Access to the PPM Journal and newsletters is FREE for clinicians.
11 Articles in Volume 7, Issue #9
CES in the Treatment of Addictions: A Review and Meta-Analysis
Chronic Cancer Pain Management
Compliant Billing, Coding and Documentation for Interventional Pain Management
Critical Transition from Short-to-Long-Acting Opioid Therapy
Dextrose Prolotherapy and Pain of Chronic TMJ Dysfunction
Dysfunction and Rehabilitation of the Shoulder
Low Level Laser Therapy (LLLT) - Part 2
Placebos in Pain Management
The Good Patient: Responsibilities and Obligations of the Patient-physician Relationship
TMJ Derangement and SUNCT Syndrome Co-morbidity
Ziconotide Combination Intrathecal Therapy

Dysfunction and Rehabilitation of the Shoulder

Considerations of muscular relationships in pain management based on Surface Electromyographic (SEMG) Studies.

It is the objective of this article to present a novel view on the diagnosis of shoulder dysfunction and rehabilitation based on the statistical analysis of muscular activity data as noted on surface electromyographic (SEMG) studies of the asymptomatic shoulder muscles.1-3

The human shoulder is a very complex musculoskeletal structure. The anatomy allows it to perform both postural and ballistic functions. Muscular, bony, and joint evolution has changed little from the epoch when early humans were still partially quadrupeds. The clavicle is one of the earliest bones to develop in the fetus.

The shoulder dexterity engram is engrained deep in our motor cortex and related structures. All that one has to do is to observe the ease of ‘four-legged’ movements of infants before they become ‘two legged’ ambulatory. No one ever had to teach infants to move about and explore the surroundings with ease on their four limbs and reach objects with their upper limbs along their way.

The primary bony structures—the scapula and clavicle—are enveloped by nineteen primary muscles.4 While it is possible to classify those muscles by their anatomic location, for a few of them the classification is only partially correct. Thus, the classification could comprise the superior, lateral, anterior. and posterior muscle groups.

The superior shoulder group is comprised of the supraspinatus, levator scapulae, and the upper trapezius. The lateral group is comprised of the deltoid (anterior, middle and posterior) components and coraco-brachialis. The anterior group is comprised of the pectoralis major and minor. The posterior group is comprised of the infraspinatus, subscapularis, teres major and minor, rhomboid major and minor, middle and lower trapezius, latissimus dorsi, and serratus anterior. During motion, these muscles are active and modulate one another’s action via the myofascial components.

The shoulder movements proper also involve myofascial contributions from the contiguous muscles of the neck, back, and upper limb muscles.4,5 Classically, the range of motion (ROM) of the shoulder is comprised of the following segments: abduction, adduction, (anterior) flexion, lateral flexion, posterior flexion, external and internal rotation.

The shoulder movements require energy. The fact that there are so many muscles that participate in any given motion allows for an easy ‘spread’ of the energy consumption, such that no muscle in particular may suffer much from fatigue due to repetitive motion. One objective means of measuring the energy consumption and effort is by SEMG assessment of the shoulder muscles through the ROM in asymptomatic and symptomatic muscles.

Table 1. Muscles of the superior shoulder region: SEMG amplitude potentials
Segments of motion
Muscles Ext Rot Lat Flex Abduction Int Rot Post Flex Ant Flex Avg
Levator Scapulae 33.2 41.3 32.8 35.0 25.5 17.8 30.9
Upper Trapezius 37.0 21.9 25.3 23.9 23.9 14.9 24.5
Supraspinatus 22.9 26.5 22.1 18.7 15.1 15.7 20.2
Average 31.0 29.9 26.7 25.9 21.5 16.1 25.2
Table 2. Muscles of the posterior shoulder region: SEMG amplitude potentials (µV RMS)
Segments of motion
Muscles Ext Rot Abduction Lat Flex Post Flex Int Rot Ant Flex Avg
Rhomboid Major 38.5 34.3 46.9 33.6 44.0 20.0 36.2
Rhomboid Minor 33 41 38.2 18.1 23.8 29.1 30.5
Middle Trapezius 40.4 26.0 31.3 20.8 32.3 13.7 27.4
Infraspinatus 30.3 26.4 22.1 23.8 16.3 27.8 24.5
Latissimus Dorsi 11.1 13.2 7.6 23.3 9.7 11.4 12.7
Lower Trapezius 33.2 28.8 17.5 11.5 9.2 18.4 19.8
Serratus Anteror 10.8 12.1 10.5 15.2 5.2 13.6 11.2
Teres Major 12.7 10.1 7.5 15.0 8.3 7.6 10.2
Teres Minor 10.0 9.4 6.3 9.2 6.2 7.5 8.1
Average 24.4 22.4 20.9 18.9 17.2 16.6 21.1

Testing Protocol

Studies of shoulder muscles were conducted within the limitations of anatomy and symmetry of motion.6 The limitations of these studies include the lack of ability to assess deep muscles such as subscapularis and coraco-brachialis with SEMG (or needle-EMG).6 Also, it is difficult to perform simultaneous symmetrical testing of the adduction movement. Thus, the data presented in this article are exclusive of these limitations.

The testing protocol (in the standing at ease position) involved acquiring data on the segments of motion performed at the level of minimal effort for 9 seconds at a time, with 5 repetitions, and interspersed with 5 periods of rest of 9 seconds each. Statistics of average amplitude of tonus activity (µV RMS) and minimal tonus at rest, standard deviations (S.D.), and coefficients of variation (C.V.) were compiled. The data have been published in textbooks and articles.6,7 Upper limb SEMG studies of symptomatic muscles were performed using similar protocols.8

Test Results

The data are presented in five tables and presents the pertinent muscles activity through the shoulder ROM in order of decreasing magnitude of total energy expenditure and per segment of motion.

Table 1 describes the muscular activity of the three superior shoulder region muscles. Of the three, the levator scapulae exerts the largest effort through the ROM. The motion that requires the greatest energy expenditure in this group is that of external rotation.

Table 2 describes the muscular activity of the nine posterior shoulder muscles. The rhomboid major is the muscle that exerts the greatest effort through the ROM. External rotation is the motion that requires the largest energy expenditure of performance.

Table 3 describes the muscular activity of the three lateral shoulder region muscles. Of the three, the middle deltoid exerts the largest effort through the ROM. The motion that requires the greatest energy expenditure in this group is that of anterior flexion.

Table 4 describes the muscular activity of the anterior shoulder muscle group. Pectoralis minor exerts a greater level of effort than pectoralis major. Anterior flexion is the segment of motion that requires most energy.

Table 5 is inclusive of all the shoulder muscles tested from the four muscle groups. Of the seventeen muscles tested, levator scapulae is the muscle that exerts the greatest level of activity through the shoulder ROM. Shoulder abduction is the segment of motion requiring the most activity followed closely by external rotation.


A review of the tables above allows for a number of quantitative observations of clinical and ergonomic relevance.

  1. Each shoulder region muscular group has its own pattern of muscular activity through the six segments of motion tested. When each group is considered as a whole, the difference of total energy consumption among the six segments of motion is quite close, varying only ± 14% from the overall average.9
  2. The pattern of energy consumption among the different muscle region groups and segments of motion recapitulates, to some extent, evolutionary realities: the shoulder was originally the site of the anterior aspect of locomotion apparatus in the quadruped ancestor and of the ‘hanging in the tree’ capability of the more advanced primate ancestor.
Table 3 . Muscles of the lateral shoulder region: SEMG amplitude potentials (µV RMS)
Segments of motion
Muscles Abduction Lat Flex Ant Flex Ext Rot Post Flex Int Rot Avg
Middle Deltoid 44.5 33.4 37.3 36.8 29.0 25.5 34.4
Anterior Deltoid 48.6 33.8 39.8 31.3 12.8 20.5 31.1
Posterior Deltoid 22.2 30.1 12.2 20.4 31.0 18.7 22.4
Average 38.4 32.4 29.8 29.5 24.3 21.6 29.3
Table 4. Muscles of the anterior shoulder region: SEMG amplitude potentials (µV RMS)
Segments of motion
Muscles Ant Flex Post Flex Abduction Int Rot Lat Flex Ext Rot Avg
Pectoralis Minor 17.6 12.8 13.2 8.9 7.7 7.9 11.4
Pectoralis Major 12.5 11.5 9.5 7.5 7.2 6.5 9.1
Average 15.1 12.2 11.4 8.2 7.5 7.2 10.2

Its posterior region is comprised of more muscles than the other regions. This serves to spread the effort of motion and to reduce the chance of fatigue and dysfunction. A large and important muscle could not be tested electrophysiologically at the present time because of its position beneath the scapula: the subscapularis. However, its relevance cannot be underestimated.4

The anterior aspect of the shoulder is comprised of larger muscles. In the quadruped, it serves to stabilize the motions of the posterior group and as accessory to breathing and the protection of the inner organs. Its contribution to the overall level of effort through the shoulder segments of motion is marginal.10

The superior region group is comprised of only three muscles. It helps to lift and move the heavy upper limb against gravity.

The lateral region group is really comprised of two muscles: the coraco-brachialis (not amenable to SEMG testing because of its deep position) and the deltoid, one of the largest and strongest muscles of the body. The latter is tested in its three components because of the different contribution that each component imparts to the different shoulder segments of motion.

As seen in Table 3, the overall contribution of energy and effort of the lateral group is 36% above that of the seventeen-muscle group as a whole. The lateral group is essential for all the steering motions required of the shoulder in its ancient and modern functions.

A review of the database shows that the overall minimal tonus during the resting periods of the muscles of the shoulder is 9 The resting tonus of shoulder muscles which are symptomatic or dysfunctional are at least 200% higher than the value above.8 This is indicative of the fact that dysfunctional muscles do not rest well or are unable to restore the potential energy required for motion.

As documented in previous articles and textbooks, dysfunctional shoulder muscles’ activity amplitude potentials tend to also be roughly 200% above the values described in the tables above. Several SEMG studies have documented that the clinical phenomena of pain and fatigue have electrophysiologic correlates of hypertonus, spasm, hypotonus, co-contraction, co-activation, loss of mirror image, etc.8, 9,11

If muscles are clinically symptomatic and show SEMG correlates of abnormal resting and activity potentials, it stands to reason that symptomatic muscles are prey to abnormal effort levels, conducive to loss of energy, difficulty in regaining of energy, and early fatigue.8 It is likely that the symptom of pain may be a signal that the involved muscle(s) cannot adapt for function due to the lack of energy.12

Ergonomic Functional Perspective

To begin with, homeostasis and related concepts require that the body utilize the least effort for any activity requiring energy expenditure. This minimizes the need to acquire energy—always an expensive solution. In the case of the shoulder, there are nineteen muscles that contribute directly to any motion and several regional contiguous muscles that contribute in a myofascial vectorial fashion.5,6 Thus, the energy needed for any segment of motion is relatively widely distributed as compared to that of other joints. The data shown above presents the levels of energy required by each muscle and region in one ergonomic condition: movement performed at the least amount of effort. The presentation of the different muscles and shoulder regions may be different for different ergonomic conditions requiring different levels of effort.

The myofascial clinician deals with the presence of painful trigger points and associated pain and dysfunction. An examination of the tables above readily finds concurrence with the fact that the superior shoulder group is very frequently the site of trigger points while the anterior and lateral groups are rarely the sites of such trigger points. Of the posterior group, clinical experience frequently finds the presence of trigger points and associated pain in the area of the major and minor rhomboids—the muscles which exhibit the largest effort of activity in the group.

The ergonomist or athletic trainer may utilize the data above to get ‘one’s bearings’ in terms of the expectations for shoulder muscular function of an asymptomatic individual, before training that individual for the required tasks. If the values of muscular activity through any segment of motion differ 20% from those expected from the database, that individual needs to receive neuro-muscular retraining such as with SEMG-biofeedback and/or other modalities. Clinical experience shows that such muscles ‘recapitulate’ stories of prior injury or dysfunction. Such dysfunction may still exist at a sub-clinical level. It is worth remembering the old adage “bones forget, muscles remember.”

The rehabilitation clinicians may benefit from realizing that they should pay particular attention, on the one hand, to the muscles that exhibit the largest component of effort in any particular segment of motion and, on the other hand, to the motions requiring the highest levels of activity.

Clinical experience shows that the process of rehabilitation, be it by individual muscle or individual motion, needs to be accomplished ‘from low to high,’—from movements that require less effort and progress to those that require greater effort. The tables above provide such a guideline. Muscular function can be improved earlier and pain can be reduced earlier when the rehabilitation process smoothly follows ‘the path of least resistance.’


SEMG studies of seventeen shoulder muscles are presented on an anatomical region and functional basis. The study documents the amount of electrical energy and effort needed by each muscle through six segments of shoulder motion. The data can be utilized for a better clinical and ergonomic understanding of shoulder function and rehabilitation/ re-education.

Table 5. SEMG amplitude potentials (µV RMS) of seventeen shoulder muscles
Segments of motion
Muscles Abduction Ext Rot Lat Flex Post Flex Ant Flex Int Rot Avg
Rhomboid Major 38.5 34.3 46.9 33.6 44.0 20.0 36.2
Middle Deltoid 44.5 36.8 33.4 29.0 37.3 25.5 34.4
Anterior Deltoid 48.6 31.3 33.8 12.8 39.8 20.5 31.1
Levator Scapulae 32.8 33.2 41.3 25.5 17.8 35.0 30.9
Rhomboid Minor 41 33 38.2 18.1 29.1 23.8 30.5
Middle Trapezius 26.0 40.4 31.3 20.8 13.7 32.3 27.5
Infraspinatus 26.4 30.3 22.1 23.8 27.8 16.3 24.5
Upper Trapezius 25.3 37.0 21.9 23.9 14.9 23.9 24.5
Posterior Deltoid 22.2 20.4 30.1 31.0 12.2 18.7 22.4
Supraspinatus 22.1 22.9 26.5 21.0 15.7 18.7 20.2
Lower Trapezius 28.8 33.2 17.5 11.5 18.4 9.2 19.8
Latissimus Dorsi 13.2 11.1 7.6 23.3 11.4 9.7 12.7
Pectoralis Minor 13.2 7.9 7.7 12.8 17.6 8.9 11.4
Serratus Anterior 12.1 10.8 10.5 15.2 13.6 5.2 11.2
Teres Major 10.1 12.7 7.5 15.0 7.6 8.3 10.2
Pectoralis Major 9.5 6.5 7.2 11.5 12.5 7.5 9.1
Teres Minor 9.4 10.0 6.3 9.2 7.5 6.2 8.1
Average 24.9 24.3 22.9 19.9 20.1 17.0 21.5


Last updated on: January 28, 2012
close X