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Greater Endurance with Aging

I’m 74 years old and ride my bicycle more than 200 miles per week, often in pace lines with younger riders. I have noticed that younger riders can easily pull away from me in short bursts, but I keep coming back on them and seem to be better able to keep up with their accelerations as the ride progresses.

The latest issue of Exercise and Sports Sciences Reviews (January, 2009) reviews the entire world’s literature to show that endurance improves as you age. Wow!

The maximal muscle contraction force occurs when you do a single muscle contraction with all your might. Even though older people are not as strong as younger ones, many studies show that they can retain maximal force after many contractions far longer than younger people can.

Here’s the theory and evidence to explain why aging improves endurance. Muscles are made up of millions of individual fibers just as a rope is made up of many different threads. Each muscle fiber is innervated by a single nerve. As you age, you lose nerves throughout your body and when you lose the nerve that innervates a specific fiber, you also lose that muscle fiber.

Muscle fibers are classified as type I endurance fibers and type II strength and speed fibers. With aging, you lose far more nerves that innervate the strength and speed fibers than those that innervate the endurance ones. So, with aging, you lose strength but you retain a greater proportion of endurance fibers.

Muscle fatigue comes from the accumulation of waste products that occurs while food is converted to energy to power your muscles. Scientists can measure fatigue by measuring the accumulation of acid (H+), Phosphate (Pi) and protonated phosphate (H2PO4) in muscle. With the same percentage of their maximal muscle force, older people accumulate far lower levels of these end products than younger people do. Therefore even though older people are weaker, they can maintain their forceful contractions far longer than younger people can and they have greater endurance. This exciting recent data will encourage me to train even harder.

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Dear Dr. Mirkin: I’m an avid runner who gets severe leg cramps; could they be caused by an over-the-counter supplement that contains beta sitosterol?

Beta sitosterol is a plant cholesterol that blocks the absorption of animal cholesterol in your intestines. It’s unlikely that these supplements would cause your leg cramps. Low salt levels are a far more common cause of cramps in exercisers. When you exercise intensely, you lose tremendous amounts of salt in your sweat. Go for a long run of at least ten miles. On the next morning have your doctor draw your blood sodium level. If it is below 130, you need more salt.

Try adding more salt to your food and eat salted peanuts or other salty snacks frequently during your rides. Salt may contribute to high blood pressure in non-exercisers, but it rarely causes high blood pressure in athletes. If you are concerned about the effect of added salt on your blood pressure, get an inexpensive wrist blood pressure cuff at your local drug store and keep a log.

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Dear Dr. Mirkin: Should I exercise when I have a cold or a fever?

Exercise may actually be beneficial when you have a cold. However, it's probably better to stop exercising altogether if you have a fever with aching muscles. When you exercise, your heart has to pump blood to your muscles to supply them with oxygen. It also must pump blood from your muscles to your skin where the heat is dissipated. When you have a fever, your heart has to work harder to get rid of extra heat.

You risk injury if you exercise when your muscles hurt at rest. When muscles are damaged, they release enzymes from their cells into the bloodstream and they fill with blood from broken blood vessels. One study reported markedly increased muscle damage during relatively minor exercise during an infection, with blood tests showing increases in muscle enzymes and ultrasound tests demonstrating hemorrhage into the muscles. You will not lose much conditioning if you take off a few days.

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Recipe of the Week:

Spanish Rice

You'll find lots of recipes and helpful tips in The Good Food Book - it's FRE

June 22nd, 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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