What are the pros and cons of exercising in water instead of on land?
As baby boomers get older and their joints get creakier, the fitness industry will need to respond to a new set of needs - and Charlene Kopansky believes she has the answer.
"Water is magical," says Ms. Kopansky, who has led the Canadian Aquafitness Leaders Alliance since 1993. "It's the place to train."
A growing pile of evidence supports the idea that water-based exercise is a good approach for people with joint pain or poor balance, offering some unique benefits that can't be replicated on land. But new research also suggests that, to get the full benefits, you need a well-designed program that emphasizes the links between your movements in the water and functional movements on land.
The differences between water and land might seem obvious, but there are some subtleties. For example, the pressure exerted by water against your body is strongest at the bottom of the pool, where your feet are, and weakest at the top. This pressure gradient helps push blood back towards your heart, making its job easier. As a result, Ms. Kopansky says, your heart rate in the water will be 10 to 17 beats per minute lower during a water-based workout.
Another key difference is the type of muscle contraction. On land, you use essentially the same muscle to lift and lower a weight, fighting against gravity in both cases. But water offers resistance in every direction, meaning you have to use the opposing muscle - triceps instead of biceps, for example - to return to your starting position.
Not only does this prevent muscle imbalances from developing, but it also helps to avoid muscle soreness, which is usually associated with the "eccentric" muscle contractions involved in lowering a weight on land.
Most importantly, water's buoyancy means that you only bear about 10 per cent of your usual weight if you're immersed to your shoulders. "Buoyancy unloads you, like there's no gravity," Ms. Kopansky says. "Compression is decreased in every joint in your body, which decreases pain of movement and the chance of injury."
So it feels good - but does it get you fit? A number of studies have compared water-based exercise programs to comparable routines incorporating aerobic exercise, strengthening and stretching, with results that are encouraging, but not earth-shattering.
"Most studies find that the gains people make in water are similar to those made on land," says Cathy Arnold, a physiotherapist and professor at the University of Saskatchewan who has conducted several aquafitness studies.
For example, a 2008 study in the journal Geriatrics & Gerontology International compared 12 weeks of water-based exercise with land-based exercise in women in their early 60s, and found similar improvements in strength, flexibility and aerobic fitness in both groups compared to non-exercising controls.
It's not that aquafitness is necessarily better, Dr. Arnold says - the advantage is that it's more accessible. For example, her research focuses on older adults with osteoporosis, who can exercise in the water without the fear of a bone-breaking fall.
Her most recent study, published last year, found that aquatic exercise twice a week improved the balance of a group of adults over 65 with hip osteoarthritis. But only those who also received education about how the workouts related to their daily lives - how doing squats in the water related to standing up from a chair, for example - were able to improve their functional performance and reduce their risk of falling.
"You need to understand why you're doing the exercises you're doing," she says. "That cognitive piece is important."
Ultimately, like any exercise program, the benefits also depend on your execution. The resistance provided by the water increases exponentially as a function of how fast you move, which makes the guidance of a good instructor crucial. Too slow, and you won't get your heart beating - and too fast is just as bad.
"People think if it's faster, it must be better," Ms. Kopansky says. "But you can't do the movements that fast in water. You end up with people jumping around but not getting the benefits."
Pool running for athletes
Pool exercise may be low-impact, but that doesn't mean it has to be easy. Competitive runners have long relied on deep-water pool running as a cross-training activity, using a flotation belt to allow them to mimic a "normal" high-knees running form without touching the bottom.
Electromyogram studies have found that pool running activates most of the same muscles as running, without aggravating injuries. Because the water's hydrostatic pressure lowers heart rate, it's difficult to sustain high heart rates for extended periods of time, so most workouts alternate bursts of "sprinting" with slower periods of recovery.
Canadian mile record-holder Kevin Sullivan relied on this gut-busting, hour-long workout to maintain fitness when he suffered a stress fracture in his sacrum (located above the tailbone) before the 2002 Commonwealth Games:
5 minutes/2.5 minutes
4 minutes/2 minutes (times 2)
3 minutes/1.5 minutes (times 3)
2 minutes/1 minute (times 4)
1 minute/30 seconds (times 5)
30 seconds/15 seconds (times 6)
15 seconds/10 seconds (times 7)
Alex Hutchinson blogs about research on exercise at sweatscience.com. His new book, Which Comes First, Cardio or Weights?, will be published in May.