Drop Some Knowledge #8: Postural and Mechanical Loading on Bones/Ligaments

If you did not know, I am a kinesiology major (exercise and movement science… pretty much the best major ever). Currently, I am nearing the end of five weeks of a summer school class, anatomical kinesiology! Love the class and I am learning so much about the human body and what it takes to raise your hand, walk up the stairs, or sit in a chair (like every single muscle and what kind of contractions at each segment).

I thought it would be important to drop some knowledge on the subject of one’s posture, as well as the kind of loading the bones take during everyday activities (and why it is important to keep bones healthy).

First, let’s start with the different characteristics between muscles and ligaments:

Muscles have the ability to contract, be stretched, and recoil (they can also get excited by electrical charges and conduct them… but those two characteristics are not that relevant here). That means the muscle fibers can shorten (like in the up phase of a biceps curl), lengthen (like in the wind up phase before throwing a ball), and recoil (come back to a neutral position after it is stretched). 

Ligaments are fibrous bands of tough connective tissue that serve to hold bones together and add stability to the joints (attaching bone to bone with ligaments allows for these articulations). Since ligaments are for stability, they are not capable of stretching very much. In fact, once they go passed a certain amount of stress (or stretch), they will be strained through tearing and cannot go back to the normal length. 

Okay, so here are two pictures of me… first one with good(ish) posture and the second one with poor/”fatigued” posture.
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Okay, I was actually trying to exaggerate my fatigued posture by jutting out my hips and letting my shoulders fall forward with a curved upper back. It didn’t really work, but you can see that my head is jutting out in the second one yeah?

Proper loading on the spine means that your muscles are very slightly engaged in keeping you upright (muscles should not be working TOO hard unless you’ve been bedridden). If you completely let go and don’t let your muscles do anything, it will resemble that second picture where your ligaments are now trying to hold an upright posture. 

Remember how ligaments that are stretched and stressed too much can have tears and will NOT go back to the normal length? Well let’s continue on to see the other times we may be stressing the ligaments.

I assume you are sitting at a desk and reading this from a computer, yes?

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Kind of doubt that you are sitting as upright as the first picture, but let’s hope you are not slouched down with “computer neck” like the second picture. 

Here’s the breakdown of compression force on the lumbar spine in different positions:

Laying down: less than 30%.

Standing up (good posture): 100%

Sitting down (good posture): 140%

Standing up (fatigued posture): 160%

Sitting down (fatigued posture): 200%  <— HOLY CRAP.

So I suggest for all of you to sit up straight right now! If you are able to have a standing desk, please get one, as long as you promise to stand with good posture, too. 

The compression on the lumbar spine is twice as much when you are sitting with POOR posture than when you are standing with GOOD posture. Not only that, you are letting your core disengage, meaning your poor ligaments are going to get strained and forever be in a stretched out position (which will most likely get worse with age, leading to a hunched back position). 

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You’ve all heard about how we need to pick things up off of the ground with good posture, yes?

Here’s exactly why:

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^When you flex the trunk (bend forward), the erector spinae muscles that run down your spine (posteriorly) are engaged and working eccentrically. 

Researchers still don’t know why, but at a certain point when you continue in trunk flexion, the erector spinae muscles completely shut off:

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So what is it that is holding you up from that point? Your ligaments!… and that’s a bad thing. 

Imagine how many times we pick up something off of the floor with poor posture and add all of that stress to the ligaments in our spine. Add that up over the weeks, months, and years. That’s a LOT of stress to the ligaments, and it can lead to permanently stretched ligaments (btw, ligaments do not get as rich a blood supply as muscles, so if a strain does occur, it can take a pretty long time to heal). 

So when you are going to pick something up, be sure to bend your legs and don’t let the ligaments in your back do all of the work.

[Sidebar: when doing deadlifts, it is REALLY important that your break at the hips in order to get the weight up. If it becomes purely a back workout, you are doing it wrong and your are going to break].

That’s all my talk about posture and loading on the spine. Stand up/sit up straight! I know some people my age who can no longer stand straight because of their permanently stretched ligaments from poor standing and sitting posture. Not cute!

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Did you know… that when you are walking, there is a portion of the time when one leg is supporting all of the weight?

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Here’s the interesting part. You can see in the above picture that my left leg is in the “stance” phase and supporting the weight as my right leg gets reading to go into the swing phase. Well the left leg is actually going to take on 2-3 times more body weight at that moment in time! Then when my right leg steps and my left leg swings, the right leg takes on 2-3 times my body weight.

Guess what happens when you run?

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It turns into 5-10 times your body weight on the leg that is on the floor! If you are jogging, it is around 5 times… if you are sprinting (or overweight and jogging), it is more like 10 times your body weight. 

Your bones, when healthy, are able to deal with 12-15 times your own body weight in compressive forces and such (generally from top to bottom loading, not side to side. Most of our bones do not like to encounter forces that come from the sides). 

When you have osteoporosis, your bones can only handle 8 times your own body weight. 

Guess how much the impact of falling is? [Picture is supposed to be of me falling, not doing some kind of single leg exercise, haha]:

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The impact from a fall is 8 times your body weight. So now you see how many people who have osteoporosis break a bone whenever they fall down? Work on your bone health, friends!
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I am currently building my bone strength by building up my muscles. I have been consistently doing leg days and upper body days, working up slowly to lifting heavy. 
 
Leg days always consist of 5 x 8 squats and deadlifts, with lunges or stuff with the cables if I have time. Upper body days are pushups, pullups (lots of work on the negatives), and shoulder work. I want to gain a good amount of muscle and be able to lift pretty heavy by the time winter comes around (again, I am building up SLOWLY). 
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Here are my other “Drop Some Knowledge” posts, if you are interested:
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Got my triathlon this Sunday! Excited and nervous. It’s totally just for fun because I know I didn’t train as much as I should have 🙂
 
What do you have planned for the weekend?
 
Bone health… is it something you think about often?
 
Do you use a standing desk at work? [I do not… I sit quite a bit while I drive and when I am in class. I need to be really conscious of my posture to make sure weirdness isn’t happening].

3 responses to “Drop Some Knowledge #8: Postural and Mechanical Loading on Bones/Ligaments

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