Fox and I are about to ascend up from 'Testosterone Gap'. To non-hikers here is something counterintuitive at first blush: we prefer the climbs much more than we like the descents.
Going up is putting one foot in front of the next, pushing up body and load, maybe with some help from the hiking stick. Going DOWN is torture. Every step is phenomenal biomechanical insult: all of the force of the body as well as the pack presses on the joints.
The knee works as a hinge, with quads and hamstrings managing the flex through a couple of tendons. Toss in some discs of cartilage and a few ligaments evolved to limit bone-on-bone bash and grind. All that momentum of going down has to be absorbed by the cartilage, with control of the flex by the muscles and their tendons attached to the bones, with the ligaments guiding the proper range of motion. The repetition of going down fatigues the muscle as well as stretching or smashing these tissues. Every step, the alternate body part puts up with it -- arresting the pull of gravity to keep hiker and pack from tumbling forward.
That's just the knees. We could touch on the hips, the ankles, metatarsels and toes. But you get the idea. Going up, in relation, is a breeze: it's just lungs, legs and sweat.