If your backpack fits properly, you should be carrying roughly 80 percent of the weight on your lower body, 20 percent on the front of your shoulders, and exactly 0 percent on top of your shoulders. After dialing in waistbelt fit, you need to make sure that the shoulder straps fit—and are adjusted—properly. What should you be looking for?
In my last post, I discussed the essentials for proper fit of a backpack's waistbelt—the most essential component for a winning backpack. After correctly positioning the waistbelt (the bony knob of your iliac crest in the middle of the waistbelt), take a look at the top of the shoulder straps. They should attach to the backpack at, or slightly below (an inch or so), the top of your shoulders. If they're higher or lower than this, adjust the straps up or down accordingly. If you can't move the straps far enough, move on to the next size pack up or down, as appropriate.
Once you've got this sorted, it's time to adjust the shoulder straps. Most packs feature two adjustment buckles. You'll find one below the armpit that attaches the padded portion of the shoulder strap to the rest of the pack. The other is just behind your ears and connects a separate strap—the load stabilizer strap—from the top of your backpack to the shoulder straps.
If you tighten the lower buckle, it will bring the shoulder straps tighter across the front of your shoulders. If you tighten the load stabilizer, it will lift the shoulder straps off the top of your shoulders. Achieving a good fit is a game of balance between the two: The shoulder straps should fit snugly across the front of your shoulders, but then lift just off the top of your shoulders.
If adjusted properly, the straps won't even touch the top of your shoulders. You should be able to shrug your shoulders freely and fit about a finger between the straps and the top of your shoulders. The main idea here is that you want zero weight pulling downward on your shoulders—all the downward force should be translated to the much stronger bones and muscles of your lower body. (The weight on the front of the shoulders is caused by the backwards tilt of the pack's weight.)
If you're on comparatively level, smooth terrain, you can use the load stabilizer straps to lift the shoulder straps well off the top of your shoulders. The big drawback to this, however, is that the pack will swing more side-to-side if you're maneuvering around quite a bit more. If you're traveling over rough, uneven terrain where there's significant side-to-side motion, you'll want to cinch the straps up as close to the shoulders as possible, keeping a minimal space between them and your shoulders.
There are a few final components to shoulder strap fit. First is the sternum strap, a short strap that connects the two shoulder straps together across the front of your chest. Most packs provide you the ability to move this up or down—adjust it so that it passes right across your sternum (hence the name), the bony plate right in the middle of your chest.
Second is the male/female chest fit. Most women prefer that their more substantial breasts don't get squashed underneath the shoulder straps. For this reason, women's shoulder straps are cut with more curve to wrap around the breasts and avoid chest squash.
“Equipped” is an AMC Outdoors blog, written by Matt Heid.