Runners cross-train to prevent injuries. so it's ironic, if not unjust, to get injured when cross-training. Last year, I was in a Pilates class, struggling to master a hamstring-curl maneuver with my feet up on a stability ball. Although the instructor suggested a less difficult move for beginners, I forced the motion until my right hamstring felt like it was being torn to shreds. It was. My injury sidelined me from running for nearly two months.

Of course, Pilates and alternative forms of exercise can improve your fitness, prevent and rehabilitate injuries, promote recovery, and revive a stale routine. The trick is to approach them as a runner. Runners have their obvious strengths: power, endurance, tenacity. But within those strengths lies the potential for weakness: quads that overpower our hamstrings, neglected upper bodies, and poor flexibility--qualities that could lead to problems. "Running makes you a fit runner," says Jason Karp, a running coach in Albuquerque, New Mexico. "But your running fitness may not translate to other activities that use your muscles and joints in different ways. Taking on too much too fast can make you vulnerable to injury." Understanding the three most common problems for runners will help you cross-train safely, so you can benefit without incident.