Most coaches and researchers are convinced that you
have to do a series of short bursts of very fast speed training [interval training] to
improve long-term endurance, but they do not know why. The
most offered explanation is that muscle fatigue caused by many
hours of cycling is associated with a reduction in muscle fibers'
ability to contract with force. Now a study from France shows
that short bursts of very fast cycling improve endurance for
cycling competitions that take many hours, because the stronger
you are, the less of your maximal effort is needed to get the
same pressure on the pedals (Medicine and Science in Sports
and Exercise, January 2005.)
Muscles are made up of millions of individual fibers. Each fiber is a single muscular thread that functions independently. When you contract a muscle, you contract only a small percentage of these fibers at the same time. As each muscle fiber fatigues, you lose the ability to contract that fiber. When enough of these fibers stop contracting efficiently, you lose strength and your muscles feel tired. However, stronger fibers take longer to fatigue because they are being worked at a lower percentage of their capacity. So stronger muscles can be exercised for longer periods of time.
Making each muscle fiber stronger and bigger allows it to exert force for a longer period of time and therefore, increases endurance. The only way to make a muscle stronger is to exercise that muscle against progressively greater resistance, and that applies to each muscle fiber also. It is impossible to put great pressure on a muscle for a long time. When you do all-out fast bursts for a short time, you exert so much pressure that you have to back off after several seconds or a minute. All-out sprints for a short period followed by resting and then repeating the sprint is called interval training. It makes the entire muscle stronger and delays fatigue.
Athletes in all sports use long and short intervals [for interval training]. Short intervals take less than 30 seconds and because you do not build up significant amounts of lactic acid in that time, you can do hundreds of repeats in a single workout. Long intervals take two to three minutes and are very damaging to your muscles. Because you feel burning in your muscles and become very short of breath for a longer time, you can do only a few of these in a single workout. So athletes in all sports that require endurance do both long and short intervals to help them exercise longer.
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