Assuming equivalent fitness, what is a fair way to split up weight when hiking as a couple/in mixed gender groups? Should the weight be proportional to the individuals weight? Are there other considerations?
Someone who has overweight isn't normally able to carry more, so weight isn't as important. The height would be more adequate... Muscle strength isn't much important when you go on long hikes... Strength doesn't translate directly to endurance, often it's the opposite - people with smaller muscles are more endure and are actually able to carry more on greater distances.
The most practical solution is not to care about fairness, but of outcome for the group. If someone is doing poorly that day, stays behind etc., take some things from them and give it to the best performing members of the group. As the result, the fastest will go a bit slower, the slowest will go a bit faster and the group as the whole will perform much better.
When I go hiking with my wife or a mixed group, we don't use any fixed figures of x%. Instead we all know roughly what weight we are happy carrying, and if someone feels they could carry more they will offer to help out someone who appears to be struggling or overloaded.
In general, if you are experienced hikers, you will have your pack size/type pretty well nailed for most occasions so shouldn't need to do this, but you may have an inexperienced member of the team, or someone may be a bit off their usual strength through illness etc.
My advice - just work it out between you.
Dividing loads between your trek-mates is a good idea. A better one would be to pack light. In case you want to collectively gain by individually reducing the weight, then my suggestion would be to go by the comfort level of the individuals involved. A strong person might be able to haul more load. But the same one might not be able to perform the same way the next day. The division of labor changes dynamically during treks.
For example: I am used to carrying heavier backpacks during my treks. I usually carry over 15Kg over treks spanning 7 days or more in the Himalayan regions. Whereas, during a weekend trek, with a backpack of just over 3Kg, I whined like a baby (my bad).
Hence, when deciding on dividing your backpacks, think about the current day and the stronger person(s) on the given day. DO NOT go by weight or gender. That never works out.
I face this problem not for hiking, but for portaging. That is a slightly different situation, but not entirely different. We often went as two families: 4 adults and as many as 5 children. Not only could some children not carry anything, some of them had to be carried. Our approach:
- separate kitchen packs from tent packs. Kitchen packs have food and cooking equipment and never go in the tents. Tent packs have tents, bedding, clothes etc. Typically the kitchen pack is much heavier than the tent pack.
- each family was responsible for their own tent pack, but we split supplies across the kitchen packs
There is a LOT of stuff you only need one of. Pots and pans, stoves, firewood saw, first aid kit, repair kit, water treatment, binoculars, ... we either worked out in advance who would bring something, or moved things between packs before setting off. (And often, the family with less children had some shared stuff in the tent pack, too, like books for bedtime stories.) We also balanced out the food as it got eaten. For ease of finding things, one bag held snack foods (jerky, nuts) and an entirely different bag held meal ingredients. Knowing where to find things when 4 or more bags might hold it sounds like a nightmare to me.
As the children got larger we had bags for them to carry, but could always clip those things to an adult pack if we needed to. In the same vein, a mixed-strength group should pack in a way that makes it easy to take something (the [whatever] kit, the rope bag) out of one person's pack and put it on someone else's.
Remember, too, that you may start a trip as a matched group, then a minor injury or illness may afflict one of you, and splitting up the load may be the key to continuing.
Different people have vastly different styles. Some people carry a lot more weight, some a lot less. If you're interested in cutting your pack weight or moving toward a more lightweight or ultralight style, a good web site to check out is http://backpackinglight.com .
I think many people do use percentage of body weight as a criterion, e.g., I recently hiked with a very small woman (who, BTW, kicked my ass hiking), who had a certain strict figure -- she never carries more than x% of her body weight. (This was just her rule for herself. The three of us on that trip all brought our own gear and didn't re-divvy-up anything.) If you want a figure for comparison, I practice a pretty lightweight/ultralight style for summer in the Sierra, and my normal setup has a base weight (i.e., not counting food or water) of about 9% of my body weight.
I try to make my wife as comfortable as possible so she will continue to go with me on these backpack trips. Our last weekend backpack trip was in the Big South Fork in Tennessee in January. The low temperatures were 22 degrees. We had a total of 39 pounds. My pack weighed 22 pounds 56% and Alice's pack weighed 17 pounds 44%. She stated at 17 pounds it felt like her pack was empty. With this weekend's low temperatures expected to reach 9 degrees our gear will probably weigh about 50 pounds. I can see my pack weighing 27-28 pounds and my wife's about 22-23 pounds.