Gist
Stationary cycling can improve leg strength and muscular endurance, especially in beginners and at higher resistance, but it does not replace progressive resistance training for maximal strength.
Cycling can improve leg strength and muscular endurance, especially in beginners and with hills or resistance, but it does not replace progressive resistance training for maximal strength or muscle growth.
Spinning can improve leg muscular endurance and cycling-specific strength, especially with resistance and standing climbs, but it does not replace progressive resistance training for maximal strength.