When you begin to increase your mileage, you may create muscle imbalances that can lead to injuries. Often, the remedy is strength training. “Those two words strike fear in many runners, but it’s even more important than getting the right shoes,” says Mike Keohane, a running coach and sports pedorthist at Foot Locker in New York City and a former 2:16 marathoner.

The most helpful exercises for runners focus on the upper legs, hips, buttocks, and lower back. “These muscle groups work together to carry us forward, and if any of them is weak, it will cause trouble,” Keohane says. He recommends two to three 30-minute strength workouts per week, with a gradual buildup in repetitions and sets. Focus on proper execution of each exercise, rather than on increasing the weight.

• Upper legs: Leg extensions strengthen your quadriceps, says Jim Wharton, president of Wharton Performance. “The front quad muscle that attaches to the knee joint takes the most impact when you're running,” he says. To strengthen the hamstrings (backs of the thighs), good exercises include dumbbell lunges and prone leg curls.
• Hips: “Strengthen the muscles on the outside of your hips with single-leg lifts, walking lunges, and ‘monster walks’—walking sideways in a squat position with a resistance band around your ankles,” advises Polly de Mille, PhD, an exercise physiologist and a registered nurse at the Hospital for Special Surgery (HSS) in NYC. She adds that studies show correlation between weak hip muscles and common runners’ complaints such as iliotibial band syndrome, knee pain, and tibial stress fractures.
• Buttocks: Weakness in these muscles can cause hip and knee pain, says Jim Cavin, manager of Lower Manhattan Physical Therapy. To strengthen them, he recommends prone hip abductions: Lie face-down with your legs straight. Lift your legs a few inches off the floor, then open them as wide as you can. Hold briefly, and return.
• Lower back: Core exercises help here, says running coach Jimmy Lynch: “Doing abdominal work, like crunches, and exercising the hip flexors and buttocks, can strengthen your back.” He advocates “strengthening the little intrinsic muscles that you're not always aware of when you run, so they can support the major muscles that you notice. During a long run, those core muscles kick in.”

Other exercises, such as push-ups and bench presses for the chest muscles, can help round out your routine. “I’m a big believer in strength training,” says Jordan Metzl, MD, of HSS. “The biggest mistake runners make is not doing it.”