Aerobic system: interval training to improve fitness

You can vary the intensity, the work period and the rest period but which combination is most effective?

Interval training is a well-known method for improving fitness. Technically, it is defined as high-intensity intermittent exercise. In an interval session, high-intensity periods of work are interspersed with rest intervals. In this way athletes can cover more distance at a high intensity than they could if they worked continuously. Because interval training is intense, it is a great method for improving both aerobic and anaerobic fitness.

Interval-training sessions can be different in composition, as there are three variables that can be altered: the intensity (speed), the work period and the rest period. For example, a running interval session could comprise 200 metre efforts in 25 secs with 60 secs recovery. Another session could be 200m in 35 secs with 20 secs recovery. In the first session, the athlete runs fast with a moderately long recovery, whereas in the second session the athlete runs only moderately fast but has a shorter recovery. Each session would end with the athlete being unable to continue at the desired pace. As many readers will know, one session may be faster than the other but by the end of the workout both sessions will feel pretty tough.

However, without accurate analysis of the aerobic and anaerobic energy demands of each session, it is impossible to say which session is the more effective, or whether the sessions place the same demands on the energy systems. With this is mind, Izumi Tabata and his colleagues at the Japanese Institute of Fitness and Sport designed an experiment to measure how two different types of interval training sessions taxed the aerobic and anaerobic energy systems ('Metabolic Profile of High-Intensity Intermittent Exercises', Tabata, I, Irishawa, K, Kuzaki, M, Nishimura, K, Ogita, F, & Miyachi, M, Medicine & Science in Sports & Exercise, 29(3), 390-395, 1997).

Tabata et al obtained the aerobic energy demands directly, by measuring the amount of oxygen used during exercise in millilitres of oxygen used per kilogram of body weight per minute. This score can be presented as a percentage of the VO2max of the subject, which is the maximum amount of oxygen per kg per min the subject can use. Unfortunately, the anaerobic demands cannot be measured directly in the same way. This is because ATP produced anaerobically is fuelled from the breakdown of phosphates and glycogen stored in the muscles and so it is impossible to measure directly exactly how much energy has been released. However, some researchers have argued that it is possible to estimate accurately the anaerobic demands from the 'accumulated oxygen deficit', and this is the method Tabata et al chose to use.

How to work out the deficit

At rest, we use a certain amount of oxygen simply to function. If we start to walk around, we use more. Breaking into a jog, we use more still. As exercise intensity increases, so does the use of oxygen, and the relationship between the two has been shown to be linear. At fairly high intensities, fast running, for example, energy will also be produced anaerobically, but the oxygen use will still increase until it reaches its limit at the VO2max. From then on, any further increases in exercise intensity will be fuelled by anaerobic sources. However, it is possible to predict a theoretical amount of oxygen required to work higher than the VO2max by extrapolating from the linear relationship between intensity and oxygen to intensity levels about the VO2max. The difference between the theoretical level and the actual maximum must represent the anaerobic energy demands. This anaerobic demand is expressed as an oxygen equivalent. The difference between actual and theoretical over the period of the exercise is called 'the accumulated oxygen deficit'. This is the method Tabata and colleagues used to measure the anaerobic demands of exercise. They are among the first researchers to employ this technique, so their findings from this study are very useful and informative.

Tabata and his team used nine undergraduate sportsmen as their subjects. The exercise was performed on a a static bike, which enabled the exercise intensity, in Watts, to be easily controlled. First, they established the subject's relationship between exercise intensity and oxygen demands between 35% and 87% of the subject's VO2max. This was done so they could predict the theoretical oxygen demands at intensities above VO2max. Then the subject's VO2max and anaerobic capacities were measured as reference points. The mean VO2max of the group was 57 ml/kg/min. The anaerobic capacity was obtained from the accumulated oxygen deficit during a high-intensity 2-3 minute exhaustive exercise bout. The accumulated oxygen deficit in one bout is the difference between the predicted oxygen demand in ml of O2 per kg and the actual ml of O2 per kg used. The researchers found that the mean anaerobic capacity of the group was 69 ml/kg.

Now to the intervals

On a different day the subjects performed two different kinds of interval workout. The first session (I1) comprised bouts of 20 seconds with 10 seconds rest at an intensity equivalent to 170% of their VO2max. The subjects performed six or seven bouts each until reaching exhaustion, ie, they could no longer continue at the prescribed intensity. The second session (I2) comprised bouts of 30 seconds with two minutes rest at an intensity of 200% of their VO2max. The subjects managed four or five of these bouts. The oxygen used was measured directly as usual to give the aerobic demands of the interval sessions. The anaerobic demands were calculated as the accumulated oxygen deficit. The accumulated oxygen deficit for bouts with rest intervals is the difference between the theoretical oxygen demand of the bouts and the actual oxygen used during both the bouts and the rest periods.

Tabata et al found that the anaerobic demands of I1 were significantly higher than I2, with the accumulated oxygen deficit being 69 ml/kg compared to 46 ml/kg. This means that on the I1 workout the subjects had reached their anaerobic capacity. In other words, the session was equivalent to a maximal anaerobic effort. On the other session, I2, the anaerobic demand was below the subjects' capacity.

Tabata et al do not report the overall oxygen consumption for the two interval sessions but they do report that the peak VO2 for I1 is 55 ml/kg/min and for I2 is 47 ml/kg/min. This suggests that the I1 workout places greater aerobic demands on the subjects than I2, with peak VO2 reaching the subjects' VO2max values.

The conclusion from these findings seems to be that the I1 workout, the 20-second bouts with 10 secs recovery at 170% VO2max, is a better training stimulus for aerobic and anaerobic systems than the I2 workout of 30-second bouts with two mins recovery at 200% VO2max. In support of this, Tabata et al found that a six-week regime of I1 resulted in a 13 per cent improvement in VO2max.

Although I2 does not stress the anaerobic or aerobic systems as much as I1, the actual total amount of anaerobic work done during the I2 workout was greater than that for I1. This is because during I2 the subjects performed 4-5 x 30 sec bouts at 200% of Vo2max, an average of 126 seconds at 200% VO2max. In contrast, on I1 the subjects performed 6-7 x 20 sec bouts at 170% VO2max, an average of 126 seconds at 170% VO2max. Therefore on I2 subjects performed more anaerobic work in total.

The reason subjects didn't reach their anaerobic capacity on I2, even though they did more work, is due to the differences in the rest periods used. During each bout, phosphocreatine (PCr) is broken down, oxygen stores used up and lactate is produced from anaerobic glycolysis. During a two-minute rest period, as on I2, oxygen stores in the muscles can be replenished and the PCr stores used during each bout will be significantly recovered. Therefore the oxygen store and PCr contribution to each bout in I2 will be high. Because of this, more work can be done until lactate reaches the level whereby the subject cannot continue. In addition, although more TOTAL anaerobic work is done on I2, a two-minute recovery time allows the aerobic system to contribute more. Thus, PROPORTIONATELY less anaerobic work is performed and so the subjects do not reach anaerobic capacity.

In contrast, the rest intervals in I1 are very short. Therefore the PCr and O2 contribution will be insignificant after the first or second bout, as little oxygen and PCr store recovery will occur during 10-second rest intervals. PCr and O2 stores are quickly used up, and so the anaerobic energy must be mainly supplied by anaerobic glycolysis. This results in faster accumulation of lacate and earlier fatigue. Also, with short rest intervals there is proportionately less aerobic contribution and so subjects must reach anaerobic capacity to achieve the workout. Interestingly, even though proportionately less aerobic work is done, the aerobic demand on I1 is higher than on I2.

What it means to you

The conclusion must be that I1, with high-intensity bouts and very short rests, is a very intense workout that maximally stresses both aerobic and anaerobic systems. I2, with longer rest periods, does not stress both the anaerobic and aerobic energy systems as much, and so more work can be done until fatigue.

The results of this research by Tabata et al clearly show that two different intervals workouts have different demands and therefore training effects. I1, with 20-second bouts with 10 secs rest at 170% VO2max places the aerobic and anaerobic systems at peak stress. Therefore it would be a fine session for improving both aerobic and anaerobic capacity. Events where both aerobic and anaerobic demands are high are, for example, 400m, 800m and 1500m running, sprint cycling, canoeing, rowing and speed skating. This kind of workout would be great for these sports. Games players may also want to use the I1 workout as an intense training method for improving aerobic and anaerobic fitness.

The 12 workout doesn't put either system at peak stress. However, it does allow more high-intensity work to be done in total.With the longer recovery, I2 has a greater contribution from the PCr energy stores. So this kind of session will be better for developing the PCr system, improving maximal power. In addition, by allowing greater rest periods, the session can help improve recovery mechanisms.

Professor Craig Sharp, in a lecture at an International Coaching Conference on anaerobic exercise, recommended longer rests for anaerobic recovery training, as the body can learn to buffer the acidosis and mobilise the anaerobic enzymes during the rest period ('Some aspects of anaerobic exercise and training', Sharp, N C C. Transcripts of a lecture from the 18th International Coaches Convention, hosted by the Scottish Amateur Athletic Joint Coaches Committee). This I2 workout will be useful for games players, who need the ability to repeat short maximal efforts, with low-intensity recovery periods, throughout a match. However, I2 will not bring about the same improvements in anaerobic capacity as I1, so games player could complement I2 with I1. By the same token, if only I1 was used, the athlete would not develop the PCr system and recovery mechanisms as much as if I2 were included.

I recommend that for anaerobic training, both types of interval sessions are used, one with very short rests, another with long recoveries. However, the athlete's sport will determine which type of session is most important. Incidentally, if you want to use interval training, remember that to get the kind of benefits described you must perform the workouts to exhaustion. Interval training is about setting a demanding intensity level and working at that level for the prescribed work/rest ratios until you cannot continue. If you do that, you have reached overload and the training will be effective. Without overload, there is no adaptation.

Raphael Brandon