First, know that your concern is not unfounded. Research has shown that novice runners with a BMI of 25 or higher have a higher risk of injury than their lighter counterparts, likely because the extra pounds increase impact forces on your body. Dropping your BMI below 25 can decrease your risk of injury by 10 percent.
If it’s early on in your training, say four or fewer months out, substituting the elliptical for one or two runs a week while you drop some pounds is fine, says Dr. John Porcari, an exercise scientist at the University of Wisconsin La Crosse.
At the turn of the century, when elliptical machines were exploding in popularity, Porcari studied the difference between ellipticizing and treadmill running and found that heart rate and oxygen consumption were nearly identical when exercising at a similar rate of perceived exertion. The impact forces on the elliptical, however, were more comparable to walking than to running, making the elliptical a good alternative for cardiovascular conditioning without the pounding.
But, Porcari points out, cardio is only part of the marathon prep equation. “The problem, especially if you’re a recreational runner, is not so much your cardiovascular endurance as your musculoskeletal endurance,” he says. “When you’re running a marathon, you’re not really taxing your cardiovascular system, you’re taxing your musculoskeletal system.” Most recreational runners race a marathon at a fairly relaxed effort of 60 to 65 percent of their VO2 max, Porcari says.
THE BOTTOM LINE
Yes, you can certainly use the elliptical to keep up your cardio conditioning and shed some weight. But only running will properly prepare your body—bones, joints, muscles, tendons, ligaments—to pound out 26.2 miles. Once you’re within four months of race day, try to get your scheduled runs in.
(Bonus: Still worried about injury? Increasing your cadence by just five percent can help lessen impact forces on your knees, and therefore your risk of injury at that joint, according to a study published last year. If you take 170 steps per minute, for example, try upping that to 178.)