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What keeps our arms from falling down?


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I'm reading this anatomy book for artists, and there's a question that made me curious which probably a book for artists will not answer. I'm working on the skeleton now, and have just learned about the spine, and we're moving on to the upper limbs. Now, as I understand, the only bones connecting the arm bones (the humerus) to the rest of the skeleton, are the collar bones (clavicles), is that correct? I see there are the shoulder blades (scapulae) as well, but these do not seem to be connected to the rest of the skeleton, they are floating. Furthermore, the clavicles aren't really connected directly to the spine, but instead to the sternum, which is in turn connected to the spine via the ribs.

Now, if this was the only thing holding our arms connected to the rest of our body/skeleton, then it surely looks like a flimsy construction! Also, when looking at closeup drawings of it, it looks like the clavicles aren't even "trying" to keep the humerus from falling down. The clavicles just seem to sit on top of the scapulae and humerus without anything to hold them up. But a weight lifter can lift up to 260 kg, that's four times my own weight, maybe three times an average male's weight. So clearly the construction can withstand a lot of downwards force.

So, I haven't gotten to the muscles yet. Is it the muscles on top of our shoulders (I don't even know what they're called) that holds the whole thing up, even when we're not flexing them? How does this construction really work?

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Basically, yes. Tendons and muscles. The bones are just the structures underlying it all. They’re like pieces of wood coming together at common points. The muscles, tendons, and ligaments then act like giant rubber bands pulling this direction and that, stretching across those bones, and often opposing each other between resting and flexing states. 

Also, as you mention... it all really is pretty flimsy. This is why so very many people experience shoulder and knee trouble as they age, and hips too... or why were so easy broken after accidents and impacts.

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Even as a pre-teen I wondered these questions about the six million dollar man. Here he can lift a truck with this bionic arm, but no explanation as to how that force gets transferred through his torso to the bionic legs below.  He'd need an I-beam from shoulder to hips to do that, and said beam would prevent any ability to move like a normal human.

The arm wrestling thing (6 vs. 7 million dollar man) had similar issues.  From where comes the reaction force to this theoretical insane pressure/torque between the two guys? I was a kid and couldn't handle the blatant disregard for physics suggested.

Point is, you can make the arm as fictionally strong as you want and the flesh shoulder connection is still going to limit what you can do with it.

A pro weight lifter can exert incredible stress on those shoulders, and his bones are not appreciably stronger than mine, but the muscles and tendons and such are, so the answer lies there, as described by the prior posters.

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5 hours ago, Bautastein said:

I'm reading this anatomy book for artists,

I don't know your purpose in asking this question however a small, but important point is worth making espcially if you are an arts person.

A force is a push or a pull.

In the body the muscles can only pull. There is no mechanism for directly exerting a push.

To obtain a push Nature causes the pulling muscle to pull on a stiff object ie a bone. This causes the other end of the bone to push against something.

Muscles always work in opposing pairs and the balance between these two muscles determines which way the pull occurs.
Often one half of the pair is (much) weaker than the other so for isntance a crocodile has very powerful closing muscles onhis jaw but much weaker opening ones.

In relation to your question, arms hang down so they are pulling on the rest of the skeleton.

So only body parts (ie muscles etc) that are involved in pulling are needed to stop them falling down.
That is why there no direct connection to other bones is needed.

1 hour ago, dimreepr said:

What keeps our knees from falling up???

In contrast our knees are held down by the weight of the upper body pushing down on the pelvis/thigh and eventually knee bones.

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  • 2 months later...
On 2/6/2021 at 8:38 PM, Bautastein said:

I'm reading this anatomy book for artists, and there's a question that made me curious which probably a book for artists will not answer. I'm working on the skeleton now, and have just learned about the spine, and we're moving on to the upper limbs. Now, as I understand, the only bones connecting the arm bones (the humerus) to the rest of the skeleton, are the collar bones (clavicles), is that correct? I see there are the shoulder blades (scapulae) as well, but these do not seem to be connected to the rest of the skeleton, they are floating. Furthermore, the clavicles aren't really connected directly to the spine, but instead to the sternum, which is in turn connected to the spine via the ribs.

Now, if this was the only thing holding our arms connected to the rest of our body/skeleton, then it surely looks like a flimsy construction! Also, when looking at closeup drawings of it, it looks like the clavicles aren't even "trying" to keep the humerus from falling down. The clavicles just seem to sit on top of the scapulae and humerus without anything to hold them up. But a weight lifter can lift up to 260 kg, that's four times my own weight, maybe three times an average male's weight. So clearly the construction can withstand a lot of downwards force.

So, I haven't gotten to the muscles yet. Is it the muscles on top of our shoulders (I don't even know what they're called) that holds the whole thing up, even when we're not flexing them? How does this construction really work?

Heyyyy! Yes you are correct with the clavicle and the scapula. What holds our upper extremities together are muscles, tendons and ligaments. There are numerous ligaments and muscles that you will encounter when you proceed with your study. You will encounter Sternoclavicular Ligaments, Interclavicular ligaments, Acromioclavicular Ligaments and many many more!. Upper extremity is a fun topic to study, especially if you are into challenging memorization and analyzation. oh, you will encounter Kinesiologies as well so, keep it up! 

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