MARK GRABINER has spent most of the past 15 years tripping people up. It鈥檚 not that he鈥檚 a prankster. He has simply been trying to solve a problem that all of us face from time to time: how to avoid falling when you trip.
It has been known for decades that older people fall more than younger ones, and falls are responsible for the majority of injuries to people over the age of 65. But to understand why, Grabiner reasoned, why not study those people who usually manage to avoid it: the youth? While people in their 20s and 30s trip very regularly, they nearly always manage to stay on their feet. And it turns out a large part of this is down to what might be called a sleight of foot.
When Grabiner joined the Cleveland Clinic Foundation in Ohio in 1986, he decided to study young, healthy people鈥檚 responses to being tripped. But first, he had a problem. If volunteers knew they were going to be tripped 鈥� and they had to so they could give their permission 鈥� how could he surprise them? Grabiner, now at the University of Illinois at Chicago, worked out a way to do it.
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For his experiments, Grabiner straps the volunteers into a harness attached to a sliding track mounted in the ceiling. This catches them just before they hit the floor if they do fall over. 鈥淭hey have to be able to throw themselves as hard as they can at the ground 鈥� and miss,鈥� he says. Once the hapless subjects have got used to their new outfits, the researchers warn them they are going to be tripped. But they don鈥檛 say when or how. 鈥淲e have to be pretty devious,鈥� he says.
An assistant uncoils a rope, lays it across the lab and instructs the volunteers to walk over it normally. The victims approach the rope cautiously, but relax once they have passed it, thinking they are safe. 鈥淎nd then we trip them,鈥� Grabiner says. An aluminium box pops out of the floor, catching the unsuspecting victim鈥檚 foot as they swing it forward.
This work produced puzzling results. When people trip, the upper body pitches forward. Because older people have weaker muscles, they are less stable when they walk. So Grabiner expected to find that weak people would pitch further forward than stronger people. But this was not what his results showed. Weak or strong, people always tilted to about the same angle when they were tripped. He also found it as difficult to get strong young people to fall down as their weaker cohorts.
C茅cile Smeesters of the University of Sherbrooke in Quebec, Canada, took a more robust approach to making people fall, with slightly different results from Grabiner鈥檚. To ensure that the young subjects in her experiments fell down, she attached a rope to a hapless volunteer鈥檚 ankle and let it play out as they walked. After a random delay, a spring-loaded clamp would stop the rope, catching the leg and holding it. When she made the device hold on for long enough, anyone would reliably fall flat. Her work showed that stronger people need a greater force to trip them up. So having strong legs clearly does help keep you on your feet.
Easily floored
But Smeesters鈥檚 study didn鈥檛 entirely answer Grabiner鈥檚 question. How did young people avoid falling down when he tripped them? To try to find out, he decided to include older, unsteadier people in his work. He beefed up the harness to make it safe even for older people 鈥� including, in one case, an 86-year-old. The volunteers recruited for this study, carried out with Michael Pavol, now at Oregon State University in Corvallis, included older people who were mainly strong and in good health.
Unlike the younger participants, most of the elderly volunteers were easily floored by the box-out-of-the-floor trick. But there was a complication. The study found there were two categories of older faller: the slow and the fast. The first group knew they were weak, and tended to shuffle along slowly. In this way they gave themselves extra time to react after they tripped 鈥� though not enough, it turned out, to prevent a fall. More unexpected were the fast fallers. These were elderly people with strong back muscles and more confidence. They walked as quickly as young people, but were far easier to bring down. Why should this be?
The answer eventually came from Mirjam Pijnappels at the Free University, Amsterdam, in the Netherlands. And there seems to be more to it than brute strength.
Pijnappels improved on Grabiner鈥檚 set-up by hiding not one but 21 pop-up obstacles in the floor, making it easier to trip people up by surprising them more. She also measured the distance and height that her victims moved when they were tripped. From this, she found that people used the leg that had not been tripped to win them some extra time to recover. When one leg was tripped, they immediately pushed off with the other leg, as if hopping, but also bending their knee (see Graphic, page 48). This pushed them 40 per cent higher into the air than in a normal step, and gave them 63 per cent more time in the air before they landed again.
According to James Ashton-Miller, at the University of Michigan, these studies point to one crucial factor: the importance of muscle power, particularly in the calf. By power he means not merely the force a muscle can exert, but also how fast it can produce that force. Even allowing for differences in reaction time, calf muscles in elderly people take twice as long as to reach their maximum force as young peoples鈥� muscles do, which prevents them using their supporting leg in the same way.
And this observation could be put to practical use. 鈥淭he good news is that at any age muscle is trainable,鈥� Ashton-Miller says. For older people in good health, the potential fast fallers, exercises like dancing and skipping can increase muscle power. Staying active should make it easier to push off on that supporting leg to recover from a trip.
However, there is another important risk factor. In the labs, people who weren鈥檛 paying attention fell more often than people who were. You may have the agility of a spring lamb, but it won鈥檛 help you if you are too drunk or too engrossed in your cellphone to react before you fall on your face.
