Any source for that? (study, huge amount of anecdotal evidence, etc.)
GoRuck challenges don't go above +30lbs/13,6Kg.
No source for that number, it was an observation that I made amongst a loosely knit community of mountain hunters. I noticed a trend that many of them would run the weight up on training hikes and invariably end up lowering it due to knee issues and they seemed to settle on 40 pounds more often than not.
To give you an idea of the demands of mountain hunting, one gentleman has been logging his forays on a forum.
Duration - 10 hours
Calories burned - 5316
Steps walked - 38,345
Miles - 14.6
This is just one day. As you can see it is possible to out train a bad diet, if you are stubborn enough.
He did this every weekend for a month, this is just in the spring for shed hunting. He will do this almost daily from September through November with a few days off here and there to return home to clean gear and resupply food. He trains all year round in a gym near his house with weekend jaunts into the mountains for shed hunting and scouting prior to hunting season. His base endurance training is done on a treadmill set on maximum incline with enough pack weight to get his heart rate into the correct zone.
It is difficult to put into words just how steep and rugged the mountains of North Idaho are, here weight is your enemy, body fat, excess gear and even muscle. Most hardcore mountain hunters run under 190 pounds, there is just too much of a penalty for extra weight.
It has been my observation that adding more weight doesn't really work at raising heart rate on flat ground, at least not for me. I can push my HR into MAF zone by walking past 4 mph, but I find this pace annoying to maintain, I would rather jog than walk that fast. In the mountains it is a different matter entirely. The real issue is carrying that weight back down the hill, too much is very hard on your knees. Alpinism suggests using water jugs, dumping the water out before you head back down.
For me and other mountain hunters the whole point of carrying a weighted pack is to condition our bodies to it's presence. Under normal circumstances by the time you have your camping gear, food, water, rifle or bow and other basics you are pushing 40 to 50 pounds, some go lighter, but very few go heavier and stay there.
So as you can see, after just one trip into the mountains, when you get back to camp or home, you look like a badger digging all the stuff out of your pack that you don't really need.
For a mountain hunter, the pain starts once we succeed, take the above load and add 100 pounds of meat and you suddenly have a 140 pound pack on your back and on steep rugged terrain, it's not a walk in the park. Just getting back to a trail feels like a paved highway.
I carried a 160 pound load up the side of a mountain once and swore I would never do it again.
Now if like us you have a reason to go heavier, don't do it very often, certainly not daily and not more than once a week. My ruck weight in around town is in the 25 pound range, no more. As summer wears on I will begin doing a once per week heavy short ruck, one or two miles with up to 100 pounds, but I keep that to a minimum as it's just too hard on the feet, knees and legs. Backpacking and scouting trips are only what I need for the trip, no sand babies.