Why do humans swing their arms while walking?

[Yashbhatt]

I have read a number of answers to this question. All have answers like it helps us keep balance etc. . . But there can be a very fundamental reason. Humans evolved from apes. Our ancestors used to walk on on four legs. If we observe a four-legged animal, it walks moving its limbs alternately. So, can we reason that as we evolved from four-legged animals, we have still haven't forgotten moving our limbs alternately?

 

[Simon Bridge]

That would suppose that walking is mostly about reflexive movements of the fore and hind limbs - like a wind-up toy. We can check this by observing animals which normally move on four limbs but occasionally on two.

i.e. - when you see monkeys and apes balance on their hind legs to "walk", do they normally wave their arms as well?
What about performing dogs?

Observation seems to support the idea that arm-movements while walking is something that got picked up after a more upright stance was developed. However, there are no single causes for things in evolution.

 

[Yashbhatt]

Have we found any exact cause for the thing?

 

[Simon Bridge]

Define "cause". Recall - there is no such thing as a single cause in evolutionary systems.
But the simple answer would be "no".
Why would anyone need an exact cause?

 

[Yashbhatt]

Okay. Then let's say need instead of cause. There should be a reason for swinging arms.

 

[Simon Bridge]

In evolution? - it needs only be passed on to offspring and not actually hurt having offspring.
What's wrong with stability and balance?

 

[russ_watters]

I'm not sure there needs to be a reason: when you walk, your body twists from side to side, which makes your arms swing on their own unless you stop them.

 

[Borek]

http://en.wikipedia.org/wiki/Arm_swing_in_human_locomotion

 

[adjacent]

I'm not sure there needs to be a reason: when you walk, your body twists from side to side, which makes your arms swing on their own unless you stop them.

I agree

 

[Borek]

I'm not sure there needs to be a reason: when you walk, your body twists from side to side, which makes your arms swing on their own unless you stop them.

I agree

I am not convinced they will swing in a synchronized way on their own. Try to walk keeping your arms fully relaxed - IMHO they start kind of a chaotic dance (and mostly forearms are moving).

 

[Pythagorean]

On my mobile now, but I'm pretty sure it's just a result of the way the control system evolved in a way that allows limbs to all work together when they have to (not all terrain is flat in the world we evolved in).

So my guess is that it's just a result of one of our driving control systems: central pattern generators, and the lack of perfect isolation in motor cortex. Will look more in depth when I'm at a computer.

edit: ah, my proposition is discussed in Borek's wiki link under "Evolution", considering remnants of quadruped locomotion:

The inter-limb coordination in human locomotion, questioning whether the human gait is based on quadruped locomotion, is another major topic of interest. A recent research indicates that inter-limb coordination during human locomotion is organized in a similar way to that in the cat, promoting the view that the arm swing may be a residual function from quadruped gait.[7] Another work on the control mechanisms of arm movements during walking corroborated the former findings, showing that central pattern generator (CPG) might be involved in cyclic arm swing. However, these findings do not imply vestigiality of arm swing, which appears to be debateful after the 2003 evidences on the function of arm swing in bipedal locomotion.[8]

[7] http://www.ncbi.nlm.nih.gov/pubmed/12183207
[8] http://www.ncbi.nlm.nih.gov/pubmed/12610695

 

[russ_watters]

I am not convinced they will swing in a synchronized way on their own. Try to walk keeping your arms fully relaxed - IMHO they start kind of a chaotic dance (and mostly forearms are moving).

Your wiki says that at least at low speed it is completely passive. Certainly due to geometry there is a natural frequency: Perhaps that's how we find a comfortable walking speed - anything faster requires forced synchronization?

The wiki says "The contribution of active muscle work increases with the walking speed." [above the sync speed]

I would also consider that a separate issue from motion for stability, which almost certainly must be active.

 

[Borek]

Certainly due to geometry there is a natural frequency

This is double pendulum, my understanding is that they don't have a natural frequency. Or perhaps they do, but only for very low energies (which can support the idea of arms swinging in a passive way for low speeds).

Looks like it is quite complicated system.

 

[Simon Bridge]

Anyone here done any military training/service?
One of the things new recruits have to be trained to do is swing their arms in counterpoint to their legs.
Watch people in a crowd and see how complicated the passive motion can be.
Just the idea that there is a single style for humans to walk seem a bit off.
As usual, real life is messy.

Some notes:
Everything on two legs does something extra for balance.
There is an extent to which basic walking motions are built-in - iirc compare Coelacanth swimming motion and land quadrupeds.

 

[Yashbhatt]

Hey Borek, the wikipedia article you mentioned also mentions the point I mentioned under the section "evolution". It says it is possible that we might have picked it up from quadrupeds.

 

[harborsparrow]

The army teaches men who are marching to swing their arms regularly. It makes walking over long distances more efficient. The arms should be swinging opposite to how the feet are going. The opposing movements help the trunk remain stationary relative to the limbs, making the forward motion smoother and using less energy.

 

[Simon Bridge]

@Yashbhatt: careful of the confirmation bias: it mentions a lot of other things too doesn't it?
Evolution results in a web of cause and effect, not a chain.

Nobody has been saying that there is zero built-in factor for the arm-swinging.
You just may want to be less invested in the idea that you appeared to be in post #1.

You have left a number of questions outstanding...
Why would anyone need an exact cause? (for arm swinging in humans)
What's wrong with stability and balance? (as a reason for favoring arm-swinging)

But I think your question has been answered by now.

 

[Yashbhatt]

@Simon Bridge Okay. I agree that there might not be a particular reason. There may be various reasons as the Wikipedia article mentioned.

But there needs to be some stimulus to cause it. If there were no external factors affecting it, then all the random kind of mutations would have survived instead of a few.

 

[Simon Bridge]

But there needs to be some stimulus to cause it. If there were no external factors affecting it, then all the random kind of mutations would have survived instead of a few.

Sure there is a feedback mechanism - but there does not need to be a single outside stimulus.
Anyway - a wide variety of different gaits, for two-legged motion, is exactly what you do see in Nature.
Even within humans, different people walk differently - just sit by a busy street and watch.

People's arms swing for a variety of reasons - they all boil down to getting simians to have an upright stance, so the question changes into "what are the evolutionary pressures favoring an upright stance?" Always remembering that an upright stance is unusual - so we don't expect it to be an optimal solution.

 

[Yashbhatt]

I think an upright stance was needed for hunting. We needed to hold weapons in our hand.

Or was it the other way round? We started walking upright and then we invented weapons because we had our hands free.

 

[Chronos]

With arms swinging in cadence with legs you are bettered prepared to avoid a head injury when you fall.

 

[Simon Bridge]

I think an upright stance was needed for hunting. We needed to hold weapons in our hand.

You mean it was needed for tool use, weapons being among the tools to be used. There are many animals which hunt but which do not have an upright stance... they seem to manage.

But we do not advance personal theories here - do you have references to back up this opinion? ;)

Maybe humans started out as scavengers and gatherers so reaching is useful - then you need to be able to run away with your booty ... see what I mean. You get a choice of possibilities - probably all of them have a part.

Please see:
http://humanorigins.si.edu/human-characteristics/walking
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2819487/
http://www.thenakedscientists.com/forum/index.php?topic=4623.0

 

[Simon Bridge]

With arms swinging in cadence with legs you are bettered prepared to avoid a head injury when you fall.

Interesting - do you have a reference for that?

But that would put a feedback mechanism in place with the big head wouldn't it?

 

[Chronos]

Here is one http://phys.org/news168027773.html. It does not entirely affirm my point, but, provides a reasonable explanation. Brain protection is a side benefit.

 

[OCR]

http://en.wikipedia.org/wiki/Arm_swing_in_human_locomotion

If you continue to the page bottom, to "See also" you can read a bit more information about "walking with two legs"...

http://en.wikipedia.org/wiki/Bipedalism

However, this Cheetah uses four... "structures?"

 

[Yashbhatt]

@Simon No, just casual thinking. I forgot that is against the site rules.

 

[Simon Bridge]

@Simon No, just casual thinking. I forgot that is against the site rules.

Not a problem - we all have opinions. The trick is to support them.
Notice I asked Chronos about a reference and got some form of substantiation back?
So we can at least be assured about where he's coming from there.

I'm not going to always absolutely insist on peer-reviewed references - that would get onerous very fast. Though there are situations where this would be appropriate.

 

[Pythagorean]

You mean it was needed for tool use, weapons being among the tools to be used. There are many animals which hunt but which do not have an upright stance... they seem to manage.

Weren't the immediate ancestors of man mostly vegetarian (+ insects)?

 

[Yashbhatt]

I think the ancestors only ate meat and roots of plants and maybe some fruits.

 

[Pythagorean]

I think the ancestors only ate meat and roots of plants and maybe some fruits.

Early humans ate meat, but I'm pretty sure their ancestors did not:

http://www.nature.com/scitable/knowledge/library/evidence-for-meat-eating-by-early-humans-103874273

I think the transition to a meat diet is often considered part an important step in human evolution.