Both machines are effective tools for at-home aerobic exercise. They can be used for high-intensity and highly effective workouts, as well as more casual cardiovascular fitness sessions. But generally speaking, exercise bikes are better for people with joint pain, injuries, or other problems, as the seated pedaling motion places far less impact on the hips, knees, and ankles. "While using a treadmill is still softer than running outdoors, there is still a lot of impact absorbed every time our foot strikes the ground," Karp says. "With an exercise bike, our legs move in a fluid motion with constant contact between our feet and the pedals—resulting in a smoother transfer of power and less abrupt force generated with each pedal stroke," he adds. The low-impact nature of cycling also makes it a safer choice for beginners and older adults with longevity in mind.