Electromyography of the coracobrachialis—Construct validity and implications for rehabilitation of anterior shoulder dislocation

Abstract

Background

The coracobrachialis (CB) is an important anterior shoulder stabilizer, yet no study has previously reported its electromyography (EMG). The purpose of this study was to validate a novel technique for surface EMG (sEMG) of the CB.

Methods

Thirty-two (21 females) healthy individuals participated in this cross-sectional study. A surface landmark for recording the CB was identified within the medial aspect of the proximal arm using real-time ultrasound imaging and cadaver dissection. The EMG from this landmark as well as from the biceps brachii (BB) were simultaneously recorded during shoulder horizontal adduction (SHAd) (a CB-dominant test), and during elbow flexion (EFlx) and forearm supination (FSup) (BB-dominant tests). Differences between CB and BB activation (% maximal voluntary isometric contraction) within each test were assessed.

Results

CB activation was significantly greater than BB during SHAd (36.51% versus 4.09%, P < 0.001). BB activation was significantly greater than CB during FSup (34.85% versus 27.70%, P = 0.045). No difference between CB and BB activation was noted during EFlx (41.82% versus 28.64%, P = 0.061).

Conclusions

The moderate activation of the CB with simultaneous minimal activation of the BB during SHAd supports this novel sEMG technique. This technique may assist the design of exercises to train the CB.

Level of evidence

III.

Introduction

The coracobrachialis (CB) shares a common origin with the short head of the biceps (SHB) from the coracoid process and courses distally and laterally to insert on the medial aspect of the proximal half of the humerus. ¹ Although classically considered an accessory shoulder flexor and adductor, ¹ Bassett et al. ² suggested the CB is an effective horizontal adductor of the abducted and externally rotated shoulder, and as such is well suited for providing anterior shoulder stability in the apprehension position. ² Itoi et al. ³ found the largest moment arm of the CB was in the direction of shoulder horizontal adduction (SHAd), ³ while Giles et al. ⁴ reported that applying tension through the conjoint tendon increases resistance to anterior translation of the humeral head when the shoulder is abducted. ⁴ Finally, Kobayashi et al. ⁵ found shoulder horizontal abduction to effectively stretch the CB. ⁵ Given that excessive shoulder horizontal abduction has been implicated in the mechanism producing anterior shoulder dislocation (ASD),^6,7 it seems the CB may enhance anterior shoulder stability particularly in the abducted and externally rotated position. Following the Latarjet procedure the CB becomes a more potent shoulder adductor and internal rotator particularly in elevated shoulder positions, thus further enhancing its potential to provide anterior shoulder stability. ⁸

Given its implied role in anterior stability of the native shoulder, as well as following the Latarjet procedure, further investigation into the role of the CB in dynamic shoulder stability seems warranted. Both indwelling and surface electromyography (sEMG) is often used to study muscle function in the context of joint stability, however, to the best of our knowledge no study has previously reported the electromyography (EMG) of the CB.

Although a method to record the activation of the CB has been described, ⁹ this method involves the use of indwelling EMG with fine-wire electrodes insertion into the CB. Such a technique may jeopardize the in-lying musculocutaneous nerve which is perhaps why no study has previously reported its use.

Based on its course from the coracoid process to the proximal humerus, the CB may lend itself available for sEMG within the medial aspect of the proximal arm just posterior to the SHB (Figure 1). The proximity of the CB and SHB raises concerns of “cross talk” when picking up EMG signals from the proximal arm, but differentiation between the signals of these muscles may be possible given their respective functions over the shoulder and elbow.

Figure 1.

Surface landmark of coracobrachialis.

In the absence of a prior report of CB EMG and given its apparent role in providing anterior shoulder stability,^2,4,5 the purpose of this study was to describe and preliminary validate a new method to record CB activity using sEMG from the medial aspect of the proximal arm. We hypothesized that if the EMG signal obtained from this landmark represents that of the CB: (1) This signal would be greater than the signal from the biceps brachii (BB) during resisted SHAd, and (2) The signal from the BB would be greater than that of CB during resisted elbow flexion (EFlx) or forearm supination (FSup).

Materials and methods

Design

A cross-sectional study was conducted within the Physical Therapy Department of Ariel University. The study was approved by the Ethics Committee of Ariel University and all participants were given an informed consent form to review before the initiation of the study. All testing procedures included in the study were performed by a physical therapist with over 25 years of experience in managing musculoskeletal conditions.

Identification of a surface landmark for collecting CB EMG

Identification of a surface landmark to record CB EMG was assisted by real-time ultrasound imaging (RUSI) in the following manner: With the shoulder abducted 90° a flat US transducer (E2, Sonoscape Medical Corp, Shenzhen Guadong, China) was placed perpendicular to the long axis of proximal arm immediately distal to the pectoralis major (Figure 2). The transducer was adjusted until a clear view of the CB and SHB was obtained (Figure 3). Differentiation between the SHB and CB was achieved through a biceps isometric contraction, FSup, and SHAd, respectively (Supplemental Videos 1, 2 and 3). Digit compression of the skin overlying the CB verified the skin surface overlying the subcutaneous portion of the CB over which the EMG sensor was to be positioned (Supplemental Videos 4 and 5). These procedures were repeated on three volunteers prior to data collection. The location of the surface landmark was further validated using cadaver dissection in the following manner: A 19-gauge hypodermic needle was inserted into the CB under the previously described RUSI technique (Figure 4(a)). With the needle in situ the proximal arm was dissected by an anatomist with over 15 years of experience. Following skin opening, epidermal and subcutaneous fat removal (Figure 4(a) and (b)), the location of the needle within the CB was verified (Figure 4(c) and (d)). This procedure was repeated on a second cadaver before data collection. Incidentally, during one of these procedures the needle was found to have pierced the musculocutaneous nerve (Figure 4(d)) confirming our hypothesized concern of violating this nerve with a fine wire technique.

Figure 2.

Transducer orientation.

Figure 3.

Ultrasound imaging of the proximal medial arm.

Figure 4.

(a) Beginning of dissection following insertion of a hypodermic needle into CB. (b) Skin and subcutaneous fat removal around needle within the CB. (c) Deep dissection, note the hypodermic needle has pierced the musculocutaneous nerve (arrow). (d) Verification of needle placement within the CB.

Study population

A convenience sample of 32 (21 females) healthy volunteers was recruited for the study. Inclusion criteria were age 18 years or older and the ability to understand and read in the Hebrew language. Participants were excluded if they experienced any neck, upper back, or upper extremity symptoms over the previous year, or if they had a history of any spine or upper extremity surgery. A-priori sample size analysis (GPower, version 3.1.9.7) indicated 29 participants would need to be recruited using a paired T-test with an effect size (d) 0.7, P ≤ 0.05, and power of 95%.

Strength testing for determining individual resistance level

Maximal strength of EFlx, FSup, and SHAd was measured to customize the intensity of all EMG tests to each participant's maximal strength. All strength tests were performed in a supine-lying position with the EasyForce hand-held dynamometer (HHD) (Meloq AB, Stockholm, Sweden) which is designed to assess pulling force with a 1% error according to the manufacturer's specifications.

EFlx test: The arm was rested on the table with the shoulder abducted 20°, the elbow bent 90° and the forearm supinated. The HHD accessory belt was wrapped over the distal forearm while the examiner pulled the opposite handle attempting to extend the participant's elbow (i.e., “break test”) (Figure 5(a)). The test was terminated when the elbow began to extend.

Figure 5.

(a) Elbow flexion strength test. (b) Forearm supination strength test. (c) Shoulder horizontal adduction strength test.

FSup: The arm was rested on the table with the shoulder abducted 20°, the elbow bent 90° and the forearm supinated. The participant held one end of a 24 cm dowel while the accessory belt was anchored at a 90° angle to the opposite end of the dowel. The examiner pulled the dowel through the accessory belt attempting to pronate the forearm (Figure 5(b)). The test was terminated when the forearm began to pronate.

SHAd: The shoulder was flexed 90°, the elbow bent 90° and the forearm fully pronated. The accessory belt was wrapped over the distal arm. The participant was asked to maintain the test position while the examiner applied a lateral force attempting to horizontally abduct the shoulder (Figure 5(c)). The test was terminated when the arm began to horizontally abduct. Although classic manual muscle testing references suggest shoulder flexion, or flexion combined with adduction for testing the CB,^10,11 we used SHAd for several reasons. First, previous work demonstrated the greatest moment arm of the CB was for SHAd. ³ Second, pilot testing consistently produced greater raw EMG signal during SHAd compared with shoulder flexion, or combined shoulder flexion and adduction, and third, it is the SHAd function of the CB which seems most relevant in the context of anterior shoulder stability. The elbow was bent 90° and the forearm fully pronated to maintain activation of the triceps and pronator teres, respectively, thus promoting reciprocal inhibition of the BB. ¹²

A submaximal familiarization test was given prior to each strength test (EFlx, FSup, SHAd) followed by two 5-s maximal test repetitions with 30 s of rest in between. The repetition yielding the highest value was used for determining the 50% strength threshold of each participant during all subsequent EMG testing. The reliability between the 2 maximal repetitions was assessed using intraclass correlation coefficient with 95% confidence interval and found to be excellent (EFlx: 0.98 [0.95, 0.99]; FSup: 0.93 [0.85, 0.97]; SHAd: 0.94 [0.87, 0.97]).

Electromyography

Skin preparation and sensor positioning: The skin was shaved when needed, lightly abraded and cleaned with alcohol swabs. A 23 × 37 mm wireless EMG sensor (Trigno Avanti, Delsys Inc, Natick, MA) with a fixed interelectrode distance of 10 mm was placed over the middle of the BB belly in parallel to the muscle fibers (Figure 6). A smaller (25 × 12 mm) wireless sensor (Trigno Mini Sensor, Delsys Inc) with a fixed 10 mm interelectrode distance was placed over the CB based on the previously described pilot work for identifying a landmark for sEMG collection from this muscle. With the shoulder abducted 90°, the examiner identified the SHB and followed it proximally to its intersection with the overlying pectoralis major. The CB sensor was placed just posterior to the SHB in parallel with the course of the CB (Figure 6).

Figure 6.

Electromyography (EMG) sensor setup.

EMG data collection: Three maximal voluntary isometric contractions (MVIC) of EFlx and SHAd from which the highest root mean square (RMS) amplitude within a 250-ms window represented 100% EMG activity for the BB and CB, respectively. The MVICs were performed using the maneuvers previously described, however resistance was given manually in place of the HHD. Following MVIC testing each participant performed the 3 isometric tests (EFlx, FSup, SHAd) against his/her pre-determined 50% strength level in a random order. The procedure and setup were identical to the maximal strength tests except that the intensity of each contraction was controlled by the examiner in the following manner: The examiner began to apply tension through the handle of the HHD while the participant maintained each test position (i.e., EFlx, FSup, SHAd). The examiner monitored the force output displayed on the HHD until the 50% resistance threshold was reached and triggered data collection for 5 s for both muscles assuring muscle forces stayed at the designated force for the entire trial. This was repeated a total of three times with a 30-s rest interval between repetitions.

Data processing

The EMG signal was collected using Delsys Trigno System and analyzed using EMGworks ^™ acquisition software (Delsys Inc). All raw EMG data was collected at a sampling rate of 2148 Hz, band-pass filtered at 20–450 Hz, and further smoothed using a RMS moving window of 125 ms and a window overlap of 62.5 ms. The normalized muscle activity (%MVIC) recorded during the middle 3 s of each 5-s repetition was averaged and served for data analysis.

Data analysis

Descriptive statistics were used to summarize the data with measures of central tendency and dispersion. Non-parametric analyses were performed as assumptions for normal distribution were not met based on Shapiro–Wilks tests. To assess the two hypotheses of the study, separate Wilcoxon Signed Rank tests were performed to compare activation (%MVIC) of the CB and BB during EFlx, FSup, and SHAd. All analyses were performed using SPSS version 26 (SPSS, Inc, Chicago, IL) with an a priori level of significance of P ≤ 0.05.

Results

Thirty-two participants (21 females) were enrolled in the study. The mean ± SD age, height, and weight of the participants were 25.4 ± 6.7 years, 166.0 ± 9.0 cm, and 62.3 ± 9.8 kg, respectively. Thirty participants (93.8%) were right-hand dominant. The median (interquartile range) %MVIC of the CB and BB during the three muscle tests are summarized in Table 1.

Table 1.

Median (interquartile range) %MVIC during the 3 muscles tests.

	Coracobrachialis	Biceps brachii	P-value
Shoulder horizontal adduction	36.51 (22.00, 53.46)	4.09 (2.72, 7.82)	P < 0.001
Forearm supination	27.70 (14.89, 38.60)	34.85 (21.40, 45.14)	P = 0.045
Elbow flexion	41.82 (21.87, 67.42)	28.64 (21.94, 43.49)	P = 0.061

MVIC: maximal voluntary isometric contraction.

Activity of the CB was greater than that of the BB during SHAd (36.51% versus 4.09%, P < 0.001). Activity of the BB was greater than the CB during FSup (34.85% versus 27.70, P = 0.045). No difference was noted between CB and BB activation during EFlx (41.82% versus 28.64%, P = 0.061).

Discussion

The findings of this study support the ability to record CB activity through sEMG from the medial aspect of the proximal arm. The CB was significantly more active than the BB during SHAd, thus confirming our first hypothesis. The minimal activation of the BB during SHAd along with a simultaneous clear signal from the CB suggests the sensor placed over the proximal medial arm most likely recorded activation of the underlying CB rather than a crosstalk effect from the SHB.

The second hypothesis of this study was partially confirmed. Activity of the BB during FSup was greater than that of the CB, but not during EFlx. Several reasons may explain the limited ability to isolate BB activation from that of the CB during these tests. First, as the EFlx test was performed with the arm by the side, resistance to EFlx likely exerted an inferior-directed force over the humerus. Given that the CB possesses a superiorly directed vector over the humerus when the shoulder is adducted,^13,14 the EFlx test may have resulted in reactive activation of the CB to prevent inferior humeral translation. In that sense co-activation of the CB during resisted EFlx may reflect a natural synergy to promote glenohumeral stability. Second, we often observed a visible contraction of the pectoralis major and a tendency to adduct the arm against the ribcage during the EFlx and FSup tests, respectively. This tendency, which is supported by previous studies noting increased activation of the pectoralis major during resisted biceps curls and FSup,^15,16 may reflect a natural strategy to promote arm stability during these tests. Because the CB is also an arm adductor, ³ its’ co-activation during these tests may reflect a role in this strategy as well. Finally, the testing maneuver for FSup may have exerted an abduction moment over the shoulder thus producing compensatory activation of the shoulder adductors including the CB.

The activation of the CB during SHAd lends further support to previous anatomical and biomechanical evidence for its potential to provide anterior shoulder stability.^2–4 Given that shoulder horizontal abduction has been implicated in the mechanism of ASD,^6,7 the horizontal adduction capability of the CB may play an important role in protecting the shoulder from excessive stretch of the anterior capsuloligamentous complex.

The sEMG described in this study may have several clinical and research implications. From a clinical perspective, resisted SHAd may be used to train the CB in its anterior shoulder stabilization function. Doing so while maintaining the elbow bent and the forearm pronated, has demonstrated clear activation of the CB with almost no BB activation. This may be particularly beneficial to throwing athletes as it replicates elbow and forearm position, as well as the low BB activation characteristic of the acceleration phase of throwing.^17,18 From a research perspective, the technique described in this study may be used to further explore the effect of the Latarjet procedure on the CB. The change in lever arm of the muscle following this procedure may be associated with changes in activation as well. ⁸

Our study has several limitations. First, as with any sEMG study, crosstalk from adjacent muscles (i.e., SHB) cannot be completely ruled out. Given the almost absent BB signal during SHAd, and the 9-fold greater activation of the CB, the source of the EMG signal from the proximal medial arm is likely to be the CB. The greater than expected activation of the CB during EFlx and FSup may have been due to crosstalk from the SHB. Additionally, based on its orientation relative to the glenohumeral joint, this activation may reflect a synergy to promote glenohumeral stability. Perhaps recording the EMG of other shoulder adductors (e.g., pectoralis major) would have helped shed more light on a possible synergy between the shoulder adductors and the BB during these tests. Likewise, the use of a more abducted shoulder position, particularly during the EFlx test could have resulted in a better isolation of the BB from the CB through reciprocal inhibition or due to a reduced need to counteract any inferior translation forces imparted to the humerus.

Conclusion

The greater activity of the CB relative to the BB during SHAd supports the first hypothesis of this study and lends validity to this new sEMG technique. Resisted FSup, and particularly resisted EFlx with the arm by the side, may not be the ideal tests to isolate BB from the CB. Alternatively, CB activation during these tests may reflect a synergy to promote glenohumeral stability. The sEMG technique described in this study may assist the design of exercises to train the CB, or in the exploration of the effects of surgical techniques on the function of the CB.

Supplemental Material

Supplemental Material

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Supplemental Material

Supplemental Material