It's the ninth inning, you lead by one run and your starting pitcher, who usually has very good control, has just walked the first two batters. Is it time to bring in a relief pitcher?
I would say yes. The old pitcher has thrown nine innings and prolonged exercise has weakened his muscles. Fatigue has already reduced his power and accuracy. His effectiveness is waning, and it's time to take him out. The same principle applies to all team sports. Teams that substitute freely early in a game have fresher players late in a game.
There's a physiologic reason why tiredness weakens muscles. Muscles are made up of thousands of fibers. Each fiber stores sugar, and when it runs out of its stored sugar, it cannot contract effectively. So, as you tire, your muscles have fewer fibers to contract, and as a result you become weaker and less coordinated.
That means that a pitcher who is warmed up and fresh will have more active fibers in his muscles and be able to throw more accurately and faster than when he is tired. A fresh football player can kick further and more accurately than when he is tired. The same principle applies to boxers and wrestlers who are weaker toward the end of bouts. You lose both accuracy and strength as you become tired. So, when you compete in a sport that requires endurance and you cannot use a substitute, take it easy early in your competition.
Q: I can't get through a whole 30-minute aerobic dance session. What can I do to become more fit?
A: Fitness refers to your heart, so scientists ascertain fitness by determining an athlete's VO2 max, a measurement that reflects heart strength. "VO2 max" is the maximum amount of oxygen your body can take in and use over a specific period of time. To increase your VO2 max, you should exercise vigorously enough to increase your pulse rate by at least 20 beats above what it would be if you were resting and keep it there for several minutes three times a week.
You are not alone. Almost 80 percent of American adults cannot exercise vigorously for 30 continuous minutes, so many feel they shouldn't bother to exercise at all. The new exercise guidelines announced by the American College of Sports Medicine and the President's Council on Physical Fitness tell us that you don't have to engage in vigorous, continuous exercise to gain substantial health benefits. They recommend that you get a total of 30 minutes of exercise or vigorous activity throughout the day.
Use several different sports or activities, and stop exercising when you feel the least discomfort. Resume exercise in another activity when you have recovered. For example, go to an aerobic dance class and drop out when you feel the least bit tired, even if you have to quit after just two minutes. Rest, and then ride a stationary bike until your legs start to feel heavy. Later in the day, walk for a while until you feel tired. Try for a combined total time of 30 minutes of exercise per day, five days a week. You can count any physical activity that keeps you moving constantly, such as fast walking, climbing stairs or gardening.
Dr. Mirkin is a practicing physician in Silver Spring specializing in sports medicine and nutrition.