As have been mentioned, it's easy to determine the source of discrepency for someone trained in this area (namely physical therapists, etc. ). Below are the basic steps if you think you have someone whom may be able to help.
Finding Leg Length Discrepancy 1. Determine if pelvis is aligned: Stand flat footed, have someone palpate (feel) your hips and located a boney reference point (Typically the PSIS). See if these two sides are even with each other. If a difference is seen, then you have one hip rotated compared to the other. This is not structural, it can be fixed with mobilization / therapy by a trained individual.
2. Lay down flat on a table (on your back). Do a bridge or two, then have someone grasp your ankles and pull gentle. As they have slight traction, see if you ankles line up or if one is longer than the other. If a difference is seen, move to step 3.
3. While laying on back, bend knees to approximately 90 degrees, keeping feet flat on surface. Have helper look at knees in relation to each other to see whether the discrepancy is coming from. (e.g. find the femur or tibia that is shorter than it's counterpart on the opposite leg. Think of this like legs of a triangle and how they would move the apex of the triangle if a side were shorter).
Correcting for Leg Length Discrepancy 4. You always want to affect the shorter limb. Fit for the longer limb and then make corrections to adapt the shorter.
5. If you tibia is shorter, then you need to add more height because the tibia is *mostly* vertical during pedaling. this is done with shims between the cleat and the sole of the shoe. Multiple sources online
6. If you femur is shorter, then you need to shift your cleats because the femur is *mostly* horizontal during pedaling. Different shoes / cleat systems can accommodate this to different degrees. There are adaptor made for speedplay cleats that can shift back further than the standard adjustment.
When making corrections on the bike, only attempt to correct for half of the discrepancy. e.g. if a femur or tibia is 10mm short, then you typically want to only correct for 5mm of difference.
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