If you want to compete in sports that require speed, such as running, cycling, swimming, skiing, skating, or team sports such as football, basketball, baseball or hockey, you have to train at a pace fast enough to exceed your lactic acid threshold. Lactic acid threshold is the pace that causes lactic acid to start to accumulate in your bloodstream. The limiting factor to how fast you can move is the time it takes for oxygen to get inside your muscles. The only way that you can increase the rate that your muscles can use oxygen is to train intensely enough to run an oxygen debt. When you exercise so slowly that you do not accumulate lactic acid, you will not improve your speed.
Lactic Acid Makes Muscles Burn During Exercise When you exercise, your muscles use food for energy. As long as you can meet your needs for oxygen, your muscles convert food to energy and the food is broken down into carbon dioxide and water which you can blow off from your lungs. However when you run so fast that you gasp for breath and can't get all the oxygen that you need, the chemical reactions slow down and you start to accumulate lactic acid in your muscles. Since lactic acid is acidic, an accumulation of lactic acid makes your muscles burn and you have to slow down. It is the acidity that burns.
When lactic acid makes your muscles burn, you have to slow down to catch up with your oxygen debt. This allows muscles to use lactic acid as their main fuel. Blood levels of lactic acid go down, the muscles become less acidic, and your muscles stop hurting. A pace that you can hold breathing fast and deeply, but not gasping for breath, is called the lactic acid threshold and is the minimal training level for most competitive athletes.
All athletes learn sooner or later that spending a lot of time exercising at a slow pace does not help them to move faster. Slow training is called "junk miles" because it does not help you to move faster since it does not increase the rate that muscles can take in and use oxygen.
Pick Up the Pace Again and Run Even Faster As soon as you slow down, you catch up on your oxygen debt and turn the lactic acid into carbon dioxide and water. This allows you to run even faster since lactic acid is the body's most efficient fuel for muscles, even better than sugar. (Sports Med, 2006;36(4):279-91). When you exercise as hard as you can, lactic acid helps you to go faster.
Electric currents cause muscles to contract. This electricity is generated by cell membranes causing potassium to move inside cells and chloride ions to stay outside. With vigorous exercise, potassium ions accumulate outside cells. As large amounts of potassium ions accumulate outside cells, electricity is not generated and the cells cannot contract. Another ion called chloride accumulates outside cells and prevents potassium from getting back inside cells. Lactic acid removes the chloride, so it is easier for potassium to get back inside cells. Therefore lactic acid increases the ratio of potassium inside cells to the amount outside, and this helps the muscle contract with more efficiency.
Lactic acid buildup in muscles does not make muscles tired and may even make muscles contract more efficiently, which increases your endurance. This concept contradicts what most instructors used to teach to their students. The old incorrect theory was that lactic acid interferes with a muscle's ability to contract, so you feel tired.
Interval Training Teaches your Muscles to Use Lactic Acid as Fuel Interval training means that you move over a fixed distance at a very fast pace, then slow down for recovery and then repeat exercising at a fast pace followed by a slow recovery. You stop the workout when your muscles start to tighten or feel stiff. Interval training causes you temporarily to exceed your lactic acid threshold, so you can increase the rate that muscles can take in and use oxygen and you can exercise at a faster pace.
Interval training has been used in all endurance sports since the 1920's. George Brooks of the University of California at Berkeley showed why interval training makes you a better athlete (American Journal of Physiology-Endocrinology and Metabolism, June 2006). Inside each muscle cell are mitochondria, the little furnaces that burn fuel for energy. A major fuel for your muscles during exercise is the sugar, glucose. In a series of chemical reactions, glucose is broken down step by step, with each step releasing energy. When enough oxygen is available, the glucose releases all of its energy until only carbon dioxide and water remain; these are blown off through your lungs. However, if not enough oxygen is available, the chemical reactions stop at lactic acid which accumulates in the muscles and spills over into the bloodstream. Since lactic acid is the most efficient source of energy for muscles, anything that helps muscles to break down lactic acid faster will make you a better athlete because it will increase your endurance and allow you to move faster when you are tired.
Since lactic acid is burned for energy in the mitochondria, anything that enlarges the mitochondria builds a bigger furnace and helps to increase endurance. Lactic acid is carried from the cells into the mitochondria by special proteins called lactate transporter molecules. Anything that increases these molecules will build endurance. An enzyme called lactic acid dehydrogenase is needed to start the reaction, so anything that increases this enzyme will also help. Interval training does all three: it enlarges the furnace (mitochondria), increases lactic acid transporter molecules, and increases the amount of lactic acid dehydrogenase. These improvements from interval training have been shown in several studies to help improve heart function, even in people who have had heart attacks (Br J Sports Med, Oct 21, 2013).
Types of Interval Training Intervals are a fixed number of repeats of a fixed distance at a fixed pace with a fixed recovery time. There are two types of intervals: long and short. A short interval takes less than 30 seconds and does not build up significant amounts of lactic acid in the bloodstream, so an athlete can do lots of repeat short intervals in a single workout.
Long intervals take two minutes or more and are very tiring. In interval training, a runner may run a quarter mile 12 times, averaging one minute, with a 110-yard slow jog between each. A weightlifter may lift a heavy weight ten times in a row and then repeat another set of ten. Runners run intervals as fast as they can and recover enough to run the same fast pace several times. Runners need very short recoveries between intervals, usually only about 30 seconds; but weight lifters need much longer recoveries, at least two and a half minutes. Runners become short of breath and feel a burning in their muscles when lactic acid starts to accumulate in muscles, but it takes only a few seconds for a trained athlete to recover between each hard run. On the other hand, weight lifters feel burning caused by tearing of the muscle fibers and it takes a much longer time for the pain to disappear so they can lift very heavy weights again.
What This Means For You You can apply the concept of interval training to your program at any level of fitness. When you start a new exercise program, exercise for 30 seconds, stop for 30-60 seconds, longer if you need it. Alternate exercising and resting until you feel tired or your muscles feel heavy. Then stop for the day. The stronger you get in your sport, the more intense your intervals can become. You work at your maximum capacity for 30-60 seconds, then take 60-90 seconds to recover, then go very hard for another 30-60 seconds. Do this vigorous interval workout once a week until you get tired. At first you may only be able to do two or three intervals, but your muscles get stronger and you build up the number of intervals you can complete. Go easy the next day or take a day off if you feel any discomfort.
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