It's a little difficult for me to give an easy answer. Let's just say my knees would prefer that they're not loaded with much more than 200 lbs if they're going to be given a lot of work. Unfortunately I'm 215 lbs, so I have to do things differently than when I was younger and lighter.

Anyway, I've always carried a lot of water because I have always sweated a lot no matter what kind of shape I've been in, so I rarely have a pack less than 40 lbs. Even on a day hike my pack can approach 30 lbs.

If muscles are all that mattered, I'd probably be comfortable with as much as 60 lbs....I just need to drop to 140 lbs again so my knees will be comfortable too!