subjects main effect of AGE for males or females. This indicates that the relative ratios of m u s c u l a r performance m e a s u r e d between the younger and older elite junior players did not significantly differ from one another.
Source
Males (p-value)
Females (p-value)
Between Subjects Factor Age
.013
.287
Within SubjeCts Factors Dominance Rotation Parameter Dominance X Rotation Dominance X Parameter Rotation X Parameter Rotation X Dominance X Parameter
.001 .001 .001 .001 .001 .013 .001
.001 .001 .001 .001 .001 .001 .001
Table 1:
Resultsof ANOVAperformed on 147 elite iunior tennis players.
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Age specific isokinetic glenohumeral..,
Parameter
Age 12-17 (11=50) Dom Arm
Age 18-21 (n=31)
Ndom Arm
DOm Arm
Ndom Arm
Torque/Body Weight Ratios (%) * ER210 33.6_+6.3 ER300 29.6_+5,6 IR 210 49.7_+12.5 IR 300 44,0_+11.0
32,0_+6.25 28.5_+6,0 40.8_+9,2 35,5_+8.9
37,8_+9.1 32.9_+8.1 55.1_+14,6 46.5_+13,2
35.8_+6,8 40.0_+7.4 47.4_+13.4 40.3_+11.1
Work/BOdy Weight Ratios (%) ** ER 210 58.3_+14.5 ER 300 49.7_+13,2 IR 210 93.6_+26,7 IR 300 79,4_+24,4
58.3_+14.3 49.2_+13.2 74,6_+21,1 60,9_+18.9
67.7_+17.9 57.6_+16.8 108.4_+32.0 90,1_+32.3
67,6_+15.5 57.8_+15.2 92.2_+30,0 77.4_+26.6
* Values are expressed in Nm/Kg of body weight (Conversion 1 ft Ib/Ib = 2.98 Nm/Kg) ** Values are expressed in Joules/Kg (Conversion 1 ft Ib/Ib = 2.98 Joules/Kg)
Table2/1- Isokinetic peak Torque~Body Weight and Work~Body Weight Ratios for the male elite junior tennis players, Parameter
12-17 (n=31) Dom Arm
Ndom Arm
Age 18-21 (n=35) Dora Arm
Ndom Arm
Torque/Body Weight Ratios (%) * ER 210 29.0_+4.1 ER 300 22.4.+6.1 IR 210 42.3_+8,0 IR 300 34,2_+7.8
27.6+4.5 21,2.+5,3 34,7_+6.6 27.1 _+6.6
24.7_+6.1 21.2.+7,3 33.2_+8,6 28.8.+9.4
23.7+5.8 20.7_+6.4 23,4_+7.6 20.0_+8,5
Work/BOdy Weight Ratios (%) ** ER 210 49.1.+8.7 ER 300 34.1.+12.4 IR 210 80.5_+20,7 IR 300 57.2_+20,6
49.5_+7,4 32.8+11.4 61.6_+11.9 43.4_+14.4
47.1.+13,2 37,0.+14.9 64.0_+19,0 51.1_+19.6
46.1.+12,7 37.4_+14,0 43,1_+5.0 34.2_+15.8
* Values are expressed in Nm/Kg of body weight (Conversion 1 ft Ib/Ib = 2.98 Nm/Kg) ** Values are expressed in Joules/Kg (Conversion I ft Ib/Ib = 2.98 Joules/Kg)
Table 2B: Isokinetic peak Torque~Body Weight and Work~Body Weight Ratios for the female eite junior tennis players.
Significant within-subjects m a i n effect differences were identified (p<.001) for dominance, rotation, a n d parameter. Significant interactions were identified and are listed in Table 1 for m a l e s and females. Post-hoc d e p e n d e n t t-tests identified significantly greater (p<.001) d o m i n a n t a r m internal rotation strength in b o t h males a n d females in both age groups at 210 a n d 300 degrees per second. This difference w a s m e a s u r e d for both p e a k torque and work p a r a m e t e r s . No significant difference was m e a s u r e d in external rotation strength between extremities in a n y group. Tables 2A & 2B list the p e a k torque and work to body weight ratios from the 147 elite junior tennis players tested in this study. D a t a are p r e s e n t e d for the 12-17 y e a r olds and 18-21 year olds separately for internal and external
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Age specific isokinetic glenohumeral...
Parameter
Peak Torque ER/IR (%) ER/IR 210 ER/IR 300 WOrk ER/IR (%) ER/IR 210 ER/IR 300
Age 12-17 (n=50)
Age 18-21 (11=31)
Dora Arm
Ndom Arm
DOm Arm
Ndom Arm
69.7_+12.7 69.3_+11.3
81.5_+16.6 82.1_+16.2
70.4_+14.0 72.7_+11.9
80.5_+16.4 84.0_+18.4
81.4±18.4 83.3±17.3
64.5_+13.1 67.3_+14.7
77.6_+19.3 79.1_+20.6
64.3_+12.47 65.5_+14.5
Table 3,4: IsOkinetic external/internal rotation unilateral strength ratios for male elite junior tennis players.
Parameter
Age 12-17 (n=31)
Age 18-21 (n=35)
Dom Arm
Ndom Arm
Dom Arm
Ndom Arm
Peak Torque ER/IR (%) ER/IR 210 ER/IR 300
69.1_+11.0 66.8_+13.3
81.6_+13.8 80.5_+20.0
76.1_+13.0 76.5_+12.9
106.4_+26.4 111.0_+34.6
Work ER/IR (%) ER/IR 210 ER/IR 300
63.7_+13.8 61.1_+13.2
82.7_+16.1 77.5_+19.8
75.2_+14.1 75.2_+19.8
117.0_+38.5 120.4_+40.3
Table 3B: Isokinetic external/internal rotation unilateral strength ratios for female elite junior tennis players.
rotation. Tables 3A & 3B contain the e x t e r n a l / i n t e r n a l rotation (ER/IR) ratios t h a t are derived directly from the Cybex 6000 software and provide a m e a s u r e of m u s c u l a r b a l a n c e between the h u m e r a l rotators. Male E R / I R ratios range between 64-72% on the d o m i n a n t a r m a n d 77-84% on the n o n - d o m i n a n t a r m . Female ratios for the d o m i n a n t a r m were similar to those m e a s u r e d in the male s u b j e c t s , r a n g i n g b e t w e e n 61-76%, a n d w e r e slightly higher on t h e n o n d o m i n a n t a r m , ranging between 77 a n d 120%.
DisCussion The results of this study s u p p o r t earlier r e s e a r c h performed on smaller subject populations of junior and senior tennis players t h a t identified significantly greater d o m i n a n t a r m glenohumeral internal rotation strength, with no bilateral difference in external rotation strength (1,2,4). This s t u d y utilised a substantially larger subject population t h a n earlier studies a n d provides normative p e a k torque and w o r k / b o d y weight ratios t h a t are age and gender specific. No statistically significant difference was found between age groups in the traditional p e a k torque a n d work variables w h e n normalised to body weight in either the m a l e s or females. E x a m i n a t i o n of the normative ratios in tables 2A and 2B do show small increases in the t o r q u e / b o d y weight and w o r k / b o d y weight ratios in the older players b u t these differences were not statistically significant at the (p<.01) level. Apparently, as players increase in age a n d develop, b o d y m a s s and shoulder strength increase at similar levels in healthy,
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Age specific isokinetic glenohumeral... uninjured players, leading to similar isokinetic internal and external rotation strength ratios across age ranges. The E R / I R ratios m e a s u r e d in this s t u d y are substantially lower on the d o m i n a n t a r m , as c o m p a r e d to the n o n d o m i n a n t arm. The unilateral increase in d o m i n a n t a r m internal rotation without m a t c h e d development of the glenohumeral external rotators creates a lower E R / I R ratio on the d o m i n a n t a r m . The E R / I R ratios on the d o m i n a n t a r m do c o m p a r e with other studies of n o r m a l uninjured male a n d female subjects (9,14/and ranged between 61 a n d 76%. Warner et al (151 m e a s u r e d E R / I R ratios in patients with glenohumeral joint instability a n d i m p i n g e m e n t a n d found alterations of the E R / I R ratio. The r e c o m m e n d e d E R / I R ratio typically ranges between 66-75%, s u c h t h a t the external rotators are at least 2 / 3 r d s the strength of the internal rotators, to provide m u s c u l a r b a l a n c e (9,8). Application of the normative d a t a from this s t u d y provides valuable insight regarding d o m i n a n t / n o n - d o m i n a n t differences, p e r f o r m a n c e of the internal a n d external rotators relative to body weight a n d finally, the n o r m a l unilateral e x t e r n a l / i n t e r n a l m u s c l e balance in healthy elite junior tennis players. Utilisation of the ratios provided in this s t u d y can serve to guide clinicians a n d r e s e a r c h e r s in developing r e h a b i l i t a t i o n a n d p r e v e n t a t i v e conditioning p r o g r a m s for the shoulder in elite junior tennis players. The m e t h o d for direct application of this data c a n b e s t be described via discussion of bilateral comparison, normative relative ratios and finally unilateral (ER/IR} strength ratios. For example, isokinetic testing of a 16 y e a r old male elite junior tennis player results in significantly greater d o m i n a n t a r m internal rotation strength, a n d mildly greater external rotation strength. This bilateral c o m p a r i s o n is c o m p a r a b l e with the e s t a b l i s h e d s t a n d a r d s of published d a t a on elite junior m a l e tennis players (2,4/. However, without knowledge of the degree of n o n - d o m i n a n t extremity strength, the bilateral c o m p a r i s o n c a n create a n i n a c c u r a t e representation of d o m i n a n t u p p e r extremity strength. Therefore, application of the normative d a t a presented in Table 2A allows the clinician to determine w h e t h e r the degree of external or internal rotation is consistent with t h e n o r m s from the elite populations of players tested in this investigation. Peak t o r q u e / b o d y w e i g h t ratios of 30-35 N m / K g would be expected for the external rotators on the d o m i n a n t arm, a n d ratios between 50-60 N m / K g of b o d y weight for internal rotation. Regardless of the bilateral comparison, the application of the n o r m a t i v e torque a n d w o r k / b o d y w e i g h t ratios e n s u r e s t h a t d o m i n a n t a r m strength is consistent with other high level players in this population. Players testing well below these s t a n d a r d levels of strength adjusted for body weight would be placed on a p r o g r a m of resistive exercise to a d d r e s s the m e a s u r e d deficiencies. Finally, even ,in the p r e s e n c e of acceptable levels of d o m i n a n t a r m internal a n d external rotation strength, m u s c u l a r imbalance is often p r e s e n t in elite athletes of unilaterally d o m i n a n t sports s u c h as baseball and tennis (8). To e n s u r e t h a t this 16 year old player h a s optimal m u s c u l a r b a l a n c e on the d o m i n a n t a r m , e x a m i n a t i o n of the e x t e r n a l / i n t e r n a l rotation unilateral strength ratios is r e c o m m e n d e d . Often, the ratio of E R / I R strength drops below the 66-75% range a n d a p p r o a c h e s 50%. This 50% ratio indicates t h a t the external rotators are only one-half as strong as the internal rotators. Alterations in the E R / I R ratio have b e e n described by Warner et al (151 in
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Age specific isokinetic glenohumerai... individuals with glenohumeral joint instability and impingement and are a n important parameter to apply from the normative data presented in this research. Players with E R / I R ratios below normal s t a n d a r d s would be placed on specific resistive exercise programs to target the weaker muscle group (external rotators in this example) to improve the ratio and s u b s e q u e n t balance of rotational strength. Application of specific exercises (16,17) to increase external rotation strength appears warranted, based on the lower external/internal rotation ratios on the d o m i n a n t a r m and no concomitant external rotation strength dominance m e a s u r e d in this population. Increases in the E R / I R ratio have been correlated with serving performance in elite players (18) a n d specific evidence based r e c o m m e n d a t i o n s for increasing external rotation strength in tennis players using exercises to improve external rotation strength have been reported in the
literature {19,20,21). Several cautions m u s t be followed w h e n utilising the normative data presented in this research. These data are relevant for elite junior tennis players m e a s u r e d concentrically on the Cybex 6000 isokinetic dynamometer. Caution m u s t be used when extrapolating normative data profiles to other a p p a r a t u s and subject populations (9,22). The use of the larger and h o m o g e n o u s subject population of elite junior tennis players in this study serves to provide the most accurate representation of glenohumeral rotational strength patterns in elite j u n i o r tennis players. Finally, this s t u d y measured only concentric isolated glenohumeral joint internal and external rotation and does not take into effect the complex interplay and important contribution of the scapular m u s c u l a t u r e in the u p p e r extremity kinetic chain.
Conclusion The results of this s t u d y show similar peak torque and w o r k / b o d y weight ratios in male and female elite junior tennis players across two age ranges. Significantly greater d o m i n a n t a r m internal rotation was consistently m e a s u r e d in both males and females, with no significant difference between extremities identified in external rotation. The normative data presented in this s t u d y are designed to assist clinicians and researchers in the interpretation of isokinetic glenohumeral internal and external rotation strength tests during injury rehabilitation and performance e n h a n c e m e n t evaluations.
References (1) Ellenhecker TS (1991). A total arm strength isokinetic profile of highly skilled tennis players. Isokinetics and Exercise Science 1:9-21. (2) Ellenbecker TS (1992). Shoulder internal and external rotation strength and range of motion of highly skilled junior tennis players. Isokinetics and Exercise Science 2:1-8. (3) Ellenbecker TS, Roetert EP (1999). Testing isokinetic muscular fatigue of shoulder internal and external rotation in elite junior tennis players. J Orthop Sports Phys Ther 29(5):275-281. (4) Chandler TJ, Kibler WB, Stracener EC, Ziegler AK, Pace B. (1992) Shoulder strength, power and endurance in college tennis players. A m J Sports Med 20:455-458. (5) Kennedy K, Altchek DW, Glick IV: Concentric and eccentric isokinetic rotator cuff ratios in skilled tennis players. Isokinetics and Exercise Science 3:155-159, 1993 (6) Koziris LP, Kraemer WJ, Triplett NT et al. (1991). Strength imbalances in women tennis players (abstract) Med Sci Sports Exercise 23(Suppl):253 (7) Kibler WB, McQueen C, Uhl T (1988). Fitness evaluation and fitness fmdings in competitive junior tennis players. Clin Sports Med 7:403-416. (8) Ellenbecker TS (1995). Rehabilitation of shoulder and elbow injuries in tennis players. Clin
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Age specific isokinetic glenohumeral... Sports Med 14:87-110. (9) Ellenbecker TS, Davies GJ (2000). The application of isokinetics in tesing a n d rehabilitation of the shoulder complex. Athietic Training 35(3):338-350. (10) Timm KE, Genrich P, B u r n s R, Fyke D (1992). The mechanical a n d physiological reliability of selected isokinetic dynamometers. Isokinetics and Exercise Science 2:182-190. {11) Durall CJ, Davies GJ, Kernozek TW, et al (2000). The effects of training the h u m e r a l rotators on a r m elevation in the scapular plane. J Sport Rehab 10(2):79-92 (12) Wilhite MR, Cohen ER, Wilhite SC (1992). Reliability of concentric a n d eccentric m e a s u r e m e n t s of quadriceps performance using the Kin-Corn dynamometer: The effects of testing order for three different speeds. J Orthop Sports Phys Ther 15(4): 175-182. {13) Baltzopoulos V, Williams JG, Brodie DA (1991). Sources of error in isokinetic dynamometry. Effects of visual feedback on m a x i m u m torque m e a s u r e m e n t s . J Orthop Sports Phys Ther 13:138-142. (14) Ivey FM, Calhoun JH, Rusche K, et al: Normal values for isokinetic testing of shoulder strength. Med Sci Sports Exerc 16:127, 1984. (15) Warner JJP, Micheli IJ , Arslanian L E e t al (1990). Patterns of flexibility, laxity, a n d strength in n o r m a l shoulders a n d shoulders with instability a n d impingement. Am J Sports Med 18:366-375. (16) Moncrief SA, Lau JD, Gale JR, Scott SA (2002). Effect of rotator cuff exercise on h u m e r a l rotation strength in healthy individuals. J Strength Conditioning Research 16(2):262-270. (17) Ellenbecker TS, Roetert EP (2002). Effects of a 4 m o n t h season on glenohumeral joint rotational strength a n d range of motion in female collegiate tennis players. J Strength Conditioning Research 16(1 }:92-96. (18) Cohen BD, Mont MA, Campbell KR, Vogelstein BN, Loewy J W (1992). Upper extremity physical factors affecting tennis serve velocity. Am J Sport Med 22(6):746-750. (19) Ellenbecker TS, Davies GJ, Rowinski MJ (1988). Concentric versus eccentric isokinetic strengthening of the rotator cuff. Objective data versus functional test. Am J Sports Med 16:64-69. (20) Mont MA, Cohen DB, Campbell KR, Gravare K, M a t h u r SK (1992). Isokinetic concentric versus eccentric training of shoulder rotators with functional evaluation of performance e n h a n c e m e n t in elite tennis players. Am J Sports Med 22(6):513-517. (21) Treiber FA, Lott J, D u n c a n J, Slavens G, Davis H, (1998). Effects of t h e r a b a n d a n d lightweight dumbbell training on shoulder rotation torque and serve performance in college tennis players. Am J Sports Med 26(4):510-515. (22) Francis K, Hoobler T (1987). Comparison of peak torque values of the knee flexor a n d extensor muscle groups u s i n g the Cybex II and Lido 2.0 isokinetic dynamometers. J Orthop Sports Phys Ther 8:480-483.
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Ellenbecker, T, & Roetert, EP (2003). Age specific isokinetic glenohumeral internal and external rotation strength in elite junior tennis players. Journal of Science and Medicine in Sport 6 (1): 6370. The high incidence of overuse shoulder injuries in elite junior tennis players is often attributed to both the high repetitive stresses inherent in the game, as well as muscular imbalances in the humeral rotators. The purpose of this study was to isokinetically measure concentric glenohumeral joint internal and external rotation strength, to develop a bilateral descriptive profile in elite junior tennis players across two age ranges. One hundred forty seven elite junior players between the ages of 12 and 21 were bilaterally tested on a Cybex isokinetic dynamometer with 90 ° of abduction. Players were analysed in two groups (12-17 and 18-21 years of age), using isokinetic variables of peak torque and work relative to body weight. Data analysis consisted of ANOVAs, with dependent t-tests used for post-hoc testing where main effect differences were identified. Results indicated no significant main effect difference in the isokinetic relative parameters between the two age groups (p>.01). Significantly greater (p<.001) dominant arm internal rotation was measured in both groups for both males and females, with no significant difference in external rotation strength measured between extremities in either age group. These data show specific adaptations in shoulder strength and identify a relative muscular imbalance between the internal and external rotators on the dominant arm of elite junior tennis players. Normative data presented in this study can assist clinicians and researchers in interpreting isokinetic test results in elite junior tennis players.
Introduction Repetitive muscular exertion in the upper extremity, required during p e r f o r m a n c e of t e n n i s specific m o v e m e n t p a t t e r n s , l e a d s to t h e d e v e l o p m e n t of s p o r t specific m u s c u l a r a d a p t a t i o n s i n elite level t e n n i s p l a y e r s (1-6). M u s c u l a r i m b a l a n c e s in t h e r o t a t o r c u f f a n d s c a p u l a r m u s c u l a t u r e , c o u p l e d w i t h inadequate muscular endurance and improper stroke biomechanics, can lead to o v e r u s e i n j u r y in t h e g l e n o h u m e r a l j o i n t of elite level t e n n i s p l a y e r s (7,8} P h y s i o l o g i c a l profiles of elite level t e n n i s p l a y e r s h a v e b e e n p r e v i o u s l y p u b l i s h e d . M u s c u l o s k e l e t a l profiles d o c u m e n t i n g r a n g e of m o t i o n a n d m u s c u l a r s t r e n g t h h a v e i d e n t i f i e d u n i l a t e r a l d o m i n a n c e in s e l e c t m u s c l e s in t h e u p p e r e x t r e m i t y of elite j u n i o r (2.3) a n d collegiate p l a y e r s (4,6}, a s well a s in adults{ 11. I n c r e a s e d d o m i n a n t a r m i n t e r n a l r o t a t i o n s t r e n g t h , w i t h o u t a c o n c o m i t a n t i n c r e a s e in g l e n o h u m e r a l j o i n t e x t e r n a l r o t a t i o n , h a s b e e n a c o n s i s t e n t f i n d i n g in t h e s e s t u d i e s . T h e s e p r o f i l e s h a v e p r o v i d e d d e s c r i p t i v e profiles u s i n g s m a l l e r s a m p l e sizes t h a t c a n l i m i t effective a p p l i c a t i o n i n clinical rehabilitation or physiological performance enhancement evaluations(9). T h e p r i m a r y p u r p o s e of t h i s s t u d y w a s to p r e s e n t a d e s c r i p t i v e a g e
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Age specific isokinetic glenohumeral...
specific profile of concentric isokinetic glenohumeral joint internal and external rotation strength in a larger sample of elite junior tennis players.
Methods and Procedures Subjects Male and female elite junior tennis players were isokinetically tested in this study. Subjects were divided into 2 testing groups; 12-17 and 18-21 years of age. Total subject population included (n=50) 12-17 year old males, (n=31) 1821 year old males, (n=31) 12-17 year old females, and (n=35) 18-21 year old females. All subjects were free from a glenohumeral joint injury and did not have an upper extremity surgical history. Additionally, subjects were excluded from the study if they had experienced glenohumeral joint pain that limited tennis play or practice for three consecutive days in the year prior to testing.
Instrumentation A calibrated Cybex 6000 isokinetic dynamometer (Cybex Inc., Ronkonkoma, NY) and upper body testing table (UBXT) were used for testing all 147 subjects in this study. The mechanical and physiological reliability of the Cybex isokinetic dynamometer's concentric mode has been previously established and published (10,11).
Isokinetic testing protocol An informed consent was completed by all subjects prior to the performance of isokinetic testing. A five minute warm-up on an upper body ergometer (UBE) (Cybex Inc., Ronkonkoma NY) was performed at 90 kpm in the clockwise direction (right hand view) prior to isokinetic strength testing. Extremities were measured in a random stratified sequence to minimise the effects of a learning bias. Testing was performed at 210 and 300 degrees per second, in that order. Randomisation of testing velocity was not followed to enhance the reliability of data acquisition (12). Subjects performed 4 submaximal warm-up repetitions at
Figure 1: Isokinetic set up for glenohumeral joint internal/external rotation with 900 of abduction
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Age specific isokinetic glenohumeral.., /
each testing speed, followed by five maximal repetitions for data generation. A thirty second rest between testing speeds was followed, based on tennis specific physiologic stress patterns. Subjects were placed on the UBXT in a supine position, with stabilisation straps applied at the pelvis and mid-thoracic region to minimise substitution (figure 1). The glenohumeral joint was abducted 90 ° in the coronal plane, with the d y n a m o m e t e r input axis aligned with the centre of rotation of the glenohumeral joint. The range of motion used for testing included (0-90 °) of external rotation and (0-65 °) of internal rotation. Range of motion stops were used to e n s u r e that all subjects performed identical ranges of motion during testing. Testing was initiated with the shoulder in a position of 90 ° of external rotation. Standardised visual and auditory feedback was given by the test administrator during all tests (9,13). Data analysis
The p a r a m e t e r s of peak torque and single repetition work relative to body weight were analysed for the male and female subjects in both age groups. Unilateral strength ratios (external/internal rotation ratios) were computed by the Cybex 6000 software and were not subjected to further statistical analysis. SPSS for Windows (SPSS Inc., Chicago, IL) statistical software was used to perform two repeated measures ANOVAs; one for males and one for females. A 2 (age) X 2 (dominance: dominant, nondominant) X 2 (rotation: external, internal) X 2 (parameter: peak torque, single repetition work) ANOVA was applied with statistical significance set at the (p<.01) level. Post-hoc testing was performed using dependent t-tests in areas where main effect differences were initially identified. A Boneferroni a d j u s t m e n t was utilised to minimise the chance of type II error with the application of multiple t-tests. Statistical significance was set at the (p<.001) level for the post-hoc tests.
Results Table 1 lists the results of the two ANOVAs performed on the male and female isokinetic data. No significant difference was found for the between-subjects main effect of AGE for males or females. This indicates that the relative ratios of m u s c u l a r performance m e a s u r e d between the younger and older elite junior players did not significantly differ from one another.
Source
Males (p-value)
Females (p-value)
Between Subjects Factor Age
.013
.287
Within SubjeCts Factors Dominance Rotation Parameter Dominance X Rotation Dominance X Parameter Rotation X Parameter Rotation X Dominance X Parameter
.001 .001 .001 .001 .001 .013 .001
.001 .001 .001 .001 .001 .001 .001
Table 1:
Resultsof ANOVAperformed on 147 elite iunior tennis players.
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Age specific isokinetic glenohumeral..,
Parameter
Age 12-17 (11=50) Dom Arm
Age 18-21 (n=31)
Ndom Arm
DOm Arm
Ndom Arm
Torque/Body Weight Ratios (%) * ER210 33.6_+6.3 ER300 29.6_+5,6 IR 210 49.7_+12.5 IR 300 44,0_+11.0
32,0_+6.25 28.5_+6,0 40.8_+9,2 35,5_+8.9
37,8_+9.1 32.9_+8.1 55.1_+14,6 46.5_+13,2
35.8_+6,8 40.0_+7.4 47.4_+13.4 40.3_+11.1
Work/BOdy Weight Ratios (%) ** ER 210 58.3_+14.5 ER 300 49.7_+13,2 IR 210 93.6_+26,7 IR 300 79,4_+24,4
58.3_+14.3 49.2_+13.2 74,6_+21,1 60,9_+18.9
67.7_+17.9 57.6_+16.8 108.4_+32.0 90,1_+32.3
67,6_+15.5 57.8_+15.2 92.2_+30,0 77.4_+26.6
* Values are expressed in Nm/Kg of body weight (Conversion 1 ft Ib/Ib = 2.98 Nm/Kg) ** Values are expressed in Joules/Kg (Conversion 1 ft Ib/Ib = 2.98 Joules/Kg)
Table2/1- Isokinetic peak Torque~Body Weight and Work~Body Weight Ratios for the male elite junior tennis players, Parameter
12-17 (n=31) Dom Arm
Ndom Arm
Age 18-21 (n=35) Dora Arm
Ndom Arm
Torque/Body Weight Ratios (%) * ER 210 29.0_+4.1 ER 300 22.4.+6.1 IR 210 42.3_+8,0 IR 300 34,2_+7.8
27.6+4.5 21,2.+5,3 34,7_+6.6 27.1 _+6.6
24.7_+6.1 21.2.+7,3 33.2_+8,6 28.8.+9.4
23.7+5.8 20.7_+6.4 23,4_+7.6 20.0_+8,5
Work/BOdy Weight Ratios (%) ** ER 210 49.1.+8.7 ER 300 34.1.+12.4 IR 210 80.5_+20,7 IR 300 57.2_+20,6
49.5_+7,4 32.8+11.4 61.6_+11.9 43.4_+14.4
47.1.+13,2 37,0.+14.9 64.0_+19,0 51.1_+19.6
46.1.+12,7 37.4_+14,0 43,1_+5.0 34.2_+15.8
* Values are expressed in Nm/Kg of body weight (Conversion 1 ft Ib/Ib = 2.98 Nm/Kg) ** Values are expressed in Joules/Kg (Conversion I ft Ib/Ib = 2.98 Joules/Kg)
Table 2B: Isokinetic peak Torque~Body Weight and Work~Body Weight Ratios for the female eite junior tennis players.
Significant within-subjects m a i n effect differences were identified (p<.001) for dominance, rotation, a n d parameter. Significant interactions were identified and are listed in Table 1 for m a l e s and females. Post-hoc d e p e n d e n t t-tests identified significantly greater (p<.001) d o m i n a n t a r m internal rotation strength in b o t h males a n d females in both age groups at 210 a n d 300 degrees per second. This difference w a s m e a s u r e d for both p e a k torque and work p a r a m e t e r s . No significant difference was m e a s u r e d in external rotation strength between extremities in a n y group. Tables 2A & 2B list the p e a k torque and work to body weight ratios from the 147 elite junior tennis players tested in this study. D a t a are p r e s e n t e d for the 12-17 y e a r olds and 18-21 year olds separately for internal and external
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Age specific isokinetic glenohumeral...
Parameter
Peak Torque ER/IR (%) ER/IR 210 ER/IR 300 WOrk ER/IR (%) ER/IR 210 ER/IR 300
Age 12-17 (n=50)
Age 18-21 (11=31)
Dora Arm
Ndom Arm
DOm Arm
Ndom Arm
69.7_+12.7 69.3_+11.3
81.5_+16.6 82.1_+16.2
70.4_+14.0 72.7_+11.9
80.5_+16.4 84.0_+18.4
81.4±18.4 83.3±17.3
64.5_+13.1 67.3_+14.7
77.6_+19.3 79.1_+20.6
64.3_+12.47 65.5_+14.5
Table 3,4: IsOkinetic external/internal rotation unilateral strength ratios for male elite junior tennis players.
Parameter
Age 12-17 (n=31)
Age 18-21 (n=35)
Dom Arm
Ndom Arm
Dom Arm
Ndom Arm
Peak Torque ER/IR (%) ER/IR 210 ER/IR 300
69.1_+11.0 66.8_+13.3
81.6_+13.8 80.5_+20.0
76.1_+13.0 76.5_+12.9
106.4_+26.4 111.0_+34.6
Work ER/IR (%) ER/IR 210 ER/IR 300
63.7_+13.8 61.1_+13.2
82.7_+16.1 77.5_+19.8
75.2_+14.1 75.2_+19.8
117.0_+38.5 120.4_+40.3
Table 3B: Isokinetic external/internal rotation unilateral strength ratios for female elite junior tennis players.
rotation. Tables 3A & 3B contain the e x t e r n a l / i n t e r n a l rotation (ER/IR) ratios t h a t are derived directly from the Cybex 6000 software and provide a m e a s u r e of m u s c u l a r b a l a n c e between the h u m e r a l rotators. Male E R / I R ratios range between 64-72% on the d o m i n a n t a r m a n d 77-84% on the n o n - d o m i n a n t a r m . Female ratios for the d o m i n a n t a r m were similar to those m e a s u r e d in the male s u b j e c t s , r a n g i n g b e t w e e n 61-76%, a n d w e r e slightly higher on t h e n o n d o m i n a n t a r m , ranging between 77 a n d 120%.
DisCussion The results of this study s u p p o r t earlier r e s e a r c h performed on smaller subject populations of junior and senior tennis players t h a t identified significantly greater d o m i n a n t a r m glenohumeral internal rotation strength, with no bilateral difference in external rotation strength (1,2,4). This s t u d y utilised a substantially larger subject population t h a n earlier studies a n d provides normative p e a k torque and w o r k / b o d y weight ratios t h a t are age and gender specific. No statistically significant difference was found between age groups in the traditional p e a k torque a n d work variables w h e n normalised to body weight in either the m a l e s or females. E x a m i n a t i o n of the normative ratios in tables 2A and 2B do show small increases in the t o r q u e / b o d y weight and w o r k / b o d y weight ratios in the older players b u t these differences were not statistically significant at the (p<.01) level. Apparently, as players increase in age a n d develop, b o d y m a s s and shoulder strength increase at similar levels in healthy,
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Age specific isokinetic glenohumeral... uninjured players, leading to similar isokinetic internal and external rotation strength ratios across age ranges. The E R / I R ratios m e a s u r e d in this s t u d y are substantially lower on the d o m i n a n t a r m , as c o m p a r e d to the n o n d o m i n a n t arm. The unilateral increase in d o m i n a n t a r m internal rotation without m a t c h e d development of the glenohumeral external rotators creates a lower E R / I R ratio on the d o m i n a n t a r m . The E R / I R ratios on the d o m i n a n t a r m do c o m p a r e with other studies of n o r m a l uninjured male a n d female subjects (9,14/and ranged between 61 a n d 76%. Warner et al (151 m e a s u r e d E R / I R ratios in patients with glenohumeral joint instability a n d i m p i n g e m e n t a n d found alterations of the E R / I R ratio. The r e c o m m e n d e d E R / I R ratio typically ranges between 66-75%, s u c h t h a t the external rotators are at least 2 / 3 r d s the strength of the internal rotators, to provide m u s c u l a r b a l a n c e (9,8). Application of the normative d a t a from this s t u d y provides valuable insight regarding d o m i n a n t / n o n - d o m i n a n t differences, p e r f o r m a n c e of the internal a n d external rotators relative to body weight a n d finally, the n o r m a l unilateral e x t e r n a l / i n t e r n a l m u s c l e balance in healthy elite junior tennis players. Utilisation of the ratios provided in this s t u d y can serve to guide clinicians a n d r e s e a r c h e r s in developing r e h a b i l i t a t i o n a n d p r e v e n t a t i v e conditioning p r o g r a m s for the shoulder in elite junior tennis players. The m e t h o d for direct application of this data c a n b e s t be described via discussion of bilateral comparison, normative relative ratios and finally unilateral (ER/IR} strength ratios. For example, isokinetic testing of a 16 y e a r old male elite junior tennis player results in significantly greater d o m i n a n t a r m internal rotation strength, a n d mildly greater external rotation strength. This bilateral c o m p a r i s o n is c o m p a r a b l e with the e s t a b l i s h e d s t a n d a r d s of published d a t a on elite junior m a l e tennis players (2,4/. However, without knowledge of the degree of n o n - d o m i n a n t extremity strength, the bilateral c o m p a r i s o n c a n create a n i n a c c u r a t e representation of d o m i n a n t u p p e r extremity strength. Therefore, application of the normative d a t a presented in Table 2A allows the clinician to determine w h e t h e r the degree of external or internal rotation is consistent with t h e n o r m s from the elite populations of players tested in this investigation. Peak t o r q u e / b o d y w e i g h t ratios of 30-35 N m / K g would be expected for the external rotators on the d o m i n a n t arm, a n d ratios between 50-60 N m / K g of b o d y weight for internal rotation. Regardless of the bilateral comparison, the application of the n o r m a t i v e torque a n d w o r k / b o d y w e i g h t ratios e n s u r e s t h a t d o m i n a n t a r m strength is consistent with other high level players in this population. Players testing well below these s t a n d a r d levels of strength adjusted for body weight would be placed on a p r o g r a m of resistive exercise to a d d r e s s the m e a s u r e d deficiencies. Finally, even ,in the p r e s e n c e of acceptable levels of d o m i n a n t a r m internal a n d external rotation strength, m u s c u l a r imbalance is often p r e s e n t in elite athletes of unilaterally d o m i n a n t sports s u c h as baseball and tennis (8). To e n s u r e t h a t this 16 year old player h a s optimal m u s c u l a r b a l a n c e on the d o m i n a n t a r m , e x a m i n a t i o n of the e x t e r n a l / i n t e r n a l rotation unilateral strength ratios is r e c o m m e n d e d . Often, the ratio of E R / I R strength drops below the 66-75% range a n d a p p r o a c h e s 50%. This 50% ratio indicates t h a t the external rotators are only one-half as strong as the internal rotators. Alterations in the E R / I R ratio have b e e n described by Warner et al (151 in
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Age specific isokinetic glenohumerai... individuals with glenohumeral joint instability and impingement and are a n important parameter to apply from the normative data presented in this research. Players with E R / I R ratios below normal s t a n d a r d s would be placed on specific resistive exercise programs to target the weaker muscle group (external rotators in this example) to improve the ratio and s u b s e q u e n t balance of rotational strength. Application of specific exercises (16,17) to increase external rotation strength appears warranted, based on the lower external/internal rotation ratios on the d o m i n a n t a r m and no concomitant external rotation strength dominance m e a s u r e d in this population. Increases in the E R / I R ratio have been correlated with serving performance in elite players (18) a n d specific evidence based r e c o m m e n d a t i o n s for increasing external rotation strength in tennis players using exercises to improve external rotation strength have been reported in the
literature {19,20,21). Several cautions m u s t be followed w h e n utilising the normative data presented in this research. These data are relevant for elite junior tennis players m e a s u r e d concentrically on the Cybex 6000 isokinetic dynamometer. Caution m u s t be used when extrapolating normative data profiles to other a p p a r a t u s and subject populations (9,22). The use of the larger and h o m o g e n o u s subject population of elite junior tennis players in this study serves to provide the most accurate representation of glenohumeral rotational strength patterns in elite j u n i o r tennis players. Finally, this s t u d y measured only concentric isolated glenohumeral joint internal and external rotation and does not take into effect the complex interplay and important contribution of the scapular m u s c u l a t u r e in the u p p e r extremity kinetic chain.
Conclusion The results of this s t u d y show similar peak torque and w o r k / b o d y weight ratios in male and female elite junior tennis players across two age ranges. Significantly greater d o m i n a n t a r m internal rotation was consistently m e a s u r e d in both males and females, with no significant difference between extremities identified in external rotation. The normative data presented in this s t u d y are designed to assist clinicians and researchers in the interpretation of isokinetic glenohumeral internal and external rotation strength tests during injury rehabilitation and performance e n h a n c e m e n t evaluations.
References (1) Ellenhecker TS (1991). A total arm strength isokinetic profile of highly skilled tennis players. Isokinetics and Exercise Science 1:9-21. (2) Ellenbecker TS (1992). Shoulder internal and external rotation strength and range of motion of highly skilled junior tennis players. Isokinetics and Exercise Science 2:1-8. (3) Ellenbecker TS, Roetert EP (1999). Testing isokinetic muscular fatigue of shoulder internal and external rotation in elite junior tennis players. J Orthop Sports Phys Ther 29(5):275-281. (4) Chandler TJ, Kibler WB, Stracener EC, Ziegler AK, Pace B. (1992) Shoulder strength, power and endurance in college tennis players. A m J Sports Med 20:455-458. (5) Kennedy K, Altchek DW, Glick IV: Concentric and eccentric isokinetic rotator cuff ratios in skilled tennis players. Isokinetics and Exercise Science 3:155-159, 1993 (6) Koziris LP, Kraemer WJ, Triplett NT et al. (1991). Strength imbalances in women tennis players (abstract) Med Sci Sports Exercise 23(Suppl):253 (7) Kibler WB, McQueen C, Uhl T (1988). Fitness evaluation and fitness fmdings in competitive junior tennis players. Clin Sports Med 7:403-416. (8) Ellenbecker TS (1995). Rehabilitation of shoulder and elbow injuries in tennis players. Clin
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Age specific isokinetic glenohumeral... Sports Med 14:87-110. (9) Ellenbecker TS, Davies GJ (2000). The application of isokinetics in tesing a n d rehabilitation of the shoulder complex. Athietic Training 35(3):338-350. (10) Timm KE, Genrich P, B u r n s R, Fyke D (1992). The mechanical a n d physiological reliability of selected isokinetic dynamometers. Isokinetics and Exercise Science 2:182-190. {11) Durall CJ, Davies GJ, Kernozek TW, et al (2000). The effects of training the h u m e r a l rotators on a r m elevation in the scapular plane. J Sport Rehab 10(2):79-92 (12) Wilhite MR, Cohen ER, Wilhite SC (1992). Reliability of concentric a n d eccentric m e a s u r e m e n t s of quadriceps performance using the Kin-Corn dynamometer: The effects of testing order for three different speeds. J Orthop Sports Phys Ther 15(4): 175-182. {13) Baltzopoulos V, Williams JG, Brodie DA (1991). Sources of error in isokinetic dynamometry. Effects of visual feedback on m a x i m u m torque m e a s u r e m e n t s . J Orthop Sports Phys Ther 13:138-142. (14) Ivey FM, Calhoun JH, Rusche K, et al: Normal values for isokinetic testing of shoulder strength. Med Sci Sports Exerc 16:127, 1984. (15) Warner JJP, Micheli IJ , Arslanian L E e t al (1990). Patterns of flexibility, laxity, a n d strength in n o r m a l shoulders a n d shoulders with instability a n d impingement. Am J Sports Med 18:366-375. (16) Moncrief SA, Lau JD, Gale JR, Scott SA (2002). Effect of rotator cuff exercise on h u m e r a l rotation strength in healthy individuals. J Strength Conditioning Research 16(2):262-270. (17) Ellenbecker TS, Roetert EP (2002). Effects of a 4 m o n t h season on glenohumeral joint rotational strength a n d range of motion in female collegiate tennis players. J Strength Conditioning Research 16(1 }:92-96. (18) Cohen BD, Mont MA, Campbell KR, Vogelstein BN, Loewy J W (1992). Upper extremity physical factors affecting tennis serve velocity. Am J Sport Med 22(6):746-750. (19) Ellenbecker TS, Davies GJ, Rowinski MJ (1988). Concentric versus eccentric isokinetic strengthening of the rotator cuff. Objective data versus functional test. Am J Sports Med 16:64-69. (20) Mont MA, Cohen DB, Campbell KR, Gravare K, M a t h u r SK (1992). Isokinetic concentric versus eccentric training of shoulder rotators with functional evaluation of performance e n h a n c e m e n t in elite tennis players. Am J Sports Med 22(6):513-517. (21) Treiber FA, Lott J, D u n c a n J, Slavens G, Davis H, (1998). Effects of t h e r a b a n d a n d lightweight dumbbell training on shoulder rotation torque and serve performance in college tennis players. Am J Sports Med 26(4):510-515. (22) Francis K, Hoobler T (1987). Comparison of peak torque values of the knee flexor a n d extensor muscle groups u s i n g the Cybex II and Lido 2.0 isokinetic dynamometers. J Orthop Sports Phys Ther 8:480-483.
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