How interval training works Although more attention is usually given to the period of activity during an interval workout, the training stimulus occurs during the recovery period. It is during this phase that the heart rate declines at a proportionally greater rate than the return of blood to the heart, resulting in a brief increase in stroke volume (SV), or the amount of blood that the heart pumps with each beat.7 The effect of this is two-fold. The first effect puts a natural overload on the heart muscle, and the second effect enables the muscles to be quickly cleared of waste products (lactic acid).5 Since SV is highest not during exercise but during the recovery period, and since interval training has many recovery periods, the SV reaches its highest level many times.1 By comparison, continuous training has only one recovery period, immediately after the workout is over. Over time, repeatedly attaining peak SV values from interval training provides a much greater stimulus for improving maximum SV and the capacity of the oxygen-transport system than continuous training does.3 Since the recovery periods provide time for physiological adaptation, their duration is a very important part of interval training. A very short recovery period may not allow the body to recover sufficiently to perform the next work interval at the desired intensity. Conversely, a very long recovery period may allow the body to recover too much and some of the training effect would be lost. The duration of the recovery period depends on the length of the work interval and the specific energy system that is being trained. Using interval training A caution. When first training a sedentary client, the cardiovascular training program should be very general, focusing on increasing basic fitness and endurance. Because of its physically stressful nature, interval training should only be used as a method of enhancing cardiovascular fitness after a relatively long period of continuous training. Keep in mind that your members are not Olympic athletes and most commonly join gyms when they are out of shape. They first need to get their bodies used to continuous exercise. However, there is substantial evidence that interval training indeed improves cardiovascular performance, both in trained and sedentary individuals.

Implementing interval training. Interval training can be employed in a number of ways. For individuals who, for various reasons, cannot run (orthopedic problems, poor physical condition, cardiac disease, etc.), alternate periods of fast and slow walking. For individuals who can run but cannot sustain a run for at least 20 minutes, alternate periods of running and walking. On a stationary cycle, the resistance or the cadence (revolutions per minute, RPM) can be increased and decreased to elicit the interval training effect. Increasing the resistance will have a greater effect on muscle force, while increasing the RPMs will have a greater impact on the velocity of muscle contraction.

During continuous exercise, intensity is typically based on a target heart rate that is kept constant for a period of time. Interval training, however, abruptly raises and lowers the heart rate. Think of it as manipulating the heart's responses by increasing the heart rate to meet the increasing demands of the activity, decreasing the rate to recovery and repeating this cycle a number of times. For example, have clients warm up by moving easily for five minutes on the cardiovascular equipment of their choice. Then have them run for five minutes, followed by five minutes of walking. Repeat this sequence -- run five minutes, walk five minutes --three times. Finally, finish the workout with a five-minute cool-down.

For a more creative workout, have them perform a five-minute warm-up at 60-percent max HR, then increase the speed or resistance of the machine to elicit 70-percent max HR for two minutes. Follow that with two minutes of recovery at 60-percent max HR, two minutes at 80-percent max HR, two minutes of recovery at 70-percent max HR, two minutes at 85-percent max HR and finally a five-minute cool-down at 60-percent max HR. Add it up and your client just finished a 20-minute interval workout. Table 1 shows other examples of interval workouts.

Targeting energy systems

Possibly the greatest use of interval training lies in its ability to target individual energy systems. Performing long work periods at a moderate intensity has a much different physiological effect than performing short work periods at a high intensity. Knowing the needs and goals of your clients and their exercise histories will help to decide what types of interval training to perform. The optimum way to train any of the energy systems is to repeatedly call upon the targeted system through periods of work and rest.

The ATP-CP system. To train the most immediate source of energy, the ATP-CP system, use activities of very high intensity and short duration. For example, very short intervals lasting six to 10 seconds, performed at close to maximum intensity, will increase the amount of stored ATP and CP in the muscles. Strength training with heavy weights and few repetitions (one to three) may also be used to train the ATP-CP system.

Use inactive recovery periods when training the ATP-CP system to allow the muscles to be replenished. If active recovery is used, part of the ATP produced is used to provide energy to perform the activity during the recovery period; therefore, this ATP is not available to replenish the CP or ATP stores. If the ATP-CP system is not replenished during the recovery period, it will not be available for use during the next work interval.

The longer the recovery interval, the more ATP-CP will be restored and the more this system can be used as an energy source during subsequent work bouts.3 To restore all of the ATP-CP, recovery periods should be about three to five minutes. In general, a work-to-rest ratio of approximately 1:3 to 1:5 (20 seconds work, 60 to 100 seconds rest) will effectively train the ATP-CP system. Power athletes, including sprinters, jumpers, Olympic-style weightlifters and shot putters are most interested in training the ATP-CP system.

Anaerobic glycolysis. Similar to training the ATP-CP system, anaerobic glycolysis is best developed using intense workloads that last 30 seconds to two minutes. For this system to be effectively trained, the body must become accustomed to the discomfort and fatigue resulting from high levels of lactic acid in the muscles and blood. This is accomplished by using active recovery periods that will remove some, but not all, of the lactic acid accumulated during the work intervals. The concentration of lactic acid will, therefore, be slightly greater than normal at the start of each successive work interval and will rise again as the work is performed. This process of increasing and decreasing the concentration of lactic acid and keeping its concentration above normal for the duration of the workout forces the body to adapt to greater and greater concentrations of lactic acid. In response, more lactic acid can be produced by the anaerobic glycolytic system before the same amount of discomfort and fatigue results.

Lactic acid is removed more quickly when active recovery periods are used. Active recovery also blocks the replenishment of the ATP-CP system so it is not available as an energy source during the work intervals. This causes the glycolytic system to be the principle contributor of energy. The length of the rest periods will depend on the duration of the work, but should be about twice as long as the work period (1:2 work-to-rest ratio). The number of times the work period is repeated will depend on the individual's fitness level, but should be repeated at least three times, with more advanced individuals performing six to eight repetitions.

Individuals who are interested in training the anaerobic glycolytic system are those who need to sustain physically-intense efforts for up to a couple of minutes, such as construction workers, furniture movers, and athletes who compete in events lasting from 30 seconds to about two minutes, including 400- and 800-meter runners, hockey players and downhill skiers.

Aerobic system. Interval training can also be used to improve the capability of the largest of the three energy systems, the aerobic system. The key to training the aerobic system with interval training is the volume of work performed, rather than the intensity. The work periods should last at least two minutes, the work-to-rest ratio should be 1:1 or less, and the number of repetitions should vary from about three to five repetitions, to eight to 12 repetitions, depending on the duration of the work intervals and your client's fitness level.

If the duration of the work intervals is at the low end of the range (about two minutes), inactive recovery should be used. With inactive recovery, any lactic acid accumulated during the work intervals will not be removed as quickly and, as a result, the glycolytic system will not be able to supply the large amount of the ATP required for the work intervals. This places a greater demand on the aerobic system to supply the required ATP. If the duration of the work intervals is long (four to six minutes), however, active recovery should be used. As the duration of the work intervals increases, the intensity of work decreases, resulting in less contribution from the glycolytic system and, therefore, less lactic acid accumulation. With active recovery, the oxygen transport system does not completely recover, since this system will still be active to support the activity during the recovery period. Therefore, the aerobic system is stressed to a greater extent.

With some proper planning and creativity, incorporating interval training into your clients' programs can help them overcome their plateaus and improve their fitness dramatically.

REFERENCES

1. Cumming, G.R. Stroke volume during recovery from supine bicycle exercise. Journal of Applied Physiology 32: 575-578, 1972.

2. Faulding, R. Origins of interval training. New Zealand Journal of Health, Physical Education, and Recreation 10(2): 46-49, 1977.

3. Fox, E.L., and D.K. Mathews. Interval Training: Conditioning for Sports and General Fitness. W.B. Saunders: Philadelphia, Pa., 21-30, 1974.

4. Novich, M.M., and B. Taylor. Training and Conditioning of Athletes. Lea & Febiger: Philadelphia, Pa., 1983.

5. Paish, W. Track and Field Athletics. Lepus Books: London, 1976.

6. Reilly, T., N. Secher, P. Snell and C. Williams. Physiology of Sports. E. & F.N. Spon: N.Y., 1990.

7. Reindell, H., and H. Roskamm. A contribution to the physiological foundations of interval training under specific considerations of circulation. Cited by Down, M.G. Interval training: An appraisal of work rest cycle applications to training for endurance running. Loughborough University of Technology, 1966.

Table 1. SampIe Interval Workouts
Beginner	Intermediate	Advanced
1	1	1
* 5 minutes warm-up	* 5 minutes warm-up	* 5 minutes warm-up
* 5x1:00 at 70-75% max HR with 2:00 recoveries at 50-60% max HR	* 5x2:00 at 75-85% max HR with 3:00 recoveries at 55-65% max HR	* 5x2:00 at 85-90% max HR with 2:00 recoveries at 60-75% max HR
* 5 minutes cool-down	* 5 minutes cool-down	* 8-10 minutes cool-down
2	2	2
* 5 minutes warm-up	* 5 minutes warm-up	* 5 minutes warm-up
* 3x5:00 run with 5:00 walk recoveries	* 2x10:00 run with 5:00 walk recovery	* 5x5:00 at 80-85% max HR with 3:00 recoveries at 60-70% max HR
* 5 minutes cool-down	* 5 minutes cool-down	* 5 minutes cool-down

 

Jason R. Karp has a Ph.D. in exercise physiology, and is director and coach of REVO2LT Running Team, a freelance writer and competitive runner.

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