Exercise equipment gets cheaper as technology improves. Although very high-end exercise bikes remain slightly cheaper than the highest-end rowers and treadmills, all three types of cardio exercise machines are now within relatively easy reach of most middle-class Americans. Each has its unique advantages and disadvantages, although in the end the best exercise machine is always the one you enjoy enough to use consistently.
Upper or Lower?
You might, very rarely, see an exercise bike or treadmill with moving handlebars that allow you to work your upper body. The vast majority of these treadmills and bikes work your lower body only. A rowing machine, on the other hand, works your upper and lower body together. The rowing machine's upper-body component primarily works the large muscles in your back and shoulders, plus the pulling muscles in your arms.
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Hold Your Own Weight
Weight-bearing exercise, such as walking on a treadmill, helps to slow mineral loss in your legs, hips and lower spine. Neither rowing machines nor exercise bikes provide this benefit of a weight-bearing workout, because the seat supports your weight instead of your legs and feet.
What's the Impact?
Walking on a treadmill is considered a low-impact activity; your feet leave the workout surface, then return to it, but there's not much impact to each step. Running on the treadmill, on the other hand, is a high-impact activity. Although a low-impact walk can be beneficial if you have osteoporosis, you should avoid high-impact exercises such as running if you have osteoporosis or injuries that keep you from tolerating the repeated pounding of a high-impact workout. This limits your exercise intensity when using a treadmill.
With a rower or bike, however, you can work out as hard as you want, since both machines provide a low-impact workout, even when used at a high intensity.
Proper technique on a rower requires a powerful thrust with both legs, followed by a quick upper-body tilt and even faster hand movement with the handle. You can master this technique, and put it to work for a punishing workout, with just a few minutes of instruction and practice. But the rower still requires a greater learning curve than the treadmill and bike, which both mimic motions that have probably been familiar to you from a very early age.
Most high-end rowers are constructed for training and conditioning competitive rowers. That doesn't mean that you shouldn't use rowers if you're a non-rower, but it does affect the sort of features you're likely to see. If you want features such as pre-programmed workouts and built-in cooling fans, you should opt for a treadmill or exercise bike. Most rowing machine manufacturers focus on performance features such as a race mode, that lets you race against computer-simulated opponents or, in some cases, another rower on a nearby rowing machine.