I ride fixed, not single-speed, and multispeed bikes (derailleur and IGH). Fixed gear is a different kind of work out. It works different muscles, and works the same muscles differently. It trains you to pedal smoothly regardless of cadence and develop power regardless of cadence. You can't really slack off, since whenever the bike is moving, the pedals are moving. You can choose to pedal fast or slow, hard or soft, but you're always pedaling.