With waist packs it's all in the fit. It is strenuous enough to run several miles through a canyon, wilderness, or whatever destination you venture to, so don’t add to your stress by wearing an uncomfortable running waist pack. While most packs hardly differ in overall size and shape, some will fit you better than others. Your height, upper body mass, and the shape of your spine dictate the comfortability level. In addition, the straps, waist straps and back length vary depending on the size and volume of the best running belt.