Paul is 46 and runs two to three marathons each year.

He hasn’t been running as much as usual this winter, but signed up for the Boston Marathon, and has been working on a strict regimen and increasing his miles each week.

Part of his weekly workout is to take a long run each weekend, and last week he ran 12 miles. Since then, Paul has had pain in his right hip that extends to his knee. He fears he may not be able to run long distances again, or have arthritis.

Mary is 15. She plays high school lacrosse. She began having pain in her kneecaps after two-a-day workouts to start the season, and although she uses ice and takes it easy in practice, the pain keeps getting worse.

Mary sometimes cries on the way home from practice. She has to take pain medication to sleep most nights. Even though her team is doing well and she’s become a pivotal player, Mary is considering quitting because her knees hurt so much.

Brandon is a college baseball pitcher. There were already three seniors in the rotation, but the coach liked Brandon’s pitches and work ethic.

Things began to fall apart after the first few weeks. Brandon began having elbow pain, first only in the evenings after he threw a lot of pitches, then later while pitching. He lost velocity and control. His coach told him to forget about starting. Brandon came to me frustrated, upset, and thinking his career might be a bust.

What is the same about each athlete? They are different ages, compete in different sports, and have injuries in different parts of their bodies. It turns out that their injuries are all caused by the same muscle imbalances, and for them it’s all about the core.

Let’s take a closer look at each athlete. Paul always runs facing traffic, and his route takes him over Maine’s crowned roads. This means that his body is constantly running on an angle. Because his back and abdominal muscles aren’t as strong as they could be, his back slumps to the side with each stride. The slumping, over months of running and miles of road, puts strain on one hip, causing inflammation and pain.

Mary has recently just grown 4 inches. The muscles of her hips, back and thighs haven’t kept up with her height. Since they are weak, her knees are absorbing all the strain of each stride on the lacrosse field. The repeated impact is leading to her pain.

Since he got to college, Brandon has been trying to impress by throwing harder. His velocity is up, but he’s not using his hips and legs to generate extra power. Since his core isn’t conditioned, he tries to get more from his arm. Brandon’s arm is strong, but the small ligaments in his elbow aren’t designed for the forces he’s now exposing them to. He’s literally throwing his arm out.

The solution for each patient is to work on the injured joint AND make their cores stronger. It seems strange – why look at an athlete’s back when the elbow or knee hurts? These examples show how important the core is to connecting the upper and lower body. Most sports require an athlete to generate tremendous forces through the lower body. If the core isn’t strong enough, the forces don’t end up where they belong. Injuries can follow.

Over the years, doctors have discovered that conditioning the core takes a lot more than doing crunches. The core muscle group is composed of the back muscles, muscles in the sides of the abdomen, and the “six-pack” abdominal muscles in front. These muscles have to work with other small and large muscles in the hips, middle back, and even inside our abdomens. If even one of these muscles isn’t doing the right thing, or if it’s doing the right thing at the wrong time: presto – the athlete has problems.

So the next time your doctor diagnoses your athletic injury by looking at your back, don’t fret. They’re not in need of a GPS to figure out how to locate your problem. Instead, your doctor is giving you a state-of-the-art biomechanical analysis. Chances are they will have suggestions about improvements you can make.

Each week in my office I discover core strength imbalances that even the most fit athletes need to work on. After a few weeks with a physical therapist, these athletes are back on track and recovering. They have learned that it’s all about the core.

Dr. James Glazer is a sports medicine physician for Coastal Orthopedics and Sports Medicine in Freeport. He serves as a consultant for the Portland Pirates and the U.S. ski team.