While switching up your routine and utilizing the different benefits from varying machines, the treadmill has been proven to provide the best overall cardio workout. The American Council on Exercise conducted a study on cardio machines that includes the treadmill, stationary bike, elliptical, and rower. Scientists looked at the rate of perceived exertion, heart rate, and oxygen consumption of participants on each machine. The treadmill ended up producing the highest heart rates and the most calories burned by the users. A stepmill came in second place, with a recumbent stepper and arm ergometer making the bottom of the list.