Most of us that hike and enjoy photography should probably access what we carry. About 5 years ago I did some hiking in the Swiss Alps and went to a store to rent/buy some basic hiking equipment beyond my photography needs. On the wall of the store was something published by a Swiss organization that stated that when a backpack exceeded about 15% of body weight, then it was highly probable that balance was affected and the risks of falls and back injuries went up considerably. 15% translates to about 30# for a 200# person; about 22# for someone at about 150#. The recommendation was to not exceed 10% of body weight if hiking in rough terrain or for extended distances.
Interesting. Percentages don't really make a lot of sense in all cases. We all need to carry safety gear etc and because I'm small and fairly light it's a huge percentage of my weight >15% when I have my minimum camera gear. However, if a man weighing 200lbs was carrying my pack it would be less than 10% of his weight. No wonder I struggle up the hills! BTW I tend to hike on very rough terrain and generally extended distances but it's the amount of ascent that really determines how hard a hike it is!