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Endurance and Heart Size

What laboratory test is the best predictor of finishing times for a running race of 100 kilometers (62 miles), or more than twice the distance of a marathon)? A study from Yokohama, Japan suggests that it is an echocardiogram to measure the size of your left ventricular heart chamber (British Journal of Sports Medicine, Volume 40, 2006), which determines how much blood your heart can pump with each beat.
This would be expected to predict how fast you can run for short distances in which you have to move so fast that the limiting factor is lack of oxygen. This study is surprising because most athletes believe that the major limiting factor for running very long distances is the amount of fuel you can store in your muscles.

Now we know that the limiting factors for ultra- endurance competitions are similar to those of shorter distances: the time it takes to move oxygen from blood in your lungs to your muscles. This is determined by how much blood your heart can pump and how much oxygen your blood can carry. Since 98 percent of the oxygen in your blood is carried by the hemoglobin in your red blood cells, the higher your red blood count, the more oxygen you can circulate. However, a more important factor is how rapidly your heart can pump blood to your muscles, and this is determined by the strength of your heart muscle. The longer and harder you train by running, the stronger your heart, and that’s what a thicker left ventricle means. The runners who ran the most miles in training had the strongest hearts and the best finishing times. So if you want to compete in any sport requiring extremes of endurance, you have to spend a lot of time training and you also need to exercise very intensely once or twice a week to strengthen your heart.


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Dear Dr. Mirkin: Is loss of strength with age inevitable?

The older you become, the more you need to exercise. Researchers at Slippery Rock University of Pennsylvania measured grip strength in older men at baseline and an average of seven years later (Aging Male, September-December 2005). The men squeezed a machine that measured the force that they could exert. They lost 20 percent of their grip strength in seven years. The older they were, the more they lost. Those who lost the most height or weight, those on calcium channel blockers for high blood pressure, and those who took in the most caffeine had greater losses of strength. Loss of height is linked to osteoporosis, which is associated with loss of muscle. High blood pressure and unintentional weight loss indicate other health problems. No explanation was offered for the association of caffeine with loss of muscle strength.

These results are expected. Muscles are made of millions of individual muscle fibers. A single nerve innervates each fiber. With aging, a person loses nerve fibers that cause loss of each connected muscle fiber. However, you can continue to build strength in the remaining muscle fibers into your 90's and beyond. Perhaps all people over 50 should get a stress electrocardiogram as a screening test to see if exercise is likely to harm them. If they pass the test, they should start or continue an exercise program that includes some form of strength training such as lifting weights or using strength-training machines.


Dear Dr. Mirkin: What would cause a leg clot in an otherwise healthy person?

Leg clots occur without warning with sudden pain and swelling in a leg muscle, usually the calf. This is a particularly dangerous condition because the clot can break lose from the veins in the leg, travel to the lungs and block blood flow to kill a person. In a report in the British medical journal, Lancet (April 1, 2006), doctors at the London School of Hygiene and Tropical Medicine showed that infections may cause sudden clotting in the leg muscles called Deep Vein Thrombosis. They showed a 20 percent increase in infections, particularly urinary and respiratory, one to two weeks before a person develops clots. This report supports the current theory of inflammation causing heart attacks, strokes, and clotting. Your immunity is good because it is supposed to kill germs when they enter your body. However, if your immunity keeps on being active, it attacks your own body to damage arteries and other tissues.

People at the highest risk for clots are those who are sedentary for a long time, such as in long distance plane flights, and those who suffer cancers. Since infections are common and deep vein clots are not, you should not worry about clots every time you get an infection. However, if after a urinary or respiratory infection, you suffer sudden pain in a leg without any other explanation, check with a doctor immediately to rule out a clot.


Recipe of the Week

Ratatouille with Baby Potatoes

You'll find 100 recipes, and lots of helpful diet tips, in The Good Food Book - it's FREE List of Diana's Healthful Recipes

June 26th, 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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