To begin with, an external frame pack does a good job of carrying heavy loads over long distances if the trail is clear and well-maintained. If the equipment you are carrying is awkward and unbalanced, an external frame will carry it better than an internal frame. Furthermore, an external frame allows for more ventilation, so it is cooler to carry. On the other hand, internal frame packs are ideal for traveling over uneven terrain where balance is important. The suspension on an internal frame pack is flexible and can be formed to follow the contours of the spine. In addition, the load rides closer to the back than in an external frame, therefore, the pack’s center of gravity is close to your center of gravity and you and your pack are more stable as you move. Other factors to consider when selecting a pack include (1) the season you will be traveling, (2) the trip length and (3) the environment in which you will be hiking. A winter trip will require more space for additional clothes and gear to deal with the weather. Some packs have an extended collar to provide extra space for this purpose. Pack size, in general, corresponds to the length of trip you will be taking. Packs are usually measured in cubic inches. A one to five day trip requires about 50 liters, 3000 cubic inches. Anything over five days requires about 65-85 liter, 4000-5100 cubic inches.
Pack size is, mainly, determined by torso length. Measure your torso between the bottom of your neck and your lower back. To do this, hold the top of a soft measuring tape at your seventh vertebrae. (This is the bony protrusion at the base of your neck.) Hold the other end of the measuring tape at the low point between your hipbones. This is the point at which your thumbs point inward, when hands are placed on each hip, touch. See Osprey’s photos below to illustrate this. In addition, for efficient carrying, it is important for hip-belts to be the proper size. Hip-belts, when properly fitted, are snug and ride squarely on your hips. The hip-belt should most of the pack weight.
Fitting a Pack:
Once you have found the correct pack size and packed it well, begin by cinching down the hip-belt and shoulder harness. The hip-belt should rest squarely on the hip bones right at your belly button. The shoulder straps are tightened by pulling the straps at the lower end of each strap. Then, connect the sternum strap and move it so that it snugly lies two inches below your collar bones. Next, bring the load against your back by pulling on the load lifter straps above your shoulders. Ideally, this strap should create a forty-five degree angle. Do not allow the load lifter straps to lift the shoulder straps any more than ½ inch off your shoulder. Check the shoulder straps to be sure they wrap, with no gaps, around your shoulders. If there is space between the shoulder straps and your back or shoulders, take the pack off and adjust the shoulder harness for a better fit. When a pack is fit well, it will be easier to carry, more comfortable and allow you to carry it longer with less discomfort.