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Stress Fractures Caused by Weak Muscles and Over-Striding

One of the most common injuries in runners is a stress fracture of the lower leg (tibia) because running fast causes the foot to hit the ground with tremendous force that can shatter bones. A study from the University of Minnesota shows that women with stress fractures do not have weaker bones, they have smaller and weaker calf muscles (Medicine & Science in Sports & Exercise, December, 2009). Another study from Iowa State University in Ames, in the same journal, shows that longer strides cause the greatest foot strike forces that increase bone fracture risk.

Strong muscles may help to prevent bones from breaking by absorbing more force from the foot hitting the ground during running. Most distance runners do not use weight machines to strengthen their leg muscles. They strengthen their calf muscles by running very fast no more often than three times a week.

In the Iowa study, reducing stride length by ten percent reduced force of the foot striking the ground and therefore reduced force on the tibia.

Shortening your stride will not slow you down. When your foot hits the ground, your Achilles tendon contracts to store up to 60 percent of your foot strike force. Then when you step off that foot, your Achilles tendon releases the stored energy to drive you forward. Over-striding deprives you of some of this stored energy. Since many runners take strides that are too long, shortening stride length usually allows them to increase cadence and will help to increase speed and endurance.

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Dear Dr. Mirkin: Which sports are best for the knees?

A study from the University of California in San Francisco shows that high-impact weight-bearing exercises, such as running and jumping, often damage the knees of middle-aged people; while low-impact sports, such as swimming and cycling, can help to protect healthy or already-damaged knee cartilage (Annual meeting of the Radiological Society of North America. November 30, 2009). 236 people without knee pain were classified into low-, middle-, and high-activity groups based on their responses to the Physical Activity Scale for the Elderly (PASE) questionnaire. MRIs of their knees showed that the more-active people were more likely to have knee damage, with the highest incidence in runners or those who participated in sports requiring jumping or running.

Cartilage covers and protect the ends of bones. When the cartilage is gone, bone rubs on bone to cause pain all the time, increasing your likelihood of needing a knee replacement. When you run or jump, your foot strikes the ground, causing your lower leg to stop suddenly and your upper leg (femur) to slide forward (at the knee) over the lower leg (tibia). The harder you hit the ground, the greater the force to push your upper leg forward over your lower leg, which can shear off cartilage in your knee. On the other hand, you cycle with a smooth rotary motion with no sudden stopping and no jarring to push your upper bone of your knee over the lower one. This strengthens the muscles around the knee and stabilizes the knee joint. Low-impact exercises such as cycling and rowing are among the best ways to strengthen your knee joints.

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Dear Dr. Mirkin: Do all sugared beverages increase risk for diabetes?

Young women who drink five sugared colas per week are 22 percent more likely to develop diabetes during pregnancy (Diabetes Care, December 2009). In this study, no association was found for other sugar-sweetened drinks. However, most studies show that all sugared drinks increase risk for diabetes. For example, several studies have shown that eating fruit is associated with protection from diabetes, while drinking fruit juice increases risk (Diabetes Care, July 2008; Archives of Internal Medicine, July 2008; American Journal of Clinical Nutrition, July 2008).

When food enters your stomach, the pyloric valve at the end of the stomach closes and prevents solid material from entering your intestines. Then your stomach contracts and mixes the food until it is turned into a soup. Only the liquid soup is allowed to pass into the intestines. So solid sugared foods stay in your stomach for a while, but liquid sugar passes immediately into your intestines and subsequently into your bloodstream to cause a high rise in blood sugar, which increases risk for diabetes.

On the other hand, during exercise, it is usually safe to take sugared drinks. Your exercising muscles draw sugar so rapidly from your bloodstream that blood sugar levels usually do not rise too high. So my rule is to drink sugared beverages only while exercising.

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Recipe of the Week:

Mexican Vegetable Stew

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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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