Friday, February 17, 2012

Age and Peak Athletic Performance

Sarcopenia - age-related muscle loss - becomes more common
and more severe after age 60, but even for 80 year olds a supervised
program of resistance exercise can add strength and improve quality of life

Peak athletic performance is reached at age 30, with remarkably little decline for the next ten years. After 40 there is a slow, steady decline to age 60, accelerating thereafter. The mid-life decline is a consequence of less oxygen being delivered to muscles; the late-life decline to a complex web of less oxygen delivery plus loss of muscle mass, compounded by disuse, obesity and osteoarthritis. But there is hope.

At the direct physiological level, theories span loss of function of nerves that control muscle movement, less oxygen uptake in the lungs, decreased oxygen delivery by blood, lessened availability of fuel, and poorer fuel use efficiency. While lung function can decline with age, especially in smokers or people with occupational exposure to airborne chemicals, the main cause is ever-decreasing oxygen delivery. With age there is a decline in maximum heart rate (roughly estimated as 220 minus age), less blood volume per heartbeat and progressive loss of capillaries delivering oxygenated blood to muscle cells. 

Fuel availability and usage are not part of the problem - muscle cells have adequate access to fuel and do not require more calories to contract. This aspect, known as "exercise economy," is assessed by looking at the metabolic calorie cost of sustained submaximal exercise.

Other changes factor into sarcopenia. Beyond 60 years, sensitivity is lessened to the signals which in younger people will trigger muscle enlargement, for example, vigorous exercise or consumption of a high protein diet.

Indirect contributors to the age decline include disuse, obesity and osteoarthritis. After college age, few people have physical labor jobs or sufficient leisure time for the 20 to 30 hours of training per week needed to maintain a level of physical activity necessary for peak performance. Recovery from sports injuries take longer. Body fat, especially torso fat, is pre-inflammatory, contributing to muscle cell insulin resistance; without this anabolic signal being received, muscles shrink. Osteoarthritis has a chicken-or-egg-first relationship with fitness, as exercise slows the progression of osteoarthritis, but this disease is a major reason people stop being physically active.

Certain sports appear to contradict the "peak at 30" hypothesis, but on closer examination, may not. For many years women's swimming and tennis were dominated by teenagers. These individual-effort sports channeled young girls into early commitment and intense training. The typical result was an early-age peak followed by mental burn-out and/or career-ending physical injury. With wider access to competitive sports via Title IX, a broader pool of women athletes across a wider range of sports finds peak ability reached in close to 30 years, just like men. In the New York City marathon, women in their mid-40s are competitively close to women ten and twenty years younger.

The net result of the direct and indirect consequences of aging? For healthy, well-trained endurance athletes there is a 20 to 25% decline in performance from age 40 to age 65. Evidence comes from bicycling time trials, and 5K and 10K runs. Decline over time is faster for occasional athletes and the sedentary.

There is hope. For the approximately ten million U.S. adults who have sarcopenia, i.e., age-related muscle loss, a supervised program of resistance exercise can partially reverse muscle weakness and improve quality of life, even when started late in life. Vitamin D at 1,000 IU/day (many elderly are vitamin D deficient) has been proven to reduce the risk of injury from falls by 20 percent. Flavonoids, chemical compounds found in foods such as red wine, dark chocolate, green tea, nuts, and some types of dark-colored fruits, and are thought to contribute to artery health.

"Article Directory by Category" lists eight other health-related topics, such as recovering from donating blood (March 2011).

Thursday, February 9, 2012

Hibernators Large and Small

Winter strategies for species without central heating span migrating, toughing it out and hibernating. That last option - damping down the metabolism - has been taken up by species ranging in size from large to small.

Black Bears: In the late summer and early fall these New England neighbors double to triple their food intake in order to add an inches-thick layer of body fat. Come fall the bears find a confined space for a den such as under the trunk of a fallen tree. The den offers some physical protection from the elements, but is not warmer than the outside temperature. Body temperature drops to about 90F degrees and heart rate slows to ten beats per minute (bpm). This semi-hibernating state last for months, but if disturbed a black bear can achieve a fully roused state within minutes. One large puzzle is how bears manage the winter-long rest without needing to eat, drink, urinate or defecate, nor suffer from the bone loss and muscle atrophy humans would undergo from the equivalent months of bed rest.  

Woodchucks: Also known as groundhogs, these mid-sized mammals are true hibernators. They gain thirty percent in body weight, almost entirely as fat, before entering a den in late October to begin a months-long state of torpor: body temperature dropped to 40F degrees, heart rate dropped to about five bpm and breathing rate decreased to less than one per minute. Roughly every two weeks the hibernating animals rouse to full awareness, go to the bathroom and undergo a day or two of normal sleep in order to catch up on their dreaming (confirmed by rapid-eye-motion type sleep). If this coincides with February 2nd then it is Groundhog Day. One puzzle: what late summer signal triggers the beginnings of over-eating to gain all that weight?

Chipmunks: These ground-living relatives of squirrels add some body fat in the fall and store caches of food – primarily seeds – in their underground burrows. The burrows extend below the frost line. Chipmunks enter an intermittent hibernating state. Body temperatures approach 40F degrees. Heart rate slows from 200-300 bpm to under 10. Every few days the chipmunk warms back up to close to 100 degrees and becomes active. Separate areas of the den serve as food larder, bathroom and bedroom. Come late winter the males will begin to surface-wander in search of dens of females. One consequence - spring litters tend to have only one father, whereas summer litters are likely to have multiple fathers.  

Yellow Jacket Wasps: In the fall the virgin queens leave their mother's nest to mate. Like honey bees, yellow jacket queens will mate with 3-10 males. The males die. The fertilized females hibernate under tree bark on in another dry place. Insects have a few hibernating strategies to choose from. Some synthesize glycerol (antifreeze) in order to lower their freezing temperature. Some select a dry place to stay and supercool without ice crystal formation. Others allow themselves to freeze, and in fact synthesize freeze-promoting proteins, with the idea that freezing s-l-o-w-l-y and then remaining frozen all winter reduces risk of large ice crystal formation which would poke holes in cells. Yellow jacket queens employ glycerol and supercooling. Coming out from under hibernation is not triggered by an internal signal. Rather, the yellow jacket queens depend on the warming temperatures of spring to bring them back to life.

Warmer winters in Alabama and other south eastern states have led to an interesting phenomenon. Rather than the nest dying from the cold there is enough food and warmth to get through the winter as an active colony. Come spring, the colony re-expands, and related daughter queens set up housekeeping next door. So instead of scattered annual colonies there is a multi-year, condominium-like complex making up a super-colony of tens of thousands of related but not genetically identical yellow jacket wasps.