Initiator vs. Synergistic: A Clarification On the Role of Supraspinatus Muscle

A few days days ago, I published six articles that are medically irrelevant. Now allow me to transition to the scientific aspect of this blog. But before we begin, I shall say, that I am not a know-it-all. I am a nobody who happens to have a fondness for teaching — even with the little I know. 

On that note, let’s begin! *enter the propeysyunal me*


Whether you are a pre-medical or a medical student, chances are, you have already read of the obscure role of the supraspinatus muscle in terms of shoulder function. Let I, Doktora Undone,  shed some light on the topic that raised a thousand eyebrows.

Let’s begin by defining the suprasinatus muscle:

The supraspinatus (plural supraspinati, from Latin supraspinatus) is a relatively small muscle of the upper back that runs from the supraspinatous fossa superior portion of the scapula (shoulder blade) to the greater tubercle of the humerus. It is one of the four rotator cuff muscles and also abducts the arm at the shoulder. The spine of the scapula separates the supraspinatus muscle from the infraspinatus muscle, which originates below the spine. [1]

Malabo ba, ‘teh? Basta ang sinasabi lang sa taas e ‘yung anatomical description ni supra. Now, let’s cut the chase and get down to business: what then is its REAL function?

CLARIFICATION ON THE ROLE OF SUPRASPINATUS IN SHOULDER FUNCTION

SUPRA BABY.png
Clinical Anatomy by Regions,by Snell, 9th Ed., pg. 360

It is for a fact that professors base their lectures using different references, thus resulting to varying information. Personally, I can no longer put a finger on how much I have sat and listened to contrasting lectures re: its function— which is why I had to make this article in the first place. Some said it acts as an initiator for shoulder abduction, while others have said that is synergistic with deltoid. Perplexed as I was, I did a little digging which led to my discovery of several Easter eggs. The references are as follows:

BRUNNSTROM, 6th Ed: Initiator (pg. 197)

“In normal abduction, the supraspinatus initiates motion. However, in the persons with supraspinatus paralysis or debilitating injury, the deltoid is able to abduct the humerus throughout the range of motion if the other other functioning rotator cuff muscles are able to counteract the deltoid’s translatory force. Even though deltoid-only abduction occurs, it is produced with less than normal strength.”

To better explain the description above, I will be writing in my native language.

TAMBAY TRANSLATION:

  1. Kung ang pinag-uusapan ay normal shoulder abduction, si supraspinatus muscle po ay INITIATOR
  2. Pero kung PARALISADO si supra, magte-take over si DELTOID bilang shoulder abductor sa buong ROM (0-180°).
  3. Ngunit ‘yun po ay mangyayari lamang kung NORMAL o walang diperensiya ang iba pang bahagi ng rotator cuff muscles. Ibig sabihin ay okay ang ITS muscles.  (see note below on point #3)
  4. At kahit posible ang deltoid-only abduction, MAS MAHINA ito kumpara sa kung okay o wala diperensiya ang supraspinatus.

On point #3:  Ang pinag-uusapan po dito ay ‘yung tinatawag na translatory forces. Mahaba pong diskusyon ito. Kailangan mong aralin ‘yung translatory forces ng SITS muscles (supraspinatus, infraspinatus, teres minor at subscapularis) para maintindihan mo kung bakit kinakailangang normal ang mga ito para mangyari ang deltoid-only abduction. Gets pa ba? Text niyo ako kapag hindi na ah.

Susubukan kong i-explain  ito in details sa ibang blog post. Pero kung gusto mo nang palitan si Jollibee sa pagiging bida e mabuti pang aralin mo na si Norkins, 5th ed., pg 263. Ayan na ha. Binigay ko na ‘yung page. Magbabasa ka nalang.

NORKIN, 5th Ed: Initiator (pg. 263)

“The supraspinatus muscle is considered an abductor of the humerus. Like the deltoid muscle, it functions in all planes of elevation of the humerus. Its role, according to MacConaill and Basmajian, is quantitative rather than specialized. The pattern of activity of the supraspinatus is essentially the same as that found in the deltoid. The moment arm of supraspinatus is fairly constant throughout the ROM and is larger than that of the deltoid for the first 60° of shoulder abduction. When the deltoid is paralyzed, the supraspinatus alone can bring the arm through most, if not all, of the glenohumeral range, but the motion will be weaker.”

TAMBAY TRANSLATION:

  1. Ha?! Ano daw?!
  2. Ang dami niyang sinabi pero ang tinutumbok niya talaga e INITIATOR pa rin ang function ni supraspinatus muscle.
  3. Opposite scenario naman ang binigay niya dito. Imbis na supraspinatus ang paralisado (katulad nang ibinigay ni Brunnstrom sa taas), si deltoid naman ang paralisado dito. Essentially, ang sinasabi niya e kaya ring mag-abduct ni supraspinatus sa buong SH ABD ROM if deltoid is paralyze, pero WEAK lang din.
  4. May kinaliman pa rin sa translatory forces dito. Explain ko sa ibang post pag sinipag ako.

SNELL, 9th Ed: Initiator (pg. 360)

Sup.png
Woah! Ang daming words! Ang intense!

To sum it up, Snell basically said supraspinatus acts as initiator. In case of its paralysis, shoulder abduction is still possible provided that the arm be assisted in the first 15° of abduction. Thereafter, the deltoid takes over to a right angle, or simply 90° abduction.

Now, here lies the problem: both Brunnstrom and Norkin said that deltoid can take over as the shoulder abductor in case of supraspinatus paralysis. But here comes Snell saying that deltoid can take over up until 90° of shoulder abduction ONLY. 

‘Yung totoo?! Anong problema niyong tatlo?! Suntukan nalang kaya tayo?!

The answer lies in the study of translatory forces of the rotator cuff muscles. Yet again. I will discuss this should my schedule allow. Tuuhhhrayyy.

At this point, you should now be seeing it as an integrated function of the shoulder complex. In other words, you have to “look at the forest and not just the trees.” 

Did I make myself clear? Well, at least I hope, I did.

Now most professors will stop after giving you these references, but since I’m not like the others, I’ll give you more… for free. 

This part is where it gets exciting as I will be challenging everything you think you know.


GRAY’S ANATOMY, 40th Ed: Synergistic with Deltoid

“The conventional view is that supraspinatus initiates abduction of the shoulder and assists deltoid in abduction thereafter. However, there is evidence that both supraspinatus and deltoid are involved throughout the range of abduction, including initiation of the movement. As part of the rotator cuff, supraspinatus helps to stabilize the head of the humerus in the glenoid fossa during movements of the glenohumeral joint. With the arm dependent, even when moderately loaded, supraspinatus and tension in the upper capsule prevent downward displacement of the humerus.”

TAMBAY TRANSLATION:

  1. Una sa lahat, walang  akong page number na binigay sa taas kasi Vishal ang version ng Gray’s Anatomy na meron ako. Baka may pdf version kayo. Pahingi naman. Pretty please?
  2. Anyway, ang sabi ng kapitbahay na si Gray ay synergistic daw si supra with the deltoid. Ibig sabihin, si supraspinatus ay hindi nag-tatrabahong mag-isa bilang isang shoulder abduction initiator. Nag-tutulungan silang parehas ni deltoid throughout the entire motion. Gets pa ba?

 LIPPERT, 5th Ed: Synergistic with Deltoid (pg. 138)

uyuu.png
Read the supraspinatus part at the leftmost.

TAMBAY TRANSLATION:

  1. Supraspinatus is active throughout abduction. Still synergistic with deltoid. Self-explanatory.
  2. Kung PT ka at reviewee ka dati sa Gold Rank, eto po ‘yung reference na ginamit ni sir ken– yung nagturo mismo ng UE ortho conditions. He didn’t mention it though, but I figured since he used the exact same words.

GOOGLE SCHOLAR: Synergistic with Deltoid

hahaha.png

It is imperative to note however, that the research above had been published way back 1986, meaning it could be obsolete nowadays.Why did I use such old research, you ask? This is because I failed to find anything that’s more recent than this. The rest are dated 1940s.

With that said, let us now recap everything that’s been written here so far:

  1. Brunnstrom, 6th Ed: Initiator (pg. 197)
  2. Norkin, 5th Ed: Initiator (pg. 263)
  3. Snell, 9th Ed: Initiator (pg. 360)
  4. Gray’s Anatomy, 40th Ed: Synergistic with Deltoid
  5. Lippert, 5th Ed: Synergistic with Deltoid (pg. 138)
  6. Google Scholar, 1986: Synergistic with Deltoid

There you go. That’s a three on three.

Now, who has the monopoly to truth? I will leave you with the analysis.

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Initiator vs. Synergistic: A Clarification On the Role of Supraspinatus Muscle

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