Left and Right Muscle Imbalances on a Body Composition Scan

Most people think of body composition as one number: body fat percentage. But a segmental scan also breaks lean mass down by region, so you can see each arm, each leg and the trunk on their own. That is where left and right differences show up.

An InBody scan sends a small electrical current through electrodes at the hands and feet. Lean tissue holds water and conducts the current well, while fat resists it. By measuring the current through each limb separately, the device estimates lean mass in the right arm, left arm, trunk, right leg and left leg. It takes about a minute standing on the device.

What is normal

Nobody is perfectly symmetric. Most people have a slightly larger dominant arm, and right handed people often show a bit more lean mass on the right. A small gap between legs is common too. These differences are not a problem and do not need fixing.

What we pay attention to is a gap that is clearly larger than the rest of the body would suggest, or a gap that grows from one scan to the next.

What a larger imbalance can suggest

When one limb carries noticeably less lean mass than the other, the cause is usually something that changed how that limb gets used:

• An old injury or surgery. After a knee surgery, for example, the thigh muscles on that side often shrink and may never fully recover without targeted work.
• Favoring one side. Someone with a sore hip or ankle may shift load to the other leg for months without noticing.
• Training habits. A lot of single arm sport, or always leading with the same leg, builds one side more.
• Less commonly, a nerve problem. Muscle loss on one side with weakness or numbness is worth a medical evaluation.

Imbalances matter because they change how load moves through the body. A weaker leg means the stronger one, and the hip and back above it, take more of the work. That may contribute to overuse injury in runners and lifters, though research on how much asymmetry matters is still mixed.

A 44 year old man who had his right knee scoped years ago might see his right leg lean mass sitting a pound or two below his left. That gives us a starting point. Adding single leg work, such as split squats, step ups and single leg presses, lets the weaker side do its share. A repeat scan about monthly shows whether the gap is closing.

One caution: hydration, a recent meal and a hard workout can all shift readings, so scans are most comparable when done under the same conditions each time.

Asymmetry is easy to feel and hard to quantify. That's the kind of thing we measure.

Book an InBody scan at HYBRD Health ($39)

This article is educational and is not medical advice.