Shoulder problems are quite common, with 30% of people suffering pain in this part of the body at some moment in their lives. Besides the high prevalence, shoulder dysfunction is often persistent and recurrent, with 54% of patients manifesting some symptom 3 years after the first occurrence of shoulder pain (1).

The subacromial impingement syndrome (SIS) or shoulder impingement is no longer defined as a pathology related to the decrease in subacromial space only, but as something more complex. It is the most frequent cause of shoulder pain and it is multifactorial in origin, deriving from multiple underlying pathologies and dysfunctions. A precise diagnosis of SIS requires a systematic evaluation consisting of a complete anamnesis of the patient and the performance of different challenging manoeuvres. Shoulder impingement occurs mainly with pain, decreased muscle strength and range of motion of the shoulder, along with loss of limb functionality, which will negatively impact the patient’s life quality. These signs and symptoms are believed to be produced as a consequence of the loss of musculature motor control, weakness of the periscapular, axioscapularis, and rotator cuff muscles. Besides the possible presence of rotator cuff tendinopathy, or of the tendon of the long portion of the biceps brachii, dyskinesia of the scapula or muscle imbalances, instabilities, or labrum injuries can all be a cause or a consequence of SIS.

The capacity of the scapula and the glenohumeral joint to maintain functionality depends on proper motor control, determined by the order and quantity of muscle activation. Most of the studies on muscle activation focus on the quantity of activation and not so much on the activation time, being equally important. The evidence in relation to the order of activation of the shoulder musculature is inconclusive. In this sense, individuals with SIS tend to show an altered order of muscle activation. There are studies that reveal muscle activations started by the upper trapeziums, delay the activation of the serratus anterior and lower trapeziums during the abduction movement, although their interaction with the rest of the shoulder muscles is unknown. According to these results, it could be inferred that people affected with this pathology seem to tend to delay the activation time of some periscapular muscles (2), which may produce a scapular dyskinesia. On the other hand, studies involving healthy people seem to describe an early activation sequence of the anterior deltoid, upper trapeziums, and supraspinatus, followed by an undetermined interaction of serratus anterior and lower trapeziums, as well as the rest of the rotator cuff during movements of the shoulder (3, 4, 5).

As in the diagnosis of SIS, the underlying mechanisms that result in altered activation patterns are not yet clear, and there is a dichotomy of whether the alteration of these patterns is a consequence or a cause of pathophysiological changes associated with SIS. In this sense, clinical differences between asymptomatic and SIS individuals remain unclear. For this reason, a study recently published by the staff from FC Barcelona’s Physiotherapy Department (6) has tried to identify the muscle activation pattern of the shoulder muscles (periscapular, axioscapularis and rotator cuff muscles) during the physiological movements used in a shoulder assessment, in addition to the combination of different speeds and loads. For this, electromyographic records were taken during flexion, escapement and abduction performed at fast, medium, and slow speeds, both with load (3 kg) on the arm and without load among 68 people. In order to identify and describe the altered activation order and its clinical relevance, it is first necessary to know the activation order in healthy individuals, so 34 healthy individuals were compared with 34 subjects diagnosed with SIS.

The main result was that there were no differences in the order of activation of the periscapular, axioscapularis and rotator cuff muscles between healthy people and those diagnosed with SIS. All muscles were activated before the start of a movement in all studied conditions. The first muscles to activate were anterior deltoid, upper trapeziums, and mid deltoid in most of the assessed conditions. The rest of the muscles studied (serratus anterior, lower trapeziums, posterior deltoid, pectoralis major and, especially, supraspinatus, infraspinatus, and subscapularis) did not present a specific order of activation. These results question the stabilising function of the rotator cuff since its activation was subsequent to that of the rest of the muscles. Consequently, as explained by Dr. Silvia Ortega Cebrián, member of FC Barcelona and first author of the study: “We are taught that the rotator cuff has a stabilising and centring function to optimise joint movement, but according to our results and other previous results in both healthy and pathologically affected people, the cuff is activated after the muscles with a more dynamic function. That is why we have to continue studying the role of the rotator cuff, as it may not have a function as mechanically relevant as we initially believed.”

The results also question the re-education trends for activation patterns that some rehabilitation programmes use, as at the moment, no differences in activation times are identified between healthy people and those diagnosed with SIS. In the results it was also observed that the speed and load component did show changes in the activation times, being the movements at medium speed and the movements with load the ones that were activated before.

In summary, the similarity in muscle activation patterns in both people diagnosed with SIS and healthy people forces professionals to contemplate the need to assess other functional movements, as well as to consider the individualised evaluation of motor schemes instead of generalised patterns. To finish, Dr. Ortega Cebrián leaves us with this reflection: “these results make me wonder about some methods of neuromuscular re-education or resolving scapular dyskinesia. Maybe I have given up worrying so much about activation times and this idea of synchronisation between muscles, and I am trying to focus more on the amount of activation.”

BIHUB team

References:

1. Lewis JS. Rotator cuff tendinopathy/subacromial impingement syndrome: is it time for a new method of assessment? Br J Sports Med. 2009;43(4):259-64.
2. Cools AM, Witvrouw EE, De Clercq GA, Danneels LA, Willems TM, Cambier DC, Voight ML. Scapular muscle recruitment pattern: electromyographic response of the trapezius muscle to sudden shoulder movement before and after a fatiguing exercise. J Orthop Sports Phys Ther. 2002 May;32(5):221-9.
3. Wattanaprakornkul D, Halaki M, Boettcher C, Cathers I, Ginn KA. A comprehensive analysis of muscle recruitment patterns during shoulder flexion: an electromyographic study. Clin Anat. 2011;24(5):619-26.
4. Phadke V, Ludewig PM. Study of the scapular muscle latency and deactivation time in people with and without shoulder impingement. J Electromyogr Kinesiol. 2013;23(2):469-75.
5. Reed D, Cathers I, Halaki M, Ginn KA. Does load influence shoulder muscle recruitment patterns during scapular plane abduction? J Sci Med Sport 2016, 19(9): 775-60.
6. Ortega-Cebrián S, Bagur-Calafat C, Whiteley R, Navarro R, Monné-Guasch L, Girabent-Farrés M. Subacromial Impingement Syndrome does not alter muscle onset activation patterns during shoulder cardinal movement at different speed and load. Musculoskelet Sci Pract. 2020;48:102161