It can be, depending on how great the difference is and what physical condition the person is in. Researchers recently attempted to determine how much discrepancy could become a problem using a set of healthy participants age 55 to 86.

Small shoe lifts of varing heights were placed in the participant's shoes. Then they walked on a treadmill. Heart rate, muscle activity, oxygen use, and air exchange were monitored.

Researchers found that a 2 cm of leg-length difference had a considerable effect on oxygen use and how hard the walkers felt they were working. Between 2 and 3 cm of difference in leg length made a big difference in most of the factors tested.

This led researchers to conclude that elderly patients with significant cardiac, pulmonary, or musculoskeletal problems might have trouble walking with even 2 cm of difference in leg length.