Concentric Internal and Eccentric External Fatigue Resistanc of the Shoulder Rotator Muscles in Female Tennis Players

Abstract

Background

Shoulder muscle imbalance is a potential shoulder injury risk factor in athletes performing overhead sports. While normative functional peak strength of concentric external to concentric internal shoulder muscle fatigue data is available, comparisons of functional eccentric external to concentric internal shoulder rotator muscle fatigue resistance, which impacts muscle imbalance throughout the duration of play, have not been studied in this population.

Objectives

To assess fatigue resistance of the internal and external shoulder rotator muscles in female tennis players.

Methods

Fifteen female collegiate tennis players were tested bilaterally for shoulder concentric internal and eccentric external peak torque production throughout 20 maximal repetitions on a Kin-Com isokinetic dynamometer. Twelve t - tests were conducted to evaluate for differences in peak torque, relative fatigue ratios, and functional peak torque ratios between extremities and mode of activation during the first, as well as, last five repetitions that were conducted.

Results

Non-dominant concentric internal and eccentric external peak torque production significantly decreased throughout the twenty repetitions. Neither dominant concentric internal peak torque decrements and eccentric peak torque decrements were not significantly different across the twenty contractions.

These changes in peak torque upon subsequent repetitions resulted in relative fatigue ratios of dominant eccentric external rotation that were significantly greater than non-dominant eccentric external rotation. Relative fatigue ratios of dominant concentric internal rotation did not differ from non-dominant concentric internal rotation.

Conclusions

The data suggest that eccentrically activated external shoulder rotator muscles could possibly adapt to overhead activities by becoming more fatigue resistant.

INTRODUCTION

Shoulder muscle imbalance, indicated by a low external to internal shoulder rotator muscle strength ratio, has been observed in patients with glenohumeral joint instability and impingement^1,2 and is considered to be a shoulder injury risk factor for athletes performing overhead activities.³ Early studies on amateur, elite junior, as well as professional tennis players have isokinetically assessed the shoulder rotator musculature concentrically, showing that shoulder muscle imbalance often occurs as the result of an adaptation to frequent overhead motions, which causes greater increases in concentric internal rotator strength than in concentric external rotator strength.^4–7

While assessment of concentric internal and concentric external shoulder rotator strength provided early isokinetic normative values, researchers increasingly acknowledged the need to test the strength of the external shoulder rotator musculature eccentrically, which is the dominant mode of activation of the external shoulder rotator muscles during overhead motions.⁸ McCarrick and Kemp⁹ showed that while mean peak torque of the internal rotator muscles was similar when tested eccentrically as compared to concentrically, eccentric external rotation mean peak torque was significantly greater as compared to concentric external rotation mean peak torque. Consequently, functional muscle eccentric external to concentric internal rotation strength ratios were found to be significantly greater than concentric external to concentric internal strength ratios in athletes performing overhead activities. Data by Noffal¹⁰ further supported these findings leading to the suggestion that functional eccentric assessments of the external rotation muscles rather than concentric assessment might be a better identifier of possible muscular imbalance. Normative data of eccentric and concentric peak strength of internal and external rotation, as well as functional ratios have since been published for various groups of athletes. Results vary depending on sex, angular velocity, range of motion and testing position, as well as the type of athletic involvement of the subjects.^8,10–15

In contrast to the growing body of normative peak strength data, very few studies have attempted to assess isokinetic muscular fatigue of the shoulder internal and external rotator musculature.^4,16,17 Chandler et al,⁴ as well as Ellenbecker and Roetert¹⁶ assessed concentric internal and concentric external fatigue of the shoulder rotator muscles and found that concentrically activated external rotator muscles fatigue faster than concentrically activated internal rotator muscles. These authors suggested that shoulder muscle imbalances increase upon prolonged activity, thereby, potentially increasing the risk of injury to the athlete throughout the duration of play.

Furthermore, only one study is currently available that assessed relative fatigue of eccentrically activated external shoulder rotator muscles on athletes who did not perform overhead activities.¹⁷ Similar to the studies that assessed concentric external shoulder rotator muscle fatigue, the study by Mullaney and McHugh¹⁷ showed no significant difference in fatigue of the concentrically activated internal rotator muscles versus eccentrically activated external rotator muscles in athletes participating in recreational sports. Data on eccentric fatigue of the external rotator muscles in comparison to concentric internal rotator muscle fatigue in athletes performing overhead activities, however, was not reported. Since such a functional fatigue ratio potentially provides a better indicator of the change in shoulder muscle imbalance throughout the duration of play and, hence, is a potential predictor for sustaining shoulder injuries, normative data on functional relative fatigue ratios in athletes performing overhead activities are warranted. The purpose of this study was to assess the effects of fatigue on concentric internal and eccentric external shoulder rotation strength in a group of female collegiate tennis players.

METHODS

Subjects

Fifteen collegiate Division II and National Association of Intercollegiate Athletics tennis players without a history of previous shoulder injury were recruited for this study (Table 1). All subjects completed a brief personal history form including age, years played, and arm used to serve. Weight and height measurements were also recorded. The study was approved by the Institutional Review Board of Indiana at the University of Pennsylvania. Informed consent was obtained and the rights of all subjects were protected prior to and after the data collection process.

Table 1.

Subject demographics (n = 15)

Subject	Mean ± SD	Range
Age	19.5 ± 1.5	18 – 23
Years Played	7.0 ± 3.3	4 – 16
Weight [kg]	67.2 ± 13.4	49.1 – 98.4
Height [cm]	164.1 ± 6.5	152 – 179
BMI* [kg/m2]	24.9 ± 4.4	19.8 – 36.6

^*body mass index

Assessing Isokinetic Shoulder Strength

Assessment of muscular strength was conducted using the Kin-Com AP Muscle Testing System (Chattecx Corp., Hixson, Tennessee). Testing employed maximum contractions during concentric internal and eccentric external shoulder rotation. An angular velocity of 120°/second was selected to minimize variance as well as to reduce the risk of injury to the subjects.^18,19 The subjects completed 10 submaximal contractions to familiarize themselves with the procedure and to aid in a specific neuromuscular warm-up.

Throughout the assessment, the subjects were seated without the legs touching the ground and the trunk secured to the chair. To approximate shoulder and elbow positioning throughout an actual overhead motion and in accordance with previous studies, the shoulder was abducted to 90° and the elbow was flexed to 90°.^6,7,11,20 The elbow was then secured in a custommade support. The range of motion completed during the test was between 90° external rotation and 30° internal rotation. While the external range of motion stop was chosen based on prior studies,^11,21 the internal range of motion stop was chosen based on established passive range of motion limits that were as low as 30° of internal rotation for some of the subjects. Hence, greater internal rotation range of motion stops during isokinetic testing as employed in prior studies^7,11,21 would have increased the risk of injury to the subjects and were, consequently, avoided. Imitating the sequence of muscle involvement in a serve, concentric internal rotation was decided upon to always be tested first, immediately followed by eccentric external rotation. Each subject performed 20 maximal contractions during the test. To calculate a relative fatigue ratio, the total peak torque produced in the last five repetitions was divided by the total peak torque produced throughout the first five repetitions.

Data Analysis

Means (± standard deviations) of isokinetic peak torque on the dominant and non-dominant extremity were calculated for each condition. Twelve t - tests were conducted to evaluate for differences in peak torque, relative fatigue ratios, and functional peak torque ratios between extremities and mode of activation during the first, as well as last five repetitions that were conducted. A Bonferroni adjustment was used to correct for the multiple comparisons. Originally, the data were considered significantly different at the 0.05 level if the p value was less than 0.05 / 12 = 0.00416 within any of the subsets.

RESULTS

Peak Torque Protection

The results of eccentric external and concentric internal isokinetic peak torque on the dominant and non-dominant extremity are summarized in Table 2. The data of the following t-test discussion are provided in Table 3.

Table 2.

Mean concentric internal rotation and eccentric external rotation peak torques, and eccentric external to concentric internal peak torque ratios, of the dominant and non-dominant extremity during the first and last five repetitions.

Peak Torque [Nm]	D		ND
	Mean ± S.D.	Range	Mean ± S.D.	Range
IR 1 – 5	13.98 ± 3.05	8.51 – 18.48	15.58 ± 2.78	10.12 – 20.46
IR 15 – 20	12.18 ± 2.80	7.36 – 17.25	12.75 ± 1.75	10.40 – 16.80
ER 1 – 5	15.42 ± 4.46	8.51 – 26.62	17.78 ± 2.94	13.20 – 25.52
ER 15 – 20	15.15 ± 4.41	8.80 – 27.50	14.57 ± 2.48	10.08 – 18.04
ER / IR Ratio 1 – 5	1.11 ± 0.24	0.70 – 1.73	1.19 ± 0.41	0.78 – 2.50
ER / IR Ratio 15 – 20	1.27 ± 0.37	0.75 – 0.37	1.17 ± 0.25	0.60 – 1.48

D = dominant extremity, ND = non-dominant extremity, IR = concentric internal rotation, ER = eccentric external rotation

Table 3.

t-test evaluations of peak torque, relative fatigue ratios and functional peak torque ratios between extremities and mode of activation during the first and last five repetitions that were conducted

Measurement		t stat	p
Peak Torque
DER 1-5	NDER 1-5	2.946	.0053
DIR 1-5	NDIR 1-5	1.464	.0826
DIR 1-5	DIR 15-20	2.670	.0092
NDIR 1-5	NDIR 15-20	3.900	.0008*
DER 1-5	NDER 15-20	.0409	.3444
NDER 1-5	NDER 15 -20	4.954	.0002*
Relative Fatigue Ratios (15-20 / 1-5)
DIR	NDIR	0.590	.2822
DER	NDER	3.151	.0035*
DIR	DER	2.387	.0158
NDIR	NDER	0.210	.4184
Functional Peak Torque Ratios (Eccentric External/Concentric Internal)
D 1 - 5	D 15 - 20	2.515	.0124
ND 1 - 5	ND 15 -20	0.257	.4005

^*Significant difference at p< 0.00416

DIR = dominant internal rotation, DER = dominant external rotation, NDIR = non-dominant internal rotation, NDER = non-dominant external rotation, * = eccentric external contractions, † = concentric external contractions).

Subjects showed a tendency to reach a lower peak torque during eccentric external rotation on the dominant extremity (15.42 ± 4.46) than on the non-dominant extremity (17.78 ± 2.94; t = 2.946; df = 14, p = 0.0053) but the difference was not significant. Concentric internal rotation peak torque was not significantly different between extremities (dominant internal rotation 13.98 ± 3.05; non-dominant internal rotation 15.58 ± 2.78; t = 1.464; df = 14, p = 0.0826).

Throughout the twenty repetitions, concentric internal and eccentric external peak torque production on the non-dominant extremity significantly decreased (p ≤ 0.008). Concentric internal peak torque decrements on the dominant extremity were not significant (Mean ± standard deviation: dominant internal rotation 1-5 = 13.98 ± 3.05, dominant internal rotation 15-20 = 12.08 ± 2.80; p = 0.0092), and dominant eccentric external peak torque did not significantly change upon subsequent repetitions, as well (dominant external rotation 1-5 = 15.42 ± 4.46. dominant external rotation 15-20 = 15.15 ± 4.41; p = 0.3444).

Relative Fatigue Ratios and Functional Peak Torque Ratios

While the relative fatigue ratios for concentric internal rotation of the dominant extremity (88.78 ± 20.03) were not significantly different from the relative fatigue ratios for concentric internal rotation on the non-dominant extremity (84.08 ± 18.13; t = 0.590; df = 14, p = 0.2822), relative fatigue ratios for eccentric external rotation on the dominant extremity (100.48 ± 21.76) were significantly less than relative fatigue ratios for eccentric external rotation on the non-dominant extremity (82.71 ± 13.25; t = 3.151; df = 14; p = 0.0035). Additionally, dominant extremity eccentric external rotation peak torque (100.48 ± 21.76) showed a tendency to decrease less than concentric internal rotation peak torque (88.78 ± 20.03; t = 2.387; df = 14; p = 0.0158). Decrements in eccentric external rotation peak torque on the non-dominant extremity (82.71 ± 13.25) were not significantly different from decrements in concentric internal rotation peak torque (84.08 ± 18.13; t = 0.210; df = 14; p = 0.4184). As a result, eccentric external rotation to concentric internal rotation peak torque ratios on the dominant extremity during the last five repetitions tended to increase from 1.11 ± 0.24 to 1.27 ± 0.37 (t = 2.515; df = 14; p = 0.0124) whereas eccentric external to concentric internal rotation peak torque ratios on the non-dominant extremity did not change (non-dominant external rotation / internal rotation Ratio 15-20: 1.17 ± 0.25; non-dominant external rotation / internal rotation Ratio 1-5: 1.19 ± 0.41; t = 0.257; df = 14; p = 0.4005).

DISCUSSION

The subjects in this study exhibited weakness of eccentric external rotation on the dominant extremity compared to the non-dominant extremity. This weakness is in accordance with findings of previous studies on eccentric external peak torque differences between the dominant and non-dominant extremity in athletes performing overhead activities and is thought to be an adaptation to frequent overhead motions.^10,22 Increased concentric internal peak torque in the dominant extremity, another common adaptation to frequent overhead motions,^6,10 was not observed in this study. Conversely, athletes in this study tended to display increased fatigue resistance of the eccentrically activated external rotator muscles compared to the concentrically activated internal rotator muscles on the dominant extremity. Moreover, fatigue resistance of the eccentrically activated external rotator muscles on the dominant extremity was significantly increased as compared to the non-dominant extremity.

The present study is the first available study to observe greater fatigue resistance of the eccentrically activated external rotator muscles of the dominant extremity in athletes performing overhead activities and stands in contrast to previous findings on fatigue resistance of the shoulder rotator muscles.^4,16,17 Previous studies by Ellenbecker et al¹⁶ and Chandler et al,⁴ however, differed from this study as their assessment employed concentric instead of eccentric contractions of the external rotator muscles as well as several other varying parameters of angular velocity and range of motion being assessed.

Employing concentric external contractions at an angular velocity of 300°/sec, the study by Ellenbecker et al¹⁶ demonstrated decreased fatigue resistance of the concentrically activated external rotator muscles as compared to the concentrically activated internal rotator muscles. Furthermore, no difference in concentric external fatigue resistance of the contralateral extremity was observed.¹⁶ Similarly, the study by Chandler et al4 which assessed fatigue of the shoulder rotator muscles on the dominant extremity only, also employed concentric contractions of the external rotator muscles at an angular velocity of 300°/sec. That particular study observed no difference in fatigability between the concentrically activated external and internal rotator muscles.

The only available study on fatigue resistance of concentrically activated internal rotator muscles together with eccentrically activated external rotator muscles by Mullaney et al¹⁷ focuses on a group of 10 non-athletes. In that study, fatigue resistance was determined by the change in peak torque between the first and last five of a total of 32 repetitions at an angular velocity of 120°/sec. Like Ellenbecker et al¹⁶ and Chandler et al,⁴ Mullaney et al¹⁷ did not observe a significant difference in fatigue of the eccentrically activated external rotator muscles versus the concentrically activated internal rotator muscles. Since Mullaney et al¹⁷ accessed the dominant extremity only, no statements could be made regarding fatigue resistance of the eccentrically activated external rotator muscles on the dominant as compared to the non-dominant extremity.

Considering that fatigue ratios for concentric internal rotation in the present study were similar to those observed in prior studies, the authors hypothesize that the greater relative fatigue ratios for eccentric external rotation observed in this study were primarily due to the eccentric instead of concentric mode of activation of the external rotator muscles, rather than varying ranges of motion, angular velocities, or positioning of the body. This hypothesis is supported by previous studies that showed that muscular adaptations are contraction specific. McCarrick et al⁹ for example, showed that mean peak torque of the external rotator muscles is increased after a 12 week resistance training program if tested eccentrically but not if tested concentrically. Based on this finding, as well as the present data, the authors suggest that eccentric fatigue measures instead of concentric fatigue measures of the external rotator muscles might provide rehabilitation professionals with further functional data of shoulder muscle fatigue relevant to athletes performing overhead activities.

Furthermore, the authors hypothesize that the difference in fatigue of the eccentrically activated external rotator muscles on the dominant extremity observed in this study compared to the study by Mullaney et al²² could be attributed to specific adaptations of the shoulder rotator musculature in response to frequent overhead motions among the present group of tennis players versus the group of non-athletes studied by Mullaney et al.²² Such an eccentric external rotation fatigue resistance in these athletes performing overhead activities would be in accordance with previously reported data on eccentric fatigue resistance of the knee extensors, plantarflexor and the dorsiflexor muscles. Tesch et al²³ showed a 34 –47 % decrement in strength during 96 concentric contractions of the knee extensors without any fatigue occurring during 96 eccentric contractions. Hortobagyi et al²⁴ found fatigue of the plantarflexors during 50 maximal isometric and concentric contractions to be 41% and 32%, respectively, but found no change in force during 50 eccentric contractions. Eccentric fatigue resistance was also observed in dorsiflexsors with a strength decrement of 31.6 % during concentric contractions but only 23.8% during eccentric contractions.²⁵ All of these cases of eccentric fatigue resistance have been found in the lower extremity and concern segments that are trained in daily activities such as walking, jogging, or biking. Eccentric fatigue resistance could be hypothesized to be an adaptation to regular eccentric activation of a given muscle, suggesting that it might also be a prevalent adaptation in athletes performing overhead activities which is absent in athletes who do not perform overhead activities.

Assuming that the present data provides an accurate reference of functional muscle fatigue during repetitive overhead motions, the authors make the following conclusions. First, since muscle fatigue of the eccentrically activated external rotator muscles have not been found to be greater than muscle fatigue of the concentrically activated internal rotator muscles, perhaps, muscle balance is not exacerbated throughout repetitive overhead motions, and appears that no need exists for further exercises to improve fatigue resistance of the external rotator muscles in healthy athletes. Second, peak torque muscle strength imbalance assessments during only a few repetitions can potentially be used to assess a healthy athlete's possible risk of injury due to muscle imbalance without having to consider a potentially increased risk due to differential fatigue throughout prolonged overhead activities. Finally, increased fatigue resistance of the eccentrically activated external rotator muscles might be an adaptation to frequent overhead activities that protects the athlete performing overhead activities from overuse injuries to the shoulder. More research is warranted regarding the role of increased fatigue resistance of the eccentrically activated external rotator muscles in injury prevention. Potentially, athletes returning to overhead activities after shoulder injury could benefit from specific strength training to increase fatigue resistance of the eccentrically activated external rotator muscles. Lastly, testing of relative fatigue ratios of concentric internal and eccentric external rotator strength could possibly be applied to evaluate a rehabilitating athlete's readiness to return to the sport.

CONCLUSION

Contrary to previous studies, this study was the first to show increased fatigue resistance of the eccentrically activated external shoulder rotator muscles in adult and uninjured athletes performing overhead activities. The authors hypothesize that this adaptation might protect athletes performing overhead activities from sustaining overuse shoulder injuries. Athletes returning to play after injury might benefit from specific strength training exercises to increase fatigue resistance of the eccentrically activated external rotator musculature. Isokinetic testing of fatigue resistance of the shoulder rotator musculature might also be useful to determine an athlete's readiness to return to competition.