For more than forty years, fitness instructors have based exercise prescriptions on the maximum heart rate formula of 220 minus your age. A study from Oakland University in Rochester, Michigan shows that this formula may be wrong (Medicine and Science in Sports and Exercise, May 2007). The researchers found that the original formula overestimated the maximum heart rate for younger exercisers and underestimated the maximum rate for older ones. The new formula they recommend is 206.9 – (age x .67) = maximum heart rate.

Athletes train by taking a hard workout on one day, feeling sore on the next day, and then going easy for as many days as it takes for the soreness to go away. Then they take another hard workout and repeat the cycle. Most exercise physiologists and coaches tell their conditioned athletes to raise their heart rates to 80 to 100 percent of their maximum when they take a hard workout. For people who exercise for fitness, a hard workout usually means exercising at 60 to 80 percent of maximum heart rate.

However, the maximum heart rate formulas are set by averages of large populations. Your own maximum heart rate is determined by your fitness level as well as your age. Your legs drive your heart, not the other way around. When you start to exercise, your leg muscles contract and squeeze blood from your veins near them. Then when your leg muscles relax, your veins open and fill with blood. This alternate contacting and relaxing of muscles pushes extra blood toward your heart. The increased return of blood to your heart speeds up your heart. People with stronger muscles pump more blood towards their hearts and therefore can get a faster heart rate.

Since there is huge variation between individuals ranging from competitive athletes to novice exercisers, you would be better off setting your workout level by “perceived exertion”, rather than by any formula based on averages. “Perceived exertion” means that your brain interprets how hard your are exercising, and you can respond to these signals. As you exercise more intensely, you become short of breath and your muscles start to burn and hurt. You can interpret your own effort and discomfort levels to decide how hard you should work on a hard day or an easy day.

People who are just starting an exercise program or who do not exercise regularly should use much lower levels of effort. They should never try to get to their maximum heart rates because they are the ones most likely to suffer heart attacks during exercise. Start any new exercise program slowly and build up your level of fitness gradually.


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Dear Dr. Mirkin: How does caffeine affect endurance during a long race?

A major source of energy for muscles during hard exercise is the sugar in muscles. When muscles run out of their stored sugar supply (glycogen), they hurt and are more difficult to coordinate. Caffeine stimulates the body to produce large amounts of adrenalin which raises blood levels of a fat called triglyceride. This causes muscles to burn more fat and spares the stored glycogen to help you to exercise longer.

A study from the University of Georgia in Athens confirms many other studies that show that caffeine-laced drinks help to improve endurance, especially in hot weather (International Journal of Sport Nutrition and Exercise Metabolism, Volume 17, 2007). In this double-blind study, caffeine increased work performance by about 20 percent, increased strength of measured muscle contractions, and decreased feelings of fatigue.


Dear Dr. Mirkin: How can my doctor tell which weight-loss diet is best for me?

You may want to have your insulin levels checked. Researchers at Boston Children’s Hospital showed that people with high insulin levels lose weight more effectively on low-refined- carbohydrate diets (Journal of the American Medical Association, May 16, 2007). The researchers measured blood insulin levels one half hour after eating. They then placed overweight patients on either low-fat or low-refined-carbohydrate diets. Those with insulin levels above average at 30 minutes after eating lost more weight on the low-refined-carbohydrate diet.

When blood sugar levels rise too high, the pancreas releases too much insulin. This converts sugar to triglycerides. Then you use up the good HDL cholesterol carrying triglycerides from the bloodstream to the liver. Patients on the low-refined- carbohydrate diet had a much greater rise in the good HDL cholesterol and a much greater lowering of triglycerides. Those on the low-fat diet had a greater lowering of the bad LDL cholesterol.


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