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  • Writer's pictureA L

The 3mm causing your shoulder pain

The Scenario: Ongoing shoulder pain. I’m mindful of my posture, I roll out my pecs, I keep my rotator cuff good and strong. Why does it still hurt?

Alignment. It’s a tricky word that I have a love-hate relationship with. The phrase “out-of-alignment” often conjures up a really dramatic image to me – like a picture hanging on your wall that’s noticeably crooked, or a wobbly restaurant table with one leg shorter than the rest – the idea of being out of alignment often feels a little exaggerated.

That is until it applies to the human body. Alignment issues in our structure doesn’t have to be that big. Our upper thoracic spine vertebrae only flex 2 – 6°. Our SI joint has roughly 1 – 4° of rotation. Your skulls ability to rotate? 5 – 7°. These are inherently very small movements – you remove 3° from your thoracic spines 6°, or 3° from your SIs 4°, and these small changes can have massive consequences.

Let’s explore this concept as it applies to your rotator cuff.

Your rotator cuff is essentially a “skirt” of blended muscles that hold the head of your arm within your socket. Below – it’s a messy picture, but we’re looking at your shoulder socket with the humerus – aka your arm – removed and out of the way. The 4 muscles of your rotator cuff – the supraspinatus, infraspinatus, teres minor and subscapularis – make a near continuous circle around the socket. When all 4 work well, the head of your arm stays in a nice, centred, and secure place.

It’s a beautiful design really. But not one without some flaws. Mainly, the subacromial space and your supraspinatus.

The subacromial space is located exactly as the name suggests – sub, or underneath, the acromion. The rotator muscle that occupies this space is your supraspinatus – which occupies the space above (ie. supra) the spine of your shoulder blade (ie. spina).

How much space does the supraspinatus have to operate?

1984, Claes Petersson and Inga Redlund-Johnell from the University of Lund in Malmo, Sweden set out to find out.1 The idea was pretty straightforward: take a whole bunch of people without any history of shoulder pain or injury (175 people to be exact), xray their shoulders, and measure out the subacromial space.

From Petersson et al 1984

On average the width of this space turned out to be about 9.7mm in men, and 9.2mm in women.

Ok – so now how much of this space is occupied by the supraspinatus tendon?

In a 2003 study, Jan Bjordal and his colleagues from the University of Bergen, Norway performed a very similar study, except this time instead of measuring the width of the subacromial space, they measured the thickness of the supraspinatus tendon.2 This time it was 40 people with no history of shoulder pain or injuries, and an ultrasound as opposed to an xray was used to look at the tendons thickness. Their findings? On average the supraspinatus tendon was about 6.6mm thick.

A 6.6mm tendon occupying a 9.7mm space. That is not a lot of room for error.

Changing that space by 3mm will essentially put your supraspinatus within a compressed environment, which we in the physio world called impingement. Movement while impinged means greater wear and tear. I tell my guys that an impinged supraspinatus is like a brake caliper that never fully disengages from the brake rotator – it’s only a matter of time before the components wear out.

So what’s reducing your subacromial space? Is it tightness in muscle groups that can draw your humeral head forwards (pecs, teres major, lats, subscapularis)? Is it weakness in muscle groups that are allowing the humeral head to drift forwards (middle/lower traps, serratus anterior)? Is it a stiff thoracic spine pushing your shoulder girdle forwards? Is it you hanging out in poor posture during your day job that’s closing off this space? Or is it the swimming that you do every other day to keep in shape that’s repetitively impinging this space on every stroke? Maybe it’s everything I’ve listed.

Alignment’s a tricky word. When it comes to the body, it doesn’t have to be an overly obvious glaring trait. Is your shoulder position off by 3mm? Sometimes that’s all it takes.


  1. Petersson CJ, Redlund-Johnell I. “The subacromial space in normal shoulder radiographs.” Acta Orthopaedica Scandinavica 1984;55:57-58.

  2. Bjordal JM, Demmink JH, Ljunggren AE. “Tendon thickness and depth from skin for supraspinatus, common wrist and finger extensors, patellar and Achilles tendons. Ultrasonography study of healthy subjects.” Physiotherapy 2003;89(6):375-383.

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