The Centers for Disease Control and Prevention recommend doing 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity per week. If you're extremely overweight or have joint injuries, low-impact cardio machines are one of the most comfortable ways to satisfy this recommendation. Since your feet neither leave the pedals at all nor re-contact the pedals with great impact, you don't have to endure the repeated pounding that characterizes high-impact activities.
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You'll find banks of elliptical trainers in most large gyms. Using these machines is relatively simple -- step into the pedals and walk or run. The pedals move through an elliptical path intended to approximate your natural stride. Most mid- to high-end elliptical trainers and those of gym-quality have magnetic resistance. You set your own pace, then use push-button controls to adjust the machine's resistance against your pedaling. Almost every elliptical trainer also comes with moving handlebars that you can push and pull for an upper-body workout.
Exercise bikes come in upright models, which closely resemble regular bikes, or recumbent versions with a wider, flatter seat and backrest. An upright bike's seat is wider and better-padded than a real bicycle seat, but if you're still uncomfortable on the upright seat, try a recumbent. Like elliptical trainers, most high-end exercise bikes offer magnetic resistance against your pedaling. Lower-end home models may have direct-tension resistance or fan-based resistance.
The rowing machine, also called a row ergometer, works your lower body and the pulling muscles of your upper body. In order to get a proper workout from this low-impact machine, you must use it properly. Start with a strong leg drive to push you away from the resistance mechanism, then hinge back slightly from the hips and pull the handle in toward your ribs. Reverse these steps to reset for another stroke. Air-resistance rowers are very common, but you might also encounter magnetic-resistance models.
You will always experience at least some impact when using a treadmill, because you repeatedly lift your feet off the treadmill deck, then place them back on the deck. Although running on a treadmill produces high impact forces, walking is a relatively low-impact activity. You can increase your workout intensity without increasing the impact by setting the treadmill to its highest incline setting. Some so-called "incline trainer" models only adjust up to a 40-percent incline.
Air gliders, also known as arc trainers, function as a non-impact cross between a stair stepper and an elliptical. You step into the two hanging pedals, which are supported by a basic metal frame. By consistently stepping on the pedals you swing your legs back and forth in an arcing, pendulum-like motion. These machines are equipped with variable resistance and incline. The higher the incline the more the movement emulates a stair stepper. The lower the incline the more the motion imitates running.