system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the body of a 400-lb person able to carry around 200 lbs extra day after day, but a healthy 200-lb person who weight trains extensively would be exhausted carrying 200 lbs for an hour?

I know a man who weighs in at a little over 400 pounds.  He is at least 200 lbs overweight and has gained the weight over 4 years. He leads a pretty normal life.

On the other hand I know a guy who has weight-trained almost every day for 4 years. He could pick up a bar with 200 lbs of weight, but an hour of carrying it around would leave him absolutely exhausted.  In the same 4 years he could have gained 200 lbs like the other guy and be carrying the weight around every day.

What is going on in each person's body to cause the difference?

Thanks!
gpt
A 400lb man's weight is centralized to his frame, spread throughout his entire body. There would probably be excess around his mid frame, which causes a lot of overweight men to have back problems.
because of this increase in mass, the overweight person has more blood with which to oxygenate his muscles, making it possible for him to heft his heavier body around.
 

I postulate that a 200lb man that is in peak physical condition could indeed carry 200lbs around with him all day if the weight was closely tied to his frame, such as with a weight vest. Because the weight would not throw off his center of balance, fatigue would be much lower.

Carrying around a 200lb backpack is not about the extra weight, but the rotational forces it places on your spine due to changing your center of gravity so much.