RENEW OR SUBSCRIBE TO PPM
Subscription is FREE for qualified healthcare professionals in the US.
14 Articles in Volume 19, Issue #5
Agonism and Antagonism of the Muscles of the Shoulder Joint: An SEMG Approach
Analgesics of the Future: The Potential of IV Formulations for Post-Op Treatment of Pain
Blood Biomarkers Show Promise for Precision Pain Management
Can I Call Myself a “Pain Specialist?”
Cases in Urine Drug Monitoring Interpretation: How to Stay in Control (Part 2)
Fear-Avoidance and Chronic Pain: Helping Patients Stuck in the Mouse Trap
How to Avoid Patient Alienation When Discussing Stress
Managing Phantom Limb Pain with Medication
Nerve Blocks Lead to Improved Quality of Life
Sacroiliac Joint Dysfunction: New Methods in Evaluation and Management
SCS Therapy in a Patient with Advanced Bilateral Kienbocks
Thoracic Epidural Abscess with Cord Compression Following a High-Frequency SCS Trial
What is the evidence to support clonidine as an adjuvant analgesic?
What’s In A Name? In This Case, That Which We Call Addiction Is Not Dependence

Agonism and Antagonism of the Muscles of the Shoulder Joint: An SEMG Approach

The author shares 10 years of clinical evidence for the use of surface electromyography to restore a healthy and functional range of motion–using the shoulder as an example.
Pages 41-44

The Shoulder Joint

For physical medicine and rehabilitation clinicians (PM&Rs), the shoulder joint is one of the most complex joints of the body. Nineteen different muscles share different components and together participate in any given motion. The shoulder joint and muscles transition from the quadruped position to the bipedal position, hanging loose in the standing, prone, or supine positions. In the upright position, the shoulder muscles modulate and maintain the neutral position of the neck and head. While shoulders are largely independent of each other, they usually work in tandem. However, each shoulder may sustain an independent action simultaneously. They sustain and impart momentum to the elbow muscles and indirectly to the distal myotatic units of the wrist and hand.1

The shoulder joint has several anatomic components. Whereas these components exist in the quadruped position, they transition in function for the bipedal position. A number of muscles envelop the shoulder joint, and some muscles overlap the strict anatomic definition of the position on the shoulder:

  • the superior area: levator scapulae, supraspinatus, middle deltoid, upper trapezius, coracobrachialis
  • the anterior area: anterior deltoid, pectoralis major and minor
  • the posterior area: subscapularis, posterior deltoid, middle and lower trapezius
  • the posterior area, lateral aspect: infraspinatus, teres major and minor, latissimus dorsi
  • the posterior area, medial aspect: rhomboid major and minor
  • the inferior area: serratus anterior.

Classically, the shoulder joint has several segments of motion that together comprise the range of motion (ROM):

  • abduction
  • adduction
  • anterior flexion
  • lateral flexion
  • posterior flexion
  • internal rotation
  • external rotation.

A proper understanding of the physical principles of momentum, inertia, and vectoral activity is paramount to the understanding of ROM. The phenomenon of co-activation or co-contraction is exemplified by the presence of low-level active potentials in the resting muscle, while the homologous contralateral muscle is active and moving.2 In a healthy individual, active motion amplitude potentials during movement of a muscle of one limb is not met with any active potentials in the homologous muscle of the other limb while that limb is resting. A balanced relationship among the muscles of a joint is conducive to normal function, such as the ability to conduct movements for a long span of time without fatigue and pain.

If even one muscle of a joint is dysfunctional, that muscle will affect the function of the whole joint by limiting motion, utilization of energy, resistance, and strength. Consequently, voluntarily or involuntarily, that joint could become underutilized (“splinted”) and the contralateral joint will exhibit protective guarding and become overutilized. If there is a larger imbalance, the joint that is overutilized may eventually become dysfunctional and develop fatigue, trigger points, and pain.

This paper addresses how a PM&R provider may restore healthy function and ROM in a patient who has undergone myofascial injury resulting in muscular pain and trigger points. The scope will further focus on the use of surface electromyography (SEMG) in pain management for myofascial dysfunctions as well as acute and chronic pain from injuries based on the author’s clinical experience over a period of 10 years.1-9

SEMG dynamic testing is typically completed in under 15 minutes (seven motions, each at 90 seconds). (Source: 123RF)

Restoring Healthy Function with SEMG

Methods

Shoulder muscles may be trained within a few days of surgery after sutures are removed, or when the muscles are no longer in danger of tearing. Physical therapy should be started in an incremental manner: first using muscles and motions which require less energy and progressing gradually to all motions. The training needs to be done first without added resistance (besides gravity). It may progress to add resistance as tolerated and eventually to the level of ergonomic or athletic needs.

SEMG dynamic testing, that of testing a joint through classic ROM, consists of repeated muscular motions performed at the minimal level of effort (activity and rest) through the classic ROM of any joint. Data is typically collected in units of microvolts root mean square (RMS), and considered only when the coefficients of variation (CV) during motion and rest are 10% or less.6 The use of SEMG dynamic testing allows not only for the finding of amplitude potentials underlying the concept, but also for the statistical correlation coefficient.6 The results, both positive and negative, form the basis for agonistic and antagonistic values and relationships (see the sidebar “Clinical Refresher: Agonism vs Antagonism and the Shoulder”).5

Training may begin with SEMG biofeedback alone and then be done in combination with other modalities, always progressing from “easy” to “hard.” The final aim is optimal functioning of the patient.3-5

SEMG dynamic testing is noninvasive, fatiguing, or painful. Testing is typically completed in under 15 minutes; in the shoulder there are seven motions and testing for any motion normally takes 90 seconds.7 The testing is best performed by a qualified clinician or under one’s supervision, using SEMG equipment which includes a statistical package. The statistical package needs to include the ability to evaluate the means (or average) amplitude during muscular activity and rest, as well as the parameters of standard deviation, coefficient of variation, and regression analysis. All these parameters are necessary to evaluate the statistics underlying the amplitude domain. The testing may be done in the frequency domain, the median frequency being the parameter of choice. The testing which underlies the present article was done in the amplitude domain.

Most statistical packages allow the results to be read in a positive feature (not in raw SEMG), which is the result of the Fourier transformation of positive and negative amplitude results onto positive-only values. Only testing that may show the parameters of the means, coefficient of variation, standard deviation and, where necessary, regression analysis are compatible with the requirements of the Daubert Rule of Evidence needed for verifying the validity and scientific value of testing.

Clinical Evidence

The author’s studies of SEMG dynamic testing have been based on approximately 6,800 shoulder muscles from approximately 850 patients, tested through the classic ROM segments of motion noted above, according to established protocols.2,5-7 Two of the 19 shoulder muscles, the subscapularis and coracobrachialis, could not be tested, however, due to their deep location (at present, the SEMG electrodes do not show consistent readings if the muscles tested are deeper than 1.5 cm). Data was collected from consenting patients with a similar number of male and female patients. Ages varied between 21 to 75 and the data did not differ based on gender or age.2,5,6

The data reflect only the results from asymptomatic muscles. The amplitude potential values (microvolt RMS) were treated statistically for correlation coefficients. The positive correlation coefficients represent agonistic relationships and the negative values represent antagonistic relationships.5 These studies have shown that muscles are active during any segment of motion; there did not appear to be a “silent” muscle during any motion.

The overall average activity shown as amplitude potentials was 29.2 uV RMS. This amplitude marks the highest activity potential for all joints and average segments of motion tested with SEMG in minimal voluntary contractions (MVC), detailed below. Therefore, it may be concluded that the shoulder joint is the highest joint in terms of muscle energy use, early potential for fatigue from overuse, and subsequent pain.6

In descending order of energy utilization, the shoulder joint muscles perform as follows:

  • shrugging
  • abduction
  • lateral flexion
  • external rotation
  • posterior flexion
  • internal rotation
  • anterior flexion
  • adduction.6

The general homeostatic principle holds that the less energy used by muscles for a task, the less chance it has to develop repetitive or overuse fatigue, pain, and/or dysfunction. Within the shoulder joint, the 19 muscles contribute directly to any motion and as such there is less of a chance of fatigue than in other joints.

The author’s SEMG dynamic study data have consistently found that all muscles that subtend a given joint are active during any vectoral movement of that joint. The activity is documented by the presence of amplitude potentials that vary from muscle to muscle and from motion to motion.6 Given any sequence of the seven movements, muscles that move consistently in the same vectoral direction during the sequence are to be considered agonistic or synergistic. If they tend to be active in the opposite direction, they are considered to be antagonistic.

The overall calculations of the shoulder muscles’ intermuscular relationships are shown in Table I. A summary of the interrelationships shows that 137 are agonistic and 102 are antagonistic. The unequal numbers derive from the fact that some regression values were too close to zero to be counted as either positive or negative.6

Minimal Voluntary Contractions

The author’s dynamic SEMG testing through ROM was performed at the lowest common denominator of effort, that of MVC.7 Such utilization of minimal energy is not conducive to muscular overuse with results such as fatigue and pain. The correlation coefficient results among the shoulder muscles at the MVC level may become different when a given effort is needed for any particular motion. However, as the movements are optimized, the muscular effort will become lower and the optimal utilization of the shoulder muscles may start resembling that of the original MVC.

The overall engram, or a hypothetical permanent change in the brain accounting for the existence of memory (a trace), will be different for each shoulder function with different correlation coefficients. The aim is to format engrams to reduce the overall effort of action and therefore avoid fatigue and pain.

Conclusion

Physical medicine and rehabilitation clinicians deal with individual muscles that are injured and dysfunctional. Providers needs to understand the expected “normal” values and relationships in order to proceed with the rehabilitation process. The process of optimal functioning, either for ergonomics or athletics, may require further fine-tuning and may depend even more on quantification of expected SEMG values. An understanding of each muscle in terms of its agonist and antagonist relationship, as described at left, may be considered necessary for the mapping of this fine-tuning process.

 

A Clinical Refresher: Agonism vs Antagonism and the Shoulder

Agonism, or synergism, refers to a positive relationship in the contraction (concentric or eccentric) of two or more muscles that pertain to a given joint, all through a given set of motions, such as the range of motion. Antagonism refers to the inverse relationship. An antagonist muscle may stabilize or modify the motion of the agonist, and an antagonist muscle is not resting while the agonist contracts. These relationships are depicted within the 17 shoulder muscles tested as follows:*

  • The anterior deltoid:
    • agonistic: middle deltoid, lower trapezius, pectoralis major, pectoralis minor, serratus anterior, teres minor, infraspinatus, rhomboid minor, supraspinatus, upper trapezius
    • antagonistic: latissimus dorsi, middle trapezius, teres major, posterior deltoid, levator scapulae, rhomboid major
  • The middle deltoid:
    • agonistic: lower trapezius, middle trapezius, levator scapulae, rhomboid minor, supraspinatus, upper trapezius
    • antagonistic: latissimus dorsi, pectoralis major, pectoralis minor, serratus anterior, teres major, teres minor, posterior deltoid, rhomboid major
  • The posterior deltoid:
    • agonistic: latissimus dorsi, middle trapezius, serratus anterior, rhomboid major, upper trapezius
    • antagonistic: anterior deltoid, middle deltoid, lower trapezius, pectoralis major, pectoralis minor, infraspinatus, levator scapulae, rhomboid minor, supraspinatus
  • The pectoralis major:
    • agonistic: anterior deltoid, latissimus dorsi, teres minor, infraspinatus, pectoralis minor, serratus anterior
    • antagonistic: middle deltoid, lower trapezius, middle trapezius, posterior deltoid, levator scapulae, rhomboid major, rhomboid minor, supraspinatus, upper trapezius
  • The pectoralis minor:
    • agonistic: teres minor, infraspinatus, serratus anterior
    • antagonistic: posterior deltoid, levator scapulae, rhomboid major, supraspinatus, upper trapezius, teres major
  • The upper trapezius:
    • agonistic: anterior deltoid, middle deltoid, latissimus dorsi, lower trapezius, middle trapezius, serratus anterior, teres minor, posterior deltoid, levator scapulae, supraspinatus
    • antagonistic: pectoralis major, pectoralis minor, infraspinatus, rhomboid major, rhomboid minor
  • The middle trapezius:
    • agonistic: middle deltoid, latissimus dorsi, lower trapezius, posterior deltoid, levator scapulae, rhomboid major, supraspinatus, upper trapezius
    • antagonistic: anterior deltoid, latissimus dorsi, teres minor, infraspinatus, rhomboid minor
  • The lower trapezius:
    • agonistic: anterior deltoid, middle deltoid, teres minor, infraspinatus, levator scapulae, rhomboid major, rhomboid minor, supraspinatus, upper trapezius, middle trapezius, serratus anterior, teres major
    • antagonistic: latissimus dorsi, posterior deltoid, pectoralis major, pectoralis minor
  • The supraspinatus:
    • agonistic: anterior deltoid, middle deltoid, lower trapezius, middle trapezius, teres minor, levator scapulae
    • antagonistic: latissimus dorsi, pectoralis major, pectoralis minor, serratus anterior, posterior deltoid, infraspinatus, rhomboid major, rhomboid minor
  • The infraspinatus:
    • agonistic: anterior deltoid, latissimus dorsi, lower trapezius, pectoralis major, pectoralis minor, serratus anterior, teres minor
    • antagonistic: middle deltoid, middle trapezius, posterior deltoid
  • The rhomboid major:
    • agonistic: latissimus dorsi, lower trapezius, middle trapezius, teres minor, posterior deltoid
    • antagonistic: anterior deltoid, middle deltoid, pectoralis major, pectoralis minor, infraspinatus, levator scapulae
  • The rhomboid minor:
    • agonistic: teres minor, rhomboid major, infraspinatus, anterior deltoid, middle deltoid, lower trapezius, middle trapezius, serratus anterior
    • antagonistic: posterior deltoid, levator scapulae, latissimus dorsi, pectoralis major, pectoralis minor
  • The teres major:
    • agonistic: latissimus dorsi, lower trapezius, pectoralis major, serratus anterior
    • antagonistic: anterior deltoid, middle deltoid, middle trapezius, pectoralis minor
  • The teres minor:
    • agonistic: anterior deltoid, latissimus dorsi, lower trapezius, pectoralis major, pectoralis minor, posterior deltoid, infraspinatus, rhomboid major, rhomboid minor, supraspinatus upper trapezius
    • antagonistic: middle deltoid, middle trapezius, serratus anterior, levator scapulae
  • The latissimus dorsi:
    • agonistic: teres minor, posterior deltoid, infraspinatus, rhomboid major, upper trapezius, pectoralis major, pectoralis minor, serratus anterior, teres major
    • antagonistic: anterior deltoid, middle deltoid, levator scapulae, rhomboid minor, supraspinatus, lower trapezius, middle trapezius
  • The serratus anterior:
    • agonistic: teres major, teres minor, posterior deltoid, infraspinatus, rhomboid minor, upper trapezius, anterior deltoid, latissimus dorsi, lower trapezius, pectoralis major, pectoralis minor
    • antagonistic: levator scapulae, supraspinatus, middle deltoid, middle trapezius
  • The levator scapulae:
    • agonistic: middle deltoid, lower trapezius, middle trapezius
    • antagonistic: anterior deltoid, latissimus dorsi, pectoralis major, pectoralis minor, serratus anterior, teres minor, posterior deltoid, infraspinatus

*Correlation coefficients for each muscle can be found in Reference 6.

Last updated on: August 2, 2019
Continue Reading:
Physician Burnout: An Oldtimer’s View
close X
SHOW MAIN MENU
SHOW SUB MENU