March: Core

   Gravity is cruel. Although you may think of running as a horizontal endeavor, the world wants to pull you into the ground. This is one reason that cross training makes you a better runner.

   The instinct is to cross train upper-body muscles that are otherwise neglected in your regular workouts-shoulders, arms, and back. This type of strength training will make you a more efficient runner in modest ways, but core muscles make magic. They also prevent injury. Generally speaking, they include muscles that attach to the pelvis, abdominals, and back (technically, they're the lumbopelvic-hip complex). They distribute your body weight. They maintain balance and posture. They absorb force and transfer your own force to the ground. They help you accelerate and change direction. They move your upper body as a counterbalance to the movement beneath you. In other words, they negotiate gravity while moving you forward. They keep you on your feet.

   Not surprisingly, running builds your core muscles. Growing evidence suggests that additional strength training in this area will give you a more efficient and powerful stride. Crunches and back extensions will help, but when you add a balancing ball to these exercises, the results will astonish you. In fact, any exercise that forces you to balance yourself under exertion will strengthen these indispensable muscles.

   Why? Building your core is less a matter of increasing absolute strength than of training your nervous system to signal specific muscles to respond quickly to changes in your center of gravity. The best way to do this is by working on an unstable surface. You might be able to knock off 35 pushups from the floor. But hoist your feet on top of a balancing ball-and you'll be lucky to do half that number. What's more, you'll probably be sore the next day, because supporting muscles the floor ignores suddenly come into play. This is also why free weights tend to make you stronger than resistance machines do. A multitude of supporting muscles must balance the weight through an unrestricted range of movement. Wavering weakness can let that 35-pound barbell wrench your shoulder.

   There are other benefits to core training: it prevents overuse injuries to the hips, knees, and ankles by providing better stability to the body parts north of them. Also, the pelvis, knee, and ankle all work together to produce your magnificent stride. Stronger muscles surrounding the pelvis mean that it can share more of the load. Injuries to the knees, in particular, are often the result of a weak core.

   This is another reminder that the runner is a single piece of equipment. The specificity of training makes us focus on the muscles that give us dramatic results, so it's easy to overlook your body's supporting constituencies. Core training lets you get to know these parts. It lets you get to know yourself.