Does an elliptical trainer give you the same workout as a run?

If elliptical machines and treadmills supposedly give you the se workout, why can I "run" on an elliptical machine for 45 minutes with no trouble when I struggle to run 10 minutes on a treadmill? Seth Mountain View, CA

A: When people talk about using two methods to get the "same workout," they are often referring to the cardiovascular response to exercise. In other words, they figure that if youre using the same muscles, and your heart rate is elevated, the workouts are equal in their respective effects on the body. This isnt the case.

Running involves considerably more impact, and every time your foot strikes the ground, or treadmill, your leg experiences a force about two and half times your bodyweight. Think about this way: in the middle of a stride, your entire body is airborne. Due to gravity, youre actually picking up speed, downward, as you land on your forward leg. That leg has to catch you, completely arrest your downward motion, and then generate enough force to accelerate your body mass back into the air.

With the elliptical trainer, your body weight is constantly supported. Youre always attached to the machine, and your feet are traveling in a prescribed arc. The elliptical trainer is not exerting the force against your body that the ground or treadmill exerts as you land with each foot strike. Your heart rate might be elevated, and you might be breathing hard on the elliptical machine, but your muscles, ligaments, tendons, and bones arent undergoing the same stresses.

The bigger question, however, is whether running for ten minutes on a treadmill and using an elliptical trainer for 45 are equal in their impact on your fitness. To improve cardiovascular fitness, the longer you can sustain exercise, the better. If that means using a lower-impact machine so you can go nonstop for 45 minutes, thats better than exercising some other way for just ten minutes. The key elements in cardiovascular fitness are time and consistency.

If your goal is to finish a local charity run, a 10k, or a marathon, you have to start running for real. Right now if you can sustain ten minutes, and youre struggling by the time youre done, your body sees that unpleasant experience as a good reason to adapt, so the same effort isnt as difficult the next time. The elliptical machine might stress the cardiovascular system, and cause it to adapt, but in order to develop the muscles and technique necessary to be a runner, you need to run.

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