Biomechanics and Bike Fitting: Leg Length Discrepancy
A common problem amongst cyclists is that the pedal stroke feels uneven, one leg feels stronger, the pedal feels closer on one side or whichever way the individual expresses the problem verbally. The root of this feeling is often decided to be that one leg is longer than the other, meaning that either the shorter leg has to overreach or the longer leg doesn’t achieve a full pedal stroke. This Leg Length Discrepancy (LLD) may only be a matter of millimetres, but cycling is a sport of millimetres. The reason for this is that on a bike you are fairly locked into a certain position and doing the same movement pattern over and over – even a short ride of 2 hours will generally use somewhere in the region of 8-10,000 pedal revolutions – and therefore any imperfection in the position will become quickly apparent.
The Bike Fitter’s Solution
The solution to the problem of one leg being longer than the other is generally corrected by bike fitters by putting a shim (a thin piece of plastic or similar) under the sole of the shoe of the short leg, which essentially makes the short leg longer and corrects the problem.
This is a great solution from a bike fitting/extrinsic biomechanics (i.e. looking at how the body moves from the outside) point of view and sometimes can be exactly what the individual needs. A less hi-tech solution that can also be used is to lower the seat to the point where the shorter leg is no longer over-reaching, however this is less than ideal as it leaves the body being unevenly loaded on every pedal stroke. Many people would be able to tolerate this for short to medium distances though, and may never realise a problem exists.
The Biomechanics Coach’s Solution
A Biomechanics Coach will always start from the beginning so no matter what the problem is basic function will be checked. In this case, the Biomechanics Coach will not start by seeing one leg longer than the other and go from there looking for solutions. The BC will see one leg longer than the other and ask “Is this leg structurally longer than the other or just functionally longer, and if the latter can it be fixed?”
As you can see from the image above, it is possible to have one leg longer than the other one minute, and reduce or eliminate the Leg Length Discrepancy (LLD) the next. The two images that are side-by-side above were taken 10 seconds apart. If the assumption had been made that the situation in the first picture was permanent, an unnecessary shim of around 3mm would have been added. This client may be able to eliminate the LLD in time once the function in the hip is corrected.
Why Do People Have Different Leg Lengths?
There can be many reasons for an LLD, only a minority are due to the fact that the bones in one leg are actually longer than the other. Most times where this is the case it is due to breaking a leg and the bone not setting in the original position. A more common problem is a rotation of the pelvis (which the legs are obviously connected to) causing one leg to seem shorter than the other. Ideally, every time we lift our leg there should be a small movement of the pelvis which helps the hip joint perform the movement without jamming up. If tightness, spasm or another issue prevents the pelvic movement not only will the leg seem a different length at rest but the movement pattern will be altered, possibly causing issues above or below the hip.
How Can I Fix a Functional LLD?
The good news is that a Biomechanics Coach will have a few tools available to address the issues that commonly cause problems in the pelvis leading to an LLD and often these problems can be completely fixed. Due to the fact that this basic realigning of the body (called the ‘Normalise’ phase in the Biomechanics Coaching system) is the first step in the correction of any problems – see here for an explanation of the basic system – I would always recommend a cyclist looking to feel more comfortable on his or her bike to see a Biomechanics Coach and get normalised before getting a bike fit and addressing the specific problems you have on a bike. If the body doesn’t function off the bike it is highly unlikely to function on the bike, and if apparent issues that are actually only temporary are treated with a permanent solution in a bike fit you may end up feeling less comfortable on the bike or even getting injured.
To arrange a Biomechanics screening, please see the contact page.
For a map of Biomechanics Coaches throughout the country, please see the ‘Find a Coach’ page on the Biomechanics Education site here