Question: Will stair-climbing or cycling make my butt big?
Answer: Plenty of folks out there still think that these activities will enlarge their rears. But, like many myths out there, it simply isn't true. Your body has different muscle fibers: slow-twitch and fast-twitch. Fast-twitch muscle fibers (think of bodybuilders) are made to lift heavy weights and grow larger. The slow-twitch fibers (think marathon runners) are made for endurance and can't get very big. Remember that when you do aerobic exercise, you're relying on your slow-twitch muscles so they will NOT get bigger from cardio.
Plus, building muscle requires progressive resistance--constantly giving your muscles more weight than they can handle so they grow stronger. While there is some resistance in many cardio machines, there isn't enough there for that progressive resistance.
