Why You Need Interval Training for Both Speed and Endurance

To have great endurance for any sport, you have to do interval training: short bursts of moving almost as fast as you can, slowing down until you regain your breathe and then repeating these all-out efforts a number of times.

The old theory was that lactic acid makes the muscles more acidic which causes them to hurt and burn and interferes with their ability to contract, so you feel tired. George Brooks of the University of California/Berkeley has shown that lactic acid buildup in muscles does not make muscles tired and can make muscle contract more efficiently, which increases endurance. This research contradicts what many instructors teach in their exercise classes. When you exercise, your muscles burn sugar, protein and fat in the presence of oxygen to produce energy. If you exercise so intensely that you become very short of breath and your muscles can't get enough oxygen, lactic acid accumulates in your muscle fibers.

When you exercise, muscles need to get a lot of oxygen to turn all food sources into energy. In fact, the limiting factor to how fast you can move in sports competition is the time that it takes to move oxygen from the bloodstream in your lungs into your muscles. When you exercise so intensely that your muscles cannot get all the oxygen they need to turn sugar into energy, the series of chemical reactions slows down and lactic acid accumulates in muscles and spills over into the bloodstream.

Since lactic acid is an acid, the acidity causes muscles to burn and you have to slow down. However, as soon as you slow down enough to catch up on your oxygen debt, the lactic acid is immediately turned into more energy to power your muscles. In fact, lactic acid is a much better fuel for muscles than sugar, the second best source of energy. It is converted to energy with the lowest need for oxygen.

The major effect of regular training is to teach your muscles to use the lactic acid for energy before it accumulates in sufficient quantities to make muscles acidic to cause the painful burn and loss of strength and speed. You turn lactic acid into energy only in the mitochondria, small chambers inside muscle fibers. The more intensely you train, the greater your oxygen debt, so the larger your mitochondria become and the more mitochondria you produce in muscle cells. This helps you to turn lactic acid into energy for your muscles and requires the least amount of oxygen. Intense training is the best way to teach muscles to use lactic acid for energy. Using lactic acid efficiently for racing makes you faster and stronger and gives you greater endurance, even in competitions lasting many days.

The best way to grow new mitochondria, and to enlarge the ones that you have, is to do interval training. You move as fast as you can for a short period, become severely short of breath, slow down until you regain your breathing, and then go as fast as you can again. According to Dr. Brooks, "The intense exercise generates big lactate loads, and the body adapts by building up mitochondria to clear lactic acid quickly. If you use lactic acid up, it doesn't accumulate."

More than 20 years ago, Dr. Brooks showed that lactic acid moves out of muscle cells into the blood and enters all your organs including the liver and heart to give them an extraordinary source of energy that requires less oxygen than any other source. So the harder you exercise, the more efficiently your mitochondria turn lactic acid into energy requiring the least amount of oxygen.

In my next post I'll explain the different types of intervals and how to use them in your program, whatever your level of fitness.