Shoulder Antagonistic Strength Ratios During Concentric and Eccentric Muscle Actions in the Scapular Plane

Transcription

1 Shoulder Antagonistic Strength Ratios During Concentric and Eccentric Muscle Actions in the Scapular Plane G. Elizabeth Tata, MCIScl Linda Ng, MSc2 john F. Kramer, PhD3 P (3.7,8,11,14,16,22,24,25), atterns of shoulder musculature strength have been studied in healthy, nonathletic subjects in male baseball players (l,2,5,l O), and in elite water polo players (1 7) using concentric, isokinetic tests. Shoulder strength values have been reported for eccentric muscle actions in nonathletic males and females (8) and in female tennis players (1 8). These studies have generally reported absolute values (in Nm) using peak torque or average torque as the criterion measurement describing muscle capability. While absolute units permit comparisons with normative values one muscle group at a time, other important clinical questions center on the agonist/antagonist relationship, regardless of absolute strength. For example, two authors have evaluated rotator cuff muscle strength in baseball pitchers and have demonstrated a disparity in the balance between internal and external rotators (1.10). The external rotators have normal strength, while the internal rotators are overdeveloped due to the repetitive mechanics of pitching. This imbalance could result in abnormal shoulder mechanics, which might lead to injury. In addition, absolute values are specific to the isokinetic device used Reporting agonist/antagonist ratios is frequently more clinically applicable than reporting absolute strength values of single muscle groups. The purpose of this study was to measure shoulder abduction/adduction and external/internal rotation ratios in the functional scapular plane of shoulder movement. Ratios were calculated using peak and average torques during concentric and eccentric muscle actions, and ratios of healthy males and females were compared. Thirty-six, nonathletic subjects performed concentric-eccentric cycles at two angular velocities. No significant differences were observed in the ratios using peak and average torques as criterion measurements. Abduction/adduction ratios using peak torques were significantly greater during concentric than eccentric actions. External/internal rotation ratios using average torques were significantly greater during eccentric than concentric actions. External/internal rotation ratios at J80 /sec were significantly greater than those at 90 /sec. No significant differences were observed between male and female subjects. Ratios were higher than previously reported. This was attributed to the scapular plane testing position. The clinician should be aware that shoulder strength ratios are dependent on position and that the scapular plane may be advantageous in some cases for testing and treatment. Key Words: muscle strength, scapular plane, shoulder ' Assistant Professor, Division of Physical Therapy, School of Rehabilitation Therapy, Queen's University, Louise D. Acton Building, George Street, Kingston, Ontario, Canada K7L 3N6 ' Clinical Associate, University Hospital/University of Western Ontario Physical Therapy Clinic, london, Ontario, Canada ' Associate Professor, Department of Physical Therapy, University of Western Ontario, london, Ontario, Canada for testing. Therefore, normative data obtained with one device cannot be compared with that obtained by testing on another system (28). Also, many physical therapy clinics may not have access to sophisticated measurement systems. Conversion of absolute values to ratios allows comparison with normative data obtained from any system. The agonist/antagonist ratios for peak torque have been reported in several studies for shoulder abduction/adduction (AB/AD) ( ) and shoulder external/internal rotation (ER/IR) (1,2,4,5,10,11,14,20, 25,26). Connelly Maddux et al (4) measured AB/AD ratios of 6 1 to 74% (females) and 49 to 66% (males) and ER/IR ratios of 64 to 70% (females) and 61 to 63% (males). Abduction/adduction ratios of 50 to 54% and ER/IR ratios of 56 to 7 1 % have been reported for male baseball Volume 18 Number 6 December 1993 *JOSPT

2 players (l,2,5,l 0), but no comparable data were available for athletic females. McMaster et al (15) reported AB/AD and ER/IR ratios of male elite water polo players using angle-specific torque measurements as opposed to peak or average torques. Abduction/adduction ratios ranged from 48 to 56%, and ER/IR ratios ranged from 55 to 6 1 %. In general, these studies suggest that there is a 2: 1 ratio for AB/AD and a 3:2 ratio for ER/IR. Typically, data for agonist/antagonist ratios have been gathered with the shoulder positioned in either the frontal or sagittal planes (1,2,4,5,10,11,15,25). Disadvantages of testing in these planes, particularly with the shoulder in 90" abduction, have been addressed by some authors, who state that the rotator cuff and deltoid muscles are at a mechanical disadvantage and the risks of impingement or subluxation are increased (1 7,26). Johnston (1 2) and Saha (23) have suggested that the plane of reference for humeroscapular movements should be the scapular plane. Johnston (1 2) first defined the scapular plane as 30" anterior to the frontal plane where the center of the humeral head and glenoid are aligned. In the scapular plane, the inferior part of the capsule of the glenohumeral joint is not twisted as it would be in either the frontal or sagittal planes and the humerus is in neutral rotation (1 2). Full elevation of the arm is accomplished without rotation of the humerus in its long axis by the clavicular and acromial fibers of the deltoid and by the supraspinatus acting in this plane. Saha (23) emphasizes that the scapular plane (the position where the humeroscapular axes are aligned and coincide with the common axis of muscles controlling movements of the shoulder joint) is not fixed but changes with scapular movement around the chest wall and is approximately 45" anterior to the frontal plane when the humerus is elevated to 165". Using X-ray analysis, JOSPT Volume 18 Number 6 December 1993 Kondo et al (1 3) stated that the plane of the scapular is 40" anterior to the frontal plane. Shoulder strength testing involves testing muscles of the whole shoulder complex. Three principal muscle groups are tested: the scapular rotators (trapezius, serratus anterior, rhomboids, levator scapulae); the muscles connecting the trunk and scapula to the humerus (deltoid, pectoralis major and minor, latissimus dorsi, teres major); and the glenohumeral or rotator cuff muscles (subscapularis, supraspinatus, infraspinatus, teres minor). Testing is a measure of rotator cuff strength and strength of the trunk/scapulohu- Testing and training in the scapular plane rather than in the traditional frontal and sagittal planes may be more clinically appropriate. meral group, which cannot function normally in the absence of proper rotator cuff action. Therefore, it would seem logical to test these muscles in a position of neutral rotation of the joint with the scapulohumeral axis and common axis of the muscles aligned (23). as is the case with other joints. Donatelli and Greenfield (6) refer to the scapular plane for assessment and treatment of the stiff and painful shoulder. Furthermore, many functional activities tend to be performed in the scapular plane. Therefore, testing and training in the scapular plane rather than in the traditional frontal and sagittal planes may be more clinically appropriate. Testing in the scapular plane has been found to be highly reliable (3.7). Warner et al (26) investigated the strength ratios of the internal and external rotators in the 25" scapular plane abducted position recommended by Neer and Welsh (1 7). Subjects with shoulder instability demonstrated IR/ER ratios of 100% compared with 200% for those with impingement syndrome and % for asymptomatic shoulders (26). Clinicians are well aware that abnormal imbalances of agonist and antagonist muscles, particularly of the internal and external rotator musculature, are possible etiologic factors in glenohumeral instability and impingement syndrome. However, these assertions have been based largely on qualitative clinical observations in the past. Agonist/antagonist ratios need to be quantitatively defined in order to establish clinically useful data. Other inves:igators have used the scapular plane or similar positions for testing shoulder strength (7,9,18,24); however, positioning has not been standard, with variations in the amount of abduction and forward flexion of the humerus. No studies that investigated AB/AD ratios in the plane of the scapula were found. Another observation of clinical significance is that data for strength ratios has typically been gathered using concentric muscle actions (1,2,4,5,lO, 1 5,26). However, functional and sporting activities are characterized by eccentric as well as concentric muscle actions. As a result, knowledge of eccentric capability is important in designing and evaluating training and rehabilitation programs. A limited number of studies of shoulder capability during eccentric muscle actions is available (8,10,18). Hageman et a1 (8) provided information on eccentric peak torque capability of the shoulder rotators tested in positions of 45" glenohumeral flexion and 45" glenohumeral abduction. The authors presented male and female data separately using absolute torques (Nm). Both concentric and eccentric

3 peak torques during external rotation were greater at 45" glenohumeral abduction than at 45" flexion, while internal rotation torques tended to be similar in both positions. Hellwig and Perrin (9) reported greater eccentric than concentric shoulder rotational peak torque values. Ng and Kramer (1 8) provided information on eccentric peak and average torque (absolute values) capability of the shoulder rotators in female tennis and nontennis players. They reported no significant difference between the tennis and nontennis players. With the groups combined, average torques produced during eccentric muscle actions of the internal rotators were significantly higher than those produced during eccentric actions of the external rotators. To date, no studies have reported shoulder agonistlantagonist strength ratios using eccentric muscle actions. The major purposes of this study were to measure AB/AD and ER/IR ratios in the scapular plane and to compare ratios calculated using peak and average torques during concentric and eccentric muscle actions. The secondary purpose was to compare the ratios of males and females. Testing was confined to concentriceccentric cycles performed at 90 and 1 80 /sec angular velocities in the scapular plane in order to present a test protocol that all subjects could readily perform and that might be applicable in the early stages of rehabilitation. METHOD Subjects Twenty-one healthy males and 15 healthy females were each tested on one occasion (Table 1). All subjects gave verbal and written consent to participate before testing. No subject reported history of major upper extremity injury or shoulder pathology, and no subject participated competitively in single-shoulder Age (years) 28.3 (3.9) 27.1 (5.0) 27.8 (4.3) Height (m) 1.80 (0.08) 1.68 (0.06) 1.75 (0.10) Weight (kg) 79.3 (13.6) 56.6 (6.3) 70.6 (15.1) TABLE 1. Subject descriptive information, mean and 60. dominant activities, such as racquet sports. Procedure Testing was performed using a computercontrolled, hydraulic dynamometer (Kin-Com, Chattecx Corp., Chattanooga, TN) confined to the dominant arm, which was defined as the side used for throwing. The four movements (abduction, adduction, internal rotation, external rotation) were tested individually using continuous concentric-eccentric muscle action cycles without a pause between the concentric and eccentric phases. All tests were completed in the sitting position, and each began with a concentric muscle action. Stabilization to prevent trunk rotation and extraneous movement was provided by a strap (1 0 cm wide) placed horizontally around the u p per chest and through the axilla. The subject's thighs were oriented horizontally and the feet were uncrossed and flat on a footstool. The subject's back remained flat against the chair backrest during all testing. To achieve scapular plane movement during shoulder abduction and adduction, the chair was positioned at a 30" angle from the standard frontal plane position in relation to the dynamometer head (Figure 1). This angle was marked on the floor to ensure consistency for all subjects. By positioning in this way, the arm was required to move through abduction and adduction in the plane of the scapula, 30" anterior to the frontal plane. The rotational axis of the dynamometer was positioned to coincide with the lateral border of the acromion. The elbow was fully FIGURE 1. Subject position for abduction and adduction in the scapular plane with pad placement for adduction. extended and the resistance pad was placed against the arm, just proximal to the elbow on the lateral aspect for abduction and the medial aspect for adduction. Abduction was completed from 30 to 140". and adduction was completed from 140 to 30". During external and internal rotation movements, the shoulder was positioned in 45" abduction and at 30" anterior to the frontal plane (Figure 2). The elbow was flexed to 90" with the forearm and wrist in the neutral position. The dynamometer head assembly was tilted 45" from the vertical so that the long axis of the arm was aligned with the rotational axis of the dynamometer. The subject's elbow was secured in a V-shaped elbow support pad. The resistance pad was positioned slightly proximal to the wrist on the dorsal aspect of the forearm during external rotation and on the ventral aspect during internal rotation. The resistance pad, rather than the handle, was used in order to eliminate the action of muscles crossing the wrist. External rotation was com- Volume I8 Number 6 December 1993 *JOSPT

4 FIGURE 2. Subject position for external and internal rotation in the scapular plane with pad placement for internal rotation. pleted from 40" internal rotation to 60" external rotation; internal rotation was completed from 60" external rotation to 40" internal rotation. After each subject was positioned and given verbal instructions concerning the test maneuver, he/ she performed a series of practice contractions in order to warm up the shoulder, to become familiar with the accommodating resistance, and to allow the investigator to make minor adjustments to the apparatus and subject's position. The practice series consisted of submaximal repetitions at gradually increasing contraction intensities followed by at least one maximal repetition for the particular test movement. A 2-minute rest was required between practice and testing and between test situations. Four concentric-eccentric cycles, with no pause between the concentric and eccentric phases, were performed at 90 and 1 80 /sec for each of the four test movements. Verbal encouragement was given to the subjects to induce maximal contractions during both concentric and eccentric phases. The sequence of performance of each of the eight test maneuvers was randomized to reduce biasing effects that might result from order of presentation and fatigue. All torque records were corrected for the effects of gravity on the segment and the resistance pad of the dynamometer. Data Analysis To allow the subjects time to accelerate up to the test velocities and to compensate for impact artifacts in the torque record as the dynamometer arm changed direction, 10" of movement were eliminated from the analysis at each end of the torque record by use of windows (27). The repetition with the highest peak torque (from the four completed repetitions) was used in subsequent analysis. Peak and average torques were determined within a total range of 90" during shoulder AB/AD (40" adduction and 130" abduction) and 80" during shoulder ER/IR rotation (30" internal rotation to 50" external rotation). The values used in analysis were AB/AD and ER/IR ratios calculated using peak and average torques during each type of muscle action (concentric and eccentric) and at each angular velocity (90 and 180 "/set). Two, four-way analysis of variance (ANOVA) procedures (two genders by two angular velocities by two muscle actions by two criterion measures) with repeated measures on the last three factors were used to test AB/AD and ER/IR ratios for statistically significant differences. Following significant F ratios, a Neuman-Keuls technique was used to compare selected pairs of means (29). The 0.05 level was adopted as the criterion level for statistical significance. RESULTS All subjects completed testing without difficulty and reported that the movements were readily learned and performed. No statistically significant difference between male and female subjects was observed for either AB/AD ratios or for ER/IR ratios (p > 0.05) (Tables 2 and 3). On the AB/AD ratios, no significant difference was observed in the ratios at the two angular velocities or between ratios calculated using peak and average torques (p > 0.05). The muscle action-criterion measure interaction was significant (p < 0.01). When averaged over gender and angular velocity, the AB/AD ratios calculated using peak torques were significantly greater during concentric muscle actions than during eccentric actions (p < 0.05). On the ER/IR ratios, no significant difference was observed in the ratios during concentric and eccentric muscle actions or between ratios calculated using peak and average torques as criterion measurements (p > 0.05). The muscle action-criterion measure interaction was significant (p < 0.05). When averaged over gender and angular velocity, the ER/IR ratios calculated using average torques were significantly greater during eccentric muscle actions than during concentric muscle actions (p < 0.05). The angular velocity main effect (90 and 1 80 /sec) was significant. When averaged over gender, muscle action, and criterion measurement, the ER/IR ratios at 1 80 /sec were significantly greater than those at 90 /sec (p < 0.0 1). DISCUSSION Males and females produced similar AB/AD and ER/IR ratios regardless of the muscle action, angular velocity, or criterion measurement. Connelly Maddux et al (4), however, reported a significantly higher ER/IR concentric peak torque ratio at 60 /sec angular velocity of the dominant arm in females. They also reported a significantly higher AB/AD work ratio at JOSPT Volume 18 Number 6 December 1993

5 W/sec Muscle Strength Ratios (%) 180'/sec Gender concentric Eccentric Concentric Eccentric Peak Average Peak Average Peak Average Peak Average Torque Torque Torque Toque Torque Torque Torque Torque Male N=21 (19) (19) (17) (25) (26) (25) (29) (28) Female N=15 (27) (35) (18) (20) (37) (35) (24) (28) Groups N = 36 (22) (26) (18) (23) (30) (29) (27) (28) TABLE 2. Abduction/adduction ratios (YO) calculated using peak and average torques, mean and (SD). TABLE 3. External rotationlinternal rotation ratios (YO) calculated using peak and average torques, mean and (SD) /sec angular velocity for the dominant arm in females. In the present study, the AB/AD and ER/IR ratios for concentric muscle actions tended to be higher than those previously reported (l,2,4,5,10,26). The AB/AD peak torque ratios observed in the present study were %, whereas values from previous studies have ranged from 49-74% for nonathletic subjects (4,26). External/internal rotation peak torque ratios for concentric muscle actions in the present study ranged from 78 to 87%. These values are also higher than those previously reported (6 1-79%) for nonathletic subjects (4.26). Two factors may account for the higher ratios: the use of gravity correction and the scapular plane testing position. 1\11 torques in the present study were corrected for the effect of gravity on the arm and resistance pad. Previous studies of shoulder function have tended to not correct for the effect of gravity (I) or to not state this has been done in the description of methods. If gravity is not taken into account, torques obtained during gravity-assisted movements will be enhanced and torques recorded during gravityresisted movements will be underestimated. This will result in apparently higher absolute torque values for adduction and internal rotation and, subsequently, lower AB/AD and ER/IR ratios. This may have been a factor in the relatively high ratios in the present study. The present study tested in the scapular plane, whereas previous studies have tended to use the frontal plane (1,2,5,8,10,11,14,15, 19,25). Our higher AB/AD ratios may have been obtained because the line of action of the rotator cuff muscles was such that they could produce depression of the humeral head without also having to counteract a rotation force, which may be the case in the frontal plane. The glenohumeral joint surfaces have maximum congruence in the neutral, scapular plane or "zero-position" described by Saha (23). producing greatest stability of the joint. The muscles about the shoulder, including the deep rotator cuff muscles, deltoid, and other scapulohumeral and thoracohumeral muscles, are arranged coaxially and lose their rotary properties (23). The deltoid and supraspinatus have a more direct line of action and, thus, more efficient pull on the humerus during scapular plane abduction (1 2). Abduction torque may therefore be higher, resulting in higher AB/AD ratios in the scapular plane. The higher ER/IR ratios observed may also be attributable to testing in the scapular plane. Greenfield et al (7) reported statistically significant higher peak torque values for external rotational strength in the plane of the scapula (30" anterior to the frontal plane) than in the frontal plane but no difference between the two positions for internal rotation torque, implying higher ER/IR ratios. Hellwig and Perrin (9). however, found no significant differences in shoulder rotational peak torques when comparing scapular plane (40" anterior to the frontal plane) and frontal plane positions. The higher ER/IR ratios in our study and the study by Greenfield et al (7) are attributed to the testing position, which places the external rotators at a mechanical advantage in terms of length, tension, and line of action relative to the internal rotators, which have a greater crosssectional area and are a stronger group of muscles. The ratios obtained in this study are among the highest yet reported and may reflect a more equal balance of ER/IR torques because of the relative lengths of the muscles in the scapu- Volume 18 Number 6 December 1993 JOSPT

6 lar plane testing position. A similar rationale is put forward by those who have compared other testing positions (24.25). Although ratios were not reported, absolute torque values for internal and external rotation obtained by Soderberg and Blaschak (24) indicated that ER/IR ratios calculated from this data would range from high to low in 90" abduction, midposition (45" flexion, 45" abduction), and 90" flexion, respectively. Walmsley and Szybbo (25) observed the highest ER/IR ratios in the 90" flexion position (76-86%). followed by the 90" abduction position (73-77%), then the neutral position (61-66%). Hinton (1 0) also reported higher ER/IR ratios with the shoulder in 90" abduction than in neutral. Interpretation of reported data and comparisons between studies is difficult due to the variety of testing positions used. Even in studies using the scapular plane, there are differences in positioning. Greenfield et al (7) chose 30" as the scapular plane with 45" humeral abduction, the position used in this study. It is interesting to note that the results of the two studies are similar in the higher ER/IR ratios obtained. Alternative scapular plane positions have been 25" anterior to the frontal plane with 0" abduction (26) and 40" anterior to the frontal plane with 90" abduction (9). We chose the 30" position because it is the position originally described by Johnston (1 2) and it has been used by other studies of shoulder motion and strength (7,18,2 1 ). As pointed out by Saha (23), the scapular plane is not fixed and changes throughout elevation of the arm, as does the relationship of the line of action of the shoulder rotators with the long axis of the arm. Adjustment of the scapular plane may be needed depending on the degree of abduction. Eccentric torque is known to increase and concentric torque is known to decrease as velocity of muscle contraction increases. Al- JOSPT Volume 18 Number 6 December 1993 though this will affect absolute values, it would not be expected to affect the agonist/antagonist ratios. External/it. ernal rotation ratios were significantly higher at 1 80 /sec than at 90 /sec. A higher ratio may result from an increase in external rotation torque, a decrease in internal rotation torque, or both. As there was no significant effect of velocity on the AB/AD ratios, this finding may be spurious. The ER/IR ratios at 90 /sec of the male subjects were lower, and this seems to have produced the velocity effect, although there was no overall significant difference between male and female ratios. In the present study. AB/AD peak torque ratios for eccentric muscle actions ranged from 79 to 90%, The scapular plane is not fixed and changes throughout elevation of the arm. while eccentric ER/IR peak torque ratios ranged from 74 to 89%. Although absolute torques for eccentric muscle actions were higher than those in concentric muscle actions, the agonist-antagonist ratios were similar. Comparative data for eccentric muscle actions were not available. Concentric and eccentric ratios were similar with two exceptions: 1) during concentric muscle actions calculated using peak torques, the AB/AD ratios were greater than those during eccentric actions; and 2) ER/IR ratios during concentric muscle actions calculated using average torques were less than those during eccentric muscle actions. The authors suspect that these exceptions are aberrant, as two different criterion measures (peak and average torque) are involved and the relationship between concentric and eccentric muscle actions varied with the criterion measurement (ER/IR and AB/AD). However, further study is needed to clarify this point. We believe that comprehensive rehabilitation of shoulder strength/ mobility involves a full range of shoulder positions and planes of movement. For reasons of glenohumeral stability, avoidance of impingement, and balance of muscle action, the scapular plane may be the plane in which shoulder trauma is least likely to occur and may be the most comfortable and advantageous plane in which to begin strength training programs. The traditional test positions requiring movements in the frontal or sagittal planes may be less desirable than testing in the scapular plane. Further study should be directed toward test-retest reliability of different isokinetic protocols, completed by subjects with different types of pathology and surgical procedures as well as healthy athletes and nonathletes. In addition, further testing should establish agonist/antagonist ratios for subjects of older age groups and different sports and occupations, relating these to performance during functional tasks. SUMMARY The summary of results for this study were: 1) males and females produced similar abduction/adduction (AB/AD) ratios and similar external/internal rotation (ER/I R) ratios, whether calculated using peak or average torques; 2) ratios based on peak torque and average torque were comparable; 3) AB/AD ratios were not affected by angular velocity, but the ER/IR ratios were higher at 1 80 /sec than at 90 /sec; 4) the ratios during concentric muscle actions tended to be similar to those during eccentric actions; and 5) agonist/antagonist ratios are stable across gender and muscle action and can provide a means to assess muscle balance. Abduction/adduction and

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