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Arthritic Hips Out of Rhythm: New Rhyme for the Reason

A basic idea in science says that for every action there's an equal and opposite reaction. If someone gets pushed, the usual reaction is to shove back. The same thing happens when we walk. The foot and the ground each exert a certain amount of force on each other. What if someone has hip pain from arthritis? The natural response is to change how fast and how hard the foot hits the ground. However, the force of the ground meeting the foot doesn't change. Where does the rest of that energy go?

Interesting question! When it is necessary to change the way we walk, the body compensates by moving differently. The force goes to a different section of the body. Suddenly, the foot isn't striking the ground on the painful side of the body as hard as it is on the other. The force or action is shifted away from the hip to the pelvis and the knee.

How do scientists know that? Advances in camera technology have made it possible to study how people walk. Specialized cameras can capture movement during walking from all angles. A special plate on the floor measures foot force without the person even knowing it is there. Computer software takes the measurements and does the math. Here's what researchers have discovered.

People in early stages of osteoarthritis start to walk with shorter steps and slower speeds. They use fewer steps per minute, a measure of cadence or rhythm. Posture changes as they lean forward. This causes a flatter low back with less of an arch or curve. The force through the hip is less because the pelvis (bones between the hips) tilts down to take some of the force instead.

Not only does the pelvis adjust for the change in force through the foot, so does the knee! The knee changes the way it moves when the hip cannot absorb the full force of the ground. As the arthritis gets worse, these changes increase. Over time, these compensatory changes can cause even more problems because the back and the knee have to take more force than normal.

New technology has allowed scientists to show how (and how much) the muscles and joints compensate for early arthritic hip pain. The body has amazing strategies for handling this type of pain. Knowing these kinds of changes begin early in arthritis can help doctors and therapists develop ways to prevent future problems.

References: Eric Watelain, PhD, et al. Pelvic and Lower Limb Compensatory Actions of Subjects in an Early Stage of Hip Osteoarthritis. In Archives of Physical Medicine and Rehabilitation. December 2001. Vol. 82. No. 12. Pp. 1705-1711.

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