Pulling Against Time

Oxygen is part of the fuel that allows muscle tissue to produce mechanical energy -- to contract, in a word. Glucose (a form of sugar) or fat are the other necessary fuels. Muscles can work for short periods without oxygen -- so-called anaerobic respiration. But for sustained, long-term exertion, there is no substitute for oxygen. None.

Oxygen is carried in the blood, principally attached to hemoglobin in red blood cells but also dissolved in the blood's water, or plasma. It is put into the blood by the lungs, which are basically an elaborate mechanism for exposing an extremely thin layer of blood to air. Once it reaches muscle cells, oxygen is taken up by mitochondria, a vast archipelago of microscopic power plants floating in each cell's inland sea.

When a person commences athletic conditioning, the demand for oxygen goes up. Muscles want more oxygen as fuel. The number of muscle cells increases, and the cells already present get bigger. The number of mitochondria in each cell also goes up, in some cases dramatically. For oarsmen and marathoners, it can double.

The body's capacity to use oxygen is measurable. It's called "oxygen uptake," is designated "VO2" and is reported as the liters of gas absorbed per minute through breathing. When people train, their VO2goes up; when they become sedentary, it goes down.

But there's a limit -- maximum oxygen uptake, or VO2max. A rower or runner might enhance performance beyond that point through extraordinary effort, but the extra speed won't come from oxygen-based energy. It will require anaerobic respiration -- a process that produces lactic acid, makes muscles feel as if they're on fire, and can't be sustained for long.

Training not only raises VO2max, it also dramatically increases the level of exertion a person can sustain for long periods. This is something sedentary people realize when they try to keep up with their fit friends over a mile and not just 100 yards. Trained athletes can function at 87 percent of their VO2max for an hour and then 83 percent for a second hour. For the untrained, it is 50 percent the first hour and 35 percent the second.

In theory, many things could determine VO2max, but in practice one thing predominates -- the heart's ability to move oxygen-rich blood around the body. That is far more important than, say, the lungs' ability to put oxygen into the blood or the muscles' ability to take it out.

Endurance training enhances blood delivery in several ways. The distribution system improves; blood vessels get wider; and the number of capillaries in muscle tissue goes up. But again, one variable predominates -- it's the heart's pumping capacity, the volume of blood it can move per minute.

Training can raise this so-called cardiac output from a maximum of about 6.6 gallons per minute in an untrained person to about 10.6 gallons in a highly fit athlete. The heart achieves this by beating faster, filling fuller after each beat and squeezing harder.

And it is all those capacities (and more) that decline with age.

Maximum heart rate declines about 5 percent per decade as the heart becomes less responsive to the adrenaline-like hormones that whip it into action. VO2max declines 6 to 10 percent per decade after age 25, and this accelerates to 15 percent per decade after age 60.

At the receiving end, muscle strength declines 10 to 15 percent per decade starting at about age 30. This is because there is an actual loss of muscle fibers (and the nerves that drive them), and because some fibers usually used to generate brief bursts of power are transformed to longer-acting endurance fibers -- a change that reduces strength overall. By age 70, a person is only half as strong as he or she was in youth.