Pain may alter recruitment patterns of shoulder musculature

Research Review: Pain may alter recruitment patterns of shoulder musculature

By Stefanie DiCarrado DPT, PT, NASM CPT & CES

Edited by Brent Brookbush DPT, PT, MS, PES, CES, CSCS, ACSM H/FS

Original Citation: Scovazzo, M.L., Browne, A., Pink, M., Jobe, F.W., and Kerrigan, J. (1991). The painful shoulder during freestyle swimming: An electromyographic cinematographic analysis of twelve muscles. The American Journal of Sports Medicine. 19(6). 577-582 - ABSTRACT

Two segments of a freestyle swim stroke cycle

Why is this relevant?: Treating and preventing injury in an athletic population requires knowledge of optimal mechanics and muscle activity. This paper offers a view on muscle activity in swimmers with and without shoulder. It is recommended the reader also review the first article in this series that documents the results of freestyle swimmers without shoulder pain which will be reviewed on this site in the future (1).

Study Summary

Impingement of the supraspinatus or long head of biceps tendon is one of many swimming related shoulder injuries.

Review & Commentary:

This study is specific to a specific swim stroke, but may highlight compensatory motor patterns in those with shoulder pain and Upper Body Dysfunction (UBD) . For strong correlations to be made a study stays true to its population through subject selection, methodology, and conclusion. Scovazzo et al (1991) produced a strong study by analyzing 2 groups of muscles (see the chart above) in freestyle swimmers with current shoulder pain and signs of impingement, instability, and/or apprehension. The authors then compared that data with a previous identical study involving individuals without current shoulder pain or signs of impingement, instability and/or apprehension (1). The researchers clearly define their methodology involving data collection in two pools with underwater windows using 2 cameras - one underwater and one above the water surface. They used fine wire electrodes and obtained data for 6-12 stroke cycles per swimmer. The authors defined a stroke cycle as having 4 phases: 1. early pull through (hand entering water through humerus being perpendicular to body); 2. Late pull through (humerus being perpendicular to body through hand leaving the water); 3. Early recovery (hand leaving the water through humerus being perpendicular to surface of water); 4. Late recovery (humerus being perpendicular to surface of water through hand entering water). Fine wire EMG allowed for more accurate data collection; reducing the risk of cross talk from neighboring muscles. Surface electrodes would hinder EMG data collection of deeper muscles. The authors clearly described the equipment and settings used. However, specific needle placement was not discussed. Additionally, the authors did not define other stroke cycle terms used throughout the article such as mid-recovery. It was assumed by this author that mid-recovery is the point in the stroke cycle where the humerus is perpendicular to the water's surface.

Researchers aligned EMG data with the motion capture data to examine the relationship between osteokinematic motion and muscle activity. This relationship allowed for very pointed observational based conclusions on the impact shoulder pain has on altered shoulder mechanics during a freestyle swim stroke.

This study, as any other, is not without limitations. A small sample size, vague descriptions of shoulder dysfunction (diagnostic criteria was not described in the article), and use of pain as a primary indicator of dysfunction may represent weaknesses that should be addressed in follow-up studies. Despite the small sample size, there were statistically significant differences between symptomatic and asymptomatic groups. Pain as a primary inclusion criteria in studies examining altered mechanics and muscle activity may represent a weakness in this study and many others. As altered mechanics can exist without pain, it is not possible to determine if the AG group was a true model of optimal mechanics and muscle activity. This would reduce the strength of correlations made between pain and altered mechanics in the SG group in comparison to the AG group. As this study did, in fact, find a statistically significant difference, it may be hypothesized that the correlation between altered activity and pain is actually stronger than statistical analysis would suggest. The "chicken and the egg" relationship that may exist between pain and compensatory patterns deserves further study.

Why is this study important?

This study contributes data to our current knowledge of muscle activity in a specific athletic population including individuals with and without pain. It is important to understand how pain affects the neuromuscular systems in order to reverse and prevent movement dysfunction and injury. This particular article focused on competitive athletes - injury within this population can lead to both financial and psychological distress. As such, it is our responsibility as human movement professionals to fully understand the mechanism of injury and prepare programs to restore normal joint kinematics and muscle activity.

It is no surprise that individuals with documented shoulder pain and positive signs of shoulder dysfunction displayed altered muscle activity levels. The authors postulated that the altered muscle activity levels result from a protective mechanism in the body to avoid impingement and pain; the data supports their conclusion.

Interestingly, the deltoids showed similar patterns of muscle activation between groups but with less activity of anterior and middle deltoids  in those with painful shoulders with hand entry and just before hand exit. Both positions may create impingement with excessive superior and/or anterior translation of the humeral head. Humeral head translatory information was not provided in this study, however, increased anterior translation has been documented in those with shoulder impingement syndrome (2). Subjects with shoulder pain displayed altered rotator cuff muscle (RTC) activation, specifically in the infraspinatus and subscapularis . The infraspinatus in those with shoulder pain demonstrated increased activity at the end of the pull through portion of phase 2. This increased activity could be the nervous system's attempt at slowing internal rotation at a greater rate so as to avoid impingement. The subscapularis was less active during mid-recovery in those with shoulder pain. At this point in the cycle, the humerus is in a position of internal rotation and approximately 90o of glenohumeral (GH) elevation - a point of possible impingement. A decrease in subscapularis  activity could decrease the force of internal rotation to, again, avoid impingement.

The serratus anterior , largely responsible for optimal scapulohumeral rhythm displayed decreased activity after hand entry and during middle pull through. At the point of hand entry, the shoulder girdle has completed its movement into full elevation with full scapular upward rotation. Decreased activity in the serratus anterior could decrease the amount of upward rotation which could lead to impingement related shoulder pain (3). From that point and moving to the middle pull through position, the serratus anterior  would most likely be eccentrically controlling downward rotation and anterior tipping of the scapula. Decreased activity could indicate a lack of control of the aforementioned scapular motions which is dsyfunctional and may lead to pain. To further complete the picture, the rhomboids displayed overactivity during middle pull through. As a downward rotator of the scapula, overactivity in this muscle could contribute to inhibition of upward rotators such as the serratus anterior  and even upper trapezius . The authors theorize this overactivity may have been an attempt to decrease impingement through scapular retraction which may increase the subacromial space. Interestingly, the rhomboids and upper trapezius , an upward rotator, elevator, and anterior tipper of the scapula both demonstrated decreased activity at hand entry. The decreased trapezius activity could be a compensatory reaction as an attempt to limit anterior tipping due to the inhibited serratus anterior . Decreased rhomboid activity during hand entry could be also be compensatory as to limit the amount of upward rotator inhibition.

In syncing EMG data with visual observation of movement, the authors reported the following; during hand entry in those with shoulder pain, the arm was positioned with decreased abduction which corresponds with decreased anterior and middle deltoids  activity and which may have reduced impingement; this decreased abduction required less upward rotation and retraction of the scapula which corresponded to decreased upper trapezius , serratus anterior , and rhomboid activity. Additionally, increased rhomboid activity rather than serratus anterior  during middle pull through may have been in preparation for early hand exit which prevents the extreme internal rotation that typically accompanies the movement.

This creates an interesting topic for discussion. Is the muscle activity here compensating to avoid injury or is the altered muscle activity the reason for the injury? The answer is likely that both are occurring at the same time. Typically, in individuals that perform repetitive tasks, fatigued primary movers can result in compensatory movement patterns that eventually lead to injury. Once injured, new compensatory strategies develop to avoid pain and further damage (either consciously or subconsciously).

How does it affect practice?

Clinical practice should not only focus on rehabilitation from current injury but prevention of relapse. Further, if we are lucky enough to work with clients on an on-going basis and prior to painful patterns and injury, we should be keenly focused on opportunities to prevent initial injury. The muscle imbalances noted here combined with the osteokinematic changes noted in other studies (Altered Scapular Mechanics , Altered Glenohumeral Mechanics ) may allows clinicians to create evaluation protocols and intervention that aim to correct alterations in muscle activity and joint dyskinesis both prior to the onset of pain, and post rehabilitation in an effort return optimal mechanics and optimize performance.

How does it relate to Brookbush Institute Content?

Shoulder impingement and instability are included under the umbrella of Upper Body Dysfunction (UBD) as described by the Brookbush Institute. Muscle imbalances documented in this article such as overactivity in downward rotators (rhomboids ) and inhibited upward rotators such as the serratus anterior  and upper trapezius  support this predictive model of dysfunction and are explored further in the article describing UBD , along with a treatment strategy. The Brookbush Institute encourages detailed movement assessment, and an integrated approach to intervention. This may include, release, stretch, and mobilize of short/overactive soft tissues, mobilization of restricted joints (self mobilization techniques are available), and activation and integration of inhibited muscles. Below are videos demonstrating some release, stretch, and activation techniques specific to the muscles mentioned within this particular study.

Rhomboid SA Static Release

Subscapularis SA Static Release

Serratus Anterior Isolated Activation

Serratus Anterior Activation Progressions

Trapezius Isolated Activation

Serratus Anterior Reactive Activation

Trapezius Reactive Activation

Sources

1. Pink M, Perry J, Browne A, et al (1991). The normal shoulder during freestyle swimming An EMG and cinematographic analysis of twelve muscles. Am J Sports Med 19. 569-576.

2. Lawrence, R.L., Braman, J.P., Staker, J.L., Laprade, R.F., Ludewig, P.M. (2014) Comparison of 3-dimensional shoulder complex kinematics in individuals with and without shoulder pain, Part 2: Glenohumeral joint. Journal of Orthopaedic & Sports Physical Therapy 44(9). 646-B3

3. Lawrence, R. L., Braman, J. P., Laprade, R. F., & Ludewig, P. M. (2014). Comparison of 3-dimensional shoulder complex kinematics in individuals with and without shoulder pain, part 1: sternoclavicular, acromioclavicular, and scapulothoracic joints . journal of orthopaedic & sports physical therapy, 44(9), 636-A8.

© 2014 Brent Brookbush

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