The Effect of Asymmetry On Running Performance and Injury Prevention

Symmetry, or so we are told by science, underlies our appreciation of beauty. But what makes for beautiful—and injury-free—running?

A lack of symmetry, that is relative differences in muscle strength, motion, flexibility, balance, and mechanics between sides of the body, is one element often highlighted as a risk factor for injury. However, while it’s easy to find asymmetry in all of us—small variances in leg length, scoliosis or ankle flexibility to name a few—it’s much more challenging to determine cause and effect.

This inherent asymmetry present in every runner makes achieving complete symmetry impossible and unrealistic. Dr. Irene Davis, Director of the National Running Center at Harvard, believes that it isn’t a question of whether or not asymmetry exists, but rather how much of it. “What we don’t know is how much asymmetry is normal and how much we can tolerate before it becomes a problem.”

But how hard should runners work to achieve symmetry? Do differences in strength, flexibility and gait biomechanics on one side of the body lead to injury and diminished performance?

Jay Dicharry, a renowned physical therapist, running biomechanics expert and author of Anatomy For Runners, is adamant in support of the need for symmetry. “Running requires mobility, stability, strength, and power. While how much is enough depends on the individual, I strive to have all my athletes within 5 percent in terms of symmetry across the board.”

Dicharry believes that while it’s not as simple as asymmetry equals injury, it does lead to problems in the gait mechanics of running, which can then lead to injury. To make matters more confusing those asymmetry-related problems might even manifest on the other side of the body. “Sometimes it’s a runner’s poor right hip stability that allows their knee to rotate excessively, and that drives their chronic right knee pain. But other times, their poor right hip stability may introduce an imbalance that actually winds up in a limp and thus stressing the other leg, causing symptoms in the left side.”

But according to Dicharry, as a degree of leg dominance or asymmetry is expected, it’s difficult to pinpoint the degree of asymmetry that begins to influence the risk of injury. “Now, if you tend to be more dominant with accuracy—imagine kicking a soccer ball more accurately with your right leg—that’s OK,” Dicharry explains. “Dominance in terms of controlling your body is different. That has to do with fine motor coordination, not the ability to produce raw force and power to run symmetrically.”

But are absolute levels of strength, flexibility and mechanics more important or those relative to the other side? According to Davis, the answer may be both. “It is the degree to which strength is compromised or the degree of asymmetry that exists in either the musculoskeletal or biomechanical factors.” For instance, if both hip muscles are weak but symmetrical, there is still a problem.

Research from Davis’ lab, presented in the Journal of Biomechanics, compared gait asymmetry in female runners who had never sustained a running-related injury to those with a history of lower leg stress fractures. The study found a similar degree of asymmetry in both groups, but discovered higher impact values in both legs of the injured subjects, suggesting that the magnitude of impact stress put them at greatest risk rather than any asymmetry. The researchers concluded that asymmetry might simply influence the side on which they become injured.

An article in this month’s American Journal of Sports Medicine studied female runners experiencing mild kneecap pain and found comparable results; knee pain didn’t appear to be significantly associated with any asymmetry in hip strength.

Similarly, in the sports science literature, the role of symmetry in running performance is unclear. Glimpses into the role symmetry plays in performance came in a study showing that the degree of structural knee symmetry at 8 years old predicted sprint running performance at 20.

Though we can’t pick our parents—or our knee structure—we can control some of the other physical variables. If running faster and longer is your goal, Dicharry has this to say about the impact of symmetry on performance: “I don’t advocate running on one leg, you have two of them for a reason.”

So knowing that it’s not a question of whether or not asymmetry is present but rather how much, how do runners detect and fix asymmetry?

Dicharry believes that you don’t have to go hunting for special exercises to fix asymmetry; you just have to pay attention to your body. “Essentially, it’s not the exercise you are doing, but rather the quality you feel when doing single-leg or single-sided exercises. Drop the weight, slow down, do whatever it takes to help you feel symmetric.”

Simple observations, like an inability to twist as far to one side well or good balance on one side but not the other, can indicate an asymmetry that needs to be addressed.

It’s clear that while every runner has asymmetries, ignoring the more obvious differences may be a recipe for injury. “It’s not OK to simply take note of these differences,” Dicharry stresses. “Runners should take time to improve them if they hope to influence injury risk and performance.”

Jay Dicharry’s Single-Leg Balance Asymmetry Test

Found in: Anatomy for Runners: Unlocking Your Athletic Potential for Health, Speed, and Injury Prevention

—Stand on one leg for 30 sec; repeat on the other side. Then repeat with eyes closed.

Runners can assess a great deal of symmetry on their own, and adding vision helps more. If you can, have a buddy video the test. Some things you can feel right away but some may be more apparent when you watch the video playback. If you’ve got an issue with stability on one leg, take some time to work on it. You can get a huge improvement in balance just by spending time on your less-stable leg throughout the day.