The Effects of Aging and How Exercise Counteracts Them

• Motor neurons die, particularly from age 60 onward. This causes connections between muscle fibers to wither — and that, in turn, eventually leads to loss and shrinking of muscle fibers. As a result, muscles get smaller and a person gets weaker, says Sandra Hunter, an associate professor of exercise science at Marquette University in Milwaukee. "Physical activity can offset some of that," she says. "But there is this biological aging process going on — the neurons will die regardless of how fit you are."

• Some types of muscle are lost more quickly than others. You'll lose comparatively more fast-twitch muscle fibers (those that fire quickly and are used in activities like sprinting) than slow-twitch muscle fibers (those that contract slowly and use oxygen efficiently, making them useful for endurance activities). Slow-twitch fibers are lost more slowly because they're called on more often in everyday activities. The plus side of this: Even though aging adults have less muscle mass, their higher proportion of slow-twitch, fatigue-resistant muscle fibers can give them a leg up in endurance activities such as running or cycling.

• Beginning in the late 30s, maximal oxygen consumption, or VO2 max, decreases at a rate of at least 10% per decade, or about 1% per year, in most people other than highly trained athletes. VO2 max is dependent on heart rate, which decreases by about 5 to 10 beats per minute per decade. This reduction in aerobic capacity is one of the reasons for a decline in endurance performance with age. "You can't send as much blood and oxygen to the working muscles, and the pace slows down," says Jason Karp, a running coach based in San Diego. Hard aerobic training can offset the decline in VO2 max — up to a point.

• With age, large, elastic arteries including the aorta (which shuttles blood from the heart) and the carotid artery (which feeds blood to the brain) get stiffer. As a result, blood pressure rises and the heart has to work that much harder. In addition, the inner lining of arteries, called the vascular endothelium, loses certain functions: Signals that normally open the arteries and increase blood flow or narrow the arteries to reduce blood flow are not operating properly. As a result, the artery remains in a relatively narrow state, contributing to cardiovascular disease, says Douglas Seals, a physiologist at the University of Colorado at Boulder. Beyond the health ramifications, he adds, "if the arteries do not vasodilate robustly in response to these signals, then they cannot increase blood flow appropriately to meet the demands of increased energy metabolism of the exercising muscles — and performance will be limited."

• Wear and tear builds up on the joints. Connective tissue becomes less elastic, and lubricating fluids decline, making aging athletes more injury-prone. Cross-training — doing a mix of high- and low-impact exercises such as weight training, yoga and cycling — works different muscle groups and can reduce the risk of orthopedic injuries from overuse, says Michael Joyner, a professor of anesthesiology and an exercise researcher at the Mayo Clinic in Rochester, Minn. And (though research is limited) studies suggest that a lifelong exercise habit helps keep joints intact. In part, this could be because activity improves blood flow and other regenerative pathways and may activate stem cells that help the body repair itself, Joyner says.

A bullet point list of some of the physiological effects of aging and how exercise reverses these trends.