Like, if I pick up something fairly heavy using my legs to pick it up but need it to be at a higher elevation it’s so hard for me to bend my arms up further but I could carry that weight around for a long while even if I’d started with my arms at that higher elevation.
I hope that made sense.
Edit: I have gotten two good answers that make me feel stupid.
In: 1
Isaac Newton – Physics 101
To pick up a weight you have to apply a force to it. That force has to be enough to accelerate the weight (to motion) and counter the force of gravity.
While holding a weight just requires enough force to counter gravity.
So it requires more energy to move something, than to hold it stationary.
Its easier to pick things up assuming at a ground level because you are using your arms, back, and legs to lift an object while as when you lift it higher, your upper extrimities only does the lifting.
Holding weight is done almost by grip strength alone, and your ligaments and skeletal structure bear most of the weight. Actually lifting the weight up in the first place requires not only ligaments and bones, but requires you to engage muscles and tendons to actively change the orientation of the skeletal structure so you can eventually transfer the weight more efficiently to your ligaments and bones.
Think of a deadlift. When you start, your bones are positioned in a way that make them terribly inefficient at holding weight. It takes a lot of effort in your muscles to get the weight up. Once it’s up, your muscles can relax a little because now your spine is straight, shoulders are back, legs are locked back, arms are fully extended, etc. You’ve positioned yourself so that the weight is held on your ligaments and bones and the only tendons and muscles you’re using are for gripping and balance, which is much easier.
A few things are at play here.
First and foremost, your legs are significantly stronger than your arms. Muscle to muscle this is true. This is because the physical activity you do most often is walking, and that utilizes all muscles in the leg. And when you walk, you are moving your entire body weight – usually at least 100 lbs. From your childhood until now, that’s years of training your leg muscles daily.
For instance, your hamstring is the leg equivalent to your bicep. Both are involved in curling motions. Untrained, you can probably curl about 20lbs with your bicep. Untrained, your hamstring can curl two or three times that.
When you pick something up off the ground, your legs are doing the brunt of the work to move it from the ground into the air. Your arms (and shoulders, back, chest, and core) are stabilizing the weight. That is, they are preventing the weight from swaying to the sides and throwing you off balance. But because they aren’t fighting gravity to do so, it takes a lot less force.
Now, if you were to lift that weight further off the ground to, say, above your head, you begin utilizing your weaker arm/shoulder/chest/back muscles. And although it requires the same force that your legs used to get it off the ground, your upper body muscles aren’t as strong, so they need to apply more effort to replicate that force.
There’s also physics in play here.
When you lift a weight off the ground, you are providing force upwards that exceeds force downwards. When you hold a weight above the ground, you are providing a force upwards that equals force downwards.
TL;DR your legs are way stronger than your arms, and it takes more force to lift something upwards than it does to hold it level above the ground.
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