Unequal leg length is so common in the general population that many consider it normal. Minor differences in leg lengths will not affect most sedentary people because they do not have to adapt to the same biomechanical stress that a runner faces on a daily basis. Runners have to absorb three to five times their body weight with each foot strike. Running is a sport that requires equal and symmetrical movement, strength, and impact on both sides of the body. Running with asymmetrical leg lengths is comparable to running on the same side of a banked road. Over a few miles the difference may be hardly noticeable. However, over the course of days, weeks, months, and years the altered biomechanics and resulting stress will exceed the body's capacity to adapt and repair the damage. This is when injuries may occur. Common injuries associated with leg length discrepancies include low back pain, sciatica, hip pain, knee problems, IT band syndrome, and chronic muscle and tendon conditions.

A study conducted by the Thurston Arthritis Research Center at the University of North Carolina found that people with legs of different lengths were more likely to have arthritis in the hip and especially the knee. And those who had more severe cases of arthritis were more likely to have legs of different lengths. If unequal leg lengths can cause or worsen arthritis, imagine what it can do the muscles, tendons, connective tissues, and bones of a runner.

There are two classifications of leg length differentials: anatomical and functional. An anatomical short leg is one in which there is a measurable difference in the actual length of the bones of the two legs. This may be due to past fractures, knee or hip degeneration, deformities, or unequal growth rates of the bones over time. By contrast, a functional short leg appears shorter, but measures the same length as the other leg. Common causes can be structural imbalances of the body including a rotated pelvis, scoliosis, imbalances in the muscles from one side of the body to the other, and pronation of the foot (particularly if the feet are asymmetrical in weight bearing).

For anatomical differences, a heel lift on the short leg can balance out the difference. This can be done in conjunction with an orthotic when necessary. However, a heel lift is not the proper answer for a functional short leg as this may only serve to reinforce the altered biomechanics and resulting stress. Supporting the arch with an orthotic and doing strengthening exercises for the lower leg may stabilize pronation. Structural misalignments can be treated utilizing spinal manipulation in combination with stretching and strengthening of the hamstrings, piriformis, core abdominal, and hip flexor muscles.

Running injuries are greatly influenced by the imbalance of unequal leg lengths. If you are suffering from chronic and recurrent injuries, particularly on one side of the body, it is time have your leg lengths evaluated!