In our new monthly Q&A, we take popular questions from our forum and publish the answers. This month, we dive into recommendations for dealing with leg length discrepancies—and are the recommendations different if the leg length discrepancy is due to anatomical differences rather than functional issues?
Before correcting for a leg length discrepancy, it is critical to determine whether it is caused by functional or anatomical differences.
If it is functional in nature, the emphasis must first be placed on correcting the imbalance leading to the discrepancy. If this does not resolve the functional discrepancy, then it can be addressed with the same thinking that is applied to anatomical differences.
To correct for anatomical leg length discrepancies, the first step is to understand where the structural difference occurs and to develop a strategy that corrects for approximately half of the difference for the shorter limb.
While there are many possible reasons for anatomical discrepancies, there are three major categories—and understanding the principles behind these three can be applied across a range of specific situations, whether they are congenital or due to injury.
Diagnosing the specific cause is beyond the scope of this article. Furthermore, determining the cause of leg length discrepancies is as much an art as it is a science; it is difficult to determine what is needed to reduce injury and/or improve performance.
Leg length discrepancies: Anatomical shortness of the tibia/fibula
If you watch a cyclist pedal a bike, their lower leg (the tibia and fibula bones) stay relatively vertical throughout the pedal stroke—especially throughout the “power phase” of the stroke, which is the most important.
If there is a leg length discrepancy in the lower leg, riders typically employ two methods to compensate; both of them try to preserve the angle of the knee. In one, a rider adopts a “toe down” or “heel up” position at the ankle. This is an attempt to gain extra length on the short side, but it can lead to diminished power, calf cramping, as well as other side effects. The second way a rider will compensate involves shifting on the saddle, either by tilting the hips to reach for the pedal or shifting and/or rotating on the saddle in an attempt to get farther forward. Often, saddle sores are the result.
Because this leg length discrepancy predominantly occurs in a vertical direction, our correction needs to predominantly have a vertical component. Adding shims under the foot does just that. Typically, the initial correction would be half of the measured functional difference. Because cycling is a forefoot driven sport and the ankle is able to move freely, adding a heel lift does not correct a cyclist like it would a runner. The correction needs to be placed under the forefoot. Shimming cleats makes sense with a road pedal as a full-length orthotic or lift may affect the fit of a shoe. However, internal corrections such as these are the only ones available to those who use two-bolt “mountain bike” style pedals, as those typically do not play nicely with cleat shims.
Correcting via crank-arm length is difficult because the shorter radius affects both the vertical and horizontal positions throughout the pedal cycle. While it would correct the leg length shortness at the bottom of the pedal stroke, it would mimic a leg lengthening at three o’clock and the top of the pedal stroke.
Leg length discrepancies: Anatomical shortness of the femur
Unlike our lower leg, our thigh is mostly oriented in the horizontal direction while cycling. Shortness in the femur tends to cause a rider to twist in the saddle in an attempt to bring the hip of the shorter leg closer to the pedal. For example, a shortened right leg would cause a counter-clockwise twist. This twisting can cause saddle sore issues, and it can also result in knee pain and other joint pain as well.
To correct for this rotation, we bring the foot back toward the hip instead of making the body reach forward to the pedal. We accomplish this by shifting the cleat. Off-setting the cleat toward the front of the shoe effectively moves the foot rearward; this is useful to correct the shortness that occurs in the femur. Again, we would typically begin with a shift that is half of the measured leg-length discrepancy.
Leg length discrepancies: Anatomical shortness in the femoral neck
This structural change is interesting as it tends to move the thigh closer to the midline of the body and shorten the femur slightly. Riders may try to compensate for this by shifting their body toward the affected side on the saddle and rotating the affected side forward (just like in the shortened femur example above).
To correct for this, a fitter could perform two cleat shifts. First, they could shift a cleat toward the outside of the shoe (laterally) to offset the limb toward the midline of the bike. This helps correct for the thigh that is closer to the midline of the body and helps correct the shifting on the saddle. To correct for the slightly shorter femur, a fitter could shift the cleat of the affected leg toward the front of the shoe to offset the foot and knee backward. This will reduce the twisting on the saddle.
RELATED: Check out last month’s Fast Talk Labs Q&A, which focused on altitude training and the impact of body composition on hypothermia.