Understanding The Science Behind Interval Training: Part 3.

Part III in this series on interval training will deal with the actual mechanics of designing and implementing an interval training program.
Note: This is part three, click here for part one!
click here for part two!

Part III in this series on interval training will deal with the actual mechanics of designing and implementing an interval training program. Part I outlined the basics of exercise physiology and the specifically, the role of exercise intensity in determining fuel selection. Part II followed with a discussion of the critical role of the post-exercise period, introduced the concept of interval training and outlined the favorable confluence of these factors that combine to promote improved fat burning as a result of high intensity exercise that is best accomplished with interval training.

What Is Interval Training?

Interval training routines will now be discussed in more detail and variations will be offered to present a wide array options followed by general guidelines for customizing routines. Keep in mind, interval training (sometimes called high intensity interval training or HIIT) is not routine. When performed properly with appropriate intensity and duration, while the benefits are substantial, even trained endurance athletes may finish their workout "over a bucket" (PB Laursen, personal communication).

In addition to the clear cut HIIT routines presented, some alternative protocols that are less strenuous, but nevertheless employ some aspects of interval training that positively impact bodyfat management will be offered as well. One important aspect to note before undertaking an interval training program is that these are not physically easy routines, rather they are cardiovascularly demanding by design. One should only begin interval training after a solid aerobic conditioning base has already been developed.

In addition, for anyone with heart disease, a family history of heart disease, or over 35, a physical exam is warranted. Many of the described routines involve maximal or even supramaximal effort (maximal refers to maximum aerobic performance, while supramaximal adds an anaerobic component). Some individuals may only be able to perform submaximal exercise testing for health reasons; therefore, alternative routines that employ submaximal efforts are also presented.

The essence of any interval training routine is to maintain an intensity above the lactic acid threshold (see part II for more details) for a set period of time (usually on the order of minutes) that is somewhat less than the maximum time possible at that intensity, before failure occurs (referred to in the scientific literature as volitional fatigue).

This period is followed by a rest (also called recovery) or at least light activity to complete one cycle. This cycle is then repeated a number of times, typically 5 - 15. Obviously, since lactic acid thresholds are individual, knowing one's personal lactic acid threshold would be ideal. In fact, this is done for many athletes during training.3

For the average individual though, determination is not trivial since it requires blood drawing during exercise sessions and medical laboratory analysis. However, when the overall goal is not to specifically raise the lactic acid threshold (as is the aim for a specific athletic performance), but rather to perform training protocols that would accomplish this effect along with other desirable outcomes, routines can be selected without knowing the specific level by choosing intensity levels assured to exceed lactic acid thresholds. In addition, under these circumstances, there is no need to objectively quantify improvements in threshold levels. Other parameters of aerobic fitness will improve to show progress.

Bicycling: The Common Interval Training Exercise

Most of the studies on interval training deal with bicycling (the term 'bicycling' will be used to avoid confusion with the term 'cycling' related to the repetitions of work and rest phases of the interval training routine) and as such will comprise the bulk of recommendations, although some non-bicycling routines will also be presented. Bicycling from a research standpoint is convenient because the upper body is somewhat isolated and there is less of a concern with balance which makes cumbersome breathing tubes and blood draws easier during exercise. Before proceeding to a discussion of training routines, some aspects of stationary bikes need to be understood. Stationary bikes, depending on their price can come with either one or two modes, bicycle and bicycle/ergometer.

Cheaper stationary bikes typically only have a bicycle mode and this is the case for most home units. This mode functions exactly as it sounds, like a bicycle, meaning that the faster you pedal, the more work you are doing. If the maximum pedaling rate that can be maintained is insufficient to deliver the desired workout intensity, friction or resistance can be applied to increase the work load at a given pedaling rate, but the relationship of faster pedaling - more work still holds. Ergometer mode on the other hand, which is sometimes found on more expensive health club type models, is different from bicycle mode.

What Is A Workload?

In this case, a workload is set, (typically measured either in watts or calories per hour) and the bike's microprocessor (the basis for the higher cost) automatically adjusts the resistance in relation to the pedaling frequency to match the desired workload.

In other words, with a constant workload, slower pedaling increases the resistance, while faster pedaling lowers the resistance. With the ergometer mode, a specific workload can be selected and regardless of changes in pedaling rate over the course of the routine, the rate of work performed (which in physics is the definition of power) stays constant. If you have access to this type of exercise equipment, by all means utilize it, although recognize that not all health club employees are knowledgeable about details of their own equipment.

Another point to emphasize is that total work read by a bike in terms of calories burned is a rough estimate, especially if you are not asked to enter your weight. In ergometer mode however, the reading is quite accurate because you are performing a defined amount of work (weight is immaterial because it measures actual work performed).

For physicists and engineers who attempt to convert watts over time to total calories burned, one additional factor is required; calories burned are about 4 times the actual work performed due to inefficiency of energy production in muscles. This is why we sweat; 75% of the calories burned are lost as heat with only 25% use to produce the actual work performed. This makes our muscles about as efficient as an internal combustion engine. The initial routines described assume availability of this type of equipment.

One of the initial studies that has been extensively referenced regarding interval training for bodyfat management is by Tremblay and coworkers.10 This study compared continuous aerobic exercise with a mixture of continuous aerobic and interval training. Interval routines were performed twice weekly compared to 4 - 5 times weekly for the continuous group only. In addition, the interval group began with continuous routines, but gradually progressed to only performing interval routines.

The Tremblay interval routine is somewhat complicated consisting of both short and long routines. Both sets progressed in terms of both work times and the number of cycles. Several general features of interval routines can be delineated by examining their procedures in depth which is useful for customizing routines. The short interval workload was set at 60% of the maximum workload produced in 10 seconds, in essence an all out effort.

They performed intervals at 60% of the 10 second maximum (for example if the 10 second maximum was 500 watts, the interval routine used 300 watts) for 15 seconds for 10 cycles progressing to 30 seconds for 15 cycles over the course of several weeks. The rest period between intervals allowed heart rates to fall back to a range of 120 - 130 before starting another cycle. The long intervals followed a similar scheme with workload set at 70% of the 90 second maximum and beginning with 4 - 5 cycles of 60 seconds progressing to 90 seconds and a similar rest interval as for the short bout. The intensity of the workload was increased by 5% every 3 weeks.

The results as commonly reported were a 9-fold greater reduction in skinfold thickness of the HIIT routine versus the continuous routine. One point to emphasize is that this difference was corrected for energy expenditure and since the HIIT routine required only about half the energy output to perform, the actual measured difference of 4.5 fold becomes 9-fold. In percentage terms, the continuous exercise resulted in a reduction of 5.7% for the sum of 6 skinfold measurements (triceps, biceps, calf, subscapular, suprailiac, and abdominal) versus 14.7% reduction for the HIIT group.

One further point is that there were no significant weight changes during this period which implies these skinfold changes occurred in the absence of caloric deficits. In terms of time investment, it is difficult to assess how long these routines took, since the rest interval is based on heart rate recovery to a defined range which may vary, but can probably be estimated at about 3 - 4 minutes for the long routine at most. For the long routine, 5 cycles of 90 seconds with even 4 minutes of rest means a maximum total time of 32.5 minutes, including a 5 minute warmup.

The short interval protocol has a similar time period for cumulative high intensity phase (15 cycles for 30 seconds each). The rest interval would expected to be shorter because not only is the duration shorter, but 30 seconds is unlikely to provide enough time for the heart rate to even come close to max heart rate (maxHR). Most likely, the total exercise time would be similar.

In terms of choosing (since there was no attempt to distinguish between the short and the long bouts in terms of which is superior), the ease of identifying applicable workloads should be the deciding factor. If an ergometer is available at the gym, the long routine is probably better to perform because 5 cycles would be easier to tolerate and count off. To set the appropriate workload will require successive testing, over the course of several days prior to any cardio workouts. Obviously, interval training of this type should only be undertaken on top of a trained aerobic base.

Begin with a workload about 300 watts and determine if this can be maintained for 90 seconds. Increase by 50 watts each time if 90 seconds has been achieved over consecutive sessions (but space them out over several days with one trial per day). Once failure occurs, use the previous completed level and determine 70% of that value. That workload should be increased by 5% about every 4 weeks or sooner if the workout become easier (easily monitored by the max heart rate achieved during the 90 seconds intervals). Without an ergometer, the 10 second max workload is preferable. Simply identify the resistance required for failure after 10 seconds of an all out effort. Use 60% of that resistance for the 30 second cycles.

With the availability of an ergometer bike, there are other protocols that have been shown to also be effective. Several will be presented for variation. Most protocols involving bicycling are referenced to peak power output (PPO). PPO refers to the maximum power achieved during a graded test which differs from a single all out effort at one intensity. The test is simple to perform. Using an ergometer, begin with a warmup at 100 watts for about 5 minutes, then increase the wattage by 15 watts every 30 seconds until failure.

The highest wattage completed for 30 seconds is PPO. Many protocols then use some percentage of PPO to set conditions. For example, 175% PPO for 30 seconds followed by a 4.5 minute rest cycled 12 times.8 For those less capable of supramaximal efforts, 8 cycles of 4 minutes at 85% PPO with 90 seconds recovery works just as well8. Note that this routine is the only one with a rest phase shorter than the work phase (except for perhaps rest intervals determined by heart rate recovery) and as such, somewhat challenging in this regard.

Putting The Methods To The Test

Recently a study compared several methods head to head. The authors compared 3 training protocols, the 175% PPO described above as well as two variations of percentage of time to exhaustion at 100% PPO4. The rationale for these protocols is derived from earlier running protocols (discussed below). Basically, a PPO test is performed to identify the individual PPO. At a later time, another test is performed at 100% PPO until failure to determine the time to exhaustion (Tmax). The work interval is then set at 100% PPO for 60% Tmax for eight cycles.

For example, if PPO was determined to be 350 watts and Tmax at 350 watts was 3.5 minutes, then the interval would be 350 watts for 2 minutes and cycled 8X. Two different recovery methods were used either 2X the work interval or resting until 65% of maxHR (which can be determined relative to your heart rate at your max PPO from the previous graded test). All three protocols lead to increases in aerobic fitness in the range of 5 - 8% after only 4 weeks which is actually quite significant for highly trained endurance athletes. In addition, while the study was only conducted for 4 weeks, testing at 2 and 4 weeks showed continued improvement suggesting that further gains may be possible. In terms of overall performance, the 100% PPO tests were slightly superior to the 175% PPO protocol.

In terms of convenience, the 100% PPO protocols are cumbersome to perform solo since the time and rest intervals need to be calculated (a stopwatch that can be reset might be most useful). For most trained individuals, the Tmax times are going to be in the range of about 3 - 5 minutes. With a rest interval double the work interval, the total time to perform 8 cycles will be about 60 minutes or slightly more. With a rest interval based on heart rate, it could be slightly less, but recovery times will increase through the 8 cycles.

Using a rest interval based on heart rate has the added convenience of one less time period to follow, but does require a continuous heart rate monitor. All of these routines are quite difficult to complete. Even well trained endurance athletes failed to complete all cycles each time, so approach these protocols with caution. Also limit these sessions to no more than twice per week since these are quite demanding routines.

For running routines, the treadmill is fairly standard; however, many are limited by top speeds in the 10mph range. This upper limit should be sufficient for most conditioned athletes. Typically, VO2max (see part I for a complete description) is determined by treadmill testing, but those treadmills employ inclines that go beyond normally available. For a surrogate, begin at a brisk walking pace of about 3 - 3.5 mph.

Increase the pace by 0.2 mph every minute until failure. If using a heart monitor, maxHR should be reached (bear in mind that this form of maximal testing should only be undertaken by someone free of cardiovascular disease and already aerobically trained). Submaximal testing cannot be substituted even though protocols exist to measure aerobic fitness from submaximal results. This is because you are interested in the top speed achieved during the graded test. The top speed for a full minute is defined as your vVO2max (your velocity at your VO2max). Intervals are now set relative to this speed in a manner analogous to the bicycling routines above.

Several variations have been evaluated. In one case, you can determine a Tmax (maximum time sustainable at your vVO2max) and run intervals at 60 - 75% Tmax at vVO2max followed by a rest interval equal to the work interval and done at 60% vVO2max and cycled 5 times7. Another protocol involves cycling between 100% vVO2max and 50% vVO2max for 30 seconds for each interval for 12 cycles.2 This particular routine was not evaluated for its effects over time as in most of the other protocols presented; however, the study results do suggest that in terms of sustaining a high intensity effort (which is the goal of interval training with regards to bodyfat management), it is qualitatively similar to other protocols.

Obviously, not everyone has the capacity, determination, nor desire to engage in supramaximal or even maximal effort during a workout. While high intensity activity is clearly associated with favorable responses with regard to bodyfat management, submaximal strategies can also be employed. As discussed in part I, moderate intensity does allow for a substantial energy expenditure (in the 8 - 15 calorie per minute range) with an intermediate post-exercise fat burn relative to high intensity.

Some strategies to enhance fat burning include exercising in a postabsorptive state, in other words, on an empty stomach, typically after an overnight fast. The goal here is to eliminate any insulin effect on inhibiting lipolysis during the exercise. It goes without saying not to consume carbs during the routine as well. In a carb depleted state, fuel selection is shifted towards fat burning so that at every intensity level, more fat is burned. Assuming that intensities stay in the low - moderate range, there is no decrement to fatigue time.5

Carb depletion routines can also be employed selectively in working muscles. A simple routine on an ergometer is to alternate 2 minutes intervals between 80% (dropping to 70% when 80% cannot be sustained) and 50% PPO until failure.6 This routine can be viewed as a modified interval routine that is at the lower end of the minimum intensity to qualify as HIIT. The fact that it can be sustained for so long suggests that the intensity is insufficient to create enough lactic acid to train the threshold.

Subjects will typically require about 1 hour to completely deplete carbs in the working muscles. Alternatively, the 85% PPO 8X4 minutes with 90 seconds recovery would provide a similar effect, although probably not as significant in terms of complete carb depletion. If a moderate intensity cardio routine is performed the following day without carb replacement, the percentage of energy derived from fat will be substantially higher.12

Another Strategy

Another strategy is to perform incremental exercise from low to moderate to high intensity during the course of one session.11 In this case, the exercise performed at the higher intensity burns fat at a greater rate than a routine of just the high intensity, most likely due to the priming of lipolysis. Alternatives to this strategy are to employ interrupted sessions. This has been demonstrated for two intensity levels, moderate and moderate/high. In these cases, cardio is performed at either at 60% max HR for 1 hour or 75% max HR for 30 minutes, followed by a rest interval of one hour and then the routine is repeated.9,13 The fat burn during the second routine is higher than during the first.

Admittedly, these are not time efficient, but as discussed above, these offer routines to enhance fat burning for those not disposed to the HIIT routines. Finally, studies with excess post-exercise oxygen consumption have shown that splitting routines throughout the day provide for a greater cumulative post-exercise fat burn than if performed all at once.1 In this case, 30 minutes split into two 15 minute routines in the morning and evening is slightly better in the combined post-exercise period than all at once.

Finally, for those who wish to design their own customized routines, there are several guidelines to keep in mind. The overall concept behind interval training is to be working at an intensity level that is not sustainable for more than several minutes. This intensity level will demand a lot of effort to maintain. The higher the intensity level, the shorter the work phase should be, but 15 - 30 seconds is probably the lower limit.

Especially when using stationary exercise equipment factor in the ramp times to transition between high and low intensities. Another factor to consider is as the intensity increases, the work phase time period should decline and the rest phase can equal the work phase in terms of time. As the work phase increases to 1 - 4 minutes, the rest phase lengthens more than the work phase. More rest is required the longer the work phase, since more lactic acid will accumulate and needs to be cleared. Five minutes should probably be viewed as an upper limit to the work phase to ensure that the intensity is well above the lactic acid threshold.

HR can be used as a general guideline to follow recovery allowing a fall back to about 65% of maxHR. Also, do not simply stop activity for the rest phase, some movement will assist in clearing lactic acid more rapidly. Do not attempt these routines more than twice per week due to their demanding nature. Begin with once per week and continue with some cardio and gradually work in twice per week sessions.

It would still be advisable to maintain at least one, low to moderate cardio session as well, preferably on a day following an interval routine. Keep in mind that the goal of HIIT from the standpoint of bodyfat management is to maximize the total time spent at high intensity. It is this combination of high intensity and duration that leads to the post-exercise fat burning. Therefore, as intensity goes down, duration must go up.


In conclusion, interval training offers several attractive features to justify its incorporation into the bodybuilder's workout. It offers time efficient aerobic conditioning which is athletically useful as well as smart healthwise. Since growth hormone (GH) secretion is dependent on exercise intensity, it offers another avenue for natural GH production.

Finally, the exercise intensity effect on the post-exercise period offers substantial benefits in terms of bodyfat management relative to traditional continuous cardio workouts.

Reference List

1. Almuzaini, K. S., J. A. Potteiger, and S. B. Green. 1998. Effects of split exercise sessions on excess postexercise oxygen consumption and resting metabolic rate. Can. J. Appl. Physiol 23:433-443.
2. Billat, V. L., J. Slawinski, V. Bocquet, A. Demarle, L. Lafitte, P. Chassaing, and J. P. Koralsztein. 2000. Intermittent runs at the velocity associated with maximal oxygen uptake enables subjects to remain at maximal oxygen uptake for a longer time than intense but submaximal runs. Eur. J. Appl. Physiol 81:188-196.
3. Garcin, M., A. Fleury, and V. Billat. 2002. The ratio HLa : RPE as a tool to appreciate overreaching in young high-level middle-distance runners. Int. J. Sports Med. 23:16-21.
4. Laursen, P. B., C. M. Shing, J. M. Peake, J. S. Coombes, and D. G. Jenkins. 2002. Interval training program optimization in highly trained endurance cyclists. Med. Sci. Sports Exerc. 34 :1801-1807.
5. Phinney, S. D., B. R. Bistrian, W. J. Evans, E. Gervino, and G. L. Blackburn. 1983. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism 32:769-776.
6. Schrauwen, P., W. D. Marken Lichtenbelt, W. H. Saris, and K. R. Westerterp. 1997. Role of glycogen-lowering exercise in the change of fat oxidation in response to a high-fat diet. Am. J. Physiol 273:E623-E629.
7. Smith, T. P., L. R. McNaughton, and K. J. Marshall. 1999. Effects of 4-wk training using Vmax/Tmax on VO2max and performance in athletes. Med. Sci. Sports Exerc. 31:892-896.
8. Stepto, N. K., J. A. Hawley, S. C. Dennis, and W. G. Hopkins. 1999. Effects of different interval-training programs on cycling time-trial performance. Med. Sci. Sports Exerc. 31:736-741.
9. Stich, V., G. de, I, M. Berlan, J. Bulow, J. Galitzky, I. Harant, H. Suljkovicova, M. Lafontan, D. Riviere, and F. Crampes. 2000. Adipose tissue lipolysis is increased during a repeated bout of aerobic exercise. J. Appl. Physiol 88:1277-1283.
10. Tremblay, A., J. A. Simoneau, and C. Bouchard. 1994. Impact of exercise intensity on body fatness and skeletal muscle metabolism. Metabolism 43:814-818.
11. van Loon, L. J., P. L. Greenhaff, D. Constantin-Teodosiu, W. H. Saris, and A. J. Wagenmakers. 2001. The effects of increasing exercise intensity on muscle fuel utilisation in humans. J. Physiol 536:295-304.
12. Weltan, S. M., A. N. Bosch, S. C. Dennis, and T. D. Noakes. 1998. Preexercise muscle glycogen content affects metabolism during exercise despite maintenance of hyperglycemia. Am. J. Physiol 274:E83-E88.
13. Weltman, A., J. Y. Weltman, J. A. Kanaley, A. D. Rogol, and J. D. Veldhuis. 1998. Repeated bouts of exercise alter the blood lactate-RPE relation. Med. Sci. Sports Exerc. 30:1113-1117.

Note: This is part three, click here for part one!
click here for part two!