Understanding Interval Training
By Michael Kurilla
Most bodybuilders incorporate some form of aerobic conditioning or "cardio" into their workout routines in order to develop and maintain cardiovascular fitness for both health reasons as well as increasing endurance capacity. In addition, cardio along with dieting or caloric restriction have been the cornerstones for bodyfat management. Unfortunately, there has been much misinformation and misapplication regarding the actual impact of aerobic exercise with regards to bodyfat management.
For the most part, this derives from the fact that scientific studies upon which these ideas are based are usually part of weight loss / fitness programs for either obese individuals or those with cardiac disease.2,16,17,28 In addition, public health advocates in an effort to induce the greatest number of couch potatoes to take up exercise have typically reduced scientific information to a simplified, palatable form that is directed towards overweight, sedentary individuals with little applicability or guidance for someone who is already fit.
This three part article will review the basics of exercise physiology in order to understand the role of fuel selection (fat versus carb burning) within the context of an overall aerobic conditioning program. Next, in part two, the role of the post exercise period will be detailed.
Interval training in terms of its impact on cardio programs and bodyfat management will also be introduced. Finally, in part three, various interval routines will be presented with results from studies in order to compare the various methodologies. In addition, simplified interval routines with proven results will be outlined. In as little as 4 weeks, substantial progress in terms of aerobic fitness as well as bodyfat reduction can be achieved with the right program along with the right commitment.
Exercise Physiology - How To Burn Fat For Energy Instead Of Carbs Or Protein
In order to understand the physiology of interval training and its applications, a general outline of exercise physiology must be introduced. Energy for exercise (or any physical activity) is derived from two primary, dietary fuel sources: carbohydrates and fats. Protein can also be used, but generally, durations of less than 60 minutes involve little protein burning, although under extreme low carb conditions, this can increase.3 For each fuel, there are also two primary sources working muscle can call on: intramuscular (within the muscle itself) or peripheral (derived from the blood).
When peripheral sources are utilized, glucose (the major carb or sugar form in the body) comes either from the blood itself or the liver which releases glucose it has either stored or produced in order to maintain blood sugar levels and prevent hypoglycemia.
Peripheral fat stores are any fat depots in the body including subcutaneous (directly under the skin) and intra-abdominal (fat stored underneath the abdominal muscles) fat. The process of releasing fat from peripheral fat stores into the blood is known as lipolysis. Intramuscular stores of carbs are known as glycogen (a large complex of glucose attached to itself). Intramuscular stores of fat are in the form of triglyceride, similar to the storage form of fat in peripheral fat stores.
How A Muscle Determines It's Fuel
During exercise four fuel sources (two carb and two fat) are typically utilized. How a muscle determines its fuel mixture, both the form of the fuel and its source, is based on:
The intensity and duration of the exercise.
The training status of those specific exercising muscles
The general diet (relating to specific percentages of fats and carbs).
The time interval since the last meal prior to exercise.
In addition to the exercise itself there is also a recovery period after exercise where energy expenditure is greater than an equivalent period of time following rest. Finally, the exercise itself induces hormonal changes that impact longer term fuel selection and utilization as well as fuel (that is food) intake.
By following an exercise routine, you will burn more fat and calories even while you are at rest!
There has been a lot of confusion about intensity and duration with regards to fuel selection in terms of optimal training protocols specifically related to bodyfat reduction. A better understanding of the overall process will clear the air so stick with me! While carbs and fats supply the major energy fuels for most activities, they have dramatically different properties in our bodies. One major difference is that fats require more oxygen to burn than carbs. This implies that as exercise intensity is increased, there is a natural shift to burning more carbs because less oxygen is needed to extract the needed level of energy output to support that intensity. When oxygen is not limiting (that is at low intensity), fats are the preferred fuel. Now, the situation is not as simple a low intensity burns fat / high intensity burns carbs.
Carbs are utilized to some extent at all intensity levels with a gradual and progressive absolute as well as percentage increase as intensity is increased. Fats on the other hand, provide the bulk at low intensity and gradually increase with intensity (in this case, absolute amount of fat burning increases, even while the percentage contribution to the total is going down), but then taper off at moderate to high intensity because of oxygen limitations (during this phase, the absolute as well as percentage declines). However, the details of fuel selection get more complicated because of differences in where these fuels come from. The source of these fuels also has implications especially in the post-exercise period which in turn plays a role in bodyfat management.
Understanding The VO2max
To understand fuel selection in more detail, the concept of VO2max will be introduced. VO2max simply refers to the maximum rate of oxygen you can utilize during exercise. As intensity increases so does your heart rate and so does oxygen utilization. Eventually, at some point if you increase intensity further, your heart rate will not increase further (you've reached your max heart rate) and oxygen utilization won't go up anymore; your oxygen usage has maxed out (overall intensity can still increase, but you are going anaerobic at that point). Intensity then can be indicated by some percentage of that maximum aerobic intensity. Therefore, we can quantify intensity with some percentage of VO2max.
At rest, you are about 5-8% of your VO2max. Resting VO2 is sometimes referred to as a MET or metabolic equivalent. Some exercise equipment can relate the workload as the number of METs. Low intensity is typically in the range of 25 - 40% VO2max. A 25% VO2max effort would be a comfortable walking pace. 40% would be in the range listed as the fat burning zone on some cardio equipment. 45 - 70% VO2max is squarely in the range of moderate intensity and is labeled as the aerobic training zone.
Bonus! To calculate how many calories you are burning during over 600 exercises and activities, click here to use our online calculator! This is based on the MET calculations.
A trained individual should be able to maintain this level of intensity for 1.5 - 3 hours or more. Above 70-75% VO2max is in the range of high intensity. Duration is severely limited because at these levels, the reliance on carb burning to generate a high percentage of the total energy begins to produce lactic acid. At some point (that is, a specific percentage of your VO2max), the level of lactic acid hits a threshold and begins to skyrocket in the blood and brings about muscle failure.3,26 Once you are above your lactic acid threshold, duration can only be sustained for 3-10 minutes.3
Now that intensity levels are set, we can follow fuel selection across the intensity spectrum, since both fuel usage and source is determined by intensity.21,22 At low intensities, fat is primarily used (assuming no pre-exercise meal as been ingested). In addition, the source of the fat is circulating fat in the blood derived from release of peripheral fat stores. In other words, at low intensity, such as casual walking, calories are derived from fat coming from peripheral fat stores and supply the major fraction (85%) of total calories expended. The remainder of the calories is supplied by carb burning from uptake of sugar in the blood. However, at this level of intensity the overall level of caloric expenditure is low.
A resting value of caloric expenditure is on the order about 1 calorie per minute which scales roughly with lean body mass. The more muscle you have, the more calories you will burn at rest. Low intensity exercise like casual walking ups this value to the range of 3 - 6 calories per minute. This is the basis for recommendations for low, sustained intensity levels for fat burning, sometimes called the fat burning zone, since about 85% of the energy expended will be derived from fat burning. However, total calories burned and hence the quantity of fat actually burned will be quite low. In addition, these intensities are too low for a substantial aerobic training effect to occur which has long term impact on fuel utilization, during and after exercise.23
As an example, even at the upper range of 6 calories per minute, one hour at this intensity yields 360 calories expended. Since only 85% of these calories are fat derived, we've burned 306 fat calories or 34 grams of fat (1 gram of fat is worth 9 calories). In terms of weight control, that's not too bad since 34 grams of fat can be a substantial portion of one's daily intake, but from the standpoint of fat reduction, over 11 hours of this exercise would be necessary to burn off one pound of fat (about 3500 calories per pound).
The general rule of 20 - 30 minutes for these types of activity to "turn on fat burning" comes from the idea that as fat is pulled out of the blood by the working muscles, the level in the blood will eventually begin to drop after, surprise, 20 - 30 minutes. As this happens, there is a hormonal response to restock fat levels in the blood. These hormones (both adrenaline and noradrenaline, coming from adrenal glands and nerves, respectively) stimulate fat cells to break down their fat stores and release them into the bloodstream, through the process of lipolysis.7 This is also where diet and food composition is important, since insulin released in response to dietary intake of carbohydrate opposes the action of these hormones. Insulin is antilipolytic, shuts down fat release, and promotes fat storage in peripheral fat cells.
Increasing Exercise Intensity: More Or Less Fat Being Burned?
As the intensity of the exercise is increased, the whole process ramps up for greater energy expenditure. Both fat and carb burning are increased. Although carbs assume an increasing percentage of the total, the absolute level of fat burning still continues to increase. The major difference with moderate versus low intensity is for the source of the fat derived energy. As intensity is increased, the working muscles require more oxygen and hence more blood flow, which explains the increase in heart rate (more blood volume and hence more oxygen is pumped per minute). In addition, since muscle is only about 25% efficient in terms of work, 75% of the calories expended are lost as heat. This heat must be dissipated which is why we sweat. In order to sweat, some blood flow must be directed to the surface skin, which is different from the peripheral fat stores just below the skin.
These two factors (more blood to the working muscle and increased blood flow to the skin for sweating) combine to limit the blood flow that can be allocated to peripheral fat stores for loading the blood with released fat through.23 Thus, at some point with increasing intensity, the release of fat into the blood stream from peripheral fat stores levels off.4,21,23 Peripheral fat stores have a maximum rate of fat release into the blood which has been determined to be the primary limitation for this fuel source.7,23 However, the rate of fat burning by the working muscle continues to increase.
To meet increased energy needs for moderate intensity (relative to low intensity), the muscle begins to breakdown its own fat stores, the intramuscular triglyceride. Since this store is more limited than the peripheral fat store, the muscle prefers to preserve this store until absolutely needed. In other words, at low intensities, working muscles prefer to utilize fats from peripheral fat stores which are the most flexible in terms of fat storage (because they can become huge). But peripheral fat stores are limited in the rate at which they can release fat and so fat stores within the working muscle itself are involved when the intensity increases enough to require more fuel than peripheral fat stores can provide. Intramuscular fat stores are limited by the absolute amount of fat available.
Your Maximum Fat Burning Zone
Maximum fat burning rate occurs during moderate intensity at about 60 - 65% VO2max which corresponds to about 75% max heart rate for most people (this assumes an aerobically trained individual).1 The absolute burn rate is size dependent, but will typically fall into the range of about 0.5 - 0.8 grams of fat per minute with about equal contributions from peripheral and intramuscular sources. Total caloric expenditure for moderate intensity exercise again is size dependent (larger people expend more calories because they are moving larger masses and working larger muscles) and falls in the range of about 8 - 15 calories per minutes with fat contributing on the order of about 50 - 70% of the total caloric expenditure. With longer durations, this amount tends to the higher value, mainly due to intramuscular glycogen depletion that occurs at this level of intensity. Most trained individuals can sustain this rate of expenditure for 1.5 - 3 hours, at least, but because of the level of carb burning at this intensity, the duration will be limited by total carb stores in the body.
As intensity is further increased (to 85% VO2max), oxygen supply begins to become limiting. This causes a further shift to greater carb burning with breakdown of the muscle's intramuscular store of carb in the form of glycogen1.21,22,23 Similar to fat burning, as intensity is increased, fuel source shifts to a greater reliance on intramuscular carb stores in the form of glycogen. If the level of intensity is held below the lactic acid threshold (which varies from the high 60's% of VO2max to the 80's%), the activity can be sustained for about 45 - 60 minutes until glycogen stores are exhausted necessitating a fall back to an intensity level where fat can supply the majority of energy needs.
If on the other hand, the intensity is above the lactic acid threshold, then in a matter of a few minutes, lactic acid levels will climb intolerably high in the blood and failure results.3 As will be seen in part II, interval training is designed to improve this situation.
An important aspect to this complex pattern of fuel usage is the effect of training on fuel selection. This is vital to understand because an understanding of training effects identifies the orientation of the overall system and permits exploitation of fuel selection for maximum desired results. Simply put, aerobic training serves to enhance greater energy generation from fat sources at all intensity levels.3,12,14 The rationale for this is simple, carbohydrate stores of energy are quite limiting and can be depleted during the course of a single exercise session of sufficient intensity and/or duration.
During a marathon race, 'hitting the wall' occurs when carb stores have been depleted and underscores the focus on carb loading regimes. Under conditions of glycogen depletion, the muscle must begin to breakdown protein since branched chain amino acids present in protein can substitute for carbs, in terms of supplying energy directly as well as substituting for carbs in the process of maintaining the system for aerobic energy generation (something fats cannot support). Alanine (another amino acid in protein) released from protein breakdown can also be converted by the liver into glucose further increasing carb supplies from the blood.
Training allows for a higher sustainable level of intensity to be performed by sparing carbs in working muscle (by reducing the rate of depletion) and generating a greater percentage of energy from fat derived sources. In other words, since at a given intensity level, a specific level of energy generation is needed, endurance training allows for higher energy outputs to come from fat burning and improves duration by sparing carbs. Alternatively, training will also result in a potentially higher intensity level for a specific time period.
Typically, an aerobic training effect increases VO2max by as much as 25% in as little as 3 - 4 months of consistent training 3 - 4 times per week in the range of 60 - 85% maximum heart rate for 30 - 45 minutes per session. Fuel usage at the same absolute workload pre and post training (the post training workload is a lower relative intensity because VO2max has increased as a result of training) differs so that a greater reliance on fat for energy occurs. In addition, the greater reliance on fat for energy is derived largely from the greater utilization of intramuscular fat stores rather than peripheral sources.11,14 While this may appear counterproductive for fat loss, part two will discuss the importance of this development with regards to post-exercise effects and interval training and the relationship to bodyfat management.
What Effect Does Your Diet And Food Intake Have On All This? ***
Finally, diet and food intake need to be addressed in the context of fuel selection. Diet refers to the macronutrient composition that occurs in the range of two weeks prior to the exercise period. Unfortunately, many dietary studies typically involve a short adaptation period of as little as three days, although prior work suggests as many as 10 - 14 days are needed for complete adaptation to changes in macronutrient composition to fully manifest.18,19
Specifically, the macronutrient composition that matters is the amount of fat and carbs in the diet. Simply put, the less carbs ingested over time, the greater the reliance on fat burning.6 To achieve a faster response than changing the diet, specific glycogen depletion exercise routines can be employed.24,25,27 A sustained, reduced carb intake leads to a reduction in carb utilization at comparable work intensities. After four weeks of complete carb elimination, moderate intensity exercise can be performed with no reduction in endurance capacity, but two-thirds reduction in carb burning with a corresponding increase in fat burning.18
In other words, the less carbs you eat, the less your body will try to burn carbs while you are exercising. This means that you will naturally be burning more fat!
Food intake in the immediate pre-exercise period also affects fuel usage. Carb intake prior to exercise will result in release of insulin which retards lipolysis and fat utilization during subsequent exercise. Insulin in general promotes carb utilization throughout the body including working muscle and limits peripheral fat stores from releasing fats. Exactly how long after eating the system takes to return to baseline (defined as an overnight fast) depends on the specific meal. Studies with mixed meals suggest that effects can persist for 4 - 6 hours.8,15 Carb intake during exercise has a similar effect.5,9,10,20
Conclusion And Summary Of Points
Part I has served to introduce the basics of fuel selection and utilization during exercise along with the effects of training. Here are the main points:
There is a lot of confusion about how to really burn fat.
During exercise, your body will either burn fat (from your ugly fat stores or from inside the muscle), carbs or in some cases, protein.
Your body will decide which type to burn based on the intensity and duration of exercising, your type of diet and when you last ate, and how advanced of a trainee you are.
By following an exercise routine, you will burn more fat and calories even while you are at rest.
Fats require more oxygen to burn than carbs. This means that as exercise intensity is increased, there is a natural shift to burning more carbs because less oxygen is needed to extract the needed level of energy output to support that intensity.
The more instense the exercise is, the more carbs you will burn, in most cases.
Maximum fat burning rate occurs during moderate intensity at about 60 - 65% VO2max which corresponds to about 75% max heart rate for most people (this assumes an aerobically trained individual).
The less carbs ingested over time, the greater your body's reliance on fat burning.
Carb intake prior to exercise will result in release of insulin which stops or slows down the fat burning process.
However, the exercise period itself is only part of the story, the post exercise period also has an effect that is substantial. Understanding its role and its relation to intensity levels will serve to introduce the role of interval training and its application to body fat management in part II.
1. Achten, J., M. Gleeson, and A. E. Jeukendrup. 2002. Determination of the exercise intensity that elicits maximal fat oxidation. Med. Sci. Sports Exerc. 34:92-97.
2. Aronne, L. J. 2001. Treating obesity: a new target for prevention of coronary heart disease. Prog. Cardiovasc. Nurs. 16:98-106, 115.
3. Bassett, D. R., Jr. and E. T. Howley. 2000. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med. Sci. Sports Exerc. 32:70-84.
4. Coyle, E. F. 1995. Substrate utilization during exercise in active people. Am. J. Clin. Nutr. 61:968S-979S.
5. Fritzsche, R. G., T. W. Switzer, B. J. Hodgkinson, S. H. Lee, J. C. Martin, and E. F. Coyle. 2000. Water and carbohydrate ingestion during prolonged exercise increase maximal neuromuscular power. J. Appl. Physiol 88:730-737.
6. Helge, J. W., P. W. Watt, E. A. Richter, M. J. Rennie, and B. Kiens. 2001. Fat utilization during exercise: adaptation to a fat-rich diet increases utilization of plasma fatty acids and very low density lipoprotein-triacylglycerol in humans. J. Physiol 537:1009-1020.
7. Hodgetts, V., S. W. Coppack, K. N. Frayn, and T. D. Hockaday. 1991. Factors controlling fat mobilization from human subcutaneous adipose tissue during exercise. J. Appl. Physiol 71:445-451.
8. Horowitz, J. F. and E. F. Coyle. 1993. Metabolic responses to preexercise meals containing various carbohydrates and fat. Am. J. Clin. Nutr. 58:235-241.
9. Horowitz, J. F., R. Mora-Rodriguez, L. O. Byerley, and E. F. Coyle. 1997. Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. Am. J. Physiol 273 :E768-E775.
10. Horowitz, J. F., R. Mora-Rodriguez, L. O. Byerley, and E. F. Coyle. 1999. Substrate metabolism when subjects are fed carbohydrate during exercise. Am. J. Physiol 276:E828-E835.
11. Hurley, B. F., P. M. Nemeth, W. H. Martin, III, J. M. Hagberg, G. P. Dalsky, and J. O. Holloszy. 1986. Muscle triglyceride utilization during exercise: effect of training. J. Appl. Physiol 60:562-567.
12. Klein, S., E. F. Coyle, and R. R. Wolfe. 1994. Fat metabolism during low-intensity exercise in endurance-trained and untrained men. Am. J. Physiol 267:E934-E940.
13. Lemon, P. W. and J. P. Mullin. 1980. Effect of initial muscle glycogen levels on protein catabolism during exercise. J. Appl. Physiol 48:624-629.
14. Martin, W. H., III, G. P. Dalsky, B. F. Hurley, D. E. Matthews, D. M. Bier, J. M. Hagberg, M. A. Rogers, D. S. King, and J. O. Holloszy. 1993. Effect of endurance training on plasma free fatty acid turnover and oxidation during exercise. Am. J. Physiol 265:E708-E714.
15. Montain, S. J., M. K. Hopper, A. R. Coggan, and E. F. Coyle. 1991. Exercise metabolism at different time intervals after a meal. J. Appl. Physiol 70:882-888.
16. Nicklas, B. J., E. M. Rogus, and A. P. Goldberg. 1997. Exercise blunts declines in lipolysis and fat oxidation after dietary-induced weight loss in obese older women. Am. J. Physiol 273:E149-E155.
17. Nieman, D. C., D. W. Brock, D. Butterworth, A. C. Utter, and C. C. Nieman. 2002. Reducing diet and/or exercise training decreases the lipid and lipoprotein risk factors of moderately obese women. J. Am. Coll. Nutr. 21:344-350.
18. 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.
19. Phinney, S. D., B. R. Bistrian, R. R. Wolfe, and G. L. Blackburn. 1983. The human metabolic response to chronic ketosis without caloric restriction: physical and biochemical adaptation. Metabolism 32:757-768.
20. Rauch, L. H., A. N. Bosch, T. D. Noakes, S. C. Dennis, and J. A. Hawley. 1995. Fuel utilisation during prolonged low-to-moderate intensity exercise when ingesting water or carbohydrate. Pflugers Arch. 430:971-977.
21. Romijn, J. A., E. F. Coyle, L. S. Sidossis, A. Gastaldelli, J. F. Horowitz, E. Endert, and R. R. Wolfe. 1993. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am. J. Physiol 265:E380-E391.
22. Romijn, J. A., E. F. Coyle, L. S. Sidossis, J. Rosenblatt, and R. R. Wolfe. 2000. Substrate metabolism during different exercise intensities in endurance-trained women. J. Appl. Physiol 88:1707-1714.
23. Romijn, J. A., S. Klein, E. F. Coyle, L. S. Sidossis, and R. R. Wolfe. 1993. Strenuous endurance training increases lipolysis and triglyceride-fatty acid cycling at rest. J. Appl. Physiol 75:108-113.
24. Schrauwen, P., W. D. Lichtenbelt, W. H. Saris, and K. R. Westerterp. 1998. Fat balance in obese subjects: role of glycogen stores. Am. J. Physiol 274:E1027-E1033.
25. 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.
26. Spurway, N. C. 1992. Aerobic exercise, anaerobic exercise and the lactate threshold. Br. Med. Bull. 48:569-591.
27. 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.
28. Welty, F. K., E. Stuart, M. O'Meara, and J. Huddleston. 2002. Effect of addition of exercise to therapeutic lifestyle changes diet in enabling women and men with coronary heart disease to reach Adult Treatment Panel III low-density lipoprotein cholesterol goal without lowering high-density lipoprotein cholesterol. Am. J. Cardiol. 89:1201-1204.
***Some of this may be debatable. See http://www.bodybuilding.net/18678-21-post.html
EricT on 12-29-2006, 02:46 PM
Understanding the Science behind Interval Training.. PART 2
In part I, the basics of exercise physiology were introduced to provide a framework for understanding the role of interval training in cardiovascular conditioning as well as body fat management. Part II will discuss two related concepts, the post-exercise period in terms of fuel usage and the underlying rationale for interval training along with its impact on cardiovascular training. Part III will discuss the actual mechanics of interval training and suggest specific protocols that have been proven to be effective by clinical research studies on trained athletes keeping in mind the intent of interval training as discussed below.
While fuel selection during exercise is primarily determined by the training status of the individual and the intensity of the exercise (see part I), in terms of weight management, the calories burned during exercise do not typically contribute substantially to a caloric deficit that is required for weight loss. Much of this is simply a numbers game. Mild to low-moderate intensity does not produce sufficient caloric expenditure to result in noticeable weight loss by itself.
Walking for example, typically will result in about 100 calories per mile expenditure; thus even a pace of 3.5 miles per hour (which is a brisk pace to maintain) will yield only 350 calories after one hour. Even if 100% of the calories were derived from fat, 10 hours would be necessary for 1 pound of body fat to be lost. Alternatively, higher intensities are limited by the duration that they can be sustained as well as a lower percentage of fat burned since carb burning increases with intensity (see part I).
The Post Exercise Period
Fortunately, the effect of exercise is not limited to simply the exercise period. Just as importantly is the effect of exercise on the post exercise period. This period can be examined from the standpoint of the immediate time frame after exercise to the remainder of the entire non-exercise time period. In terms of the overall impact on weight loss, the inclusion of exercise offers a clear benefit. Typically, with any caloric restriction diet to generate weight loss, over time the rate of weight loss tends to zero and weight plateaus.
This phenomena is usually due to a fall in the basal metabolic rate (resting metabolism) that is probably related to both a slight reduction in lean body mass that can accompany weight loss as well as hormonal adaptation to the lowered caloric intake.2 Exercise in this setting, even of mild intensity will promote preservation of lean body mass (assuming the calorically restricted diet is still able to supply sufficient protein taking into account increased protein required with physical activity) and prevent or reduce the fall in basal metabolic rate due to hormonal adaptation.2,19,20,22
Burn Fat For 3 Hours After Exercising?
Of more particular relevance however, to an already physically active group of individuals is the immediate post-exercise period. Studies have revealed that following aerobic exercise, there is a period of excess oxygen consumption relative to a similar period of time following rest.14 In other words, after an aerobic session, more oxygen is consumed during rest than would be if the exercise had not occurred.
Greater oxygen consumption implies that something is being burned, either fat or carb. In fact, most studies suggest that fat burning is providing the bulk of the excess oxygen consumption.12,14,15 In addition, the magnitude of the excess consumption is directly related to exercise intensity and duration.3,8,12,16-18 Thus, the higher the exercise intensity, the greater the degree of increased fat burning in the post exercise period which can extend for well over three hours following exercise.
The presumed mechanism for this effect is directly related to the fuel utilization during the exercise period. Recall that with increasing intensity from moderate to high, the percentage of energy supplied by fat burning declines (although the absolute decline is slower to trail off), while carb burning continues to increase. Also, carb sources in the body are quite limited. Depending on the exercise, one session can significantly reduce carb stores (glycogen) in the specific working muscles. Even with just rest, carb stores can be exhausted in 2 - 3 days if not replenished by food intake. Thus, following an exercise session that has depleted the muscle's glycogen content, the body will attempt to replenish these stores.
This means that carb uptake by the muscle will go to make glycogen, rather than burned for energy needs. However, even under resting conditions, the muscle still has basal metabolic energy requirements. To meet these requirements, the muscle will burn fat for fuel and this is the basis for increased fat burning in the post exercise period, since the conversion of carbs to glycogen in muscle will require some energy itself.
Clearly, following low intensity exercise, the muscle has little to do since neither intramuscular carb or fat stores have been significantly impacted. In addition, at low intensity, energy can be supplied by peripheral fat stores slowly releasing fat into the bloodstream. These fat stores are the most flexible since they can grow and contract as needed, but the rate of mobilization (or release) of fat from peripheral fat stores is limited.10 However, once moderate intensity is initiated, the situation changes dramatically.
Under moderate intensities, intramuscular fat stores must be tapped to support the level of energy generation, since the fat contribution from peripheral fat stores has reached its limit. In essence, moderate intensity can be viewed as the point where intramuscular fuel stores begin to be utilized. Carb burning will also increase linearly with intensity and the breakdown of intramuscular glycogen stores become significant during upper levels of moderate intensity.
Thus, following moderate intensity exercise, the two major intramuscular energy stores of fat (in the form of intramuscular triglyceride) and carb (in the form of glycogen) need to be replaced. Since carbs are limiting as discussed above, carbs, in the form of blood sugar, are diverted exclusively for glycogen replacement. Fat uptake is also increased and fuels energy needs as well as intramuscular triglyceride replacement. Endurance athletes develop increased intramuscular triglycerides as a part of training.9,11
This fat, both for energy needs as well as intramuscular triglyceride replacement, is derived from fat circulating fat in the bloodstream and comes from peripheral fat stores. Given that depletion of peripheral fat stores is limited in their rate of release of fat (the speed at which they can be depleted) continuing peripheral fat store depletion following exercise offers an attractive method to enhance bodyfat management. Desirable weight loss and bodyfat loss implies depletion of peripheral body fat stores.
The Intensity Of Exercise And How It Will Affect You
In summary, the intensity of exercise, regardless of the amount of fat burned during the exercise directly influences the amount of fat burned in the post exercise period. Since this fat is derived from peripheral fat stores, the higher the intensity and longer the duration that can be sustained, the greater the post-exercise fat burn that can be achieved. In essence, the combination of intensity and duration generate a carb and fat depleted state in the working muscle that shifts fuel metabolism towards fat burning to allow the muscle to return to its baseline levels of glycogen and intramuscular fat stores, ready for another round of exercise.
Sustained moderate intensity (in the range of 8 - 15 calories per minute) exercise can create this condition, but we will now turn our attention to another training modality, namely interval training, that will achieve this same effect within a shorter time period, and also contribute an added benefit from a cardiovascular training standpoint.
Interval training arose as means for endurance athletes to improve their performance capability. We have seen how VO2max measures the level of aerobic fitness; however, maintenance of an intensity at 100% VO2max can only be sustained for a few minutes because of the accumulation of lactic acid. Lactic acid, derived from carb burning, will gradually accumulate in the blood as a result of increased production in working muscle as intensity is increased.1
Calculate Your VO2max, Click Here!
This lactic acid must be cleared from the bloodstream to avoid high levels that upset the pH balance and cause the biochemical steps that generate energy from being disrupted because these steps function in a narrow pH range. Once in the blood, lactic acid can be removed a number of ways: 1) working muscles can buffer themselves against the drop in pH as a result of the acid load, 2) the liver can remove lactic acid from the blood and convert it back into glucose (since lactic acid is produced as a result of glucose breakdown, high levels of lactic acid imply that muscles are using a lot of glucose in the first place), 3) non-working muscles can take up lactic acid and burn it for fuel, conserving glucose for working muscles, and 4) the heart, which must always operate under aerobic condition can utilize lactic acid for fuel.
Lactic acid is produced during any exercise to some extent, but increases with intensity. Lactic acid accumulates in the blood, but remains relatively steady until a specific intensity (specific to the individual) is reached, at which time lactic acid levels rise uncontrollably resulting in muscle failure. This threshold level is referred to as the lactic acid threshold. Prior to the threshold level, the body's ability to remove lactic acid from the blood allows it to balance lactic acid production with removal. Above the threshold, the removal system has maxed out and can no longer keep pace with production.
The key for endurance performance lasting greater than a 45 - 60 minute time frame is to operate at an intensity that is below the lactic acid threshold5. Lactic acid threshold is somewhat individual, but has been shown to be a better predictor of athletic performance than overall aerobic fitness (VO2max).4,6,7 For example, athlete A with a lactic acid threshold at 71% VO2max could maintain a 60 minute intensity at 70% VO2max, while athlete B with a lactic acid threshold of 81% VO2max could maintain 80% VO2max during the same period. If their VO2max levels were equal (and they were the same size), athlete B would win a competition lasting an hour since he could sustain a greater intensity over the time period of the event.
How Do You Raise Your Lactic Acid Threshold?
The question then becomes how to train or raise your lactic acid threshold. This is the intent behind interval training. While performance at events lasting 60 minutes or longer may not be everyone's goal, the adaptations that occur with interval training are valuable beyond the improvement in athletic performance and contribute to body fat management in a productive manner. In addition, interval training can result in aerobic fitness improvements and maintenance with far less time investment than the more traditional increasing volume (meaning more time) work.
The essence of interval training is to work at an intensity level above one's lactic acid threshold, thus causing an increase of lactic acid that would result in muscle failure. This intensity is sustained for a duration that is somewhat less than the maximum possible time (that is, before failure occurs), typically on the order of several minutes. This is followed by a rest period or extremely light exercise to allow the body's systems to clear the lactic acid from the blood. The cycle is then repeated.
Thus, the body is exposed to a long cumulative total duration (longer than could be produced from continuous exercise) of elevated lactic acid and will learn to adapt to improving its ability to clear lactic acid from the blood. Cycling is typically repeated 8 - 15 times. If conditions are chosen appropriately, lactic acid levels will slowly climb and each cycle will become progressively more difficult. With this general type of protocol, the lactic acid threshold can be increased so that a higher intensity can be maintained for long periods (60 minutes or longer) without failure due to lactic acid levels.
For an endurance athlete, the value of lactic training is clear. With any program of aerobic training, there is typically an increase in VO2max of about 25% relative to sedentary levels. Greater gains can only be achieved with reductions in body weight (since VO2max is measured in terms of liters of oxygen consumed per minute per kilogram of body weight). After this initial gain which occurs over 3 - 5 months of aerobic training, further increases in training volume (that is longer training sessions), do not lead to further gains. To increase performance further, interval training allows for a higher level of intensity to be maintained under conditions relevant for athletic competition. While most sporting events do not operate at 100% VO2max, athletes typically perform at or just slightly above their lactic acid threshold.
Even for those interested in non-endurance activities, interval training can achieve several goals. First, interval training is relatively time efficient in terms of producing aerobic benefits.13 Second, interval training, along with other vigorous activities of comparable intensity have been associated with reductions in body fat, specifically in terms of a reduction in skinfold thickness.21
While the physiologic basis for this effect has not been identified, the likely effect is a combination of post exercise fat burning as discussed above, along with secretion of growth hormone that occurs during intense physical activity.23 Growth hormone besides its obvious effects on muscle mass is also known to produce a lipolytic effect that would promote a loss of fat.
Finally, interval training can improve and raise the lactic acid threshold which permits a higher intensity of sustained aerobic activity for longer periods of time. Thus, for a specific time period allocated for aerobic exercise, a higher intensity level can be utilized for greater caloric expenditure.
Part II has discussed the importance of the post-exercise period in terms of bodyfat management and its relation to exercise intensity. The concept of interval training was introduced and detailed to highlight the value in an overall aerobic conditioning program. Finally, the confluence of several factors suggests that the addition of interval training provides special benefits with regards to bodyfat management because of the intensity level and the post-exercise period.
Part III will discuss various interval training protocols that can be utilized under a wide variety of conditions. Given the intensity of this type of training, some protocols are simply not suitable for all training regiments and care must taken to identify appropriate protocols. A wide range of protocols will be presented to offer options for nearly everyone.
1. Antonutto, G. and P. E. Di Prampero. 1995. The concept of lactate threshold. A short review. J. Sports Med. Phys. Fitness 35:6-12.
2. Astrup, A., P. C. Gotzsche, W. K. van de, C. Ranneries, S. Toubro, A. Raben, and B. Buemann. 1999. Meta-analysis of resting metabolic rate in formerly obese subjects. Am. J. Clin. Nutr. 69:1117-1122.
3. Bahr, R. 1992. Excess postexercise oxygen consumption--magnitude, mechanisms and practical implications. Acta Physiol Scand. Suppl 605:1-70.
4. Bassett, D. R., Jr. and E. T. Howley. 2000. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med. Sci. Sports Exerc. 32:70-84.
5. Billat, L. V. 1996. Use of blood lactate measurements for prediction of exercise performance and for control of training. Recommendations for long-distance running. Sports Med. 22:157-175.
6. Billat, L. V. and J. P. Koralsztein. 1996. Significance of the velocity at VO2max and time to exhaustion at this velocity. Sports Med. 22:90-108.
7. Coyle, E. F. 1999. Physiological determinants of endurance exercise performance. J. Sci. Med. Sport 2:181-189.
8. Gore, C. J. and R. T. Withers. 1990. Effect of exercise intensity and duration on postexercise metabolism. J. Appl. Physiol 68:2362-2368.
9. Hardman, A. E. 1998. The influence of exercise on postprandial triacylglycerol metabolism. Atherosclerosis 141 Suppl 1:S93-100.
10. Hodgetts, V., S. W. Coppack, K. N. Frayn, and T. D. Hockaday. 1991. Factors controlling fat mobilization from human subcutaneous adipose tissue during exercise. J. Appl. Physiol 71:445-451.
11. Klein, S., E. F. Coyle, and R. R. Wolfe. 1994. Fat metabolism during low-intensity exercise in endurance-trained and untrained men. Am. J. Physiol 267:E934-E940.
12. Laforgia, J., R. T. Withers, N. J. Shipp, and C. J. Gore. 1997. Comparison of energy expenditure elevations after submaximal and supramaximal running. J. Appl. Physiol 82:661-666.
13. Laursen, P. B. and D. G. Jenkins. 2002. The scientific basis for high-intensity interval training: optimising training programmes and maximising performance in highly trained endurance athletes. Sports Med. 32:53-73.
14. Maehlum, S., M. Grandmontagne, E. A. Newsholme, and O. M. Sejersted. 1986. Magnitude and duration of excess postexercise oxygen consumption in healthy young subjects. Metabolism 35:425-429.
15. Medbo, J. I. and E. Jebens. 2002. Leg gas exchange, release of glycerol, and uptake of fats after two minutes bicycling to exhaustion. Scand. J. Clin. Lab Invest 62:211-221.
16. Phelain, J. F., E. Reinke, M. A. Harris, and C. L. Melby. 1997. Postexercise energy expenditure and substrate oxidation in young women resulting from exercise bouts of different intensity. J. Am. Coll. Nutr. 16:140-146.
17. Rauch, L. H., A. N. Bosch, T. D. Noakes, S. C. Dennis, and J. A. Hawley. 1995. Fuel utilisation during prolonged low-to-moderate intensity exercise when ingesting water or carbohydrate. Pflugers Arch. 430:971-977.
18. Sedlock, D. A., J. A. Fissinger, and C. L. Melby. 1989. Effect of exercise intensity and duration on postexercise energy expenditure. Med. Sci. Sports Exerc. 21:662-666.
19. Stefanick, M. L. 1993. Exercise and weight control. Exerc. Sport Sci. Rev. 21:363-396.
20. Thompson, J. L., M. M. Manore, and J. R. Thomas. 1996. Effects of diet and diet-plus-exercise programs on resting metabolic rate: a meta-analysis. Int. J. Sport Nutr. 6:41-61.
21. Tremblay, A., J. A. Simoneau, and C. Bouchard. 1994. Impact of exercise intensity on body fatness and skeletal muscle metabolism. Metabolism 43:814-818.
22. van Dale, D. and W. H. Saris. 1989. Repetitive weight loss and weight regain: effects on weight reduction, resting metabolic rate, and lipolytic activity before and after exercise and/or diet treatment. Am. J. Clin. Nutr. 49:409-416.
23. Wideman, L., J. Y. Weltman, M. L. Hartman, J. D. Veldhuis, and A. Weltman. 2002. Growth hormone release during acute and chronic aerobic and resistance exercise: recent findings. Sports Med. 32:987-1004.
EricT on 12-29-2006, 02:48 PM
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. 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 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.
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.