A leg length discrepancy, if severe enough, must be corrected. Asymmetrical muscular growth can lead to low back pain, hip pain, and knee pain, depending on the asymmetry. A leg length discrepancy can be observed from behind by looking at the levelness of the pelvis during a deadlift or squat. A more rigorous measurement progress is outlined by Mark Rippetoe in his Leg Length Discrepancy article and will not be covered here. Correcting a leg length discrepancy is as simple as building up (shimming) the shoe on the short side. 

For the frugal, a few hours of work and $3.19 of contact cement can replace the $100 and one-week wait it will cost to have a squat shoe professionally shimmed. Having called ten shoe repair shops, I noted a price range between $69 and $150. Size of city didn't seem to be relevant. For dress shoes or other expensive shoes, I would recommend paying to have it shimmed professionally. However, for a functional shim for squat shoes, a far cheaper price can be achieved with a few tools:

• Contact cement
• Jigsaw (or utility knife)
• Belt sander
• Marker

This shim won't win any fashion awards or marathons, but neither of those activities should be done in squat shoes anyway. 

In the example following, I build up a shoe sole a half inch using horse trailer mat.  Part of the reason a shoe repair shop is more expensive is because they remove the sole of the shoe, attach the shim to the shoe, then reattach the sole to the shim. My method simply adds the shim to the bottom of the sole.

Before reading on, know that there are many ways to shim a squat shoe. There are two fundamental mechanisms happening here. First, a shim has to be formed from the raw material. Second, the shim has to be attached to the shoe. As long as these two steps are completed, the method doesn't matter – supergluing thin strips of vinyl to the shoe could theoretically work. That said, here's how I did it: 

1) Stencil marks onto the shim work piece (rubber mat) using the factory shoe.

2) Cut out the shape of the shim.

Rubber powderizes when it's machined, so I chose to wear a respirator. Leave a little extra material on the perimeter. It's easy to remove material; it's extremely difficult to add a little bit of material back. I did this with a jigsaw, but just to show that it could be done, here's what it looks like with just a utility knife.

Note: Do not clean up the edges. They'll get sanded flush to the factory shoe later anyway.

3) Expose the bare rubber on the top of the shim and the bottom of the factory shoe.

These rubbers are coated with a protective layer. This layer has to be removed or the contact cement won't adhere to the work pieces.

4) Apply the contact cement evenly on both mating surfaces.

Applying it too thick is better than too thin, because the perimeter is going to be sanded down before the shoe is done anyway. Too thin can lead to weak bond. Adhesion mechanics favor a clean surface, so vacuum the surfaces and wipe them down with a damp rag.

Because the stress is going to be in compression while used and the contact cement is tension-resisting, the shoe should be able to be used immediately after fabrication. Contact cement is essentially just synthetic rubber (butyl-rubber) dissolved in a strong solvent (<3% methylcyclohexane) with weaker solvents to allow the adhesive to flow. Unlike wood glue, which bonds wood to wood by soaking the pores of each work piece and then bonding to itself as it cures, contact cement adheres just to the surfaces of the work pieces and then to itself. The solvents in the contact cement allow the cement to flow and be applied to the work pieces. After 15-30 minutes, these weaker solvents will have evaporated and all that remains is the adhering compounds. 

As soon as the two work pieces touch, they're stuck. Technically, after 48 hours of a cool environment, the bonding strength maxes out. For our purposes, once the work pieces are pushed together, they're not coming apart. 

5) Align the shim to the shoe starting with the toe and "rolling" the shim down toward the heel.

This will ensure the highest contact surface interaction. Also, errors on the toe end are more noticeable than the heel end. If the shim is not rolled onto the shoe, it's possible the shim will curl the shoe. 

6) Just using hands, compress the shim and shoe to make sure all surfaces have bonded. 

7) Sand the edges flush.

This was easy to see, because as a consequence of stenciling, the edges of the factory shoe were all marked with red sharpie.

8) Vacuum the inside and outside of the shoe and wipe it down with a damp cloth or dry tack cloth.

9) If the shoe is too stiff, slits can be cut on the bottom to give more flexibility.

A sufficiently high LLD should be corrected. Even just stacking plywood on one side of the squat rack platform is better than letting it go and being in pain.

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