GST question about physics and weird science stuff

[Biren Patel]

Hello, people!

I have no idea where this question belongs, but since it is somewhat unrelated to GST I figure I'll put it here.

So, I was thinking about weighted dislocates, as in the KoF Challenge #2. And I was wondering, why does the weight get harder and harder to lift as your arms get closer and closer? Obviously, as the arms get closer, the weight goes farther away from the body, which makes it harder. But WHY does it become harder to lift the weight, despite the weight being same? (strength and flexbility aside. Obviously, larger weight means harder for the muscle and closer arms means more flexibility requirement.)

I guess what I'm wondering is, does the perceived amount of weight lifted change as the arms get closer? Say you hold a 10 kg barbell at arms length, parallel to the floor, with arms 100 cm apart. Now you move arms to 50 cm apart. How does the perceived weight change? Are their equations to measure this?

This is kind of like the "calculating iron cross" topic coach Sommer posted a while ago here https://www.gymnasticbodies.com/forum/topic/12531-swinging-elements-on-elite-rings/?p=122137 .But, instead of an iron cross, I am talking about an external weight

I hope this makes some sense and someone can illuminate my not-so-sciency mind!

[Joseff Lea]

The moment is slightly greater as the weight is further away but I think it has more to do with the muscles being forced to contract at the extreme end of their ROM.

You could work out a percieved effort for each exercise by first testing a muscle in isolation to work out the maximum it can support whilst under no flexion, under slight flexion and at extreme flexion. Do this for a few muscles to get an average difference between maximum under no flexion  and extreme flexion and then interpolate to get a maximum load vs degree of flexion graph. 
For a given exercise work out how flexed each muscle is calculate as percentage of how much the muscle is capable of under no flexion how much effort the muscle is giving. Convert this number to a decimal, Do this for each muscle, then average. This will give you some sort of perceived effort measure for a given exercise. Do this for a few exercises and compare to how they feel subjectively, remember smaller number means harder exercise so if your experiences roughly match then bingo you have a rough indicator of perceived effort for a given exercise.  

This would be quite a lot of effort to go to and you might have to introduce some sort of weighting by size or length of muscle or something for it to work. 

[Joshua Slocum]

Completely ignoring the effects of kinesthetics, it's a matter of leverage. Gravity is pulling down on your arms and the weight. Your arm is acting like a lever: the further out it extends, the harder it is to resist gravity's pull, because it has better leverage. 

[Alessandro Mainente]

The exercise becomes more difficult due to the muscles activation during the lift, and this is influenced by the leverage as Joshua previously said.

The arms flexion (move the arms over the head) that ends with the dislocate can be divided into 2 pieces:

-first 90° elevation

-from 90° until the head 

the muscles activation is equal under the 90° both on front and back, as the  portion over 90° is equal on the front and on the back, with some little differences (but very little).

In the first 90° the flexion is due to external rotators, then passed the 95-100° the trapezius works for the majority of the movement. you are experimenting different weight sensation (using the same weight) when you reduce the grip simply because the leverage on your traps is increased and your traps are weak. the traps becomes naturally weak as you make the grip close and closer, on the other way it is relatively strong when the grip is higher since the weight component is more horizontal then vertical.

In these situations when you feel the exercises more difficult due to different leverage is correct talk about the "perceived weight/effort" instead of the weight lifted. small and tiny difference in therms of words, big difference in therms of fact!

[Craig Mallett]

besides leverage, as you get closer and closer to your maximum current ROM, the antagonist muscles (those under stretch) begin to contract, counter acting the work done by the muscles that are actually shifting the weight. The closer you get to your maximum ROM, the harder these muscles contract, making it exponentially more difficult as you approach your limit.

[Chris Aldersley]

As before: less leverage/decreased mechanical advantage and more passive resistance (from tight structures and as Craig said, protective mechanism)

 

The trapezius works with other scapulothoracic muscles to laterally rotate the scapula to allow further range for the humerus for abduction and flexion. The closer the grip, the more abducted the humerus is during that overhead range (may be a bit confusing as at rest with arms down in front, the wider grip will be more abducted). So the trap is required to work harder to produce more scapula lateral rotation + stabilisation with a closer grip.

 

Also the length-tension relationship. If a muscle is lengthened (stretched) or shortened (contracted) towards its outer ranges, its harder to produce force. Related to the decreased mechanical advantage.

[Chris Aldersley]

Sorry, I realised I was replying in regards to a dislocate while you gave a different example.

 

For a weight held isometrically at 90 degrees flexion with varying grip width (horizontal flexion), its simply a matter of changing the perpendicular distance from the pivot point (shoulder) to the force component (the bar), that is increasing the moment arm, thus increasing the torque produced by the weight. 

 

Torque = moment arm x force x Sin theta (angle of force...in your example its 90 degrees)

 

Force (the external load) is a constant.

Moment arm changes as you change your grip width (increase or decrease perpendicular distance)

Angle of force will change if you move through range of shoulder flexion

Therefore torque must change as the variables change. 

 

I did this in undergrad a couple years ago, so its a bit rusty. I'll have more of a look and try an example haha. Also in regards to perceived effort, as you change the angle the muscle length changes which comes back to the length tension relationship. While the moment arm is furthest away at 90 degrees for e.g. the elbow, it is also the position of most mechanical advantage, with our muscles in mid range at optimum length.