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Health Information Step by Step: Energy, Motion and Shoes

When you buy shoes, you're probably thinking only about your feet. But the laws of physics tell us you should also consider your knees. It's a matter of force — and the energy that gets transferred from the ground to your knees with every step.

Wear and tear from the impact of all these steps can eventually lead to painful knee osteoarthritis, a degenerative joint disease that oc­­curs when energy-absorbing joint cartilage breaks down. Modern supportive walking shoes might seem to be a good solution. But research from Najia Shakoor, MD, a rheumatologist at Rush University Medical Center, found just the opposite.

The transfer of energy
Knee load is the phrase researchers use to describe how a number of factors affect the knee. For example, the position of your feet when they hit the ground contributes to knee load. So does the way you're built — the angle between your feet, knees and hips.

"When your foot hits the ground, the down- ward force from your step creates an upward ground reaction force that affects your leg," Shakoor says. This ground reaction force can be thought of as a force vector, and it involves a basic law of physics — Newton's third law of motion: For every action there is an equal and opposite reaction. That means that with every step you take, whatever force you exert on the ground is sent right back up, transferring energy to your knee.

While you can't control how your body's joints are shaped, some elements of knee load are entirely within your control. That includes your weight and — as Shakoor and her colleagues  discovered — your shoes.

"The very stable, supportive shoes were associated with the highest overall knee load compared with more flat, flexible, lightweight shoes," Shakoor says.

Energy in motion
This connection between shoes and knee joints came to light in the Motion Analysis Laboratory at Rush. The lab is equipped with a 3-D measurement system that records movement — energy transfer — as people walk on a special platform.

Because Shakoor's previous research showed barefoot walking decreases knee loading, she and her colleagues designed flexible footwear that mimics barefoot walking. In the lab, these "mobility shoes" significantly reduced knee load compared with conventional walking shoes.

Volunteers from Shakoor's earlier study are now testing mobility shoes during everyday use, and early results show they significantly reduce knee loads and knee pain.

Researchers at Rush are also studying whether the right choice of footwear can go beyond relieving pain and actually change the progression of osteoarthritis.

Your doctor can tell you if flat, flexible  shoes might work to reduce your knee pain. They're not the right answer for everyone — people who have foot problems may need  more support.

Although we can't escape the laws of physics, research continues to show us ways to make them work for — instead of against — our joints.

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