yes intervals work indoors or out. your body won't know the difference between running sprints on a track or the treadmill cycling on the road or on a stationary bike (indoor cycling classes are great interval workouts) or working on an elliptical or any other exercise machine. you can do intervals with swimming rowing cross-country skiing or any other sport you like. intervals work as long as you get your heart rate pumping and follow the ratios.
can i do intervals inside or outside with or without exercise equipment?