Carrying a heavy pack for long periods of time tires your trapezius muscles (the muscles between your neck and shoulders). To alleviate this problem, backpacks have waist belts which redistribute the weight from your shoulders to your hips. Now the weight is carried by the stronger muscles in your lower body. Wearing a pack that fits correctly helps your body work best at carrying alot of weight.

First, get a tailor's tape and a friend to help you measure (a regular measuring tape can replace the tailor's tape). Then locate the bone at the base of your neck, it's the one that sticks out most, (C-7 to you Med students or WFR's). That's your first measuring point.

Next, grab your love handles. This is known as the "iliac crest." In layman's terms it's the top of your hip bone. Trace this point around to your spine and that's your bottom measuring point. If you don't have any love handles, consider yourself lucky, and grab a belt and put it on over your shirt. Snug it tight just under your belly button. Where the belt sits on your back is your bottom measuring point.

Now just have your friend measure from the base of your neck to the point you've found on your lower back. That's your torso length. Now you can look for a pack that fits you perfectly or you can adjust one to your torso length.