January 30, 2015
Did it draw the evolutionary short straw?
I think we can agree that the evolutionary development of human bipedalism (walking on 2 feet!) has generally been a success, right? There are, however, a couple of structures in the human body that seem rather retro-fitted to our new upright postures. Sacrifices that must have been deemed acceptable in the pursuit of the greater good. Women may argue that the pelvis has drawn the short straw. A narrowing of the pelvis has allowed us to successfully make the transition to two feet; it allows for more efficient movement, is more durable and is less likely to break – but it’s also not as accommodating during labour. Unfortunately, as we have moved to two feet our brains have grown in order to process the vast amount of motor control required for our arms and hands – combine this increase in cranial size with a reduction in the pelvic space through which the head of the foetus passes and you can see the problem.
The shoulder has also made sacrifices. What it has gained in mobility, it has lost in stability. We ask a lot of our shoulder joints; compare their range of mobility to a knee or an elbow, for example. We can, pretty much, put our hand in any position we please. Like the hip, the shoulder is a ball and socket joint. Unlike the hip, however, the ball doesn’t so much sit inside the socket as next to it. The acetabulum (the hip socket), is rather more protective of its ball – it clasps it, like a small hand picking up a tennis ball. Our hips, as a result, are less mobile than our shoulders, but hugely more stable. The ball in our shoulder has shuffled up awkwardly beside the socket but it hasn’t been welcomed in. The ball is connected via ligament and cartilage structures; there is very little mechanical support. This freedom allows the shoulder to travel in any plane of movement with very little mechanical obstruction, which is great news for our care free arms and hands, but it does leave the shoulder susceptible to pain and injury.
Dislocation is perhaps the most obvious consequence of reduced support and stability. But it’s the injuries to surrounding musculature, tendons and ligaments which are much more common place. The rotator cuff muscles, for example, are a hot bed of pain and dysfunction. Difficulty raising your arms over your head or reaching behind your back to put on a jacket? It’s most likely the rotator cuff (and the suprispinatus muscle specifically), that are causing the problem. The muscles working on the shoulder have had to strike up a good relationship to manage the vast workload required, however, subtle variances in posture and lifestyle habits can disrupt this balance and cause certain structures to be overworked and over stretched.
Thus bringing us back full circle to our evolutionary theme and the unstoppable rise of the human desk posture. It seems that just as the shoulder was beginning to cope with us walking on two feet, we’ve gone and thrown it another curve ball by maintaining a hunched computer posture for 8 hours a day. Keeping our shoulders rolled forward for long periods disrupts the balance of supporting musculature and leads to continued micro trauma to muscle tendons. Well that’s just great, I hear you say! It’s easy and clichéd to criticise and blame desk postures for shoulder and upper back pain. I can even see how it’s frustrating for patients, as there’s no real alternative for many people. But being aware of the issue is half the battle, if you take an interest in your work posture and your desk set up then you can make the right adjustments to put the shoulders in the optimum position to maintain proper structure and function.
Friedlander, Nancy & David K. JORDAN, 1995. Obstetric implications of Neanderthal robusticity and bone density. Human Evolution (Florence) 9: 331-342.
Cailliet, R., 1991. Shoulder Pain. 3rd ed. Jaypee Brothers, New Delhi, India
Jackson-Mansfield, P., Neumann, D.A., 2009. Essentials of Kinesiology for the Physical Therapist Assistant. 1st ed. Mosby Elsevier