I used to run track and had a teammate that ran virtually identical times to me in everything we did. He was a bigger guy (5'11" 180lbs. vs. 5'8" 145lbs.) and much stronger than me in the weight room. I've always been curious about something and am not sure how the physics behind it works. If we raced eachother with 25lb. backpacks on would he now have an advantage? My intuition says yes.