The Effects of Repetitive Throwing on Shoulder Proprioception and Internal and External Rotation Strength

Transcription

1 J Sport Rehabil. 2006,15, Human Kinetics, Inc. The Effects of Repetitive Throwing on Shoulder Proprioception and Internal and External Rotation Strength Joe Nocera, Mack Rubley, William Holcomb, and Mark Guadagnoli Context: There is limited information on the effects of throwing on shoulder proprioception and strength. Objective: Examine shoulder proprioception and strength following throwing. Design: 2x3 mixed-subject design. Setting: Research laboratory and outdoor facility. Participants: 23 male college students (age = 22 ± 2.9yr, ht = 178 ± 11.3cm, wt = 72 ± 7.7kg, 22 right-handed 1 left-handed). Intervention: Subjects were pretested for proprioception, measured by active reproduction of passive positioning (ARPP). Strength was quantified using 1RM and an average peak torque at 120º/sec for internal and external shoulder rotation. Following pretesting, subjects (excluding control) completed 75 throws at 75% maximum immediately followed by posttesting. Main Outcome Measures: Pre and post ARPP absolute error and strength changes. Results: Significant difference in the pre and posttest ARPP values for throwing groups but no difference for the control group. There was no significant difference from pre to post on the strength tests for any groups. Conclusion: Results indicate that repetitive throwing affects proprioception while not affecting strength. Key Words: Active Repositioning of Passive Positioning (ARPP), strength, Isokinetic testing Due to its anatomy, the shoulder joint is inherently unstable and must rely heavily on surrounding musculature and neuromuscular control for stability. 1-7 For an overhand thrower, this dynamic stability is essential for injury prevention and performance. Together, the rotator cuff muscles (RTC) and the proprioceptive abilities of the shoulder joint provide the synergistic muscular contractions required to maintain glenohumeral joint stability. 4 The rotator cuff musculature acts as a sleeve and compresses the humeral head in the glenoid cavity, and together with the negative intraarticular pressure, creates concavity compression that is essential for dynamic stability of the joint. 1,3 This synergy of muscular contraction is dependent on proprioceptive input. 3,4 Proprioception, an essential aspect of neuromuscular control, is defined as the afferent neural input to the central nervous system from specialized nerve endings called Joe Nocera is with the Department of Kinesiology at the University of Georgia in Athens. jnocera@uga.edu. Mack Rubley, William Holcomb, and Mark Guadagnoli are with the Department of Kinesiology at the University of Nevada, Las Vegas. 351

2 352 Nocera et al mechanoreceptors. 2 Mechanoreceptors provide the body with information regarding joint position sense, muscular tension, and kinesthetic awareness. Proprioceptive information transmitted from mechanoreceptors influences reflex activity and joint stiffness to provide shoulder joint stability during dynamic activity. 5 Deficiencies in proprioception have be evaluated and attributed to joint injury and a decrease in athletic performance. 5,6,8 Furthermore, Voight et al 2 and others 6,8,9 have shown that proprioceptive abilities decline in the presence of fatigue. Laboratory studies have demonstrated both strength and proprioceptive deficiencies within the shoulder joint following fatigue exercises using isokinetic dynamometers. 2,6 However, there is limited research examining the effect of functional activities, such as overhand throwing, on shoulder muscle strength and proprioception. Because overhand throwing places a tremendous amount of stress on the shoulder joint, there may be a decrease in these vital aspects of dynamic stability. A single overhand throw produces distraction forces at the shoulder complex at 1 to 1.5 times body weight, 10 and humeral rotation velocities have been measured at over 6,000 /sec. 11 It is hypothesized that repetitive throwing may lead to short term decreases in shoulder muscle strength and proprioceptive function. Those decreases may increase the risk of injury to the shoulder complex. The purpose of this study was to quantify any declines in strength and proprioception following a single bout of repetitive overhand throwing. Methods The study was a 2 Test (pre and post) 3 Group (varsity baseball players, recreational athletes, and control) mixed model within subject design. The dependent measures of interest were joint position sense (JPS) utilizing active reproduction of passive positioning (ARPP) as measured by the mean absolute angular error, 1 repetition maximum (1RM) isotonic strength of the internal and external shoulder rotators, and isokinetic peak torque measured at 120º/sec for both internal and external shoulder rotation. Subjects Twenty-three healthy male college students (age = 22 ± 2.9 yr, ht = 178 ± 11.3 cm, wt = 72 ± 7.7kg, 22 right-handed and 1 left-handed) volunteered for this study. Six of those students were members of an NCAA Division I varsity baseball team. The remaining seventeen students were recruited from the general student population, and all were experienced throwers (defined as having played high school baseball or beyond for at least 1 season). These seventeen subjects were physically active for a minimum of 30 minutes, three times per week. All subjects were free of current and previous shoulder injury. All subjects provided informed consent and the procedures were approved by the Office for the Protection of Research Subjects. Procedures Subjects attended one familiarization session prior to testing; during this time, subjects were familiarized with the testing apparatus and all test procedures. Subjects returned for testing between 48 and 72 hours after familiarization.

3 Repetitive Throwing 353 Instrumentation JPS measurements were taken with a digital inclinometer (Saunders Group, INC. Chaska, MN). This device was chosen over the commonly used method of measuring ROM on the Isokinetic dynamometer to eliminate auditory cues, and this device has been used in previous studies to measure joint ROM. 12 Two types of strength tests were utilized in this study, isotonic and isokinetic. Isokinetic testing was completed using a KIN-COM isokinetic dynamometer (Chattanooga Group, Inc., Hixon, TN), while isotonic tested was done using standard free weights. Joint Position Sense Testing JPS tests were measured by ARPP, a common measure of proprioception. 1,2 The ARPP test was conducted with the digital inclinometer placed at the distal end of the ulna and radius just between the styloid processes of the dominant arm. The placement of the digital inclinometer was marked on the subjectʼs skin to ensure consistent placement on both pre and posttests. After instructions were given and prior to testing, subjects were blindfolded to exclude visual cues. This method of testing JPS sense was found to be highly reliable for both internal (.981) and external (.984) rotation. 13 Internal rotation ARPP sense was initiated by positioning the arm at 90 of external rotation, 90 of shoulder abduction, and 90 of elbow flexion (Figure 1). The examiner then passively internally rotated the shoulder 30 and held this position for 10 seconds. The examiner then passively returned the arm to the starting position of and asked the subject to actively replicate the 30 movement of internal rotation and hold that position for 5 seconds. Figure 1 Starting position for ARPP internal rotation.

4 354 Nocera et al External rotation ARPP sense was initiated by positioning the arm at 0 of internal rotation, 90 of shoulder abduction, and 90 of elbow flexion (Figure 2). The examiner then passively externally rotated the shoulder 30 and held this position for 10 seconds. The examiner then passively returned the arm to the starting position of and asked the subject to actively replicate the 30 movement of external rotation and hold that position for 5 seconds. Subjects were internally or externally rotated randomly for three trials; however, each subject had the same sequence from pretest to posttest. Absolute angular error (the difference between the reference angle and the angle reproduced by the subject) was measured in degrees. The average of three trials was taken. Isotonic Testing During isotonic 1RM testing, both internal and external rotation of the shoulder were tested in the dominant arm. For internal rotation, subjects were positioned supine with dominant arm at 90º of shoulder abduction, 90º of elbow flexion, neutral pronation/supination in the frontal plane and 90º of shoulder external rotation (Figure 3). Subjects then moved the weight into a position of maximal internal rotation. During isotonic external rotation testing, subjects were positioned prone with their dominant arm at 90º of shoulder abduction, 90º of elbow flexion, neutral pronation/supination in the frontal plane and 0 of shoulder internal rotation (Figure 4). Subjects then moved the weight to a position of at least 90º of external rotation. For both internal and external rotation tests, the first sets were 5 submaximal repetitions at no more than 10 lbs (4.5 kg). The second set was 3 submaximal repetitions at no more than 15 lbs (6.81 kg). The 3rd, 4th, and 5th sets were each 1 repetition, in an attempt to achieve their 1RM. If subjects successfully overcame the resistance, the weight was increased in an attempt to achieve their 1RM. The amount of weight increased was determined Figure 2 Starting position of ARPP for external rotation.

5 Repetitive Throwing 355 Figure 3 Starting position for isotonic internal rotation. Figure 4 Starting position for isotonic external rotation. by an estimation method and the subjectʼs response to the question, On a scale of 1 to 10, with 10 being the hardest, how difficult was that set? The increases in weight never exceeded 10 lbs (4.5 kg) and no subject completed more than 5 total sets. Subjects were given 3 minutes of rest between each set. Subjects were then given 3 minutes of rest prior to the next phase of testing. 14 Isokinetic Testing Each subject was seated in the KIN-COM with the dominate arm in the padded arm rest and shoulder positioned to 90º of shoulder abduction, 90º of elbow flexion,

6 356 Nocera et al 90º of internal rotation, and neutral pronation/supination (Figure 4). To ensure reliable measurements, the dynamometer was calibrated, all stabilization straps were used to prevent unwanted movement, subjectʼs hands were required to remain free, and no visual feedback was provided during testing. Isokinetic testing began with a submaximal (their perception of 50% of maximal) warm-up of 10 repetitions at 120º/sec. Immediately following the 3 minute rest period, subjects completed 3 maximal repetitions of internal and external rotation of the shoulder at 120º/sec. Peak torque values of the three repetitions of internal and external rotation were then averaged and this value was used for analysis. Throwing Protocol Following pretesting, the varsity baseball players and recreationally active groups were given 3 minutes of rest prior to participating in a throwing session. The control subjects did not throw but instead sat comfortably in the lab for 20 minutes. Throwing was done with the examiner monitoring distance and rate of throwing. The throwing session consisted of 75 throws at 75% of the subjectʼs perceived effort and was done at 60 feet 6 inches, normal distance from the pitchers mound to home plate. The pace of throwing was 1 throw per 15 seconds, or approximately 20 minutes total. Immediately following the throwing session, subjects were posttested in the same manner as the pretest. Analysis Means and standard deviations of each measure were computed. Differences between test times and groups were analyzed with repeated measures ANOVA. Appropriate post-hoc testing was conducted to determine group differences. The data were analyzed on the Statistical Package for the Social Sciences (SPSS) version 10. Significance was set a priori at an alpha level Results Mean and standard deviations for ARPP for all three groups are presented in Table 1. Analysis of JPS as measured by ARPP, quantified as absolute angular error revealed a time-by-group interaction (F 2,20 = 5.78, P =.010). Post hoc analysis revealed a significant difference in JPS from the pretest and posttest values for Table 1 ARPP Values (Means ± SD) in º Pre Post Rec. Active 2.20 ± ± 2.15* Baseball Players 1.49 ± ± 0.88* Controls 2.59 ± ± 1.51 * Indicates a significant difference pre to posttest

7 Repetitive Throwing 357 the recreationally active group (2.27º ± 1.49º of error, 103% increase) and the baseball players (1.73º ± 1.12º of error, 116% increase), but no such difference for the control group (0.46º ±.65º of error, 18% increase). Univariate analysis of variance of between-subjects for the pretest revealed no significant difference in JPS among the three groups (F 2,20 = 1.41, P = 0.267). Lastly, the between-subjects test for the posttest revealed no significant difference in JPS among the three groups (F 2,20 = 1.78, P = 0.195). No significant differences existed between pre and posttest for isotonic internal rotation 1 RM for any of the groups (F 2, 20 = 1.69, P =.210). Additionally, no significant differences existed between pre and posttest for isotonic external rotation 1 RM for any of the groups (F 2,20 = 1.90, P =.175). A univariate analysis of variance of between-subjects for the group revealed no significant difference in 1 RM strength among the three groups (F 2,20 = 2.17, P =.1405). Mean and standard deviations for these tests are presented in Table 2. No significant difference existed between pre and posttest for isokinetic internal rotation peak torque for any groups following throwing (F 2,20 = 0.72, P =.776). Additionally no significant difference existed for isokinetic external rotation for any of the groups (F 2,20 = 0.63, P =.543). However, there was a significant difference among the three groups. Using a Tukeyʼs post hoc test it was determined that the baseball group had a significantly higher mean than the recreationally active and control groups during external rotation (P 0.003) and internal rotation (P 0.004). Mean and standard deviations for these tests are presented in Table 3. Table 2 1 RM Isotonic ER and IR Values (Means ± SD) in kg IR ER Pretest Posttest Pretest Posttest Control ± ± ± ± 1.68 Rec. Active ± ± ± ± 2.27 Baseball ± ± ± ± 1.64 Table 3 3-Repetition Maximum Isokinetic Peak Torque (Mean± SD) for ER and IR in Nm IR ER Pretest Posttest Pretest Posttest Control ± ± ± ± 4.77 Rec. Active ± ± ± ± 8.11 Baseball ± 5.27* ± 6.50* ± 5.83* ± 4.21* * Indicates a significant difference between baseball players and the other groups

8 358 Nocera et al Comments Our results indicated that overhand throwing decreased proprioception of the shoulder as measured by JPS using ARPP. This was exhibited in the two throwing groups that each had a significant increase in absolute angular error (103% for the recreationally active and 116% for the baseball players), whereas the control group only increased 18%, which was not significant (Figure 5). The mechanism responsible for this reduction of proprioception following throwing is believed to be caused by a decrease in sensitivity of the muscle spindles. It is hypothesized that the muscle spindles, which is one of the mechanoreceptors responsible for joint position sense, 15 temporarily became dysfunctional following this throwing protocol. What causes the desensitizing is not completely understood; however, it may be due to increases in intramuscular concentration of lactic acid, bradykinin, and serotonin Increased concentrations of these and other contractile substances have been shown in animal studies and have been reported to strongly influence the muscle-spindle system The reduction in joint position sense measured here supports the theory proposed by Voight et al 2 that dysfunctional or compromised components of the muscle-tendon unit (muscle spindles and/or Golgi tendon organ) account for alterations in proprioceptive ability. Although joint position sense is only one aspect of proprioception, it is one of the most commonly used measures of proprioception. 1,2,5,7,19 Lephart et al 7 reported that active joint position sense stimulates both joint and muscle mechanoreceptors and is a more functional assessment of the afferent pathways. Previous studies 2,6,8 have examined the effect of fatigue, defined as a 50% decline in maximum peak torque, on joint position sense and found that shoulder proprioception was indeed impaired. The results of our study indicate that proprioception may be impaired without quantifiable muscular fatigue. This is supported by the fact that our throwing subjects had a decline in joint position sense without having significant declines in strength measured isotonically or isokinetically. The lack of muscular fatigue in the presence of impaired proprioception may be Figure 5 Starting position for isokinetic testing.

9 Repetitive Throwing 359 explained by central fatigue as opposed to peripheral muscle fatigue. Peripheral fatigue involves changes at the level of the muscle. Central fatigue, on the other hand, occurs when the central nervous system fails to drive motor neurons adequately, resulting in a reduction of activation of the muscle. 19 These decreases in spinal and supraspinal mechanisms that interfere with the descending commands have been shown during sustained motor tasks such as exercises. 20,21 For example, Sharp and Miles 22 found a decline in the ability to reproduce elbow positions after localized muscular fatigue. They concluded that the decline in joint position sense was due to central fatigue based on similar declines in reproduction sense in the opposite elbow that was not fatigued. 22 Our results indicate that there was no significant decline in the amount of weight lifted from pre to posttest after a bout of overhand throwing. These findings were not surprising for internal rotation because of the large muscle mass responsible for this movement. It was expected that the internal rotators would be able to withstand demands of 75 throws at 75% velocity. However, it was expected that there would be a decline in the external rotators based on the eccentric demand placed on the smaller external rotators due to the deceleration of the arm after an overhead throw. Surprisingly, our results indicate otherwise. After finding no significant decline, it was concluded that the demand of 75% velocity may not be intense enough to cause any decrements in strength due to a less intense eccentric, follow-through phase. The findings were similar for isokinetic testing as our results indicated that neither the internal or external rotators were impaired. The results of our study have significant clinical and practical implications because there was no significant decline in shoulder strength while there was a significant decline in shoulder joint position sense. This information is practical to throwing during rehabilitation or sports participation. Most often, the decision to allow a pitcher to continue to throw during games is based on throwing velocity as measured by a radar gun. However, the participantʼs ability to recognize joint position sense after throwing 75 throws at 75% of maximum was altered with no significant decline in muscular strength and therefore there may be no decline in velocity. This can be demonstrated in pitchers that, after a large number of throws, display no decline in velocity; however, their command or ability to throw to particular target may be impaired. The most common assessment of fatigue would indicate the appropriate time to curtail pitching is pitch velocity. Based on our results, however, we propose accuracy or command may decline prior to a decrease in velocity. As ability to recognize joint position fades, arm angle may change, which may lead to changes in mechanics, decrease in ability to control throws, and possible injury. Pitch velocity and accuracy measurements were beyond the scope of this study, however, but should be addressed in future studies. Additionally, it has also been shown that proprioception, by way of neuromuscular control, is responsible for joint stability. 2 As one continues to throw, there may be an alteration in joint mechanics caused by a decline in JPS. Because throwing was shown to hinder JPS, the neuromuscular responses required for joint stability may be hindered as well, leading to joint instability and possible injury. Pedersen et al 8 found that localized muscle fatigue decreased the acuity of the movement sense in the shoulder. They concluded the disturbances in the proprioceptive input would affect motor control and therefore place individuals at an increased risk for injury. 8 In our study we found no localized muscle fatigue; however, we did find

10 360 Nocera et al disturbances in the joint position sense and concluded that individuals with such alterations may be at increased risk for injury. Lastly, proprioception has been shown important in recognizing joint position in the extremes of range of motion (ie, full external and internal rotation or the wind up and follow-through phases of throwing). 5 At the end ranges of motion, the muscle spindle sends signals to the nervous system via afferent fibers regarding a specific muscle stretching in an effort to control and limit overstretching. 15 Efferent fibers in turn activate the agonist as well as synergistic muscles and inhibit the antagonist muscle in an effort to control both the agonists and antagonist muscle. This activation allows for contraction of the agonist muscle to prevent overstretch while concurrently relaxing the antagonist. 15 If joint position sense is limited and the muscle spindles are temporally dysfunctional, this process may be interrupted thus increasing the mechanical stress on structures responsible for joint stability and therefore increasing the risk for joint injury, particularly during throwing, which requires extremes in range of motion. It should be noted that although ARPP may provide the most functional measurement of proprioception, 1 there are alternatives measurements that were not examined in this study. For example, because the ARPP test cannot isolate JPS within the complex sensorimotor system, a passive assessment may be utilized to minimize muscle mechanoreceptor involvement and evaluate threshold to detect movement. 1,7 Muscular tension and kinesthetic awareness are also factors that influence JPS and therefore additional measurements may provide equally important measures of conscious proprioception and may be a topic for future research. Another limitation in this study can be seen in the error values reported for ARPP (Figure 6). Although statistically significant, it is not know at this time if the differences reported between the throwing groups and the control are clinically significant. Again, that may be a topic of future research. Figure 6 ARPP means and error values for the pre and posttests. The error scores for the recreational active and baseball players were significant for pre to post.

11 Repetitive Throwing 361 In conclusion, the results of our study indicate that there is a significant decline in JPS as measured by ARPP following an overhand throwing protocol. There was not, however, any significant decline in strength, as measured by a 1RM and isokinetic peak torque of the internal and external rotators of the shoulder, following the same protocol. These results support previous research 2,5 that has theorized that desensitization of the muscle spindle caused a disruption of the shoulder joints neuromuscular feedback system. Although previous research 2,5 has shown this desensitization in the presence of quantifiable muscular fatigue, our results indicated a measured disruption of the system without the presence of fatigue. The alterations of JPS may have implications for the overhand throwing athlete as it pertains to performance and injury prevention. Further studies are needed to explain the implications of JPS deficits of the shoulder, which may lead to changes in prevention and rehabilitation of throwing related injuries. References 1. Sterner RL, Pincivero DM, Lephart SM. The effects of muscular fatigue on shoulder proprioception. Clin J of Sports Med. 1998;8: Voight ML, Harden JA, Blackburn TA, Tippet S, Canner GC. The effects of muscular fatigue on and the relationship of arm dominance to shoulder proprioception. J Orthop Sports Phys Ther. 1996;23: Matsen FA III, Harryman DT II, Sidles JA. Mechanics of glenohumeral instability. Clin Sports Med. 1991;10: Wilk KE, Arrigo CA, Andrews JR. Current Concepts: the stabilizing structures of the glenohumeral joint. J Orthop Sports Phys Ther. 1997;25: Myers JB, Lephart SM. Sensorimotor deficits contributing to glenohumeral instability. Clin Orthop. 2002;1: Myers JB, Guskiewicz KM, Schneider RA, Prentice WE. Proprioception and neuromuscular control of the shoulder after muscle fatigue. J Athl Train. 1999;34: Lephart SM, Pincivero DM, Giraldo JL, Fu FH. The role of proprioception in the management and rehabilitation of athletic injuries. Am J Sports Med. 1997;25: Pederson J, Lönn J, Hellström F, Djupsjöbacka M, Johansson H. Localized muscle fatigue decreases in the acuity of the movement sense in the human shoulder. Med Sci Sports Exerc. 1999;31: Carpetner JE, Blasier RB, Pellizzon GG. The effects of muscle fatigue on shoulder joint position sense. Am J of Sports Med. 1998;26: Meister K. Injuries to the shoulder in the throwing athlete. Am J Sports Med. 2000;28: Powers ME. Rotator cuff training for pitchers. J of Sports Rehab. 1998;7: de Winter AF, Heemskerk M, Terwee CB, Jans MP, van Schaardenburg WD, Scholten R, Bouter LM. Inter-observer reproducibility of measurements of range of motion in patients with shoulder pain using a digital inclinometer. BMC Musculoskeletal Disorders. 2004; Dover CG, Powers ME. The reliability of joint position sense and force reproduction measures during shoulder internal and external rotation. J Athl Train supplement. 2002; 37(2):S Baechle TR, Earle RW, eds. Essentials of Strength Training & Conditioning Textbook. 2nd ed. Champaign, Ill: Human Kinetics; Shumway-Cook A, Woollacott MH. Motor Control. 2 nd ed. Philadelphia, Pa: Lipponcott Williams & Wilkins; 2001.

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