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This question, submitted by a reader, seems simple enough. By the age of 15 months, for example, most children have learned how to walk.

But the mechanics of walking—purposely placing one foot in front of the other—are too often taken for granted. Even young children limp. A 1985 study found that about four percent of children who visit hospital emergency departments report limping, an “asymmetric abnormality of the gait,” as their primary complaint.

It seems that walking upright, a trait that defines modern humans, is surprisingly complex. It requires continuous cooperation and information sharing between the nervous, musculoskeletal and cardiorespiratory systems. Here’s how two Austrian neurologists describe, in simple mechanical terms, what it means to walk. (Do not try this without supervision.)

“To start walking, one leg is raised and directed forward by flexing the hips and knee. Activation of the supporting contralateral leg and trunk muscles moves the body’s center of gravity over the weight-bearing leg and forward. The heel of the swinging leg is then placed on the ground. The body weight is gradually shifted to the sole and then onwards to the toes. During mid-stance, the opposite leg is lifted and moves forward until the heel strikes the ground. Meanwhile, the body is held upright, the shoulders and pelvis remain relatively level and each arm swings in the direction opposite to that of its ipsilateral leg.”

“Safe walking,” they add, “requires intact cognition and executive control.”

Anything that complicated is likely to spawn flaws, such as limping, over the course of a lifetime. Once people reach their 60s, that limping rate has more than doubled. An estimated 10 percent or more of people that age walk with a noticeable limp.

In their late 60s and 70s, walkers tend to lose speed, slowing their pace by about one to two percent per year. Gait velocity, further slowed by limping, seems to be a powerful predictor of mortality. By the time people reach their 80s and 90s, sixty percent of them exhibit some sort of chronic limp.

So we spoke with Douglas Dirschl, MD, the Lowell T. Coggeshall Professor and Chairman of the Department of Orthopaedic Surgery and Rehabilitation Medicine at the University of Chicago, to get a better understanding of why so many older people walk with a limp.

The published data on limping seems somewhat vague. How common are gait abnormalities?

Dirschl: We don’t really know. Just from teaching orthopedics for as long as I have, and being a keen observer when I’m out in public, I would estimate 15 to 20 percent of people have something in their gait you can pick up, such as pain, or weakness on one side, or a leg-length discrepancy.

Keep in mind, Olympic sprinter Usain Bolt, the fastest human on earth and recipient of eight gold medals, has a slight leg-length discrepancy.

Why is limping so common, especially in the elderly?

Dirschl: There are four basic reasons why people develop a limp. Something is painful, stiff or weak—or one leg is substantially different. That last option is responsible for only a small percentage of limps, so I will focus on the first three. Any one of these, or any combination, could cause someone to limp.

Let’s begin with pain. In younger people the primary cause is physical trauma, such as sports injuries or motor vehicle accidents. The most common cause of pain in older people, however, is arthritis, which causes pain as well as stiffness. If someone already has a limp, inflammation makes it even more pronounced.

Pain can affect any of the limping-related joints: hip, knee, ankle or foot. Discomfort in any of those locations is common and is likely to be experienced by more and more people as they age. Each specific location causes a slightly different kind of limp.

Diagnosing the cause of a limp is based on history, physical exam findings, laboratory tests, and radiological examination. If a limp is associated with pain it should be urgently investigated.

How about stiffness?

Dirschl: Stiffness can result in slightly different gaits. Someone who has a stiff lumbar spine—meaning nearly fused—will walk with a certain type of gait. Anyone with a stiff hip or knee or ankle or foot, will also walk with a fairly joint-specific limp. An injury or surgical procedure could lead to stiffness or fusion of a joint. That could lead to a limp. Each is subtly different, but an expert could tell the difference.

How can you tell?

Dirschl: People in this field see a lot of walking problems. Each painful joint presents a somewhat different limp. People find different ways to limit the discomfort. We have specific names for many of them. A coxalgic gait, for example, is a particular response to hip pain.

When you see someone walking normally, there should be a natural, balanced rhythm to their gait. For analysis, we tend to break that rhythm up into smaller segments, focusing on how each leg moves through each stage of the cycle and how it adapts to the pain of weight bearing or the limitations caused by stiffness.

How do you measure that?

Walking involves two phases: the stance phase, where the foot on that leg is in contact with the ground; and the swing phase, where the foot is raised and not in contact with the ground. People with a painful limp will shorten the stance phase on the side that hurts. They want to minimize the discomfort caused by weight-bearing.

Is there an obvious difference in how people respond to pain and stiffness?

In people with a stiff limp, you have to look a little more carefully to see which joints aren’t moving in the normal pattern. Take knee stiffness, which is probably the easiest to spot. Imagine someone who suffers from extreme knee stiffness, where the knee is kept, or may even be stuck, in a single position. If you watch this person go through the gait cycle you can see that the knee simply doesn’t move.

Normally, the knee would extend, just before the foot strikes the ground. It would be relatively straight throughout the stand phase and then, as you get to the swing phase, that knee would flex when the foot comes off the ground to make room for the leg to pass over it again. We can watch all that in a gait cycle and see if it does or doesn’t occur.

A stiff hip is a little harder to notice because the hip—unlike the leg—may rely more on rotation of the pelvis. One way to get your leg out in front of you is to rotate your pelvis around your spine.

Knees, hips, what about ankles?

A gait with a very stiff ankle is also easy to spot. Most patients will externally rotate that foot, or the whole leg, so that the foot swings wide and points out to one side. That way they can roll over the inside of the foot rather than have to bend the ankle.

In this situation, the ankle can’t move the way it needs to go during the stance phase as your body progresses from behind your foot, which has hit the ground, to in front of the foot.

Instead, they move like someone who’s in a cast or a heavy boot. Those don’t allow the ankle to move, so people walk with their foot turned out. It’s just easier.

How about weakness? That must be common in the elderly.

Yes, the third cause of limping in the elderly is muscle weakness. Once again, this can be subtle and offers multiple versions, often combined with pain.

One common example is someone who has hip pain caused by inflammation of the fluid-filled sac, the bursa, on the side of the hip. This is accompanied with weakness in the gluteus medius muscle.

This can cause what we call the Trendelenberg lurch, named after the German surgeon Friedrich Trendelenburg. Because that muscle is weak on one side, it isn’t strong enough to hold the pelvis level. So the pelvis will dip when the patient tries to stand on the weaker leg.

We also see elderly patients with significant overall deconditioning, a loss of muscle strength, often combined with late-stage diabetes. They may be weak in a lot of areas: from the hips to the ankles. Those all result in a characteristic limp.

Is there a nervous system component as well?

There is indeed. Nerve damage, such as peripheral neuropathy, balance problems and loss of coordination can disrupt a normal gait. Damage to the perineal nerve can prevent them from pulling the front of the foot up. Those people have an unusual limp, characterized by lifting the leg higher in the air than normal.

We call that a neuropathic, or steppage, gait. This was first described in people with secondary syphilis. That’s why syphilis, back in Victorian times, was called “the clap.” Your foot would clap against the ground as you walked with this odd gait. It became a generic term for any venereal disease.

So much can go wrong. How can you fix these problems?

The best thing is to prevent them, primarily through strength and balance training. If you can’t stand on one foot for at least five to ten seconds, make an appointment.

We also have many ways to reduce pain, especially the wear and tear of arthritis. Non-painful limps can be investigated and treated more gradually.

Modest stiffness can improve with stretching, physical therapy and manipulation. If it’s rigid stiffness, that might require surgical intervention. Physical therapy and weight training exercises also can help with moderate weakness.

Weakness from a neurological injury, on the other hand, is not something we can correct with exercise. Instead, we would consider ankle braces or a custom-designed shoe that could support the foot in a functional position. It could cause the ankle to be stiff, but we prefer stiffness over weakness.

How big of an industry is this?

That’s hard to say. It’s all interwoven with a variety of other industries: musculoskeletal care, physical therapy, durable medical equipment such as braces, bandages and boots, canes and crutches, heating pads and pain patches—all sorts of little things that might help you walk.

Are there new ways to approach limping?

There’s a lot of new technology in orthopedics. We have new and better joint replacements and we can now perform the operations using the surgical robot, which has much smaller incisions. This can reduce discomfort and speed recovery.

We have a vast array of new hip and knee replacements. We can replace just one side of the knee, as well as the standard total knee replacements. We have done almost 500 of those. We have the largest center and the only training center in the Midwest.

More recently, we have brought robotic assistance to spine surgery, a significant addition to our robust spine program. Instead of making a long incision along the spine and spreading the spinal muscles aside to see and reach it, the robot lets us work through little incisions and tubes at each level of the spine. That means no big incision, just a series of small incisions and entry through tubes.

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