Does Load Influence Shoulder Muscle Recruitment Patterns During Scapular Plane Abduction?

• Age: 18-49 (mean age 22.5 years old)
• Gender: 5 females, 9 males using arm used for writing
• Characteristics of the Study Design:
  • Surface EMG electrodes on upper trapezius and middle deltoid
  • Intramuscular electrodes on supraspinatus, infraspinatus, subscapularis, lower trapezius, rhomboid major, and serratus anterior
  • Scapular plane abduction, humerus externally rotated with "thumb up", opposite hand resting on hip to minimize trunk motion
  • Standardized tempo: concentric 3 seconds, isometric contractions 1 second, eccentric 3 seconds - for each condition
  • Maximum load (100% of 1RM) was determined as the heaviest load lifted while maintaining tempo and normal scapulohumeral rhythm:
    • Minimum 30 seconds rest between each trial
    • Maximum of 5 attempts to determine 1RM

  • Maximum isometric contractions were determined for each muscle group (in random order), performed in sitting, 30 second break in-between repetitions, and 2 minutes in-between tests. Test positions selected for ability to recruit maximum musculature:
    • Resisted shoulder abduction at 90° with external rotation
    • Resisted horizontal abduction at 90° of shoulder flexion
    • Resisted shoulder extension at 30° of abduction
    • Resisted shoulder flexion at 125° of flexion
    • Resisted shoulder internal rotation at 90° of shoulder abduction

  • 2 repetitions of shoulder abduction in scapular plane performed holding dumbbell corresponding to 25%, 50%, and 75% of maximal load (in randomized order) with 30 seconds in-between each load condition
  • Statistical Analysis:
    • 2-Factor Repeated Measures ANOVA utilized to compare activation levels of each of the 8 muscles during each of the different loading conditions
    • Pearson's correlation was used to determine time-normalized, group mean EMG signals, for each muscle, at each load condition, to determine consistency.
      • Weak - defined as 0.1 < r < 0.3
      • Medium - defined as 0.3 < r < 0.5
      • Strong - defined as 0.5 < r < 1.0

    • Post-Hoc Tukey's performed if significance was revealed in any of the variables investigated

• Inclusion Criteria:
  • No history of shoulder pain
  • No pain on maximally resisted rotation tests
  • Full range of motion of the shoulder in abduction with normal scapulohumeral rhythm

• Exclusion Criteria:
  • None specified

• Upper trapezius, middle deltoid, supraspinatus, infraspinatus, subscapularis, lower trapezius, rhomboid major, and serratus anterior across the different loading parameters (25%, 50%, and 75% of 1RM)
• Percent of activation in each of the different muscles during the concentric and eccentric portions of the exercise

Significant ( p < 0.05) main effects for load, muscles, and between load and muscles.

Tukey's post-hoc - revealed significantly greater muscle activity occurred at 50% than at 25% and at 75% than 50% and that the activity level of all muscles investigated had statistically significant increases in activation with increasing loads.

From Reed et al. (2015). (In Press). Summary of activation levels of each of the muscles during each loading condition

Summary of the Activation Patterns of each of the muscles:

Middle Deltoid - 50% > 25%; 75% > 50%

Supraspinatus - 50% > 25%; 75% > 50%

Infraspinatus - 50% > 25%; 75% > 50%

Subscapularis - 75% > 25%

Upper Trapezius - 50% > 25%; 75% > 50%

Serratus Anterior - 75% > 25%

Lower Trapezius - 50% > 25%; 75% > 50%

Rhomboid Major - 75% > 25%

Correlation Analysis:

• Middle deltoid activation patterns were strongly correlated ( r >= 0.67, p < 0.05) with activation patterns of the infraspinatus and subscapularis across all loads, with a moderate correlation ( 0.47 >= r >= 0.42, p < 0.05) to supraspinatus activity. Middle deltoid activity was also highly correlated ( r >= 0.59, p < 0.05) with activity of the serratus anterior and rhomboid major across all loading conditions.
• Activation patterns for all rotator cuff muscles were similar, exhibiting a strong correlation to the increase in muscle activity seen in the deltoids and axioscapular msucles. The activation pattern of the infraspinatus and subscapularis exhibited the strongest correlation with axioscapular muscle activation patterns (serratus anterior and rhomboid major ). Supraspinatus activity also exhibited a strong, but slightly weaker correlation to axioscapular muscle activity (r >= 0.45, p < 0.05).
• Activation patterns of the axioscapular muscles were similar to one another, with a strong correlation ( r >= 0.67, p < 0.05) to all musculature, across all loading conditions.

Timing Analysis Takeaways:

• The supraspinatus was more than twice as active as the middle deltoid in the beginning of shoulder abduction range of motion (~40% MVIC in 75% condition to <20% MVIC in 75% condition)
• The infraspinatus was more than twice as active as the subscapularis at all levels of abduction, in all conditions (~80% MVIC mid-way through concentric ROM in 75% condition to ~40% MVIC)

ConclusionsAs load increases in the scapular plane, so does activation of upper trapezius , middle deltoid , supraspinatus , infraspinatus , subscapularis , lower trapezius , rhomboid major , and serratus anterior .Conclusions of the ResearchersShoulder abduction in the scapular plane does not follow "the law of minimal activation" and rather follows the "law of proportional activation." Practically speaking, if strengthening of one of these eight muscles is desired, training at low loads and then progressively increasing the load over time will increase demands on the muscle in a systemic fashion and is consistent with all previous research focusing on different shoulder motions (i.e. adduction, flexion, extension, and rotation ).

http://bestperformancegroup.com/wp-content/uploads/2014/03/scaphumerorhythm.jpg

Review & Commentary:

The current study builds upon previous literature supporting the claim that the muscles controlling shoulder motion increase their activation in a systemic fashion from low load to high load conditions. Previous research on this topic looked into shoulder muscle recruitment during commonly used shoulder internal and external rotation exercises, and saw similar patterns of activation (2). Additionally, information about axioscapular muscle recruitment patterns were examined in this study.

Activation patterns of 8 different muscles were reported, using surface EMG for the middle deltoid and upper trapezius , and intramuscular EMG for the supraspinatus , infraspinatus , subscapularis , lower trapezius , rhomboid major , and serratus anterior . The authors concluded that as load increases in the scapular plane, so does activation of all the muscles examined.

It has been proposed, prior to this study, that muscles fire in accordance with "the law of minimal activation." This law states that muscles with the least synergistic activity will be recruited first, and as the load increases additional muscles will be recruited (3). However, the results of this study supports the "law of proportional activity." This law states that there is a complex coordination of muscle activity that occurs during low loads and increases in a systemic fashion as loads increases. The clinical implications of this law may be that a particular pattern will recruit all muscles with the potential to contribute force, and may be trained (in this case, scapular plane abduction) at low loads to reduce strain, force or activity on a muscle, or muscles that exhibit dysfunction. As dysfunction resolves, loads can be progressively increased to enhance strength and function.

Further, the study highlights different levels of activation between the infraspinatus and supraspinatus when compared to the subscapularis. As loads increased the activity of the infraspinatus increased nearly twice as much as the subscapularis . Both of these muscles create an inferior translatory force, and dynamically stabilize the humerus against the superior translatory force generated by the middle deltoid. However, the authors posited that the infraspinatus may have the additional responsibility of externally rotating the humerus to clear the greater tubercle and reduce impingement against the acromion during elevation. Similar conclusions can be made about supraspinatus ' activation, as the supraspinatus has the additional responsibility of assisting in abduction. Previous studies have also implicated the supraspinatus as an external rotator which may contribute to the higher levels of activation (2).

The research methods used in this study were strong. The authors precisely reported their experimental protocol - from experimental set-up to EMG set-up. All measurements taken during the study were standardized (3 seconds concentric, 1 second isometric at the top, and 3 seconds eccentric), and the order of loading was randomized for each of the participants to prevent any ordering bias. The authors chose to use intramuscular EMG for the deeper muscles (the rotator cuff, the lower trapezius , and rhomboid major), surface electrodes for the superficial muscles (upper trapezius and middle deltoid ), and provided supporting literature to support their protocol. In order to be certain of the intramuscular electrode's location, diagnostic ultrasound imaging was used to guide the insertion of the needle into the muscle of interest. The authors also performed a power analysis to determine minimum sample size needed to show statistical significance which lends credence to their conclusions.

It should be mentioned that the current study was performed on asymptomatic shoulders and the results may not be transferable to a symptomatic population. Studies have shown that kinematics are altered in those with painful shoulders (4). Further, the exclusion criteria was not noted in the publication of this study; greater care should be used to differentiate populations in future studies.

Why is this study important?

The study adds to the current body of literature that activity of muscles surrounding the shoulder complex increase in a systemicc manner. Furthermore, because this study was the first (to the author's knowledge) to investigate axioscapular (serratus anterior , rhomboid major , upper and lower trapezius ) muscle recruitment patterns with respect to the scapulohumeral muscle (supraspinatus , infraspinatus , subscapularis , and the middle deltoid ) recruitment patterns in a commonly performed strengthening and rehabilitation, there may be implications that scapulohumeral muscles follow a similar muscle activation pattern - "the law of proportional activation."

How does it affect practice?

Human movement specialists may integrate several findings from this study into practice.

First, the study supports the notion of a systemic increase in activation for both scapulohumeral muscles and axioscapular muscles as load increases. The clinical implications of this law may be that a particular pattern will recruit all muscles with the potential to contribute force, and may be trained (in this case, scapular plane abduction) at low loads to reduce strain, force or activity on a muscle, or muscles that exhibit dysfunction. As dysfunction resolves, loads can be progressively increased to enhance strength and function.

Second, activation of axioscapular muscles increases with increase load during scaption. Thus, the human movement specialist may use scaption to indirectly strengthen the axioscapular muscles.

Third, rotator cuff muscle activation will increase with increased load during scaption, but those muscles that contribute to the motion used will increase more. For example, scaption will recruit all rotator cuff musculature, but places more stress on the supraspinatus and infraspinatus .

How does it relate to Brookbush Institute Content?

The Brookbush Institute aims to optimize recruitment patterns and motion before overloading the human movement system with the intent to increase performance. This study provides evidence that all of the muscles that may contribute to shoulder elevation and scapular upward rotation are recruited in proportion to the load used. Although, further research is needed to compare the recruitment pattern in this study to the recruitment pattern used by those experiencing pain and dysfunction, it may be advisable to optimize recruitment strategies in those exhibiting Upper Body Dysfunction (UBD) before using arm elevation activities during resistance/performance training. That is, in the presence of dysfunction and altered muscled activity (over-activity and under-activity), the law of proportional recruitment may still apply and serve to reinforce movement impairment.

Further, the recruitment of the upper and lower trapezius muscles and the serratus anterior reinforces the use of scaption as an activation exercise for these muscles when exhibiting under-activity.

Brookbush Institute Videos:

Shoulder Scapular Plane Abduction Exercise

Serratus Anterior Isolated Activation

Trapezius Isolated Activation

Sources:

1. Reed D, Cathers I, Halaki M, Ginn KA. (2015). Does load influence shoulder muscle recruitment patterns during scapular plane abduction? Journal of Science and Medicine in Sport.
2. RESEARCH REVIEW: Dark A, Ginn KA, Halaki M. (2007). Shoulder muscle recruitment patterns during commonly used rotator cuff exercises: an electromyographic study. Pys Ther. 87: 1039-1046.
3. MacConaill MA, Basmajian JV. (1977). Muscles and movements: a basis for human kinesiology. New York, NY, Robert E. Krieger Publishing Co
4. Struyf F, Cagnie B, Cools A, Baert I et al. (2014). Scapulothoracic muscle activity and recruitment timing in patients with shoulder impingement and glenohumeral instability. Journal of Electromyography and Kinesiology. 24: 277-284
5. RESEARCH REVIEW: Wong CK, Coleman D, diPersia V, Song J, Wright D. (2010). The effects of manual treatment on rounded-shoulder posture, and associated muscle strength. Journal of Bodywork & Movement Therapies. 14: 326-333.© 2015 Brent BrookbushQuestions, comments, and criticisms are welcomed and encouraged.