“Cycling”: no. Not at all. It won’t even “build your glutes” like many people think it will, no matter how hard & long you ride. It can be really great cardio work, though.

bmxing (sprinting/climbing while standing up & bunny hopping as big as you can over/up onto everything you can 40-80 times per ride, at least three rides per week): yes. It’ll cut & build your entire core. Stick with it for a few years and you’ll even develop “back-abs”.

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It’s a lot cheaper to build your abs doing boring prison cell exercises than to do it via bike. You really need 48 spoke doublewall rim wheels $$$ built and bolted onto a non-kids Pro-level bmx $$$ which you’ll probably find yourself needing serious 3 piece cranks $$$. $$$+$$$+$$$=$,$$$

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You really should try to connect with an Athletic Physical Therapist (not at all the same thing as a “trainer” at a gym) to collect the whole batch of core strength exercises and how to do them correctly so you don’t over build some muscles and underbuild others in a way that has you being forced by your doctor to go see a Physical Therapist due to back pain fifteen years in the future. Ask me how I know.

These biomechanically correct exercises will build your abs and everything else really well.