The rotator cuff muscle group has an important role in providing a medial force to center the humeral head on the glenoid fossa during shoulder movements (9,13,17). Because of its vulnerable position between the humeral head and acromion, the tendon of the supraspinatus muscle is most frequently injured (8). As a result, rehabilitation exercises to improve rotator cuff strength are often specifically directed at supraspinatus.
Several exercises have been proposed for strengthening supraspinatus. All of these involve either resisted shoulder abduction/elevation in midrange or resisted external rotation. The first of these positions was described in 1982 by Jobe and Moynes (10) and is known as the "empty can" position (Fig. 1A). A modification of the "empty can" position, termed the "full can" position (Fig. 1B) (11), was subsequently promoted as a superior alternative as it is less pain-provoking. Three other exercise positions have been shown to highly activate supraspinatus: shoulder elevation in midrange in the prone position (Fig. 1C) (3,14,19,21), external rotation with the arm by the side (Fig. 1D) (6), and external rotation in shoulder midrange in prone (Fig. 1E) (1,16). Despite the fact that no study has simultaneously compared the level of supraspinatus activity during these five exercise positions, the most commonly used exercises for strengthening supraspinatus are the "empty can" and "full can" positions (5,18).
In its role as a dynamic stabilizer of the shoulder joint, supraspinatus, as part of the rotator cuff group, functions to prevent the deltoid from superiorly translating the humeral head during abduction (5,17,20). Recent researchers have therefore proposed that the optimal exercise to strengthen supraspinatus should "elicit the greatest amount of supraspinatus activity while minimizing the surrounding muscular activity, particularly the deltoid" (15).
Using the above criteria, the aim of the current study was to extend previous research into the most effective exercise to specifically strengthen supraspinatus by comparing supraspinatus activity levels in the "empty can," "full can," prone elevation, external rotation in 0° abduction, and prone external rotation exercise positions. Activity levels were compared in supraspinatus, deltoid, and infraspinatus (a prime mover of shoulder external rotation) to determine which exercise/s generates greatest supraspinatus activity while minimizing surrounding muscle activity.
MATERIALS AND METHODS
The data used in this article are a subset of those from a previous study undertaken by this research group to identify a set of tests for determining the maximal voluntary isometric contraction (MVIC) for 12 shoulder muscles (4). Different analytical tools have been used here to compare EMG activity for supraspinatus, infraspinatus, anterior deltoid, middle deltoid, and posterior deltoid muscles during the "full can," "empty can," prone elevation, external rotation in 0° abduction, and prone external rotation positions. The current paper provides a brief review of experimental arrangements and procedures, with further details being provided in Boettcher et al. (4).
Nine male and six female subjects (mean age, 28.4 yr; range, 19-47 yr) with normal dominant shoulder function were recruited from university academic staff and students for this study. The study was approved by The University of Sydney Human Research Ethics Committee, and all subjects signed a consent form before participation.
EMG data were collected simultaneously from three shoulder muscles using a combination of surface (deltoid) and intramuscular fine-wire (supraspinatus and infraspinatus) electrodes. Surface and intramuscular electrode placement was according to Kelly et al. (11) and Geiringer (7), respectively.
Surface and intramuscular electrodes were attached to amplifiers (two Iso-DAM 8 amplifiers; World Precision Instruments Sarasota, FL). EMG signals were amplified (gain = 100) and filtered (HP (high pass) = 10 Hz, LP (low pass) = 1 kHz). Data were acquired on a personal computer with a 16-bit analog-to-digital converter (1401; Cambridge Electronics Design, Cambridge, UK) at a sampling rate of 3571 Hz using Spike2 software (Version 4.00; Cambridge Electronics Design) and stored for later offline analysis.
Five exercise positions were examined in random order along with 10 other shoulder test positions as part of the larger study (4). For all test positions, the scapula was stabilized in a retracted position. The five positions considered in this article were:
- Full Can. Shoulder abduction in the scapular plane (30° anterior to the frontal plane) with external rotation and resistance applied at the wrist (Fig. 1A).
- Empty Can. Shoulder abduction in the scapular plane with internal rotation and resistance applied at the wrist (Fig. 1B).
- Prone Elevation. Shoulder elevation with the subject in prone, the shoulder abducted approximately 100°, the arm externally rotated, and resistance applied at the wrist (Fig. 1C).
- Pendant External Rotation. Shoulder external rotation in 0° abduction with the elbow at the side and flexed 90° and resistance applied at the wrist (Fig. 1D).
- Prone External Rotation. With the subject in prone, the shoulder abducted to 90° and the elbow flexed to 90° and resistance applied at the wrist to end range external rotation (Fig. 1E).
Manual resistance was applied by one of the researchers. The applied resistance was force matched to the subject's MVIC. Each contraction was held for 5 s with a gradual increase of resistance for 1 s, a sustained maximal contraction with resistance for 3 s, and a gradual release during the final second. Three repetitions of each test were performed, with a minimum rest interval of 30 s between each repetition. To ensure consistency of positioning during each isometric test, subjects were closely monitored to ensure that they did not elevate and/or protract the scapula or perform compensatory movements of the trunk. If it was determined that a test was being done incorrectly, it was ceased and repeated.
Signal and statistical analyses.
All raw EMG data were visually inspected and a confirmatory power spectral analysis performed when there was doubt regarding the validity of the signal from a channel. As a result of this inspection, 9 (0.8%) of the 1125 individual trials (15 subjects × 5 exercises × 5 muscle sites × 3 trials) were discarded because of an artifact or a channel being lost during the experiment. These data were excluded from further analysis.
The EMG signals were high-pass-filtered (20 Hz, eighth-order Butterworth), notch filtered at the line frequency (50 Hz) and its harmonics, rectified, and low-pass-filtered (3 Hz, eighth-order Butterworth). The maximum value of the resulting EMG envelope was determined for each trial. This process is broadly equivalent to finding the maximum of the averaged EMG using a moving window root mean square (RMS) of 225 ms width on the raw EMG. The maximum for each test was determined as the average of the maxima of the EMG envelopes across the three trials. Values for each muscle were normalized to the maximum obtained in the four MVIC tests (shoulder normalization tests) recently validated to generate maximum activity in all muscles tested in this study and recommended as the standard set of tests to be used for normalization in shoulder EMG research (4).
A two-way repeated-measures ANOVA (with muscles and tests as factors) was undertaken (Statistica version 7.1; StatSoft, Tulsa, OK) followed by Tukey's post hoc tests. Statistical significance was accepted at P < 0.05.
The mean activation levels (with 95% confidence intervals) for the supraspinatus, infraspinatus, anterior deltoid, middle deltoid, and posterior deltoid muscles during the "full can," "empty can," prone elevation, pendant external rotation, and prone external rotation exercise positions are shown in Figure 2.
The two-way repeated-measures ANOVA showed a strong effect for both muscles and tests as factors (P < 0.001). Tukey's post hoc analyses revealed the following:
- All exercise positions activated supraspinatus to equally high levels (P > 0.50).
- During the "full can" and "empty can" exercises, the level of activation of all five muscles tested was not statistically different (P > 0.90).
- The prone elevation exercise position activated posterior deltoid greater than all other exercise positions (P < 0.001) and activated posterior deltoid greater than supraspinatus (P < 0.001). However, the prone elevation exercise position also activated anterior deltoid to a significantly lower level than supraspinatus (P < 0.001).
- The pendant external rotation exercise:
- Activated anterior deltoid significantly less than supraspinatus (P < 0.01),
- Indicated a strong trend toward less activity in middle deltoid than in supraspinatus (P = 0.16),
- Activated anterior deltoid and middle deltoid to levels significantly less than during the "full can" and "empty can" exercise positions (P < 0.001),
- Produced no significant difference in the activation levels of posterior deltoid and supraspinatus (P = 1.00), and
- Activated infraspinatus significantly more than supraspinatus (P < 0.001).
- The prone external rotation exercise:
- Activated anterior deltoid and middle deltoid significantly less than supraspinatus (P < 0.001),
- Activated anterior deltoid and middle deltoid at levels significantly less than the "full can" and "empty can" exercise positions (P < 0.001),
- Produced no significant difference in the activation levels of posterior deltoid and supraspinatus (P > 0.80), and
- Activated infraspinatus significantly more than supraspinatus (P < 0.001).
- There was no significant difference in infraspinatus activation among prone elevation, pendant external rotation, and prone external rotation exercises (P > 0.71).
On the basis of the criteria that "the optimal exercise for the supraspinatus would elicit the greatest amount of supraspinatus activity while minimizing the surrounding muscular activity, particularly the deltoid" (16), the current study has demonstrated that, of the five exercise positions examined, those incorporating shoulder external rotation are the most suitable to strengthen supraspinatus. That is, maximal isometric external rotation exercises elicit much supraspinatus activity while minimizing deltoid activity. All exercise positions investigated activated supraspinatus to equally high levels, with both the pendant external rotation and prone external rotation exercises activating anterior deltoid, and prone external rotation activating middle deltoid, significantly less than supraspinatus. In contrast, during the "full can" and "empty can" exercise positions, supraspinatus and the three parts of deltoid are activated to high levels that are not significantly different, and the prone elevation exercise position activated posterior deltoid greater than supraspinatus.
This is the first study to compare supraspinatus, infraspinatus, and deltoid activity simultaneously during the five exercise positions examined and has built on the work of previous EMG research investigating the optimal exercise positions for strengthening supraspinatus (1,3,14-16,21,22). Not surprisingly, the results of this study support many of the findings of these earlier papers whether theexercises were performed isometrically (14,21), as in the current study, or isotonically (1,3,15,16,22). That is, there is no significant difference in levels of supraspinatus activity in "full can," "empty can," and prone elevation exercises (14,15), and there are high levels of deltoid as well as supraspinatus activity in "empty can" and "full can" exercises (14,15,19,22). Moreover, prone elevation exercise activates posterior deltoid at higher levels than the "empty can" (14,15) and "full can" exercises (15), and prone external rotation exercise activates supraspinatus at higher levels than middle deltoid (16).
The conclusion reached in this study, that the optimal exercises for strengthening supraspinatus are those involving shoulder external rotation, however, differs from other studies. Unlike the current study, many previous studies did not compare external rotation and "can" exercises (14-16,21,22), and others did not compare supraspinatus and deltoid activity levels (1,3). Using the foundation of these previous studies and robust statistical analysis, the current study has been able to more comprehensively examine all exercises known to recruit supraspinatus at high levels, resulting in these more definitive conclusions.
The results of the current study identify the pendant external rotation and prone external rotation positions as preferable exercises for supraspinatus activation and strengthening compared with the "empty can" and "full can" exercises because they recruit deltoid at significantly lower levels. However, these external rotation positions do not fully satisfy the set criterion because they also highly activate infraspinatus. This has been found in previous studies investigating "can" and external rotation exercises (1,3) and could be expected if both muscles externally rotate the shoulder as has been argued previously (6,12). Rather than being a disadvantage, the high level of activation in infraspinatus in the pendant external rotation and prone external rotation exercise positions may be beneficial because infraspinatus has a role similar to supraspinatus, that is, to centralize the humeral head on the glenoid fossa (2,9,13,17). If the goal of exercises aimed at strengthening the supraspinatus is to enhance its centralizing effect on the humeral head and thereby reduce superior translation as a way of addressing shoulder pathology such as impingement (5,15), the choice of an exercise that recruits another rotator cuff muscle at high levels could be advantageous.
Previous research evaluating the relative advantages of the two "can" exercise positions for supraspinatus strengthening have considered the risk of impingement due to a decrease in subacromial space with internal rotation required for the "empty can" exercise (5,18). If avoiding impingement is regarded as a significant factor during supraspinatus exercises, then the pendant external rotation position should be favored because this exercise is performed in the less pain-provoking adducted position while activating supraspinatus as highly as any exercise position examined in this study. As impingement symptoms resolve, supraspinatus exercises could be made more functionally specific by progressing to the prone external rotation position, which is performed in mid-shoulder range.
Although the current study has simultaneously examined the five positions currently recognized to be the most effective for supraspinatus strengthening, there may be other exercises that better satisfy the set criterion than the external rotation exercises examined in this study. However, until future research identifies such a position, the results of this study strongly indicate that exercises involving external rotation are preferable to "empty can," "full can," and prone elevation for specifically strengthening supraspinatus in subjects with normal dominant shoulder function. Future research is required to determine whether the same muscle activation responses occur in symptomatic shoulders during these exercises.
No funding was received for this work. The results of the present study do not constitute endorsement by ACSM.
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