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Why Sled Dogs Have Such Great Endurance

How can sled dogs run more than 100 miles a day for weeks on end, while humans couldn’t possibly recover from such abuse of their muscles? A study from Ohio State University shows why (Medicine & Science in Sports & Exercise, August 2005). How long you can exercise a muscle depends on how long you can keep stored sugar, called glycogen, inside that muscle. Muscles burn carbohydrates, fats and protein for energy during exercise. They get these sources from both the bloodstream and from the muscles themselves. However, when a muscle runs out of its stored sugar, it hurts, becomes more difficult to coordinate and requires far more oxygen than usual. So a limiting factor in how long you can exercise a muscle is how much sugar you can store in a muscle, how quickly you use it up, and how quickly you can restore sugar the sugar in your muscles.

Humans take a long time to restore muscle glycogen. Top marathon runners restore muscle glycogen in anywhere from a day to several days. This study shows that sled dogs can restore muscle glycogen almost as quickly as they are fed. They were able to restore more than 50 percent of their resting muscle glycogen after two consecutive 100-mile runs even when fed a low-carbohydrate, high-fat diet. Humans could never replace muscle glycogen that fast.

The only way that you can teach your muscles to store more glycogen and preserve it better is to train by running (or cycling or swimming) lots of miles and doing long depletion runs taking more than three hours at least once a week. Doing too many depletion runs will delay recovery of muscle glycogen so that you will not be able to do the very fast short interval runs that teach your brain and muscles how to run faster.

Note: It has been brought to my attention that this article has been used in discussions of dog sledding. My intent was to provide useful information about human muscles and human training techniques. I have no expertise in dog sledding, dog racing or veterinary medicine.


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Dear Dr. Mirkin: What’s the best way to treat a stress fracture?

Bone stress fractures are very common sports injuries. Any trauma to bones, such as repeated pounding when your heel strikes the ground during running, or landing on your feet after grabbing a basketball, can cause small cracks on the surface of bones called stress fractures. In healthy athletes, they can take from 3 to 12 weeks to heal, forcing an athlete to lose valuable training time. A study from Tulane University shows that intravenous pamidronate can heal these fractures quickly and keep you playing (Clinical Journal of Sports Medicine, March 2005). Bones change constantly. Calcium is carried from bones by cells called osteoclasts and carried into bones by cells called osteoblasts. Bisphosphonates such as pamidronate prevent osteoclasts from carrying calcium from bones. The athletes received five weekly intravenous infusions of pamidronate and were able to continue training.


Dear Dr. Mirkin: Will blood pressure drugs interfere with my running program?

The beta blocker drugs used to treat blood pressure and heart problems can markedly impair your ability to exercise, according to a study from Switzerland (European Journal of Applied Physiology, October 2005). How hard you can exercise is limited by the ability of your heart to pump blood from your lungs to your exercising muscles. Beta blockers markedly reduce blood flow and oxygen supply to muscles. Beta blocker brand names include Toprol, Inderal, Blocadron, Coreg, Inopran, Levatol, Pindolol, Sectral, Tenormin, Timolol Trandate, Zebeta and Bisoprol.

Beta blockers are prescribed to treat people who have had heart attacks, heart pain, heart failure, rapid heart beat and atrial fibrillation. However, even though many physicians prescribe beta blockers to treat high blood pressure, there is no data show that they prevent heart attacks in healthy people. If beta blockers interfere with your ability to exercise, ask your doctor if you can take other types of medications such ace inhibitors, angiotensin receptor blockers or calcium channel blockers. Better yet, control your blood pressure with diet.


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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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