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Intervals to Improve Both Endurance and Speed

Special issue: In last week's eZine I explained why intense exercise is necessary for both
• maximum health benefits of exercise, and
• maximum speed and endurance in athletic competition.
This week's issue describes how to use interval training for these benefits.

Cyclists, runners, and almost all other athletes have to move very fast in training to be at their best in competition. However, you can't move very fast over long distances, so athletes use a training technique called intervals in which they move very fast for a short period, move at a very slow pace until they recover their breathe, and then move very fast again and repeat these sprints a few times in a single workout. When you train intensely, you run low on oxygen to cause lactic acid to accumulate in your bloodstream. This makes your muscles more acidic to cause them to burn and you to slow down. We now know that increasing both speed and endurance for competition requires you to train so intensely that you build up lactic acid in practice sessions. This helps the mitochondria, the furnaces in muscles, to burn lactic acid more efficiently for fuel during exercise (AMAA Journal, Fall 2009). In fact, intervals markedly improve endurance for cycling competitions that take many hours and days, because the stronger you are, the less of your maximal effort is needed to get the same pressure on the pedals (Medicine & Science in Sports & Exercise, January 2005).

Duration of Intervals
Athletes in all sports use some variation of long and short intervals. Short intervals take fewer than 30 seconds and because you do not build up significant amounts of lactic acid in that time, often you can do as many as a hundred repeats in a single workout.

Long intervals usually 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 an extended period of time, you can do only a few of these in a single workout. The longer the work/rest interval, the greater the muscle damage, utilization of oxygen and sugar, and using up of muscle glycogen (Journal of Sports Science, August 2005). Athletes in all sports that require endurance usually do both long and short intervals to help them exercise intensely longer.


Short Intervals
You can do a lot more short than long interval workouts in a single workout. Intervals that last more than 30 seconds build up so much lactic acid that you get burning over a long duration that causes significant muscle damage. Short intervals between 10 and 30 seconds can markedly improve all aspects of speed and endurance (European Journal of Applied Physiology, September 2010). Since shorter intervals cause less muscle damage, beginners can start out by doing short intervals as short as six to ten seconds. Thirty-second intervals give you a better training effect and recovery during competition than six-second intervals (American Journal Physiol Regul Integr Comp, December 28, 2006).

Long Intervals
Most athletes do long intervals of two to three minutes. Intervals longer than that cause so much muscle damage that muscles take far longer to recover for the next hard workout. Many athletes do intervals lasting much longer than three minutes, but these very long intervals cause so much muscle damage that they do not do them more often than every few weeks.

Duration of Rest Between Intervals
The shorter you rest between intense intervals, the longer it takes to recover (Medicine & Science in Sports & Exercise, August 2005). Conditioned athletes doing four-minute intervals usually can recover for their next interval within two minutes (Medicine & Science in Sports & Exercise, September 2005).

Therefore most athletes slow down long enough to
• recover their breath,
• slow down their breathing, and
• relieve the burn in their muscles.

Then they do their next long interval. Most athletes start their next interval before complete recovery of heart rate and breathing rate. They should not do the next interval when muscle burning is still present. They usually terminate a workout when muscle burning or soreness persists.

Runners and cyclists often use heart rate monitors or a clock to determine when their heart rate has dropped low enough to start their next interval. Weight lifters usually wait for their bodies to "feel" recovered. Athletes learn their ideal interval rest duration during a workout through trial and error. You can use whatever yardstick for recovery from each interval you like, but if it takes you longer than two days to recover from an interval workout, you are probably exercising too intensely, doing too many repetitions, or not taking a long enough interval rest.

How Often to Do Intervals
Every time you do intense interval training, you cause a tremendous amount of damage to your muscles. Obviously it takes time to heal. If you try to do an interval workout again before your muscles have recovered, you put yourself at high risk for injuries and also impair your training because you can't train fast on damaged muscles. As a general rule, the only sport in which athletes try interval training more often than three times a week is swimming. Most athletes in most sports cannot recover faster than 48 hours from intense interval workouts. In sports such as running or cycling, competitive athletes do not improve by increasing their volume of low intensity exercise without also using intense training. Furthermore with interval training, many athletes have to decrease the volume of their slow recovery workouts done on the days after interval workouts.


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Avoiding Over-training
Going out and exercising slowly over long distances will not give you much endurance unless you also exercise intensely once or twice a week at much shorter distances. A person can run a marathon or ride a bicycle century much faster by training fast two or more times a week. The most common mistake made by endurance athletes is to exercise so much that they can't maintain their speed training on their hard interval days. This is often seen in runners or cyclists who perform so many miles per week that their fast workouts end up much slower than they should be.

Scientists used to think that the primary cause of muscle fatigue during endurance exercise was running out of glycogen, the sugar that is stored in muscles. They now know that cumulative fatigue and soreness that does not go away are caused primarily by damage to the muscle fibers. The best way to protect muscle fibers is to strengthen them by exercising against increasing resistance by running, cycling, skiing or skating faster once or twice a week. However, every time that you exercise more intensely, your muscle fibers are damaged, so you have to allow time for recovery by exercising slowly the rest of the time.

How to Include Interval Workouts in Your Exercise Program
If you want to gain the endurance to walk, run, swim, cycle, skate, ski or dance for an extended time, pick two days, say Tuesday and Thursday, for speed and the rest of the time for less intense recovery workouts or days off.

On Tuesday, warm up and then start an interval workout by doing five-second intense intervals followed by a marked slowing of your breathing and complete disappearance of muscle burning. Only then should you start your next short interval. When your muscles start to stiffen or the muscle burning takes a long time to go away, you must stop your workout. Otherwise you may take weeks to recover from that workout. After many months, you will become stronger and you can try to work up to the point where you can do lots of 30 second intervals in a single workout.

On Thursdays, start out the same way as your Tuesdays but then try to work up to the point where you can do repeat bouts of sustained exercise for two minutes. Rest until you recover your breath and the muscle burning disappears, and then repeat these fast two-minute intervals three to eight times. Of course, these long intervals will be significantly slower than your short intervals.

Before you start a new exercise program or increase the intensity of your program, check with your doctor.

Always stop exercising, particularly in interval workouts, as soon as you feel pain in one area that worsens with continued activity. Always stop a workout if you don't feel good. Never take an interval workout, or do any intense exercise, when your muscles are sore from a previous workout. On recovery days, exercise at reduced intensity.


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June 21st, 2013
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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