Here is the scoop on leg-length discrepancy:
1. The only accurate way to determine actual bone-length LLD is midline-measurment on x-ray film. This measure does not indicate effective LLD.
2. The only accurate way to determine actual greater trochanter to calcaneus LLD (effective) is vertical axis measurement on x-ray film
3. Pelvic angles alter effective leg-length. Anterior illlium roation = shorter leg; Posterior, longer leg. You can examine this principle by tilting and tucking your entire pelvis.
4. Arch height alters effective leg-length. Higher/ hard arch = longer leg; Lower/softer arch = shorter leg.
5. Heel drop alters effective leg–length. Achilles tendon tears, hamstring tears = lesser or greater extension to pedal. Heel drop= shorter; toe point = longer. Rule-out heel drop differences to accomodate actual LLD.
6. Spine curvature alters effective leg-length. Right translated spine = short left leg; left translated spine = short right leg. Rule-out actual LLD causing spine translation (longer left leg forcing left hip, up, and spine, right, etc.). Right rotated spine = long right leg; left rotated spine = long left leg (unless caused by femoral LLD or countered by opposing rotation).
7. Unequal arm length, golf, unilateral breathing (swimming), tennis, etc., can all contribute to effective LLD’s for cyclists.
8. Tibial LLD is altered by vertical correction; Femoral LLD requires horozontal correction with greater emphasis on 3pm and 9pm and less at 12pm and 6pm. There are a few crank builders who manufacturer cranks specific to the latter requirements.
9. Not all LLD’s require correction.
10. Symptoms that suggest a visit to your local bicycle fit guru-guy: Unilateral back pain, unilateral knee pain, strong weight imbalance between arms; unilateral saddle positioning (sitting off to one side (barring saddle sore)), riding in a “wind-swept” position (one knee close to the top-tube, the other far from the top-tube), etc.
Most LLD’s are easily corrected by a good fitter with the right tools.