Making HIIT a Hit Part II/II: Training Optimization? Where Do I Start? Theoretical Considerations & 10 Rules of Thumb

The stop watch is your friend, when it comes to HIIT optimization.

After the examples selected examples in the previous installment the intention of this 2nd part of the serious is to find some more general rules of HIIT program optimization with a special focus on exercise and performance on the one hand and fat loss and improvements in body composition on the other hand.

Now, if you want to optimize something, you often have one or a set of parameters you can plug into a system of linear equations to find those parameters which generate the maximal output.

Unfortunately, we are not only lacking linear, but any form of exact equations, but are also at a loss how programming is actually done. In that, Laursen's & Jenkins' definition of optimization as identifying

"[...] the optimal exercise intensity, exercise duration and number of interval bouts, in addition to the type (active vs passive) and duration of the recovery between exercise bouts."

May be helpful, but leaves us with the problem that

"[t]hese variables require manipulation according to the periodisation phase of annual training programmes, training status and the individual response that an athlete has to a training stimulus." (Laursen. 2002)

In essence, although more than 10 years have passed since the publication of this paper and the number of pertinent studies has increased, we are still (and will probably always be) at a loss when it comes to identify a single best HIIT protocol that will yield optimal results for everyone.

How do we define intensity?

In fact, even the way by which the exercise intensity is best prescribed, has as of now not been standardized. The most popular way probably is by VO2Max levels, but there are alternatives which could in fact be quite useful, e.g.:

• Your exercise selection should mirror the demands of the sport you are training for. Anyone competing in one of the classic team sports, will probably benefit most from sprints (either outside or on the treadmill). A cyclist will use the bicycle and a rower the rowing machine. Don't use equipment where you have to tweak the resistance not the speed (e.g. many of the old crosstrainers) to increase the intensity. HIIT is supposed to be fast and thus hard, not hard and thus slow!
  anaerobic threshold - intensity at which the exercise become predominantly anaerobic
• T_vent, T_lac - time to ventilatory threshold, time to lactate threshold
• OBLA - The onset of blood lactate accumulation  
• critical power - intensity that can be maintained for 30-60min
• V_max - min.speed required to reach the ventilatory threshold in an incremental exercise test
• T_max - time an athlete can run at Vmax

Now, let's be honest. How many of you know their VO2max? Or the time and speed at which you get there? If we simply assume that V_max is about 90% of what you would call "all-out" it does yet appear that the T_max value was one of the better ways to prescribe the training intensity during high intensity interval training (cf. Billat. 1998). Since there is no clearcut relation between VO2Max, V_max and T_max (25% inter-individual variation of T_max in athletes with identical VO2max), anyways, this appears to be the most reasonable and practical approach - also because measuring 60% of T_max (assuming we approximate it as the time at 90% of all out) requires nothing but a stop watch and can be repeated on a monthly basis to adapt the training intensity.

Training for increased endurance performance and conditioning

According to studies by Billat et al. and Smith et al.  (Billat. 1999, Smith. 2003) sprints lasting 50% (1:1 work to rest ratio) and 60% (1:2 work to rest ratio) of the T_max are not just sufficient, but according to Smith et al. even superior to intervals with a length of 70% of the T_max and 1:2 work to rest ratios. Although both studies were conducted in highly trained individuals, it is likely that the T_max, which is propably way lower for recreationally active, let alone sedentary individuals, provides an appropriate gauge for "optimal" training intensities in this subgroup as well. After all, this would mean that trained athletes require longer interval durations to further augment their performance than Joe Average.

Table 1: Summary of findings in high-intensity interval-training (HIT) studies in highly trained cyclists. 3-HCoA = 3-hydroxyacyl coenzyme A dehydrogenase activity; CHOOX = carbohydrate oxidation rates; CS = citrate synthase activity; HK = hexokinase activity; PFK = phosphofructokinase activity; Ppeak = peak aerobic power output; TF100 = time to fatigue at 100%; TF150 = time to fatigue at 150% of Ppeak; TT40 = 40km time-trial performance;
β = buffering capacity; ↓ = decrease; ↑ = increase; ↔ = no change. (adapted from Laursen. 2002)

And in fact, the currently available research appears to confirm this, while evidence from HIIT studies in trained cyclists suggests that the same HIIT protocols in the 5s-60s range at 80-90% of all-out performance are probably insufficient to induce further performance increases in already trained subjects (see table 1, rows marked red), unless the intensity is upped to supramaximal levels (see table 1, last row).

Optimal interval length, interval to rest ratios and recovery times

A T_max based exercise prescription with interval lengths of 50-60% (not more) of T_max, would probably mirror these increases in "optimal" interval length from non-trained over recreationally trained to elite athletes and could therefore provide a more accurate way of planning your HIIT sessions than picking a fixed work and rest duration.

Figure 1: Effects of different rest periods between 40m sprints on sprint performance (Balsom. 1992)

Let's assume you could go for just 2 min at a 80% pace of what you perceive as all-out. You would end up at a 1min interval duration, while your body, a highly trained sprinter can maintain the very same intensity for 5 minutes will have to go for intervals of 2.5min. For both an interval to rest ratio of 1:2 seems to be a happy medium, at very short submaximal intensities (in this case 80%). As the data in figure 1 goes to show, the rest requirements can be profoundly increased for all-out 40m sprints with 60s rest after a 5.6-5.7s sprint equaling a sprint interval to rest ratio of 1:11 in "seven moderately to well-trained highly motivated male physical education students" (Balsom. 1992).

The same 1:2 rule that applied to the T_max 50-60% workouts will thus maybe fail you, when you are going faster / more intense. In this context you may also argue that rest times below 60s - just as it is the case with strength training - must be regarded as an intensity technique that's actually supposed to increase the overload on the musculature by cutting rest in-between sets.

Moreover, the increased lactate during active recovery phases has repeatedly been shown to be advantageous in terms of both lower lactate accumulation and an increased likelihood of actually achieving VO2max and thus eliciting a strong enough training stimulus to trigger an adaptive response (see review in Billat. 2001; 22-23).

Can you combine intervals with a strength training routine?

Now, I would assume that most of you are no endurance athletes. So the aforementioned studies, which either were HIIT stand-alone studies or - in the case of the trained athletes - studies in which the participants replaced 1-2 of their regular aerobic exercise training with HIIT training do not answer the still highly debated question whether you can or cannot do both HIIT and strength training. I have already pointed out in previous posts that doing so on separate days doesn't seem to be a problem. However, if you "HIIT it" on the day you would usually do say chest and back. You would obviously have (a) the time and (b) the regenerative capacity to add another workout day to your week if you did not want to switch from a 3- to a 2-day split.

Baking soda could help managing the additional load (read more)

Against that background, I was not really surprised that DoesObamaEvenLift mentioned a pertinent paper by researchers from the University of Sao Paulo in his (or her?) comment beneath the first installment of this series. In this particular study (de Souza. 2013), the researchers wanted elucidate if the combination of HIIT + hypertrophy oriented strength training would hamper the benefits of one or another and whether the molecular adaptations would explain the differential effects on conditioning, strength and body composition of 43 active male physical education students who did not engage in any regular strength and aerobic power/capacity training within the last 6 months.

The good news for all muscle heads: Despite the differences in molecular adaptations between training regimens, 8 weeks of concomitant training (CT), i.e. the combination of interval training and lifting on one day did not blunt muscle strength and hypertrophy increments when compared with strength training (ST; 8-12RM for a total of leg-press 45 °, knee extension and knee flexion with 90-120s rest between sets) alone. The bad news is however that this protocol has little to do with the workout regimen of the average gymrat, who trains on 3x or more days per week and doesn't do "leg days" (without squats) on each of those days.

Figure 2: Illustration of the way the scientists tweaked the different parameters of the HIIT protocol to make it gradually more demanding from week 1-8 (left) p-AMPK and p-AKT response to different types of exercise (C: sedentary control; IT: interval training; ST: strength training; CT: concomitant training; based on de Souza. 2013)

So, while it cannot be said whether muscle or strength gains would be hampered in the long run, the way the scientists tweaked the interval part of the workout from week 1 to week 8 by slowly increasing the running speed 80-100% of the speed at which they had been running in the previous test to achieve VO2max, while increasing the rest between sets and accommodating for the additional strain of going at 95%-100% by cutting back the volume (see figure 2 left) stands in line with what has been said about the complex relation of these parameters in the previous paragraphs.

Don't forget the progression if you want to make progress

If your goal is to actually increase your conditioning and VO2max a carefully planned progression is an absolute must. To to so it would appear reasonable to re-test the initially discussed T_max after 6-8 weeks in the course of which you would progress from 80% to 95-100% in a way similar to what the participants in the study did. Based on the change in T_max you would then increase the length of the intervals appropriately while keeping the rest times similar. If your T_max for example had gone up by ~20% during the last cycle, you  would start your 2nd cycle with 72s intervals at 80% with the same progression you have used before.

HIIT for metabolic benefits and improved body composition

It's actually quite funny that up until very recently, HIIT training has been completely overlooked by most researchers who are looking for optimal exercise modalities to ameliorate. Within the last 2-5 years or so, the number of studies that suggest that high intensity interval training can do that at least as well, if not better than regular steady state aerobics is yet increasing. Here are a couple of examples, you may or may not remember from the SuppVersity news:

• 

  Previous studies have shown that a very brief interval training, even when it's performed on an empty stomach, like first thing in the morning or on an intermittent fast is not catabolic, but will in fact increase protein synthesis by 43% in men and  122% in women (read more).

  Some HIIT For Life & Less LISS For More! How to Burn 27,300 Kcal Extra W/out Losing a Single Extra Pound of Fat! (read more)
• The Iranian HIIT Solution: Three 200m Sprint Sessions per Week Double Insulin Sensitivity & Normalize Leptin Levels (read more)
• Reduced Exertion High Intensity Training - A Minimalist 2x20s HIIT Protocol For The Male Convenience Generation. (read more)
• HIIT is the Hit! Interval, not Steady State Aerobics is the Way to Go - Even for Patients with Myocardial Infarctions! (read more)
• HIIT-ing Health: Positive Effects of High Intensity Training on Symptoms of Metabolic Syndrome. (read more)

• overview of all posts with he keyword HIIT

When you go through these studies you will realize that it's impossible to identify the single-best HIIT workout for fat loss, what appears to be clear though, is that fat loss workouts do not require the same long intervals athletes with an already high VO2max need to promote their conditioning.

While the longer intervals may in the end burn more energy, the previously mentioned increase in lactate suggests that the additional energy expenditure in the course of the workout (a) does not come from your fat stores and (b) could take its toll, when you are dieting. Against that background, an "optimal" HIIT fat loss routine will probably be characterized by shorter, highly intense sprints and longer active recovery between the sets, with the latter giving your ramped up metabolism time to tap into the fat reserves, get rid of the lactate and ready for the next HIIT stimulus.

Combining HIIT and aerobic training to augment fat loss

Are You Still Burning Calories or Already Losing Fat? Study Shows: 5x15 Min HIIT Reduce Body Fat & Improve Fitness Twice as Effectively as 5x40min of Classic Cardio (read more)

A similar rationale is actually behind the idea of combining HIIT and aerobic exercises in a single fat loss workout that consists of an initial HIIT part of 3-5 intervals at 90-95% of what you could be sprinting for 30-45s interspersed by 2-3 minutes of active recovery followed by 15-20min of light intensity steady state of an intensity that would equal walking or very light jogging on a treadmill. With a total workout duration of ~30-45min this should yet be done on a day of it's own.

A viable alternative for everyone who doesn't want to give up on their current strength training routine and doesn't have room for another training day would be to alternate between the shorter varieties of the HIIT part and the LISS part of the aforementioned workout as an adjunct to a short, but intense resistance training regimen (30-40min total with no more than 18-20 sets).

How to progress when you are dieting?

As obligatory as a progression in intensity and duration of the intervals may be for someone who's trying to increase his or her conditioning and VO2max, it is nothing I would suggest on a diet. Unless you are trying to progress from overweight or obese to normal and are in real bad shape, it is not exactly likely that your performance will be increasing to an extend that will require adaptations.

That said, it is not necessary and in most cases counterproductive to increase the workload, when what you would actually have to do to drop body fat is to reduce your energy intake. Have patience and simply stop dieting, when your progress stalls for 2 weeks (sometimes even an additional refeed can reignite the weight loss).

Can HIIT help me make lean(er) gains?

8x Increase in "Mitochondria Building" Protein PGC1-Alpha in Glycogen Depleted Elite(!) Cyclists: Training Revolution or Recipe for Disaster?  (read more)

Personally, I have had greater success with very light steady state cardio like walking on an inclined treadmill either in the morning or after a strength workout, but if you take a look at the aforementioned SuppVersity post about increase protein synthesis (see image with sprinting guy and girl next to the list of examples), or take both the results of the Psilander study that was on the news in October 2013 and the latest revelations about PGC-1 A4 and how it may benefit from pre-workout glycogen depletion, a short but intense HIIT training right before your workout may not only help you to stay lean, but could actually improve your progress - assuming you don't end up seriously overtrained and are willing to eat enough.

Are there any supplements that can help?

Aside from the regular fat loss adjuvants, a combination of creatine, caffeine and amino acids, as it was used in a 2010 study by Smith et al. would be a basic stack to increase your HIIT and overall workout performance and thus augment body fat loss. If you use BCAAs this would also make a good addition to any fasted HIIT regimen in the morning for those of you who follow an intermittent fasting protocol.

Figure 3: Lactate, pH, ratio of hydrogen carbonate ions to NaHCO(3) and base excess in blood after, as well as number of total reps performed during a leg workout (data adapted from Carr. 2013 - discussed on 09/05/12)

Also, the addition of baking soda, may be worth trying, because the increased alkalinity with concomitantly increased workout intensities could provide both metabolic (=lower propensity to gain fat) and performance (=increased growth stimulus) advantages.

Too many stimulants or stim-laden fat burners, on the other hand, are probably rather counterproductive. Their ability to free fat from the adipose tissue is of no avail, when you are "bulking" - in the worst case it may even repartition the fat from the periphery to the trunk. So you better tick to one serving of stims before a workout.

10 rules of thumb to help you making HIIT a Hit

I could probably ramble on forever, but don't think that this will provide any significant insights beyond what I have written in the previous paragraphs. So let's put an end to this endless post and formulate a couple of rules of thumb:

1. Don't get overtly focused on HIIT there are benefits to steady state exercise, as well; and it goes without saying that you won't be willing to give up weight lifting completely. You can better live without HIIT and just lifting + LISS than with HIIT only.
2. If you want to improve your conditioning, you will have to train on the higher end of the 15s to 4min interval continuum.
3. If you want to improve your body composition, training at the lower end of the 15s to 4 min interval continuum and combining shorter HIIT workouts with either strength or aerobic work appear to be more promising.
4. Active rest should always be preferred over passive rest, regardless of your goals
5. The length of the rest periods should increase with increasing training intensity
6. If you are doing HIIT for fat loss, longer active rest (up to 5 min) can actually increase the amount of fat you burn and may be a viable alternative for an additional LISS workout
7. Creatine, BCAAs, caffeine and baking soda, can be used before a HIIT workout regardless of your training goals
8. Continuous progression is obligatory, if you want to maximize your conditioning
9. Stick to the same intense, but short HIIT regimen, when you are dieting in order not to fall victim to the "doing more and eating less downward spiral".
10. Know your limits and don't simply add hours of HIIT to your current regimen.

Based on these rules of thumbs and the rest of the information in this installment, as well as the examples in the previous installment you should actually be able to come up with some kind of workout you believe you can benefit from. Give it a two week trial before you decide whether it's too hard, too easy or whatever and don't expect magic results. In the end, HIIT is like a potent supplement: Those who have what it takes to make progress without it will benefit. Those who have stalled on each and every regimen they tried, will stall on hit as well (most likely because they overtrain and/or follow  a crappy diet).

References:

• Balsom PD, Seger JY, Sjödin B, Ekblom B. Maximal-intensity intermittent exercise: effect of recovery duration. Int J Sports Med. 1992 Oct;13(7):528-33.
• Billat V, Binsse V, Petit B, Koralsztein JP. High level runners are able to maintain a VO2 steady-state below VO2max in an all-out run over their critical velocity. Arch Physiol Biochem. 1998 Feb;106(1):38-45.
• Billat VL, Flechet B, Petit B, Muriaux G, Koralsztein JP. Interval training at VO2max: effects on aerobic performance and overtraining markers. Med Sci Sports Exerc. 1999 Jan;31(1):156-63.
• Billat LV. Interval training for performance: a scientific and empirical practice. Special recommendations for middle- and long-distance running. Part I: aerobic interval training. Sports Med. 2001;31(1):13-31. 
• de Souza EO, Tricoli V, Roschel H, Brum PC, Bacurau AV, Ferreira JC, Aoki MS, Neves-Jr M, Aihara AY, da Rocha Correa Fernandes A, Ugrinowitsch C. Molecular Adaptations to Concurrent Training. Int J Sports Med. 2013 Oct 8.
• Gunnarsson TP, Bangsbo J. The 10-20-30 training concept improves performance and health profile in moderately trained runners. J Appl Physiol. 2013a Jul;113(1):16-24. 
• Gunnarsson TP, Christensen PM, Holse K, Christiansen D, Bangsbo J. Effect of additional speed endurance training on performance and muscle adaptations. Med Sci Sports Exerc. 2013b Oct;44(10):1942-8. 
• Laursen PB, Jenkins DG. The scientific basis for high-intensity interval training: optimising training programmes and maximising performance in highly trained endurance athletes. Sports Med. 2002a;32(1):53-73. 
• Laursen PB, Blanchard MA, Jenkins DG. Acute high-intensity interval training improves Tvent and peak power output in highly trained males. Can J Appl Physiol. 2002b Aug;27(4):336-48.
• Lindsay FH, Hawley JA, Myburgh KH, Schomer HH, Noakes TD, Dennis SC. Improved athletic performance in highly trained cyclists after interval training. Med Sci Sports Exerc. 1996 Nov;28(11):1427-34.
• Smith AE, Fukuda DH, Kendall KL, Stout JR. The effects of a pre-workout supplement containing caffeine, creatine, and amino acids during three weeks of high-intensity exercise on aerobic and anaerobic performance. J Int Soc Sports Nutr. 2010 Feb 15;7:10.
• Stepto NK, Hawley JA, Dennis SC, Hopkins WG. Effects of different interval-training programs on cycling time-trial performance. Med Sci Sports Exerc. 1999 May;31(5):736-41.
• Stepto NK, Martin DT, Fallon KE, Hawley JA. Metabolic demands of intense aerobic interval training in competitive cyclists. Med Sci Sports Exerc. 2001 Feb;33(2):303-10.
• Westgarth-Taylor C, Hawley JA, Rickard S, Myburgh KH, Noakes TD, Dennis SC. Metabolic and performance adaptations to interval training in endurance-trained cyclists. Eur J Appl Physiol Occup Physiol. 1997;75(4):298-304.
• Weston AR, Myburgh KH, Lindsay FH, Dennis SC, Noakes TD, Hawley JA. Skeletal muscle buffering capacity and endurance performance after high-intensity interval training by well-trained cyclists. Eur J Appl Physiol Occup Physiol. 1997;75(1):7-13.

Making HIIT a Hit Part I/II: The Quest for the Optimal Interval to Rest Ratio for Your Type & Goals - Warming up: Selected Studies + Individual Take Home Messages to Set the Scene

This series will not readdress the "HIIT and/or LISS debate", but simply try to do what the title says: Help you to determine the "ideal" form of HIIT for your type & goals.

After posting yet another article about the benefits of high intensity interval training (HIIT) training on Wednesday, I thought it may be a good idea to take a look at the pertinent literature to get a better grasp of what exactly high intensity interval training actually is... "What it is? What do you mean?" While almost everyone appears to recommend you do at least some amount of high intensity interval training, these days, the definition of both the "high intensity" and the "intervals" which are more than just eponymous of this type of training are usually pretty wooly. The two questions this article is supposed to answer are therefore: How long should the intervals and rest periods be and at which intensity should they be performed.

Attention, this article turned into a two part series: I must warn you, this "quest" turned out to be hell lot of work and way more than I had expected. So I will have to split it up into a two-part series. With today's first part discussing exemplary "success stories" and the subsequent follow up on next Sunday providing a more comprehensive overview and general conclusions one may draw based on the picture that emerges.

Part I: Success stories - learning from the "best"

I decided that it may be a good idea to initially compile some examples of which you could say that they are examples of a successfully implementation of high intensity interval training. In that, I will put an emphasis on "old" studies that have not yet been discussed on the SuppVersity. This means, if you want more examples before next week's 2nd installment with a more general overview, you can simply click through the archive of articles that are marked with the keyword HIIT - either as HTML or RSS (I recommend the latter if you do not intend to read all of them anyway; opens in all good browsers without a reader).

Untrained participants and non-athletes

• 5min on, 2min off HIIT to increase fitness (VO2max) alternated with 40 min steady state cardio for 4 + 5 weeks (Hickson. 1981) -- Hard to believe but it's more than 30(!) years ago that Hickson, Hagberg and Ehsani were able to show that a combination of high intensity interval training with 6x5min cycling at 90-100% of the VO2max and 2-min at 30%-50% active rest between intervals on day one and 40 min of steady state "cardio" on a treadmill on day 2 (6 workouts per week) yielded profound increases in VO2max:
  

  

  Figure 1: Adaptation to combined HIIT + steady state protocol in "occasionally active" but not "regularly" trained men (n=8) and women (n=1) in the Hickson study (Hickson. 1982).

  For our purpose, the most significant finding of this study may however be that the t_1/2, i.e. the amount of time it takes for 50% of the adaptations to take place is only 10 days (on this intense protocol).
  
  Take home message: If VO2Max and overall conditioning is your goal you should up the intensity every three weeks, to make sure you continue to make progress. If you go by VO2max, this would be an increase in the speed you run or bike - not (!) the duration of the intervals and / or their number.

• 2 series of 5s sprint cycling (8-13x) with 55s of active rest increase force production and rids subjects of "useless" type IIb fibers (Linossier. 1993) -- The 10 students (8 men, 2 women; age 22y; VO2max = 51.2ml/kg/min, everything below 55 is still considered ) in the Linossier study performed 4 HIIT only workouts consisting of 5s all-out sprints on a cycle ergometer interspersed by 55s of active rest cycling at a heart rate of 130-140bpm per week. The number of sprints in each of the two sessions on a single training day (15 min of rest between) were increased from 8 to 13 sprints over the 7-week study period. The main results were improvements in both peak performances +25%  and in the 30-s total work +16%.
  
  

  

  Figure 2: Unexpected shift in muscle fiber type distribution (Linossier. 1993)

  What's also relevant, though not directly performance related is the 19% increase in phosphofructokinase and a 20% increase in lactate dehydrogenase, both are glycolytic enzymes and are indicative of improvements in the glycolytic pathway, which goes to show you that you don't have to be afraid of the fiber type changes hampering either your ability to handle glucose or your strength - I mean +29% in maximal force production in the sprint tests are everything else but "weak".
  
  Take home message: Shorter sprints appear to be a valid means to hammer and improve the glycolytic pathway and get rid of the useless type IIb fibers, of which not endurance athletes, but bodybuilders have the lowest amount and sedentary controls the most (cf. figure 1 in the overview of the Intermittent Thoughts on Building Muscle). To actually build muscle the 1% increase in muscle weight the scientists observed, is yet not sufficient enough.

• 7 sessions of 10x4 min cycling at 90% with 2 min rest ramp up skeletal muscle fat metabolism in young women (Talanian. 2006) -- The 8 healthy recreationally active women (22 yr old, 65.0 kg body wt) who had previously engaged in 2-3 just-for-fun sessions of various activities from weight lifting, soccer and cycling to swimming, or walking, participated in no more than 7 HIIT sessions within 2 weeks (1 day on, 1 day off) which consisted of 10x 4min sprints at 90% of their individual VO2max with two minutes of rest in between.
  
  

  

  Figure 3. Changes in substrate utilization during standardized 60 min LISS test at 60% of the VO2Max (Talanian. 2006)

  Contrary to the previously discussed studies, the scientists were in this case less interested in the performance than in the metabolic benefits, which were - as the data in figure 1 show - surprisingly significant given the short duration of the study. The increases in fat oxidation and decreases in the respiratory ratio (the ratio of glucose to fatty acid oxidation) during a standardized 60min light intensity (60% VO2max) test that was performed at the beginning and end of the study, were a consequence of increases in citrate synthase and beta-hydroxyacyl-CoA dehydrogenase (beta -HAD) activity.
  
  Take home message: In view of the important contribution of beta-HAD to the beta oxidation of (esp. medium-chain) fatty acids, the fact that an upregulation of this enzyme has been observed in response to the long duration intervals, but neither light intensitsy exercise (even w/ 2h per day, 5-6 times per week; cf. Phillipps. 1996), nor during a similar 2-week sprint training protocol (30s all-out on cycle ergometer, 4 min rest; cf. Burgomaster. 2005) points towards "long" duration intervals when mitochondrial adaptations that favor fatty oxidation and thus subsequent increases in fat loss with light activity are concerned. 

Highly trained participants and pro-athletes
 

• 3 sessions of 10 intervals at 96% VO2Max with 60-180s rest increase running economy and relative fatty oxidation during subsequent steady state exercise test (Zavorsky. 1998) -- Twelve highly trained endurance athletes volunteered for this study, four of them qualified for the 1996 Canadian Olympic Trials, and one subject was a Canadian record holder in triathlon. On three different occasions the subject ran 10 x 400-m sprints with active recovery periods of 60, 120, 180s (randomly assigned). 10 min before and afterwards standardized running economy tests were performed.
  
  

  

  Figure 4: Respiratory exchange ratio in RE test before and after the sprinting protocol (Zavorsky. 1998)

  During these running economy tests, which consisted of 2 x 5 min running on a treadmill at 12km/h or 16km/h (5 min total rest in between), the subjects exhibited, much contrary to the research hypothesis, by the way, an increase in the running economy (RE, a measure of how efficiently a person uses oxygen while running at a given pace) and - probably of greater interest for most of you an increase in fatty acid oxidation, as evidenced by the reduced respiratory exchange ratio (RER, figure 4). The latter was particularly pronounced in the "low intensity" RE test at 12km/h.
  
  As far as the differential effects of the rest intervals is concerned, significant effects were observed for the rates of perceived exertion (60s: 17.7; 120s: 16.1; 180s: 14.4), but not for the performance related measurements velocity, and the time it took the athletes to run the 400m sprints (actually the latter should be self-evident with identical velocities).
  
  Take home message: Doing ten short sprints (if you are not an athlete, shorter ones will suffice) before you go jogging (or do any other type of steady state cardio) won't hamper your running economy. And since the study at hand suggests that the relative increase in fatty acid oxidation is specifically pronounced in the lower intensities (this is supposedly even more the case if you jog at 10km/h), the combination of sprints + steady state cardio appears to be an ideal "cardio" only day, when you are trying to lose body fat.

• 8x all-out (100% VO2max) 2.5min intervals with 4 min of active rest increase VO2Max, peak aerobic power and 5k time trial performance regardless rest between sessions (Gross. 2007) -- With the unique twist with respect to the training frequency this study is somewhat unique. While other studies report similar benefits in highly trained collegiate cyclists (13 men, 4 women; VO2Max at baseline 62ml/kg/min) or other endurance athletes, this is one of the few that investigates whether it makes a difference if the athletes implement the 3-days per week HIIT regimen as a block or interspersed by one day of rest into their regimen.
  
  

  

  Percent change in TT5k velocitym TT5k power output VO2peak, and peak aerobic power output in cyclists trainin on consecutive vs. non-consecutive days (Gross. 2007)

  Interestingly enough, neither the performance outcome on the pre- and post tests, i.e. increases in VO2Max (+5.7%), peak aerobic power (+7.2%), 5k time trial performance (+6.9%), nor the actual performance during each of the workouts the subjects performed in the course of the 3-week study period suffered from doing the HIIT sessions back to back. These results refute previous speculations that doing HIIT as a block would trigger greater metabolic adaptations, specifically in athletes (e.g. Padilla. 2000). Even more, though the changes were not statistically significant, the data in figure 5 shows that - with one for endurance athletes important exception, namely the VO2Max - training on non-consecutive days produced marginally better results.
  
  Take home message: While it appears as if it would not make a difference whether you perform your HIIT workouts blocked or within your training week. The non-significant differences in figure 5 could suggest that endurance athletes intending to improve their already high VO2Max even further would be better off with the blocked training. Everybody else has the choice and I would pick the interspersed variety, whenever my schedule allows me to do this - my personal experience told me that this works better for me... apropos, take another look at figure 5 you see the narrow bars indicating the standard deviations? This goes to tell you that it is very likely that not just the personal preferences, but also the actual outcome will vary from trainee to trainee, which supports the notion that you will have to experiment to find what works best for you.

That's it for the "success stories"! Don't forget to come back next Sunday for part II and in case you have not done so already, just browse the previous SuppVersity posts on this matter, either as HTML version, post by post, or from the RSS overview (works in every modern browser, yet not in Google spyware ;-). I know you are smart enough to draw your own conclusions. And what's more, this may yield some cognitive input for the comment area of this article, where you can post questions. I will try to tackle those I can answer in the next installment.

References:

• Burgomaster KA, Hughes SC, Heigenhauser GJF, Bradwell SN, Gibala MJ. Six sessions of sprint interval training increase muscle oxidative potential and cycle endurance capacity in humans. J Appl Physiol. 2005; 98: 1985–1990.  
• Gross M, Swensen T, King D. Nonconsecutive- versus consecutive-day high-intensity interval training in cyclists. Med Sci Sports Exerc. 2007 Sep;39(9):1666-71.
• Hickson RC, Hagberg JM, Ehsani AA, et al. Time course of the adaptive responses of aerobic power and heart rate to training. Med Sci Sports Exerc 1981; 13: 17-20.
• Linossier MT, Denis C, Dormois D, Geyssant A, Lacour JR. Ergometric and metabolic adaptation to a 5-s sprint training programme. Eur J Appl Physiol Occup Physiol. 1993;67(5):408-14. 
• Padilla S, Mujika I, Orbañanos J, Angulo F. Exercise intensity during competition time trials in professional road cycling. Med Sci Sports Exerc. 2000 Apr;32(4):850-6.
• Phillips SM, Green HJ, Tarnopolsky MA, Heigenhauser GJ, Grant SM. Progressive effect of endurance training on metabolic adaptations in working skeletal muscle. Am J Physiol Endocrinol Metab. 1996: 270: E265– E272, 1996
• Talanian JL, Galloway SD, Heigenhauser GJ, Bonen A, Spriet LL. Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. J Appl Physiol. 2007 Apr;102(4):1439-47. 
• Zavorsky GS, Montgomery DL, Pearsall DJ. Effect of intense interval workouts on running economy using three recovery durations. Eur J Appl Physiol. 1998; 77: 224-30.