Body Composition for Runners: Muscle, Bone and Power to Weight

Runners pay close attention to weight, and for a reason. Every extra pound has to be carried for every mile. But weight alone is a poor guide. What matters is how much of that weight is muscle producing force, and whether the skeleton underneath can handle the training.

Power to weight, not just weight

Running speed depends partly on how much power you produce relative to what you carry. Losing fat improves that ratio. Losing muscle does not, because the engine shrinks along with the load. A runner who drops 8 pounds through hard mileage and too little food may be lighter and slower at the same time.

A DEXA scan separates the two. It reports fat mass and lean mass by region, so we can see lean mass in the legs specifically, along with ALMI, the arm and leg muscle index compared to age and sex percentiles. For a runner, leg lean mass is the number that tracks with power, and a left and right comparison shows whether one leg is carrying less muscle than the other.

Bone is the runner's blind spot

Running is weight bearing, so it is easy to assume runners have strong bones. Many do at the hip. But bone density depends on more than impact. It also depends on eating enough. When training burns more energy than a runner takes in, the body cuts back on hormones that protect bone, including estrogen in women and testosterone in men. This pattern is called relative energy deficiency in sport, and it shows up as stress fractures, missed periods in women, and low bone density in both sexes.

A DEXA scan measures bone density at the spine and hip. For younger runners we read the Z score, which compares you to people your own age and sex. Sports medicine groups treat a Z score below minus 1 in athletes as low enough to deserve a closer look, especially with a history of stress fractures.

Muscle as injury protection

Each running stride loads the legs with roughly two to three times body weight. Muscle absorbs part of that force before it reaches tendons, joints and bone. Runners who strength train two days a week tend to hold more lean mass, and across sports, strength training is one of the better supported ways to lower injury rates.

So a useful scan for a runner answers a few questions:

• Is the weight I am losing fat or muscle?
• Do my legs carry enough lean mass, and is it even side to side?
• Is my bone density where it should be for my age?

Repeating DEXA about every 6 months, across a training block or a race season, shows how each number responds to the mileage.

Fast and fragile is a common combination in endurance sport. That's why we check it.

Book a DEXA scan at HYBRD Health ($199)

This article is educational and is not medical advice.