How Does Walking Uphill Make Us Tired?

https://outdoorsfaqs.com/how-does-walking-uphill-make-us-tired/
There are several major factors that play a role when you walk uphill and that make us tired:

Extra work against gravity.
Additional muscles in action.
Elevation.
The words extra and additional here mean that we work more when we walk uphill than when we walk on a flat ground, and some less frequently used muscles are engaged. Elevation implies less oxygen in the air so you are more out of breath than when you are on a lower ground.