What exactly is happening when something is lighter than anticipated?

[happyskunky 0]

 

I wasn't sure where to post this question  and I wasn't sure the best " Title "

 

You know when you go to pick up a bag that you think is full/heavy and it's empty and there is that weird sensation.  What exactly is happening in that instant?   I went to pick up a bag and thought it was my fast food and it was empty and that little " jolt " feeling happened.  I find this very interesting. It's like in an instant your body prepares for the item your going to pick up.. 

 

I'd love to know more about what is happening 

[iNow 5344]

Mental prediction based on past experience. It’s a type of forecasting or anticipation.

You look at the clues, see the bag, it’s size, and compare that against past experiences with similarly sized objects or bags that exist in your mental model. You then have a guess or an estimate of the effort required to move the item and apply the commensurate force. When the bag then is unexpectedly empty, the resistance on your muscles is far lower than you anticipated, the applied force is too high, and consequently it jolts. 

See also: The way you stumble when going down stairs, think there’s another step even though you’ve already reached the bottom, and then step too hard into the floor which is far closer than anticipated and you practically roll your ankle. 

[Strange 3852]

I guess there is an effect from the brain having to start doing things in advance because of the delays in transmitting to and from the muscles. So the brain estimates what has to be done (in terms of force applied) based on what the weight appears to be. Then there is rapid feedback to the muscles that the force is more than needed and a resulting change to the stimulus. It is possible (I don't know enough about the nervous system) that the immediate feedback and change of stimulus doesn't go as far as the brain (that would probably be too slow and cause you to throw the object into the air, before being able to make the necessary adjustments).

[iNow 5344]

1 hour ago, Strange said:

Then there is rapid feedback to the muscles that the force is more than needed and a resulting change to the stimulus. It is possible (I don't know enough about the nervous system) that the immediate feedback and change of stimulus doesn't go as far as the brain (that would probably be too slow and cause you to throw the object into the air, before being able to make the necessary adjustments)

It does go to the brain. Our processing speeds aren't instant, but they are quick enough to account for this... on the order of a few hundred milliseconds. 

[mistermack 83]

10 hours ago, happyskunky said:

I find this very interesting. It's like in an instant your body prepares for the item your going to pick up.. 

It's worth remembering that our ancestors spent a lot of time in trees, and living like that, it's vital to be ready for the demands placed on your body when using branches for support. One slip could mean your eventual death. So we evolved to anticipate without thinking what the next demand will be. How much give will be in that branch. Will it bend to breaking point. Will my hands fit round it, or am I likely to slip if it's mossy or wet? 

We do similar things today at a much reduced level, without thinking, and it still pays off by making life easier. 

[CharonY 2056]

On 11/19/2019 at 6:16 AM, iNow said:

Mental prediction based on past experience. It’s a type of forecasting or anticipation.

You look at the clues, see the bag, it’s size, and compare that against past experiences with similarly sized objects or bags that exist in your mental model. You then have a guess or an estimate of the effort required to move the item and apply the commensurate force. When the bag then is unexpectedly empty, the resistance on your muscles is far lower than you anticipated, the applied force is too high, and consequently it jolts. 

See also: The way you stumble when going down stairs, think there’s another step even though you’ve already reached the bottom, and then step too hard into the floor which is far closer than anticipated and you practically roll your ankle. 

 

On 11/19/2019 at 7:46 AM, Strange said:

I guess there is an effect from the brain having to start doing things in advance because of the delays in transmitting to and from the muscles. So the brain estimates what has to be done (in terms of force applied) based on what the weight appears to be. Then there is rapid feedback to the muscles that the force is more than needed and a resulting change to the stimulus. It is possible (I don't know enough about the nervous system) that the immediate feedback and change of stimulus doesn't go as far as the brain (that would probably be too slow and cause you to throw the object into the air, before being able to make the necessary adjustments).

What is described is somewhat related to an interesting phenomenon called the size-weight illusion. It is a multisensory phenomenon in which, roughly speaking larger objects feel heavier, even if they are not. The interesting bit is that these studies look at the interaction between feedforward (anticipation of weight) and feedback information. In the case of OP it is fairly clear, the discrepancy between anticipated weight and experienced one is large, and as such the feedback mechanism cannot cope with the excess forces (the jolt may be a surprise reaction due to the perceived discrepancy). However, in cases where, for example two objects of same weight but different size are presented, the muscle stiffness may be different before lifting the object (as the feedforward mechanisms anticipates that the larger object is heavier) but then the feedback kicks in and adjusts closer to the actual weight. The perception, however, is still that the larger object is actually heavier.

 

It should also be noted that anticipation is common to basically all animals with an CNS, not only to arboreal ones, as relying on feedback mechanisms is typically too slow for most dynamic situations. For example, catching a ball requires anticipation of its flight path. Some simpler animals have faster feedback loops in which locally for example legs convey information regarding the status to other legs, without need for central coordination. But even then in hunting spiders one has found evidence that they may anticipate to some degree anticipate prey behaviour. 

 

 

[StringJunky 2099]

On 11/19/2019 at 12:16 PM, iNow said:

Mental prediction based on past experience. It’s a type of forecasting or anticipation.

You look at the clues, see the bag, it’s size, and compare that against past experiences with similarly sized objects or bags that exist in your mental model. You then have a guess or an estimate of the effort required to move the item and apply the commensurate force. When the bag then is unexpectedly empty, the resistance on your muscles is far lower than you anticipated, the applied force is too high, and consequently it jolts. 

See also: The way you stumble when going down stairs, think there’s another step even though you’ve already reached the bottom, and then step too hard into the floor which is far closer than anticipated and you practically roll your ankle. 

Apparently, you can also stumble on stairs that have a step that is different to the others in height.

[John Cuthber 3646]

2 hours ago, StringJunky said:

Apparently, you can also stumble on stairs that have a step that is different to the others in height.

Unless you expect it.

Castles were built with staircases with an "odd" stepp in them to trip enemies who were climbing the stairs.

I learned about this interesting fact in a talk about the safety of modern stairs where , as they pointed out, badly made stairs are still killing the unwary.

Here's a trivia question for you ( the answer is at the end of the post)

Roughly how many people are killed by falling on stairs each day in the UK?

 

Interesting fact for the day.

If you have a staircase where the steps are uneven you can always make it even by adding planks (of various thicknesses) to the steps.

This is an important result because adding to the treads is easy but taking stuff away is hard.
It's also (I think) always possible to make all the steps the same "length" as well.
So there really is no excuse for badly built stairways.

(About two, btw- did you guess right?)

On 11/19/2019 at 3:21 PM, iNow said:

It does go to the brain. Our processing speeds aren't instant, but they are quick enough to account for this... on the order of a few hundred milliseconds. 

Our brains are pretty good, but we often rely on reflexes which don't use the brain. 
I'm not sure which category this phenomenon falls into .

I'd be interested in finding out.

I'd be even more interested in knowing how they found out.

[CharonY 2056]

1 hour ago, John Cuthber said:

Our brains are pretty good, but we often rely on reflexes which don't use the brain. 
I'm not sure which category this phenomenon falls into .

Do you mean the tripping part? There is some work on walking in animals and I vaguely recall some research into walking pattern generation and how different animals behave when perturbed. For example, in walking sticks you can even pull off legs  and they recover after a part of a cycle, as there is a neuronal network that coordinates the different legs on a very basal level. In humans on the other hand bipedal walking has requires central control  to coordinate muscle activity and much of the pattern generation (responsibly for rhythmic movement) is located at the spinal level. In these cases anticipation most likely plays a minor role as once the patterns are generated, the movement sets in a somewhat predetermined way. Even if you visually see a step difference, if you are not in time to alter the pattern consciously, your leg will follow the generated pattern.

[Strange 3852]

21 minutes ago, CharonY said:

Even if you visually see a step difference, if you are not in time to alter the pattern consciously, your leg will follow the generated pattern.

Walking up a stationary escalator can be tricky even though you can see the steps are uneven and not moving.