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Methods of Endurance Training Part 5: Interval Training Part 2

On Tuesday in Methods of Endurance Training: Interval Training Part 1, I defined some basic concepts regarding interval training regarding loading parameters that typically go into deciding a given interval workout.  Today I want to look at some specific types of interval training but, since I’m overly wordy as usual, will save the series wrap-up for next Tuesday.

Again I’d mention that I’m focusing specifically on what is usually referred to as High-Intensity Interval Training (HIIT) which I’m going to be defining here rather generally (I’ll be more specific below) as interval ranging from perhaps 5 seconds to 5 minutes at intensities that are above the lactate/functional threshold.

I’d mention again that heart rate response during intervals tends to be misleading for setting intensities, intervals are typically too short to allow heart rate to hit a steady state level (some methods of interval training recommend waiting until a certain heart rate is attained during the rest interval to decide when the next interval should begin).

Rather, intensity for intervals is generally set in terms of a specific speed (e.g. runners may do repeats at their 3k or 5k best speed) or power output (e.g. 110% of functional threshold power output) or, more simply, as ‘The maximum intensity that can be maintained for the goal duration”.

Power vs. Capacity

Ok, I promise, one last topic of introduction before I get to specifics.  One distinction that isn’t often made, or that is simply confused, by a lot of people who talk about interval training is the difference between physiological powers vs. capacities (or how they should be trained or how adaptable they are).

Simply, you can think of the power of a system as the maximum output that can be generated.  Capacity represents how long you can maintain it for.  If a car analogy helps, power would be top speed, capacity would be how long that speed can be maintained.  The difference being that in a car the capacity is more determined by the size of your gas tank; in the body it’s determined physiologically.

I bring this up for two reasons.  One is that people tend to mix the two concepts and this leads to a lot of confusion about interval training and what it can and cannot accomplish.  So they might read that a given training type has a greater impact on aerobic POWER and conclude that that type of training is best for all components of aerobic performance.  But power isn’t the same as capacity and this confusion leads people down some weird paths for people who don’t understand the difference.

I’ll finish by saying that generally speaking, the power of a given system is best developed with maximal loading (for that system or duration) with complete rest periods; this allows that maximum to be hit more often stimulating an increase in the power of the system.  Sort of like maximal strength training with heavy loads and long-rest intervals.  In contrast, capacities are usually developed by using slightly lower intensities (relative to the max for that system) with incomplete rests.  Or with continuous non-interval activity.  I’ll address specifics below.


Goals of Interval Training

The goals of HIIT can vary massively and the specifics of the intensity, duration of work interval, duration of rest interval, etc. all determine the training effect sought or generated.  When interval training was first ‘invented’ or ‘discovered’ (depending on how you want to look at it), the basic idea was that it allowed athletes to work at or above their best performance level and do more total work by breaking it into more manageable pieces.

For example, an athlete working exactly at their VO2 max might be able to sustain that for 5-8 minutes (if they are very motivated) at which point they’d be exhausted.  They’d also be unlikely to repeat the effort a second time.  However, if they worked for shorter periods, say 3-4 minutes at that pace, they were able to achieve a greater total volume of training while (hopefully anyhow) generating a similar training effect  That is, while one set of 8 minutes might be exhausting, the athlete might be able to repeat a 4 minute effort 4 or 5 times, accumulating 20 minutes of total volume at that intensity.

Of course, coaches of the time weren’t looking at those kinds of physiological variables.  Rather, interval training allowed their athletes to perform at speeds (often in excess of their current best performance) but, by breaking the workout into isolated ‘bits’ they could accumulate more volume at that speed.  So a 5 minute mile runner could do interval repeats at a 4:45 mile pace, do it a bunch of times to generate a training effect and hopefully that would push performance up over time when they strung the repeats together.

In more modern times, the focus is rather on more physiologically based performance measures such as improving VO2 max or lactate threshold or buffering or what have you.  Of course, pace is still important and the original ideas of interval training still stand: it allows athletes work at a pace equal or above their maximal performance while accumulating more volume than they could get by simply working to exhaustion at that pace.

In general, I’m going to divide the potential goals of HIIT, at least as it pertains to either endurance athletes or athletes who need endurance, into the following three categories.  Each essentially represents one of the three major energetic pathways in skeletal muscle (ATP/CP for very short-term activity, glycolysis for medium term activity, aerobic for long-term activity).  Specifically, I’m going to talk about training for:

  1. Neuromuscular ability
  2. Anaerobic power/anaerobic capacity
  3. VO2 max /Aerobic Power


Neuromuscular Ability

Neuromuscular ability can be thought of semi-simplistically as ‘sprinting’.  It’s a little more complex than that but, basically, this type of training is aimed at improving the body’s neuromuscular ability to go fast.   Strictly speaking this isn’t related to endurance in terms of what I’ve been discussing in this article series if for no other reason than the ability to go fast for 4-10 seconds or so isn’t really endurance in the traditional sense of the word.  Certainly the ability for a final sprint can be important in endurance type events (e.g. many bike races end in finishing sprints) but that has more to do with race dynamics than endurance per se.

However, neuromuscular type training can benefit endurance indirectly and that’s by increasing efficiency at slower speeds.  That is, with cyclists for example, you often see individuals trying to maintain a high cadence and their upper bodies are all over the place, their hips are rocking side to side, etc. By having them work for short periods at much higher cadences (focusing to the best of their ability on maintaining form), they can train the nervous system to handle those higher speeds.  And will settle down technically at slower cadences.

For runners, neuromuscular training more often takes the form of strides.  These are short intervals done at speeds above race pace with the goal of teaching the nervous system to run more efficiently at those high speeds.  Again, it’s neural training where the goal is to teach the body to run faster/more efficiently and this tends to not only raise maximal speed potential but make runners more efficient at more realistic race speeds.

In a related vein, some sports will perform motorpacing, usually this occurs in cycling where a track cyclists will chase a maniac on a motorcycle at a speed higher than they can achieve.  The goal simply being to teach the nervous system to handle the higher speed.  Speed skaters will skate behind someone faster than they are to tow them up to a higher speed and teach their brains how to handle it.  Track sprinters will sometimes do overspeed work (with a tow cable or running a slight downhill) although watching for form breakdown or technique alterations is critical.  I think you get the idea.

Since the goal of neuromuscular training is speed, the bouts are usually kept very short (6-10 seconds perhaps up to 15 seconds maximum) since the goal is to avoid a lot of fatigue due to the buildup of waste products.  As well, full rest periods are almost always used since the goal, again, is on pure quality of movement.  You don’t want fatigue hampering the ability to work maximally so long rest intervals (which can be 3-5 minutes or more for a mere 10 second effort) are used.

A maximum number of repeats isn’t required but training would most certainly be stopped if form started to deteriorate or speed dropped off.   There’s no point in continuing this type of training if the proper neuromuscular pattern isn’t being trained.

Strictly speaking, neuromuscular capacity could also be trained but, as I noted above, the intensity would be somewhat less than maximal and shorter rest intervals would be used.  The goal of this type of training would be to improve the athlete’s ability to maintain that high neuromuscular output for long periods against accumulating fatigue since the athlete would have to work harder to maintain that speed and proper form against accumulating fatigue.  Training would be stopped at the first sign of form breakdown, mind you.

Generally muscular fatigue from this type of training is low although there can be some nervous system fatigue (especially when the body is pushed to a new level of performance previously unattained). This type of training can (and often is) be combined with other types of workouts.  So a runner might warm-up, perform some stride drills and then go do a more traditional endurance run.

As a further adaptation to this type of training, there can also be a slight increase in levels of skeletal muscle creatine phosphate levels over time.  However, the effect is not massive and the ATP/CP pathway generally shows fairly limited ability to adapt.  Again, the main focus of this type of training is neurological.


Anaerobic Power/Anaerobic Capacity

I mentioned in a previous part of this series that lactate does not appear to be causal in fatigue during high intensity work although acidosis of some sort probably is.  So rather than call these lactate buffering intervals or what have you, I’m going to refer to them more generally as anaerobic power/capacity intervals.

Above some exercise intensity (generally anything above the functional threshold I’ve discussed in previous parts of this series), the body will start to rely on so-called anaerobic processes and start producing waste products that cause fatigue.  Generally, this occurs for bouts of maximal or near-maximal activity lasting roughly 30-90 seconds.  Being able to either produce power during those types of activities, or maintain that power against accumulating fatigue, can therefore become very important.

I’d note again that many of the things that happen during ‘anaerobic’ work of this sort are actually related to the size of the aerobic engine in terms of how well the body handles it.   As it turns out, mitochondria (the powerhouse of the cell) actually metabolize many of the waste products generated by these types of activity; hence the emphasis on developing it even for seemingly ‘anaerobic’ activities.  As well, an athlete who can produce more speed or power aerobically won’t have to rely on ‘anaerobic’ processes as early.

But clearly some amount of work to improve this system is going to be important for training.  So let’s talk about loading parameters.

Training anaerobic power is pretty much what you’d imagine.  Generally work bouts lasting 30-60 seconds are used with fairly long rest intervals.  For a one minute maximum effort, a rest interval of 4-5 minutes of easy recovery might be taken.  The idea, essentially is to generate a lot of power (and a lot of waste products, some have even called this lactate production training) during the work bout and then give the body a chance to clear it during recovery.  That’s then repeated a number of times (perhaps 8-10 in total depending).

Anaerobic capacity training, as noted above, is more along the lines of repeated longer repeated efforts (60-90 seconds) with rest intervals of perhaps 1:1 (so 60 seconds rest for a 60 second interval, 90 seconds for a 90 second interval).  Intensity has to be lower here and the goal is to maintain power output against increasing fatigue (some used to call this lactate tolerance training).  So you might do 5X1’hard/1′ easy with a goal of hitting the same power output on every work bout, working harder with each one as fatigue and waste products accumulate.  After a 10 minute rest, do it again.  Others will simply perform 8-12X1’/1′ rest and that’s it.

There are about a zillion and one other ways of performing anaerobic work and athletes and coaches get endlessly creative in how they approach it from trying to hit specific paces or using a specific intensity or varying the work bouts and rest intervals.

At first glance this workout seems endlessly productive for certain types of sports.  Specific events (notably in track and swimming) work in the time range where waste accumulation is a big issue.  Even long duration endurance activities can be hampered when athletes are forced to push a pace above their functional threshold/best steady state speed.  And certainly anerobic training can benefit those.

In a previous part of this series I mentioned that aerobic training can ‘push’ the functional threshold up by improving the aerobic engine.  In a similar sense, anaerobic intervals can ‘pull’ the functional threshold up by improving buffering capacities for acid or whatever is actually causing fatigue when that intensity is surpassed.

But here’s the drawbacks.  The main one is that this energy system is not massively trainable.  Certainly buffering capacity can be improved (and science continues to understand the adaptations involved) but overall improving the power or capacity of this pathway is relatively limited; you put in a lot of very hard, very draining work for not a lot of payback.

For athletes using intervals to build endurance, this is a very real consideration.  If an exhaustive anaerobic interval session prevents proper technical or tactical work (or other high intensity work) on the next day or two due to fatigue, that’s not a very good trade off.  Especially given that the training for a lot of sports is already highly reliant on this pathway in the first place.  That is, if the sport practice is already working in this energy system, trying to add more conditioning work of the same type tends to cause more problems than it solves.

Of bigger issue is this: the adaptations to this type of training occur quickly but then stop just as rapidly.  Studies find that the improvements to this type of work typically stop after about 3 weeks.  In one study of cyclists, 12 HIIT workouts over 6 weeks generated no better improvements in performance than was seen after the first 6 workouts over 3 weeks.

In the now infamous Tabata study which I discussed in Effects of Moderate-Intensity Endurance and High-Intensity Intermittent Training on Anaerobic Capacity and VO2 Max a similar patter was seen: the major effects occurred in the first 3 weeks with far smaller effects occurring in the last three.  The short-term bang for the buck is huge, mind you, but then it goes away very quickly.

Amusingly (or perhaps amazingly), running coach Arthur Lydiard found this empirically decades ago, 2-3 weeks of anaerobic work to top off building the aerobic engine was all that was required to maximize this pathway.  His entire focus was on ‘pushing’ aerobic abilities up over time and then using a touch of anaerobic work to do the final ‘pulling’ to high-performance.

Similarly, I’ve repeatedly mentioned the 2000 German track cycling team, preparing for the 4km (an event lasting about 4 minutes); they performed miniscule amounts of anaerobic training and did so for about 8 days prior to the event.  They had found that that was all that was required to max out the anaerobic pathways.  More was neither necessary nor beneficial.

Going even further than that, there is some indication that the high acid production caused by these types of anaerobic intervals may damage mitochondria and overall endurance especially if it’s not balanced by sufficient low intensity work (Lydiard, for example, was adamant that anaerobic work never be performed while building the aerobic engine).  This was an older idea that has seen recent scientific validation: in a recent study, individuals who consumed bicarbonate prior to training obtained better aerobic adaptations, presumably by buffering acidosis and preventing actual damage to the structures involved in aerobic endurance.

If nothing else, athletes who become too dominant on anaerobic processes often find that they fatigue sooner. They get so efficient at producing energy anaerobically that they actually hurt themselves compared to if they could generate more power aerobically.

Which is all a long way of saying that anaerobic intervals clearly have their benefit but not much is required and the benefits occur quickly before athletes are putting in a lot of work for very little return.


VO2 Max (Aerobic Power)

And finally we come to VO2 max/anaerobic power intervals, perhaps one of the most misunderstood concepts around.  To understand this topic, I need to talk a little bit about what VO2 max is.  Conceptually, VO2 max represents the maximum amount of oxygen that can be utilized by the body.  As I discussed in Predictors of Endurance Performance, it was once though to be the determinant of performance although we know now that this isn’t the case.

VO2 max actually has two major determinants.  The first is the heart’s ability to pump blood which has to do with both stroke volume and heart rate.  The second has do to with the muscle’s ability to extract and utilize that oxygen.  Essentially, VO2 max has both a central (heart) and peripheral (muscle) component.

I know I said in Methods of Endurance Training Part 1 that I’d be focusing mainly on skeletal muscle but this is a place where the distinction is critical.  Not only in terms of training content but to understand why many of the claims being made regarding interval training are mistaken.

This distinction in mechanisms is actually reflected in the terminology of some sports.  In rowing for example, base endurance training is referred to as Uptake (or UT) training suggesting that it primarily impacts on the muscle’s ability to uptake oxygen from the blood.  In contrast, Vo2 max intervals, which I’ll define in a second, are referred to as Transport training; basically they improve the heart’s ability to transport oxygen in the first place.

Maximizing VO2 max means maximizing both transport (primarily determined by cardiac function) and uptake (primarily determined by muscle function and a host of adaptations that occur there).  I mentioned in Methods of Endurance Training: Interval Training Part 1 that the main benefit of interval training was thought to be on the heart.  In the 80’s this idea reversed itself, coaches felt that long duration training mainly affected the heart and intervals affected skeletal muscle.

It turns out that the early ideas were correct.  Interval training, specifically the kind that drives athletes to achieve VO2 max primarily affects heart function.  Certainly there are going to be peripheral effects in muscle but the main effect is in heart.

Typically VO2 max intervals are done at an intensity much lower than the pure anaerobic intervals I described above.  Cyclists typically use 105-110% of functional threshold power or so with runners using perhaps their best 3-5k pace.   It’s hard but athlete quickly find that if they go out too fast, they die before the full interval can be accomplished.   The primary goal is working at a rate that will allow VO2 max to be attained and held for some duration.  Working harder than that isn’t necessary or beneficial.

Since VO2 max can be sustained for 5-8 minutes at absolute maximum, generally VO2 max intervals are performed for anywhere from 2-5 minutes (with 3 minutes being a fairly standard duration).  It will generally take about one minute for the body to achieve Vo2 max such that anywhere from 1-4 minutes is performed at VO2 max.  With multiple sets, a rather large amount of training at VO2 max levels can be achieved without having to go to complete exhaustion.

Although training VO2 max is technically a power training method, rest intervals are generally not as long as you’d expect (or as athlete would like).  Then again, the intensity tends to be much lower compared to pure anaerobic power methods. Generally a rest interval that is equal or slightly shorter than the work interval is used and a fairly common VO2 max workout would be something like 3-6X3’/3′ rest @ VO2 max intensity.

Of course, there are other ways to achieve VO2 max intensity; the Tabata protocol again discussed in Effects of Moderate-Intensity Endurance and High-Intensity Intermittent Training on Anaerobic Capacity and VO2 Max lasts 4 minutes and individuals acheive VO2 max with the combination of short-intense intervals (2o seconds) alternated with very short rest intervals (1o seconds).

And studies clearly show that, in the short-term, interval training improves VO2 max far more rapidly than the steady state methods described in previous parts of this series.  This is also where a lot of the claims for the superiority of interval training is coming from.  People, misunderstanding what VO2 max represents, see that short interval sessions improve VO2 max and such in a very short time and conclude that it’s the best way to improve aerobic markers.

So what’s the drawback?

The first is one I’ve mentioned before.  While VO2 max training generates very quick results, the improvements generally slow or stop equally quickly.  Three weeks of VO2 max training is about it and the adaptations pretty much stop occurring.  So, like anaerobic work, you end up working your brains out for very little return.

I’d note that in some populations, notably untrained beginners, you often see longer term results.  In one recent review looking at training and cardiac output, interval training was found to improve cardiac output (how much blood the heart pumps) faster than traditional training.  This improvement occurred for 8 weeks and then nothing else was gained.

Tangentially, this isn’t uncommon with very high intensity training methods.  While they often generate incredibly rapid results in the short-term, those results invariably end up stopping equally quickly.  Either the athlete burns out or the same effort stops generating results.  A question I’ve asked on the site before and one I’ll come back to in the wrap-up on Tuesday is this: For coaches and trainees fascinated by all things interval related, if intervals stop working in 3-8 weeks, what are you going to do for the other 44-49 weeks out of the year?

But that’s not the only drawback to the fascination with VO2 max type intervals and this comes back to the distinction between central/peripheral adaptations and power vs. capacity.  In terms of endurance and aerobic pathways, VO2 max can be thought of as aerobic power.  It represents the top end that can be achieved.

But power isn’t capacity and given that VO2 max interval training primarily affects cardiac function, this leaves skeletal muscle function (a much bigger determinant of aerobic capacity, better described here as endurance) untouched.

One study pointed this out in spades, looking at VO2 max and citrate synthase (a marker of aerobic endurance in skeletal muscle), it showed clearly that while interval training improved Vo2 max far more than long-duration training, it had little to no impact on citrate synthase levels. In contrast, steady state training primarily improved citrate synthase levels with little impact on VO2 max.

Put differently, power isn’t capacity, VO2 max isn’t aerobic endurance and VO2 max interval training can not and does not generate the adaptations in skeletal muscle that the various types of steady state training generate.  Vo2 max intervals improve aerobic power but not aerobic capacity/endurance.  Not used in isolation anyhow.

I’d note that, if there is anything that approximates aerobic capacity ‘intervals’, it’s the threshold method I described in Methods of Endurance Training Part 4: Threshold Training.  Intervals ranging from 8-20 minutes at essentially the maximal aerobic steady state (recall that threshold intensity could technically be maintained for an hour if an athlete had to).  But at that point you’re moving away from the definition of HIIT and the way interval training are usually thought of.


Summing Up

So that’s a (not so quick) look at three of the primary uses for HIIT.  Neurmuscular function, anaerobic power/capacity and aerobic power (and sort of capacity) were discussed.  Each represents roughly the three major energetic pathways in the body and each can and should be trained differently.  I’ve summed up each method in the chart below.


Goal Intensity Work Interval Rest Interval
Neuromuscular Power Maximum 6-15 seconds Complete (2-5 minutes)
Anaerobic Power Just Below Maximum 30-45 seconds 2-5 minutes
Anaerobic Capacity High 60-90 seconds 60-90 seconds
Aerobic Power Above maximum steady state 2-5 minutes 2-5 minutes
Aerobic Capacity At/near maximum steady state 8-20 minutes 4-10 minutes


On Tuesday, I’ll wrap the series up by looking at all of the different methods I’ve discussed in terms of some actual real world training applications.

Read Methods of Endurance Training: Summing Up Part 1

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