Running speed depends on cadence and the length of your stride


April 26, 1994|By Gabe Mirkin, M.D. | Gabe Mirkin, M.D.,United Feature Syndicate

If you want to run very fast in races, you have to run very fast in training and use weights or weight-training machines two or three times a week.

How fast you run depends on two factors: the length of your stride (stride length) and how fast you move your legs (cadence). In one study, the top 70 runners in the New York City Marathon averaged 90 to 96 strides per minute during the race. The difference was that the faster runners took longer strides.

But, consciously trying to take longer strides causes you to move your legs at a much slower rate and actually slows you down. Strengthening your legs so you can take longer strides helps you to run faster.

You strengthen your leg muscles by running very fast and by exercising them against increasing resistance. Just exercise does not make muscles stronger. The faster you run, the harder you push against the ground and the stronger your legs become.

However, each time that you run fast, your leg muscles are injured and feel sore on the next day. If you run fast when your muscles feel sore, you are likely to injure yourself. Runners train by running very fast on one day and then running at a slower pace on the next days, until their muscles stop feeling sore.

You can also strengthen your legs by using strength training machines. On the days that you run fast, you can do leg presses and knee and hip extensions. Recent research shows that you should also train for strength by rotating your body against increasing resistance on a rotational strength training machine.

Q: How hard do I need to exercise to become truly fit?

A: Fitness refers to your heart. You become fit by exercising vigorously enough to make your heart stronger. Intensity is measured by how fast your heart beats. To strengthen your heart, you have to exercise vigorously enough to increase your pulse rate at least 20 beats per minute above its resting rate, but that's only the minimum intensity. If you want to achieve a reasonable level of fitness, you have to exercise more intensely than that.

Q: I'm a serious weight lifter, and I take supplements that include iron. Is it possible to get too much?

A: Having high blood levels of iron is associated with an increased risk for suffering heart attacks and cancers, particularly cancers of the esophagus and bladder. Iron helps to convert the bad LDL cholesterol to oxidized LDL, which can form arteriosclerotic plaques and cause heart attacks. Iron can bind to and damage the genetic material in cells leading to uncontrolled growth, which is cancer.

Manufacturers are encouraged to add iron to foods to help prevent iron-deficiency anemia, which is common in very young children and menstruating women. The amount of iron that is absorbed from supplements is small compared to the amount of iron that is absorbed from meat, fish or chicken. Iron in living tissue is in a chemical form called heme iron, which can be absorbed up to 10 times more readily than the iron in supplements.

You can find out if you are getting too much iron. Ask your doctor to draw blood for a test called transferrin iron binding saturation. People with a transferrin iron binding saturation of more than 60 percent are at increased risk for developing heart attacks and cancers. If your level is greater than 60 percent, you can reduce your intake of iron by cutting down on supplements and restricting meat, fish and chicken. You can get rid of extra iron by donating blood six or more times a year.

Dr. Mirkin is a practicing physician in Silver Spring specializing in sports medicine and nutrition.

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