BILL BOSTON CYCLES

Lower Limb Inequality
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One of the most challenging and difficult aspects of fit is LLI. Lower Limb Inequality, alternatively called Leg Length Inequality, presents a unique challenge to correctly measure, account for and achieve a proper fit.

The Problem

In many cases the first indication of LLI will be lower back pain caused by a constant (transverse) tilt of the pelvis. This tilt is primarily the result of the unequal pedal stroke relative to the leg length. The pelvic tilt also causes some degree of twist to the spine depending on the back angle relative to the pelvis. While this is the most common complaint, Other areas of discomfort brought about by LLI may include hip and knee discomfort caused by the misalignment.

The Goal

In an ideal situation the legs both have the same length resulting in a balanced stroke where the pelvis does not tilt, the thighs have the same degree of angular change during the pedal stroke and the center of both knees are in the same position relative to the pedal axle. LLI can be compared to a tandem situation where the riders differ greatly in leg length, but are both using the same length crank. The shorter rider will have a greater angular change in thigh angle because the crank length is a larger percentage of the thigh length. The angular change when excessive will require that the pelvis be in a more upright position, thus reducing the ability of the rider to tolerate a longer top tube length. The goal in dealing with LLI, as when setting up a couple on a tandem is to provide a pedal stroke where all aspects are balanced and in harmony, producing the greatest power output with the least amount of stress on the power plant (rider),

The Solution

How you deal with LLI depends on where the LLI exists as this will determine what needs to be done. Take your time, and determine the differences between left and right as accurately as possible. The differences can be: from the knee to the hip (upper LLI), knee to ground (lower LLI) or a combination of the two.  

Lower LLI (from the knee to the ground)

Lower LLI is the least complex situation that you will encounter. I would suggest that you make up the difference using either a pedal riser (build up the pedal platform) or with clipless pedals, a modification to the shoe to increase the thickness of the sole. It is important that the pedal to hip distance be equal to allow proper extension of both legs to prevent the pelvis from rocking with each pedal stroke as this will ultimately lead to lower back problems.

Upper LLI (differences in the femur length)

In Upper LLI , the solution becomes more complex. The first thing I would suggest is that the cranks be adjusted so that they are the same percentage of the femur length. Your Cranks lengths should be between 38% and 41% of your femur length, with 39.5% being the preferred.  A good rule of thumb is to measure the femur in inches and select a crank length in centimeters. Therefore a 17 inch femur would require a 17 cm or 170 mm crank. Adjusting the crank lengths will do two things. The first is that the angular changes of the femur will be the same, this will prevent one leg or hip joint from influencing the angle of the pelvis adversely during the full pedal stroke. The other thing that this will do is to place both knees in about the same position relative to the axis of the pedal so that both legs will be efficient and comfortable spinning the same RPM.

We have addressed thigh angle with regard to LLI of the femur. It is still necessary to address the total leg length issue with regard to the bottom of the pedal stroke. As in the knee to ground LLI above, this will require some change to the pedal platform height or shoe sole thickness. Start by taking the total LLI, and subtracting the difference between the long and short cranks. This is the amount of compensation required at the pedal or shoe sole.

Combined LLI

In some rare instances, you may identify a situation where both upper and lower LLI are present. Don't make yourself crazy, just take your time and try to figure out the best combination for the rider and use a training stand to observe the rider before and after the modifications to make sure that you have gotten the desired results. In most cases, it may not be necessary to make adjustments for minor LLI (< 1 cm or 3/8 in) as with some adjustment to saddle height, the ankle rotation should take care of the situation unless it is causing lower back problems.

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Last modified: July 09, 2014