When you carry anything extra, your body must make more of an effort. Additional weight places stress on your joints and muscles. To avoid this, try to lighten the backpack as much as you can and only carry what is necessary. If possible, keep supplemental hydration somewhere else (such as your car) and refill your backpack rather than carrying all your liquid for your entire run. Also, keep your shoulder straps tight, which will help minimize shifting of the liquids in your pack. If the contents of your running backpack are moving around, your muscles have to work harder.