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It was parents' day at my younger daughter's karate class, and those of us who dared, took off our shoes and socks and had a go at it with the kids.
The instructor showed us how to lean back on one leg and kick a rectangular plastic sheet he held with one hand.
When my turn came, I approached him cautiously, bounced and wobbled on my "planted" left leg, and kicked the sheet three times before bringing my right leg down to the floor to keep from falling over.
The blond-haired youngster next to me did it about 46 times, barely wavering as his left leg held him rock-steady and his right foot snapped out a snare-drum rhythm on the crinkly plastic film.
We switched to the other leg, with more or less the same results. OK, the Karate Kid takes lessons and I do not.
He is young and limber, and I am sure his knees do not make crackling sounds when he gets out of bed in the morning. But the difference in our ability to simply balance on one foot was something of a shock.
"When most people think about exercise and being healthy, they normally think about their cardiovascular health, about running and doing endurance exercises, or we think we have to keep our muscles strong, so we do resistance exercises," says Chanda Dutta, chief of the clinical gerontology branch of the National Institute on Ageing.
As we spend middle age sweating away kilos to ward off obesity, watching our diets to keep arteries from clogging, and strengthening our muscles to retain vigour into old age, we might want to stop and think about our balance. You take it for granted, right? You get up from your office chair, walk across the room, get a drink of water, walk back to your desk, and nothing terrible happens.
You glide across a tennis court and smack a forehand without stumbling, take out the garbage and return unscathed, step in and out of the shower without incident. And then one day, without warning, that can change.
"It isn't until we lose that ability that we realise how important these things are," Dr Dutta says.
Falls among older people are common, costly and debilitating.
More than one-third of people aged 65 or more will fall this year, according to the Centres for Disease Control and Prevention. Men are 49% more likely than women to be killed in falls.
But women are much more likely to suffer nonfatal injuries or fractured bones.
Women make up nearly three-quarters of the people admitted to hospitals for hip fractures, one of the most debilitating results of falls.
Your ability to stay upright and move confidently through space is determined by a complex combination of muscle strength, nerve function, visual inputs, the vestibular function of your inner ear and your proprioception - the work of sensors, including nerves in the soles of your feet, that orient you in relation to other objects.
These abilities can decline with age, though people age differently.
Disease, head trauma, compromised blood flow to the brain, medication problems and many other factors can affect them.
Loss of balance and mobility can be prevented or delayed if we work at keeping these abilities sharp.
Yet unless you practice yoga, karate, tai chi or a handful of other fitness regimens that emphasise balance training, you probably have not done anything about it in years, possibly decades.
"We don't put ourselves in the situations very often where we have to maintain our balance," says James S.
Skinner, a professor emeritus of kinesiology at Indiana University and a past president of the American College of Sports Medicine.
The good news, as with most things concerning health and fitness, is that you can make substantial gains, even well into old age.
After a lifetime devoid of physical exercise, Henry David arrived at Fox Hill, an upmarket community for seniors in Bethesda, Maryland, walking with the aid of a cane.
He had not suffered a stroke or a fall, or a bad reaction to a medication.
He had just grown wobbly over a lifetime of ignoring the care of his body as he pursued his career across the globe. On top of his actual deficit was a loss of confidence.
Not only was the 86-year-old psychologist in danger of a debilitating tumble, he feared falling all the time.
"I felt insecure without the cane," he says.
David has largely given up his cane after three or four months of balance classes at Fox Hill, a complex of condominiums and an assisted-living facility.
In twice-weekly balance classes, trainer Anthony Absalon takes his students, who are in their late 70s on average, through stretches and exercises designed to strengthen their muscles and make them more comfortable with being slightly off-balance.
They stand on one foot, then the other.
They walk heel to toe (think of a field sobriety test) and then reverse it.
They hold the poses looking up at the ceiling and with their eyes closed.
Each person has a chair nearby; some hold on, others do not. "I'm still not where I want to be," Mr David says.
"But I definitely feel more secure in my ability to get around without the cane. I gradually deteriorated. And now I'm coming back up."
Dr Dutta wants you to start such efforts now, in your 20s or 50s, not your 80s. (She would add flexibility exercises to the list of fitness imperatives, but that is a topic for another day.)
The exercises are quick, easy and effective, she says, even if researchers do not entirely understand how the body rewires itself to yield such improvements.
For example, when she is waiting in an airport, she practices standing on one leg for short periods.
"The earlier you start, the better off you are in adulthood and thereafter," she says.
"It's better to maintain your function than to let things go and try to improve them afterwards."
- Lenny Bernstein