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Cross-Training for Fitness

Fitness refers to your heart, and the harder you exercise, the more fit you become. But every time you exercise vigorously your muscles are injured, and the harder you exercise, the longer it takes for your muscles to heal. Muscle biopsies done the day after a person exercises vigorously show bleeding into the muscles and disruption of the Z-bands that hold muscle filaments together. You are not supposed to exercise vigorously again until the muscle soreness disappears.

Most competitive athletes set up training programs so they exercise vigorously enough on one day to make their muscles feel sore for the next day or two and then after the soreness disappears, they exercise vigorously again. You can use the same principle in your exercise program to achieve a higher level of fitness. You can exercise vigorously on one day and easy on the next few days or until the soreness disappears, or you can train in two sports. This is called cross-training, and it can make you very fit and help to prevent injuries.

Each sport stresses specific muscle groups. Cycling stresses the upper legs, while rowing stresses your back and upper body. If you cycle and row on the same day, you stress your upper legs and upper body on the same day. To reduce your chances of injuring yourself, you should take the next day off, or at least exercise at a very low intensity. If you cycle on Monday and row on Tuesday, you allow your muscles 48 hours to recover from each sport. Pick two sports that use different muscle groups and do them on alternate days. You can then exercise more intensely in each sport and achieve a higher level of fitness.


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Dear Dr. Mirkin: My daughter plays soccer and complains about being tired all the time. Will taking potassium supplements help?

Tiredness and cramps in athletes can have many causes, but lack of potassium in their diet is not one of them. Many years ago, Dave Costill of Ball State University tried to create potassium deficiency in runners. He couldn’t do it because potassium is found in all foods except refined sugar, and his athletes would not stay on a diet that consisted only of hard candy.

The kidneys and sweat glands conserve potassium so well that you don’t lose much. If an athlete develops potassium deficiency, it is usually caused by drugs, such as diuretics or corticosteroids, or by diarrhea or repeated vomiting. Some athletes try to control their weight by making themselves vomit. This is called bulimia, and the person usually denies vomiting. Their physicians can prove that they are vomiting by ordering blood and urine tests. If blood levels of potassium are low and urine levels are high, vomiting is a likely cause. Ask your daughter’s doctor to do a work-up for other causes of chronic tiredness. If none can be found, she may overtraining and should talk to her coach about changing her workouts.


Dear Dr. Mirkin: My 80-year-old mother is almost six inches shorter than she was at age 20. Is there any way to prevent getting shorter as I age?

The bones of your spine are separated by pads called discs. As you age, these discs dry out and become smaller. However, regular exercise compresses and relaxes these discs as you move up and down. This helps to keep the discs from shrinking and maintains your height. Regular exercise also helps to strengthen bones and keep them from bending or being crushed. One study from Israel showed that men who exercise regularly lose only half as much height as men who never exercise -- just 2.6 centimeters compared with 5.5 centimeters.

If you have not already started exercising to prevent heart attacks, diabetes, high blood pressure, high cholesterol and loss of mental function, and just to keep you feeling good, now we know that you should exercise to help you stand taller as you age.


Recipe of the Week
Cuban Corn Chowder
Use fresh corn kernels stripped from the cob, or convenient frozen corn; it's delicious either way.

List of Diana's Healthful Recipes

June 27th, 2013
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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