Here’s the scenario: a cyclist has back, hip, or knee pain, and ends up in the office of a local health practitioner, or in the hands of the bike fitter at the local shop. He’s told he has a difference in his leg lengths, and the asymmetrical wear and tear may be causing his pain.

It’s certainly plausible. Many of us have noticed riders ahead of us in the peloton who seem to have one hip that drops lower with every pedal stroke. That has to be originating from somewhere, right?

So what’s going on? How’d our rider get this way, and what can be done to address the problem?

The difference in leg lengths doesn’t start with the feet. The most common cause of a leg length discrepancy is displaced rotation, or torque, of the pelvis.

The two sides of your pelvis rotate forward and back when you walk, run, or turn over the pedals. When your foot goes over the top of the pedal stroke, the top of your pelvis — the ilium — has to rotate back, or posterior, to accommodate it. The opposite happens at the bottom of the stroke, where your pelvis rotates forward, or anterior.

With posterior rotation your thigh bone, or femur, gets pulled up closer to your head, making that leg temporarily shorter than the other. Due to a fall or some other kind of trauma, one side can get “stuck” in a relatively more anterior or posterior position, producing a functional leg length discrepancy.

Actual anatomical differences in leg lengths — where one or more bones are altered making one leg significantly longer than the other — are rare. You’ll see these in people who’ve had a fracture or some other significant trauma to their pelvis or legs. Knee and hip replacements are also good culprits.

I see dozens of people every week that have leg length differences, but since the vast majority are functional adaptations to some other problem, it’s rare that they can’t be fixed with treatment.

Typically, the longer someone has had the problem, the longer it takes to address. By it’s nature an athlete’s body is good at working around problems, so facilitating the release of a hard-fought adaptation like a leg length difference requires creative and thorough treatment.

A good bike fitter can be the first to catch this kind of problem. Making small positional changes or placing shims in shoes is a frequent fix for a leg length difference. These folks are wizards at getting you into a comfortable, efficient position on the bike, and reducing some of the biomechanical strain that can come from being asymmetrically positioned can be a god-send.

This is especially true if you have an anatomical difference in leg lengths. However, a functionally adapted short leg shouldn’t be propped up over the long-term with a shim or heel lift. Unless you know you’ve had trauma to your lower extremities, or you’ve had your pelvis and legs X-rayed to confirm there’s an anatomical difference, the chances are your discrepancy is functional.

Ultimately, getting yourself evened out through thorough treatment is the best way to reduce wear and tear, and increase your health and performance.