The Sciences

Fall Guys

By Mark WheelerNov 1, 1997 6:00 AM


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When i was in my twenties, I hurt myself cross-country skiing down what one would be hard-pressed to describe even as an incline. The resulting sharp pain in my knee was embarrassing enough, but it’s even more embarrassing to admit that, being the manly jock I then saw myself as, I resisted going to the doctor. Instead, returning home, I decided to tough it out. Until several days later when, still toughing, I had to pick up a friend at the airport on a rainy night.

Using a cane, I was hobbling down the sidewalk to my car, making cautious progress, when I inadvertently planted my good leg in an unseen small mound of . . . oh, let’s just call it biological waste matter.

Now, researchers who study human locomotion would describe my subsequent slip as something that occurred because the available frictional force opposing the forward movement of my foot (in this case, the minimal frictional force of--oh, all right--the dog turd lying on the sidewalk) was less than the force of the descending foot. I would describe my slip as falling flat on my arse. I’ll spare you the humbling details of the crawl back to my house and the messy but urgent cleanup. Suffice it to say that the repair to my knee--and ego--took months.

This ignoble moment came back to me recently after I heard about researchers in Texas who are marching old people around in a circle until they slip and fall. No, this is not a case of scientists run amok. This research is being conducted in the name of safety, in the Department of Industrial Engineering at Texas Tech University in Lubbock. Nor is it as cruel as it sounds--the elderly volunteers are strapped into a fall- arresting rig, a parachute harness that’s hung from a revolving boom. That way, if they do take a dive, the rig stops their fall.

Not surprisingly, falls make up the largest percentage of accidents among the elderly population, and--young bucks, take note-- overall, falling is the second leading cause of accidental death in the United States, after automobile accidents. In 1995, 12,600 Americans died from falls; of these, 9,600 were 65 or older.

While a fair amount is known about what causes falls among children and younger adults, little is known about the physiological causes of slipping and falling among the elderly. Now, to the nonscientist, the facile answer to the question, Why do the elderly fall, would seem to be: Because they’re old. As to why little research has been done, the answer also seems obvious--because you’d have to be nuts to volunteer to nose-dive for science.

It’s true, of course, that in general your sensory functions tend to degrade as you get older, says Jeffrey Woldstad, an associate professor in the Institute for Ergonomics Research at Texas Tech. The degrading, he says, is particularly true of the elderly’s proprioceptive response--the way sensory receptors in their muscles, tendons, joints, and skin detect the position and movement of their body, then signal the spinal cord and brain to make necessary muscular adjustments as needed. Put simply, the proprioceptive response helps you immediately adjust your posture and center of gravity to regain your balance after stumbling, all without conscious thought.

While reflexes and sensory and motor functions start to go to hell over time, Woldstad says it’s not understood how they affect the pattern of gait. For example, it’s known that gait changes as we age, with the result that elderly people don’t lift their feet as high as they used to and thus trip more frequently (remember Tim Conway’s slow-motion trip on the old Carol Burnett Show?). What’s not known, though, are the biomechanical measurements that make up gait--things like stride length, the velocity of the heel when it strikes a surface, vertical and horizontal foot force, and the amount of friction required between the material of the shoe and a surface to prevent slips. That’s why Woldstad and graduate student Thurmon Lockhart decided to do their study of slip. While this is nascent research, the big idea would be to develop better flooring materials or a different shoe design (a Seeing Eye shoe that can talk? Danger! Excrement dead ahead!) that would make locomotion safe for the elderly.

Surprisingly, Texas Tech’s human-subjects committee had no problem approving the study (although I imagine the university’s lawyers’ reaction to the proposal was more in the line of Whaa?); less surprising was the initial reaction of elderly people when they were approached to take a fall (Whaa?). Then Lockhart discovered a group of retirees who volunteer time aiding patients at the Texas Tech Health Sciences Center. After some spirited convincing, he recruited 12 men and 12 women to give it a try. Each was examined by a physician; all were in good health and were at least 65 years old.

On the second day of my visit to the Red Raiders’ campus, I was privy to Lockhart’s side of a phone conversation with another potential recruit; to date, eight people have dropped out of the research (figuratively speaking), so Lockhart, who is using this project for his master’s thesis, is always scouting for more volunteers. The conversation went something like this:

No, no, it’s perfectly safe. We strap you into a harness and-- pardon? A harness. It’s attached to a boom. A boom? You know, it’s like an arm that holds you up. Yes, really. No, no one’s been hurt. Come down and try it, it’s fun.

The circular track the volunteers walk on is located in a large open room on the ground floor of the industrial engineering building. The track is 248 feet long and almost 3 feet wide and is made of wood. In its center is a metal structure about 18 feet tall that holds the suspension arm overhead; the parachute harness dangles from a cable attached to the arm. Each participant has to walk on four different surfaces--plywood, stainless steel, and both ceramic and vinyl tile--that are bolted onto about a third of the track. I looked around for an ice machine or a banana tree, but none were in sight. Instead the researchers use basic, 30-weight motor oil to promote slipping, and then on only two of the surfaces, plywood and the one I got to try, vinyl tile, which is the slipperiest of all. The subjects walk on one surface on each of four days. Each surface is walked on four times, for five minutes, at four speeds: slow (84 steps a minute), average (100 steps/minute), fast (116 steps/minute), and haul ass (132 steps/minute). A loud, annoying little machine issues a constant beeping that the subjects are supposed to match with their stride.

The oil is poured on the section of the track with the bolted-on surface; what look like giant paper towels are laid down on either side of the oiled section to absorb whatever sticks to the soles of the shoes. Under the oil are two force plates, which measure how hard one’s shoes are striking the track’s surface; the results are fed into a computer that sits close by. Finally, a motion-analysis system videotapes each subject using three cameras to capture posture and stride in three dimensions. Each volunteer wears special shoes, supplied by the researchers, that have polyvinyl chloride soles and reflectors at the toes and heels. Lockhart uses duct tape to attach additional reflectors to the left ankle, knee, and hip. The cameras capture the movement of the reflectors, and the information is fed to a computer that generates a pair of stick-figure legs on the computer screen, re-creating the way each individual moves.

Before the morning’s subjects arrive, I get to try the device out. First, I put on a pair of the ugly shoes. Lockhart slaps reflectors onto my leg and hip, then straps me into the harness.

This boom isn’t perfect, and we’d eventually like to replace it, he says. But since we’re just beginning, our only funding is seed money from the university, so we can’t afford to. The problem is that the rig is motorized, the speed set each time by Lockhart. I wondered briefly if that meant that when someone slips, the machine simply drags him along the track until Lockhart hits the whoa button. It turns out, though, that if the cable attached to the rig is yanked, it comes to a stop. That’s the safety factor--if an individual does fall, he or she will only travel about six inches before being grabbed.

Despite Lockhart’s warning, the boom catches me off balance as it starts. He’s set the machine at the medium pace, but I still have to scurry to match the beep. As I approach the oil, I realize I’ve naturally shortened my stride before carefully planting my heel. It is the heel strike, says Lockhart, where most slips occur, and it has to do with friction. For each step we take, there is a coefficient of friction (cof), which can be determined by dividing the forward thrust of the leg by its downward force. On the track, these two numbers are measured by the force plates when they are stepped on; the computer then calculates the coefficient of friction. That number must be less than the dynamic cof of the surface I’m stepping on. If it’s not--splat.

When your foot comes down, there’s actually a lot going on, and it’s all happening in a microsecond, says Lockhart, walking with me on the inside of the circle and nodding his approval as I easily stride through the oil and continue my walk. When the heel strikes the surface, it actually goes a little bit backward before resuming its forward direction; at the same time, the vertical force of your leg is increasing. That’s followed by the transition--the pulling up of your heel to stride forward. So the heel strike is a critical place where slip can occur.

Maybe so, but this trekker is on a roll. Beep, beep, beep; stride, stride, stride. No problem the second time, but the third time through my shoe slides slightly before I correct it. Good adjustment, says Lockhart. Okay, why don’t we speed it up?

Lockhart sets the apparatus to the maximum speed, 132 steps a minute, about 3 miles per hour. I’m cranking now, trying to match the damn beep, and I’m thinking, Old people do this for five minutes? As I come around the curve toward the oil, the boom is moving so fast that its lateral forces are swinging me to the outside edge of the track (another reason, Lockhart tells me, they would like to get a better rig). By the time I adjust for that, I’m on top of the oil and find myself fairly mincing on my toes to get through the slick. One foot goes out from under me sideways, but I’m able to correct before falling.

Nice recovery! grins Lockhart, grabbing a paint roller and spreading more oil on the track. Wise guy, eh? This time I’m braced for the g’s from the rig, and again I’m dancing through the slick. Even though I anticipate it, my left foot slips sideways again. This time I can’t recover in time and I fall; the rig stops instantly.

Actually, this is kind of fun. Of course, if I was 75 and brittle, it might be a different story. Okay, here we go, yells Lockhart, sounding like a cheery aerobics instructor. And 3, 2, 1, go! and the boom jerks me again. Round three through the slick, and this time I’m ready, placing more weight on my right foot and hardly any on the left. Hey, no skipping, says Lockhart, and try to stay with the beep. Damn, I forgot the beep. Again I come around, but as I cross the second paper towel and step on what’s supposed to be a dry surface, my left foot slides out from under me and I’m outta there, leaving the track and doing a 360-degree turn in the harness. I come to a sheepish stop, spinning slowly.

I unstrap, and we adjoin to the computer. Lockhart punches up my numbers as measured by the force plates, and says, Remember the required coefficient of friction is the horizontal force divided by the vertical force; your cof was about .12. I interrupt to ask if that’s a good number. Yeah, it’s good, about average, but since the available cof on the floor is only .11, you exceeded that. That means you’re going to slip and fall or have a slip-grip kind of response, meaning I slipped but caught myself.

Our conversation is stopped by the arrival of Bill, 71, the first of the three suckers--uh, subjects--I will watch. (The names of the volunteers have been changed to protect their dignity.) All three are slip veterans, having already marched on the other three surfaces. Today’s surface is the toughest each will face, but Bill is unperturbed. He proceeds to mimic my performance--no slips until the fastest speed, and once again it’s the area just before the oil that catches him off guard, spinning him around.

After Bill leaves, I tell Lockhart that beyond doing science, he’s got to be having fun with this. He laughs. You know, I really do; it brings out the kid in you. Remember how funny it was when you’d see somebody trip as a kid? He also admits to having a hard time avoiding staring at people when he’s walking around campus. I’m always looking at how people walk, how they step up on curbs. . . . I guess I’m getting a little obsessive.

Bill is followed by Ed, the jock of the group, who at 78 regularly jogs and rides a bicycle. As he dons the regulation shoes, I ask if he’s been concerned at all about hurting himself. He scoffs a polite no. Later he tells me he flew B-17s in World War II; this experiment, however, is the first time he’s ever had to make use of a parachute harness. He does fine in the slower speeds but slips three times on the oil at the fastest pace.

The force plates tell Lockhart that Ed’s cof when he wasn’t slipping was .10, compared with the oil’s .11. The numbers are usually that close to each other, he notes, which implies that our bodies are that efficient in employing only the energy they need--and no more--to control their movements.

Unless the body doesn’t bother to react at all, as when mine, in its prime, apparently couldn’t handle the challenge of a bunny slope or a scat slip. But I have no time to be bitter, since Harris has arrived, along with his wife, Loretta, for his march. Harris is a small, quiet man who quickly steps onto the track as I sit, casually chatting with Loretta. She proceeds to tell me how Harris, 77, has been through eight separate surgeries, chemotherapy for cancer, and suffers from a hernia, all of this within the last ten years. I have to restrain myself from grabbing a telephone and dialing 911 as Harris begins his journey.

His trials are nearly flawless, even though Lockhart uses the paint roller several times to swab on additional oil. (Oh my, that doesn’t seem quite fair, Loretta comments when she first notices him.) Regardless, Harris doesn’t slip until the very end, and then only at the same spot before the actual slick, where oily buildup must be occurring.

After the couple leaves, Lockhart says he’s particularly interested in looking at Harris’s data. There’s something about this guy’s gait that makes him very stable, very light on his feet, he tells me. He almost always seemed to keep his center of gravity over his feet and to make corrections with very little effort. Later I ask Woldstad how the body is able to do this so efficiently, and so fast. The answer, it turns out, is Go figure. There are simply too many variables that come into play. For example, he says, People will change the way they move over a given amount of time, and those adjustments are based on parameters we don’t understand quite yet--part of it is various constraints in the environment, or we’ll devise different ways to move in order to conserve energy. Yet on top of that, there’s inherent error in our muscular systems. So figuring out how all these things fit together is what’s hard.

As I leave the lab, I’m musing to myself about how truly remarkable it is that we can navigate hill and dale without conscious thought. Suddenly I realize I’ve been staring at the long, tanned legs of a young woman who is crossing in front of me. Looking up, I meet her eyes and see she’s glaring at me. Flushing, I start to tell her I was merely admiring her biomechanical abilities, but then think better of it. Instead I turn and locomote away.

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