It isn't uncommon for total hip arthroplasty and other surgical procedures to result in one slightly shorter limb. The authors of a recent study tried to determine how much of a problem leg length discrepancy is in the elderly by studying 44 healthy men and women 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.