When you run very fast, you become short of breath and feel like you have to slow down, but if you keep pushing yourself, you will suddenly feel refreshed and be able to pick up the pace. Do you know what caused your sudden recovery?
It's called "second wind," and it is due to slowing down when you feel exhausted during exercise. When you exercise intensely, your muscles use large amounts of oxygen.
If you run fast enough, your muscles will require more oxygen than you can breathe in, and you will develop an oxygen debt that causes lactic acid to accumulate in your muscles. They hurt and the lactic acid spills over into your bloodstream, causing you to gasp for air.
You finally reach a point where you can't meet your needs for oxygen and you have to slow down, but you don't notice that you have slowed down because you are working just as hard as you did when you were running at a faster pace.
Not only must you bring in oxygen to meet your current needs, you must also use some of the oxygen to make up for the debt that you incurred when you were running at a faster pace.
Eventually, you catch up on your oxygen debt and you feel better. You pick up the pace and don't even know it was the slower running that revived you so you could run faster. That's your second wind.
Q: I'm interested in starting a weight-lifting program. Why have some of my friends become very large from lifting, while others remain scarecrows?
A: To become very strong, you have to exercise your muscles against resistance to the point where your muscles start to burn. The only stimulus that makes a muscle larger is the stretching of muscle fibers while the muscle shortens.
The first time you lift a heavy weight, you use only a small percentage of your muscle fibers, perhaps 3 percent. As you continue to lift and lower a weight, you bring in more and more fibers, until 30 to 50 seconds have elapsed, and lactic acid starts to accumulate in the muscle. This reduces the number of contracting fibers. You use the most muscle fibers when you exercise them against heavy resistance for 30 to 50 seconds. Pick the heaviest weight you can lift and lower slowly for 30 to 50 seconds through your full range of motion. That should amount to 8 to 12 repetitions in each set. If your muscles don't start to burn slightly, you need to lift heavier weights, move more slowly or do more sets. Stop lifting if you feel pain or start to lose control.
Q: Does a regular exercise program increase your chances of developing arthritis?
A: Exercise can exert a lot of force on your joints. A recent study from Finland showed former athletes are more likely to suffer from arthritis, while regular exercisers are less likely to do so. The ends of your bones are soft. If they rubbed against each other at the joint, the bones would be worn away and disappear. To prevent this from happening, the ends of bones are covered with a thick, tough, white gristle called cartilage.
Cartilage is dynamic tissue. It is constantly being removed and replaced. Lack of activity speeds up the rate that cartilage is removed, and it weakens. Exercise speeds the rate that cartilage is replaced and makes it stronger. Exercise strengthens cartilage, provided that you don't damage it. Once you break cartilage, it can never be replaced. A person who exercises regularly and stops when he feels pain can expect to strengthen his bones and joints. But if the exercise injures the cartilage in the joints, it will cause arthritis later in life.
Dr. Mirkin is a practicing physician in Silver Spring specializing in sports medicine and nutrition.