When you exercise you lose fluids via sweating. It is ESSENTIAL to put that fluid back into your body. This is equally true when lifting heavy. When you don't drink enough water, your muscles will begin to "burn out" during your workout, The reason? Not hydrated properly. Don't get me wrong here, your muscles will indeed tire after intense lifting at some point, they will just get tired. I have found personally, if I hydrate really well during the day, when I do my routine, I don't get tired as fast. Is there a direct culmination? You bet. It helps the blood deliver nutrients more quickly, which is VERY important in lifting, as well as any other high-intensity exercise. I personally have noticed if I ignore my body telling me it needs a drink (ie dry mouth), the work out is SO MUCH HARDER.Besides that, water is a fat burner, as well as the MOST essential element we need to live (after air, of course, lol). Bottom line, it's good for ya, so drink it. Lots of it....