I describe any difference between the length of one leg to the other as either true or apparent.
A true difference I leg length is anatomical and can be confirmed with a standing X-ray of the pelvis and a tape measure. In the second instance, the legs are of equal length but one appears shorter. The underlying cause is always secondary.
One if the most common contributing factors is a fallen arch. Other contributing factors include alignment, scoliosis, and muscle tension.
In either case, there is a distortion pattern created that may contribute to arch and heel pain (fasciitis), knee or lateral hip pain (bursitis), and finally low back pain.
Since 1984, I’ve provided flexible and semi-rigid arch supports as a means to support the fallen arch while stabilizing the knee, hip, and lower back.