well on a fixie you always have to pedal so pedaling is harder and it is a much better workout because unlike a regular bike where you can coast on a fixie you always have to pedal so you are always doing work and you dont get to take any breaks and you will get stronger because it is just like basically try this: get on your regular bike and ride around but you always have to pedal so it is like that but even harder because you dont have gears only one, so basically to answer your question yet fixies are very good for strength and cardio training because they constantly work your muscles unlike a regular bike where you can coast