You should be able to carry up to 15 % of your body weight for a considerable amount of time in a proper back pack. Unless you weigh less than 100 pounds, the weight you're carrying should be fine. Your problem is most likely due to your posture. It's all about leverage.

Imagine you pick up a gallon of milk. You can hold it close to your chest for a while, but if you hold your arm straight out in front of you, your muscles have to work MUCH harder. The same is true with your spine.

When you walk with a forward leaning posture, your head is carried anterior to your spine. Research has determined that for every inch of forward (anterior) head carriage, the impact on the musculature of the neck and upper back is akin to adding 10 pounds to the weight of your head. So the paraspinal muscles of your neck and back are working overtime. And you're asking your spinal muscles to do this over the course of a 3 mile commute while you're ALSO carrying a back pack. When your postural muscles can't do it anymore, the larger muscle groups try and help. The problem is that larger muscle groups aren't meant for posture. It's like asking a power lifter to run a marathon. This is a recipe for disaster. They get tight and you experience pain. I see it all the time with my patients.

People with underlying joint dysfunction in the neck and upper back are more apt to show anterior head carriage. You should consult with a good doctor of chiropractic (or another bodywork practitioner if that's your flavor but you have to be really careful with the neck, especially) to get down to the root cause of your abnormal biomechanics. Or, if funds or availability isn't there, you should consider a wheeled bag. I know, they aren't nearly as fashionable, but then again, neither is the Shaggy from Scooby Doo posture! Best of luck!

I think the biggest question is what sort of backpack are you using now? If it's a Jansport(ish) pack that your average high school kid uses to carry books, then that's your problem.

I managed a backpacking gear store for about 10 years, so I speak of this from experience. The best way to carry weight is to have the heaviest weight as close to your lumbar spine / sacrum as possible, with that weight secured tightly around the waist (to bring it as close to the body as possible.) I would recommend using a pack that might be referred to as a "technical day pack". This sort of pack looks like a small version of a full-sized backpacking pack. It has a large amount of storage with multiple ways to secure and compress gear. They have a back suspension and a full-function waist belt support system.

Some examples: Gregory Z55 (Gregory is one of my all time faves)
http://www.ems.com/product/index.jsp?productId=3800499&cp=3677338.11354705.11354709.4058670
Women's specific Osprey Aura 35
http://www.ems.com/product/index.jsp?productId=3662111&cp=3677338.11354705.11354709.4058670
Gregory Deva (Women's) 70
http://www.rei.com/product/809325/gregory-deva-70-pack-womens

The number after the model refers to volume of the pack in liters.

A larger pack may look a little silly for "hiking" to work, but we're talking about function over form here. If what you're doing now doesn't work, then there are other options. If they work and let you walk to work pain free, isn't that worth a couple of odd looks on the way? (As Paleo-ers, we are quite used to this, I'm thinking...)

As for packing your stuff, the heaviest stuff in your pack should be at the very bottom, closest to your back. Heavier bottom, lighter top. Heavier back, lighter towards outside, all the way to the top. Super light stuff at the top. Waist belt cinched as tightly as reasonably possible - you want to carry a HUGE % of the weight of your pack on your waist, not your shoulders. Shoulder straps snug, but NOT tight. Whole pack cinched in toward your back, and compression straps used to keep the load in your pack from shifting around at all.

If you have any other specific questions about packs, don't hesitate to ask, or even shoot me an email.

Babywearing is an ancient practice. Babies can get heavy quite quickly! After the newborn phase, most babies are carried on the back. I've also seen HG (on TV ;-) who carry firewood etc. On their back - it's usually high up and I think that this mat be the difference. Where the pressure is distributed..

I wonder if a backpack which distributed the weight across the top of the back (somehow) would be more beneficial?

I'm on my phone via twitter, so can't link, but Katy Bowman (Aligned & Well) is a biomechanic who blogs about our alignment from a paleo(ish) viewpoint. I wonder if she has a blog post that relates? I will have a look when I can.

I know this is never done in our culture, but balancing & carrying water containers on ones head is a very popular method in a large part of Africa.

Sorry, don't know about the mechanisms though.

I think this is a matter of keeping your back straight. I live in Africa where babies are carried on the back (usually mid to low back) and lots of stuff -- the most heavy stuff -- is carried on the head. I've also lived in the Solomon Islands (and some places in Africa) where they carry stuff with a strap of the forehead and the heavy stuff behind them on the back. The only common denominator I can see is that they all have beautiful posture.

Have you tried adjusting the straps to vary where the center of gravity of the backpack's weight falls? I've found, with numerous back problems, and we're ALL different in that realm, that a a very small change in height or weight, or even the shoes I wear can have a very substantial impact in how I feel at the end of the walk, end of the day, or in the morning when I wake up (big time fibromyalgia sufferer here, so I really notice these things and pay heed).

Here is part of what I'm getting at: I have two disks that have no mobility between them, and they weren't fused on purpose in a surgery, and I don't know when they naturally became fused sometime between my teens and 20s, I'm guessing (maybe earlier). So, that is an issue that causes me problems. I've also had lower back issues over the years, and other upper back issues.

My point is that there probably isn't one "correct" answer that will be a cure-all to back pain or strain.

I can also say with a lot of certainty that some of the paleo shoe-barefoot recommendations don't apply to me - without a slight heel, I get sharp pains thru my arches, so consider footwear that helps you thrive, not whatever someone else tells you is good or bad.

And yes, chest straps could also help, but you could easily fashion such straps with some simple stitches and straps either bought or gleaned off something else. If that is what your gut is telling you, that might be a good solution, or you need to listen to your body more closely and figure out what fix is right for YOU.

There are no ready-made answers that are exactly right for YOUR body and how it's built and how it's changed based on what it has already been through.

The Alexander Technique shows how to move properly and inhibit bad practices which otherwise lead to degeneration of the body, spine and joints in particular.

The most ergonomic bag I have found for daily (i.e. not backpacking into the woods) is the hamptons hybrid pack from bbp bags: http://www.bbpbags.com/hamptons.html

It hags on your lower back with chest and waist straps - kind of like a fancy, sort of hip-looking fanny pack. Both I (5'3") and my husband (6'4") are able to adjust it to suit our bodies. I am able to pack it pretty heavy and carry it around with almost no discomfort. Also convenient is that it can shift from backpack to shoulder bag in no time for easy access to stuff. Plus it's super sturdy and well-crafted.

Lift with your legs!

My lower back tension has disappeared as a result of keeping the knee bent and my posture straight, whenever I lift heavy objects.