Exercising in water and other types of exercise for maintaining strong bones
What are the pros and cons of exercising in water?
One of the classic problems with exercise is that those who most need its health benefits sometimes have the greatest difficulty doing it. If you have osteoarthritis, your joints hurt; if you’re battling obesity, the impact forces will put you at risk of injury; if you’re a senior, you could break bones in a fall. A common solution to this problem is to exercise in the water, often in the form of water aerobics or aquafit group classes. There’s no doubt that this lower-impact approach helps reduce some of the risks of exercise for at-risk groups, but it’s only recently that researchers have started to ask whether it really provides the same benefits as land-based workouts.
One key difference between water aerobics and swimming stems from the fact that you’re standing vertically in the water, rather than floating horizontally. Since water pressure underwater is greater than it is near the surface, the pressure exerted on your feet will be greater than it is on your chest. As a result, blood is pushed back from your extremities toward your heart with less effort than usual. This means that your heart rate will be lower at a given level of effort—meaning you have to work harder to get the equivalent cardiovascular workout that you’d get on land.
Nonetheless, a number of studies have found that water-based exercise has valuable benefits, particularly for groups with special needs. For example, a 2009 study found that a four-week aquatic exercise program consisting of a mix of aerobic, stretching, and strengthening exercise produced greater improvements in patients with lower-back pain than a comparable four-week program on land. The researchers speculated that water pressure and temperature reduced pain signals during exercise, and buoyancy reduced stress on joints and muscles, enabling a greater range of motion.
Several studies have explored aquatic exercise for patients with hip and knee osteoarthritis. A review of these studies in 2009 concluded that it likely produced short-term benefits, though further research is required to determine whether the benefits persist in the long term. More generally, older adults often face a combination of different challenges—aching joints, poor balance, weak bones—that make water a more attractive option. A 2008 study of 50 women between 62 and 65 years old found that water-based exercise produced greater improvements in cardiovascular fitness, agility, and flexibility than a land-based walking program. Of course, it’s not really fair to compare a water-based program that includes aerobic, strength, and flexibility exercise to a simple walking program. The key difference isn’t that aquatic exercise is better than exercise on dry land—it’s that people feel comfortable doing exercises in the pool that they wouldn’t do at all on land.
There is, however, one recent study that hints at the possibility that training in water might offer some subtle benefits that you can’t duplicate elsewhere. In 2009, French researchers found that a group of patients with either chronic heart failure or coronary artery disease displayed an unexpected rise in nitric oxide levels in their blood vessel linings after a three-week aquatic exercise program—a change expected to lower their risk of death. Although the findings are very preliminary, the researchers suggest that the altered circulation caused by the water’s pressure gradient may produce additional cardiovascular effects (beyond the lowered heart rate noted above). For now, though, it’s just a theory.
What type of exercise is best for maintaining strong bones?
The key word here is “maintain,” since 95 percent of your mature skeleton is already in place by the age of 17 for girls and 19 for boys. Once you reach adulthood, it’s basically one long fight against the slow but inexorable weakening of your bones. According to conventional wisdom, the key to that fight is engaging in weight-bearing activities—those in which you’re standing and supporting your own weight rather than being seated. But the latest research shows strength training can also play a key role—and in fact, lifting weights may be even more effective than some weight-bearing activities like elliptical training.
“Over the past decade, people have realized that bone is more dynamic than we thought. It’s actually a pretty responsive tissue,” says Heather McKay, a professor in the faculty of medicine at the University of British Columbia and the director of the Centre for Hip Health and Mobility. It turns out that training your bones has more in common with training your muscles than previously thought: if you stress them, they’ll get stronger. How much stronger depends on what your body is currently used to, how big a load you apply, and how you apply it. Recent studies by McKay’s team have found that short bursts of intense activity separated by brief rest periods—anything from jumping on the spot to squats in the weight room—build bone more effectively than continuous, less intense activities.
This means that weight bearing, on its own, is a bit overrated. It’s true that the skeleton gets a bit of a workout from gravity whenever you’re standing up, but you can stress your bones in a more targeted manner by training with weights. “Any time you’re increasing your muscle mass, the tension of the muscles on the bone creates a ‘bending moment’ that stimulates your bones,” McKay explains. Lifting weights also allows you to target vulnerable areas like your wrists, which get no benefit from your hours on the elliptical.
Another study by McKay’s group found that schoolchildren who jumped up and down between 5 and 15 times, three times a day (at the morning, noon, and end-of-school bells) significantly increased their bone density. Since a quarter of your adult skeleton is laid down during early puberty, it’s important to make sure children are doing the kinds of activities that build strong bones—and this study confirms that even small amounts of intense, jarring activities like jumping are more effective than simply standing or walking around.
Numerous studies over the years have found that strength-trained athletes have greater bone mineral density than endurance-trained athletes, lending support to the idea that building muscle is better for bones than weight-bearing activities like running. But a 2009 article in the Journal of Strength and Conditioning Research showed that the differences aren’t that simple. Pamela Hinton and her colleagues at the University of Missouri compared runners, cyclists, and strength-trained men. They did find that the strength group had the greatest bone density, but that was only because they had the biggest bodies. The runners were leaner, but their bones were just as strong relative to their body size.
There was, however, a significant difference between the bone density of runners and cyclists, which suggests that it’s the repeated, jarring impacts of running that produce stronger bones compared to cycling. As a result, Hinton recommends that those who engage in activities such as cycling, swimming, and rowing consider adding a dose of either strength training or a higher-impact activity like running to their regimen. That also means that elliptical trainers, which many people turn to precisely for their softer landing, suffer from the same shortcoming. “There’s no impact force, as the steps of the machine move with you,” Hinton says.
Sports like soccer and basketball (and activities like step aerobics) offer the best of both worlds, stimulating bone health through the impact of intermittent jumping and running, as well as by building muscle strength. Hinton’s research suggests that you don’t necessarily have to lift weights, and you don’t necessarily have to run and jump either—but you need to do one or the other to make sure you’re either building muscle or getting jarring impacts.
Can exercise keep my DNA from aging?
The 2009 Nobel Prize in medicine was awarded to three researchers who discovered how DNA can be copied over and over again without being damaged. Telomeres are short lengths of DNA at the end of each chromosome that serve as a protective cap, ensuring that the delicate ends aren’t snipped when the chromosome is replicated. Unfortunately, the telomeres themselves get shorter and shorter as you get older—and once they reach a certain minimum, the cell has effectively reached the end of its life. In fact, some researchers now believe that telomere shortening is the fundamental change that underlies all aging: as your cells age, so do you.
Exercise, we’ve been told for years, is a fountain of youth. It keeps your arteries supple, your muscles strong, and your mind sharp. But it hasn’t been entirely clear how it accomplishes such diverse effects. A 2010 paper by University of Colorado researchers, published in the journal Mechanisms of Ageing and Development, offers a clue. Physiologists analyzed the telomere length and aerobic fitness of four groups of people: young (18 to 32) and sedentary, young and fit, old (55 to 72) and sedentary, old and fit. The “fit” subjects did at least 45 minutes of vigorous exercise five times a week. The two young groups had essentially the same length of telomeres. The telomeres of the old, fit group were slightly but not significantly shorter. But the old, sedentary group had dramatically shorter telomeres.
The Colorado researchers also plotted telomere length versus aerobic fitness (VO2max) for the older group. Across the board, those with higher fitness had longer telomeres. The implication is clear: vigorous aerobic exercise makes your DNA look several decades younger than it is. And that’s bad news for the sedentary group. A recent study of 780 heart disease patients, for example, found that those with the shortest telomeres were most likely to die in the following four years—a mortality risk that couldn’t be explained by other known risk factors in these patients.
It’s important to note that the Colorado study can’t distinguish correlation from causation. It’s possible that a hidden underlying factor makes some people keep longer telomeres and also drives them to exercise. But another study by German researchers from the University of Saarland suggests that this isn’t likely. Like the Colorado study, the Saarland study found that older runners and triathletes had telomeres almost as long as younger subjects, while older non-exercisers had much shorter telomeres. In addition, the German study tested mice who were assigned to either run on an exercise wheel or do no exercise. After just three weeks, the exercising mice displayed higher levels of telomerase, the enzyme that stimulates the formation of telomeres. This suggests that it’s no coincidence that regular exercisers have longer telomeres—and though this effect is invisible to the naked eye, it may prove to be fitness’s most important benefit.
CHEAT SHEET: EXERCISE AND AGING
• Starting in your mid-30s, you lose 1 to 2 percent of your muscle mass each year and about 9 percent of your aerobic fitness per decade—but regular exercise slows this decline dramatically.
• Long-term studies find that runners get osteoarthritis at a lower rate than non-runners, contradicting the common belief that running wears down your knees.
• Successful masters athletes train consistently without long breaks, focus their workouts on the most essential elements, and take extra recovery time to avoid injuries.
• Endurance declines more sharply than speed as you age. Steady training may prevent your rate of decline from accelerating.
• Declining motivation may be as important as aging bodies in explaining why older athletes slow down. Ensuring that your family and friends are supportive helps maintain positive social pressure.
• Aerobics-style exercise in water can reduce the impact on joints and lower the risk of falls. The exercise benefits are similar to dry land, though your heart rate will be lower due to water pressure.
• Activities that build muscle (like strength training) or provide jarring impacts (like running or basketball) are better for building strong bones than cycling, swimming, or elliptical training.
• Exercise slows down the cellular aging process in which the caps on the end of your DNA (known as telomeres) get shorter.