Continuing from Interval Training: Part 1, I now want to look at three distinct “Types” of intervals. Each type tend to have a fairly specific goal and this will be reflected in its loading parameters.
The Three “Types” of Interval Training
As I discussed in Interval Training: Part 1, early ideas about interval training was that the heart was the primary site of adaptation. Also, the focus was more on the recovery interval than the work interval. I doubt many had much of an idea physiologically what was being targeted beyond that. As much as anything HIIT allowed athletes to accumulate volumes at higher paces than they could perform continuously.
In more modern times, the focus of interval training tends to be on improving fairly specific physiologically based performance measures. Of course, allowing the athlete to accumulate volume at a higher than steady state pace is still important.
This is especially true for extremely time based sport such as track and field, track cycling, swimming, ice speed skating and others. If you know someone has to be able to cover, say, 100m at a given speed, you have to expose them to training at or above that speed in some fashion. Interval training often allows this to be done before they can do it continuously.
Although HIIT should always be seen as somewhat of a continuum, with at least partially overlapping adaptations, I will describe three distinct types of HIIT.
- Neuromuscular ability
- Anaerobic capacity
- VO2 max
Each more or less falls into a specific energetic pathway. I’ve shown this sort of schematically below, how each pathway produces maximal amount of energy in different time frames.
So neuromuscular ability training will fall into the ATP/CP pathway. This pathway can generate a lot of power rapidly but for a very limited amount of time, roughly 5-10 seconds.
As the duration of maximal exercise increases, you get into anaerobic power/capacity training. Here energy is being produced primarily via anaerobic glycolysis, the breakdown of glucose under anaerobic conditions that produces various waste products. This will maximally occur in the 30-90 second range.
As the duration of exercise continues to and past the 2 minute mark (and up to about 8 minutes), the body is re-entering aerobic metabolism although it is working at a near maximal level. This is realm of VO2 max/aerobic power training. Now let me look at each in more detail.
For all practical purposes you can consider this type of interval training as sprinting or speed work. It’s a little bit more complex than that but the goal is to improve the body’s neuromuscular ability to go fast. This involves a host of adaptations in both the muscle and the nervous system.
As you might imagine, this type of training is utilized heavily by athletes who need to increase top speed. Short and even middle distance track and field athletes, sprinters in track cycling and others need to improve their maximum speed as a major part of their sport.
But what about endurance athletes? In the strictest sense, neuromuscular training doesn’t improve endurance in the sense that being able to go extremely fast for 4-10 seconds isn’t endurance in the sense of the word (sprinters refer to speed endurance, the ability to maintain maximum speed over their race distance but here we are talking abut races lasting 10-20 seconds or so).
Certainly the ability to launch a final sprint at the end of an endurance race can be the key to victory. Most bike races end in finishing sprints as do many long distance running events. Even having the ability to launch an attack during the race, cranking up the speed briefly to drop a competitor can be useful. But this has to do more with race dynamics and real world performance than endurance per se.
That said, neuromuscular training can indirectly help endurance by increasing an athlete’s efficiency of movement at slower speeds. In cycling for example, as athletes try to go faster you often see their riding mechanics breakdown. Their upper bodies are moving everywhere with their hips rocking side to side. By working above their normal cadence or speeds, they can “teach” their body to go faster while maintaining form at slower speeds.
In running, strides are often done. These are short intervals, perhaps 50-100 meters at speeds above race pace. It might not be full out sprinting but it is faster than the athlete’s steady state speed. Again the idea is to teach the body to run efficiently at the higher speed. Not only does this improve top speed but it will make the runner more mechanically efficient at slower speeds.
It’s important to realize that speed, by and large, is not enormously trainable. It can be improved but in sprint sports, athletes will work for an entire year to make only small improvements. For an endurance athlete, the training should be done but a great deal of focus on it would be a poor use of training time.
Loading Parameters for Neuromuscular Training
Training neuromuscular power is about high quality and effort and the loading parameters used this reflect that. Since the intensity is maximal or near maximal, the durations must be short. In general, 6-10 seconds is the maximum amount of time that top speed can be maintained as this is the longest that the ATP/CP system can realistically provide energy for. Most of the ways that sports approach neuromuscular training is based around that fact.
As examples of how this is done, a runner might do a rolling start where they are jogging relatively slowly before running 50-100 m at a speed higher than their typical threshold pace. Running guru Jack Daniels recommended that strides be done for 20 seconds at the runner’s current goal race pace or perhaps 95% intensity. Generally the athlete would take about half the time to get up to the goal speed and then maintain it for the rest of the time. Hence the total duration being longer than 10 seconds.
Swimmers typically swim about 15m at high speeds or even all out since that takes about 10 seconds or so. Cyclists would typically go up to 20 seconds aiming for power output of 180% of FTP. As with running it takes about 10 seconds to get up to maximum speed which can only be held for an additional 10 seconds. Other endurance sports would work in that general range.
The goal, ultimately is to perform 6-10 seconds above the athlete’s threshold, approaching maximum speed. But only while using proper technique. Here it is better to work at 95% properly than 100% and lose technique.
To ensure maximal quality, rest intervals are relatively long. Rest:work ratios of 6-10:1 are not uncommon. While this sounds enormous, it only represents a recovery of 60-100 seconds for a 10 second effort. So a runner doing strides would do their 20 seconds of higher speed running, rest 60-90 seconds and then do it again.
There are exceptions, typically when athletes are working on true maximum speed. Track cyclists may ride a 20 second all-out effort prior to resting for 30 minutes while a track runner might rest 10 minutes between maximal effort. When I was speed skating, we would do flying 400m laps all out and then rest 10 minutes between repetitions.
In general, passive rest is preferred for neuromuscular work. Track sprinters will basically stand around doing nothing in-between maximum speed runs. At most they might walk to keep some blood moving. Track cyclists will at most sit on rollers spinning very easily. Road cyclists might return to basic extensive endurance level pace between their efforts but it’s a bit hard to stop completely during a road ride. The key is that the recovery must be passive or as low intensity as possible to ensure maximal quality during the work bout.
The total number of repetitions is also not very high. A runner might do a total of 5-10 total “strides” while other sports might do twice as many. So a cyclist might perform a hard 15-20 second effort every 3-5 minutes during an hour ride, amounting to 15-20 total bouts. The goal with neuromuscular training should never be on the maximal number but rather the optimal number. As soon as the goal pace cannot be reached or technique deteriorates, the workout is over.
Neuromuscular workouts might be done up to 3 times per week albeit in fairly low volumes. And they can technically come before or during an individual workout. So a runner might warm-up before doing 5 20 second strides at their goal race pace. With one minute rest that’s only 7.5 minutes time. This might be done before a 30-60 minute extensive endurance workout.
Other athletes would be more likely to do their neuromuscular power work during a workout. As noted, cyclists could simply put 15-20 second high speed efforts into a longer workout. This more accurately represents race dynamics anyhow.
Repeated Sprint Ability
While neuromuscular power is related to the top speeds or power outputs that can be generated, there are times when the ability to repeat short high speed efforts is important. In research this is referred to as Repeated Sprint Ability (RSA). RSA refers to the ability to repeat high speed “sprints” with a relatively short rest interval.
This tends to be primarily relevant for team/mixed sports which may alternate between short-all out efforts, a short period of low intensity recovery and then the need to perform another high-intensity effort. Even in endurance sports, this ability may be called upon from time to time. A runner or cyclist trying to drop a competitor might perform a short high-speed effort, rest briefly and then perform another one to put distance between themselves and the competitor.
There is some debate over the best way to train RSA. In premise it could be trained by alternating short all-out efforts with longer rest intervals in an attempt to train the system. So an athlete might do 5X10″/30″ at maximum power. This type of thing is often used to test RSA looking at both top speeds and the rate of decline in speed over the series of efforts.
Some use the sport itself in some form or fashion. So soccer players will play small-sided games that force the athletes to work harder repeatedly with short breaks. Doing rounds of sparring in boxing or MMA would achieve the same basic concept.
Some suggest that just doing straight neuromuscular power/sprint work is the better approach. Presumably a higher top speed would allow any submaximal speed to be completed more easily to begin with. Hence it could be repeated more easily. Most seem to suggest doing that along with improving the aerobic engine through more traditional methods of endurance training or the anaerobic HIIT methods discussed next.
Certainly just doing straight sprint/neuromuscular power work seems to help in that it allows less than maximal efforts to be repeated more easily. So that would generally be part of the training. Most seem to advocate improving the aerobic engine through the other endurance training methods. The logic here is that recovery between maximum efforts is being driven by the aerobic system. The more well developed it is, the faster the athlete will recover between sprints. Hence the easier it will be to repeat that sprint.
In addition to that, some advocate also developing a better anaerobic capacity with the methods described next.
As I’ve mentioned previously in this series, once you move above the functional threshold, the body starts to rely more heavily on anaerobic metabolism. By this I mean that biochemical pathways that do not require oxygen (which is no longer present in sufficient amounts) become dominant in the body. Those biochemical pathways generate large amounts of metabolic waste products that eventually cause fatigue.
I’d mention again that lactate per se is not involved in causing fatigue. Rather, the production of H+ ions (and others) is far more involved here. Quite in fact, lactate helps the body to buffer H+, helping to prevent fatigue. This occurs during maximal activities lasting 30-120 seconds or so.
This makes it critically important to events such as the 400m and 800m (lasting ~1 and 2 minutes respectively) in track and field. In ice speed skating, the 1000m race is just over 1 minute long while the 1500 m is around 2 minutes. The 1500m is notorious for causing athletes to vomit or have a day-long hacking cough. It’s short enough to be all out but long enough to hurt.
Even for longer distance endurance events in running, cycling, cross country and others, anaerobic capacity can become relevant. So a cyclist might have to put in a hard 90 second effort to catch another rider who has broken away and the same might hold for a runner. After that effort, both will have to drop back to threshold intensity or lower to recover as the speed simply cannot be continued for long durations.
Being able to either produce power during those types of activities, or maintain that power against accumulating fatigue, can therefore become very important. And anaerobic capacity is a direct way of training and improving that aspect.
So the purpose of anaerobic capacity training is to deliberately cause the body to generate large amounts of those waste products as that will stimulate the body to improve its ability to handle or buffer them. These occurs through numerous mechanisms. One of these is an increase in the body’s natural buffering compounds such as bicarbonate. There are also changes in something called Monocarboxylate Transporters.
Anaerobic capacity training also causes adaptations in mitochondria themselves, improving their ability to metabolize waste products.. Importantly it does this in Type II fibers, something that lower intensity steady state methods do not do. This makes anaerobic capacity training a critical adjunct (in a performance sense) to methods such as intensive/extensive endurance, tempo, sweet spot and threshold training.
I’d mention here that the adaptations to aerobic training do play a role in anaerobic performance. First they allow a higher power or speed output to be maintained before anaerobic metabolism starts to predominate. Second, the increase in mitochondrial function helps to buffer the waste products produced during this type of exercise. Hence the importance of at least some aerobic development for seemingly “anaerobic” sports.
Loading Parameters for Anaerobic Capacity Training
In the most general sense, anaerobic capacity training involves working at high intensities, above threshold level (but lower than neuromuscular training) for durations that will increase the production of waste products. Generally speaking this means that repeats of 30 seconds to 2 minutes will be done.
In general, the rest interval might last from 1.5-2:1 so 1-3 minutes depending on the intensity and goal. This makes it possible to set up literally endless combinations by varying the work interval, rest interval and intensity. Recovery in this case should be active, usually at a low intensity as this will help to reduce acidosis level in the bloodstream.
When the goal is increasing how much absolute power the athlete can generate anaerobically, the intervals tend to be shorter and utilize near full rest intervals. So a rest:work interval of 2:1 might be used. The idea here is to generate lots of power and waste products and then allow full recovery so it can be done again. So a workout might be 5-10 repeats of 1′ all out with up to a 3′ rest. And this would likely be done as 2 sets of 5X1’/3′ with a 5-10′ break-in between. Alternately, 4 sets of 5X30″/2′ with a 5′ rest between sets might be done.
When the goal is to improve the athlete’s ability to resist fatigue during anaerobic activity, the rest interval tends to be shorter perhaps 0.5-1:1 since this allows more waste products to build up. So a 1 minute interval would have 30-60 seconds rest. Here the athlete might do 6X2’/1′ or 6X1’/1′ at a slightly lower intensity so that there would be a gradual accumulation of waste products throughout the length of the set. A brutal workout would be 10X30″/30″ with the goal being to maintain power or speed on every 30″ interval against increasing fatigue.
Once again, in sports where distances are an easier metric than time, you tend to see those used. In track running, anaerobic capacity workouts are done at 200-800m distances which is roughly 30-120 seconds or so. Swimming would use distances of 50m (~30 seconds) to 200m (2 minutes).
As a general rule, a total of 15-25 minutes of total “on-time” is more than sufficient for an anaerobic capacity workouts. As always, runners tend to be on the lower end of this due to the sheer pounding of their sport while other athletes are on the higher end in terms of duration. As with neuromuscular power, if the athlete’s power output or speed fall by more than 10% off the goal, the workout should be ended.
Due to the intensive stress of anaerobic workouts, they are usually limited to a maximum of 2 sessions per week if that. The workouts are extremely intensive, generating a massive amount of fatigue.
Issues with Anaerobic Capacity Training
For some time, the fitness industry became incredibly enamored of HIIT, especially anaerobic training. And while it is important to many sports and has powerful effects, it’s important to realize its limitations.
One is that the workouts are extremely challenging and exhausting, requiring a great deal of recovery. This gives them the potential to impair other workouts due to fatigue which may make them a poor choice.
Second, the anaerobic system is not massively trainable. Yes, improvements can be made but, in the big picture, the improvements tend to be limited. You can put in a staggering amount of work for relatively little payback. As well, the adaptations tend to occur quickly and then stop. Studies generally find that this type of training generates maximum benefits for about 3 weeks and that’s about it. The adaptations are rapid but ultimately short-lived.
As an example, in the now infamous Tabata study, the improvements to the training were maximized after about 3 weeks of training with very little improvement occurring over the following 3 weeks. The adaptations were huge for those 3 weeks but that was it.
Somewhat amazingly, famous running coach Arthur Lydiard had found this to be true empirically decades before. He only gave his runners roughly 3 weeks of anaerobic training to maximize those pathways. Finally, I’d mention again the German Track Cycling team that did a mere 10 days of high-intensity training on a base of huge aerobic training volumes for their record setting ride.
To all of this I’d add that many sports have no real need for this type of training to begin with. This is especially true of team and mixed sports where the intensities tend to be all out for short periods of time or low intensity with nothing in the middle. In soccer or basketball or lacross you simply don’t see an athlete running at top speed for 45 seconds continuously. Rather it’s a sprint to the ball, recovery, perhaps another sprint.
Boxers will never throw punches continuously for 30-60 seconds either. In MMA, the closest thing might be when the fight goes to the ground but even this is more about local muscular endurance than anything else. Those athletes found out the hard way that doing too much anaerobic work actually caused them to gas sooner. Their bodies became too reliant on anaerobic processes which ended up being a bad thing.
In a sporting context, only pure endurance athletes really have much need for this type of training. And even here, most endurance athletes do very little of their total training at this level. It doesn’t take much to maximize the system to begin with so they tend to save it for short period of intensified training. The rest of the time is spent building the aerobic engine.
For the general public, this type of HIIT may have some role as it can generate similar fitness and health adaptations in much shorter periods of time than continuous training. The problem being that, done properly, this type of training hurts which can potentially impair adherence. This is especially true in the early stages of training. After a base of fitness has been generated, HIIT can be brought in in small amounts but no more than 1-2 sessions per week should ever be done.
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 Intervals
The final type of interval to be discussed is VO2 max intervals which is often misunderstood. Conceptually VO2 max represents the maximum amount of oxygen that can be utilized by the body. And while it used to be thought that VO2 max was a primary determinant of performance, we now know this isn’t the case. Endurance athletes need to have a high VO2 max but other factors such as threshold power/speed and efficiency are equally critical.
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.
And while I said at the beginning of this series that I’d focus primarily on skeletal muscle, this is a situation where the distinction is critical since the different components of VO2 max require different types of training.
This distinction in mechanism is actually reflected in the terminology of some sports. In rowing, for example, basic endurance (extensive/intensive) training is referred to as Uptake Training (UT) to indicate that is increasing the muscle’s ability to uptake and utilize oxygen. In contrast, VO2 max intervals are referred to as Transport Training to indicate that they increase the heart’s ability to transport oxygen in the first place.
Maximizing VO2 max ultimately means maximizing both oxygen transport, primarily determined by heart function, and uptake, primarily determined by muscular function and other adaptations. In this vein, I also mentioned that early ideas of interval training were that it primarily impacted on heart function and that the focus was on the recovery interval to begin with. In the 80’s this reversed itself and coaches felt that long duration training mainly affected the heart while intervals affected skeletal muscle.
Note: this latter idea is certainly true to some degree. You can’t ever separate central from peripheral effects in training. And anaerobic intervals clearly impact on muscular function. Even with VO2 max training, it’s not as if there are absolutely distinct effects. It’s just a matter of proportions.
In any case, many of those early ideas seem to be more or less correct. Interval training, at least the type that tends to improve VO2 max is primarily impacting on heart function. Again, there are certainly going to be peripheral/muscular effects but the main effect is in the heart.
Loading Parameters for VO2 Max Intervals
Typically speaking, VO2 max intervals are done at an intensity much lower than for anaerobic or neuromuscular intervals described above. In fact, they are typically only a little bit over threshold to begin with. Cyclists typically use 105-110% of their FTP while runners might use their best 3-5k pace. Other systems use a percentage of maximal aerobic power (MAP) but this requires specific testing. This is a hard effort to be sure but athletes quickly find out that if they go out too fast they blow up before the full interval can be accomplished.
The goal here is working at a rate that will cause the athlete to attain their VO2 max and hold it for some duration. The premise essentially being that “stressing” VO2 max will stimulate adaptations. Working harder than this is neither necessary nor beneficial. If anything it simply puts the athlete into the anaerobic interval range which limits the duration of training.
Generally speaking, true VO2 max can be sustained for 5-8 minutes although this will require an absolutely maximal effort from the athlete. For this reason, VO2 max intervals are generally done for shorter periods of time so that a higher total volume can be performed.
Typical interval lengths are 2-5 minutes with 3 minutes being common in the modern era. It takes about 1 minute for the body to achieve VO2 max so that will give 1-4 minutes at VO2 max. With multiple sets of less than maximal duration, a rather large amount of training at that level can be accomplished without having to go to complete exhaustion. During the interval rate, the athlete will be breathing like a freight train and heart rate may very well hit maximum by the end of a 3+ minute interval.
Generally speaking, the athlete tries to maintain the same power output or speed during the entire interval. However, a recent study found that using variable intensity during the interval generated more time at VO2 max with less perceived effort.
Perhaps surprisingly, the rest interval for VO2 max intervals are not extremely long with a rest:work ratio of 1:1 or less being used. Mind you, the intensity is certainly lower and the idea is to stress the aerobic system maximally. So performing the workout in this fashion is possible.
Early Vo2 max workouts involved working at VO2 max intensity for 6X5’/2′ rest. I’ve done them and this is an absolutely brutal workout. More current approaches might be something like 3-8X3’/2-3′ rest at VO2 max. This is not only more tolerable as a workout but allows for more high quality repeats to be done.
There are other ways to achieve VO2 max intensity as well, usually involving shorter near maximum efforts with short rest intervals. What happens here is that VO2 is achieved more gradually over the repeats but is eventually achieved. Perhaps one of the most famous studies here is the Tabata protocol.
The protocol involves 8 repeats of 20 seconds at essentially all out intensity with a mere 10 second rest, for a total of 4 minutes of work. By the end of the series, VO2 max will be achieved and the study found that the protocol improved VO2 max and anaerobic capacity as both were stressed.
Issues with VO2 max Intervals
There is no doubt that, in untrained individuals, interval training improves Vo2 max far more rapidly than more continuous training methods. Even for highly trained athletes, the inclusion of some VO2 max intervals generates adaptations that steady state methods simply can’t.
And this is where a lot of the claims for the inherent superiority of this type of training comes from, the fact that it is highly effective in short period of, admittedly, very intense work. But it also comes from a misunderstanding many have of what VO2 max represents, often concluding that it is the best way to improve aerobic markers, endurance or performance.
The problem with this goes to the distinction in effect I mentioned above. VO2 max is a function of both central and peripheral factors. And VO2 max intervals primarily target the first. Developing the peripheral (muscular and other) adaptations takes longer duration methods such as the extensive/intensive, tempo, etc. methods I described before.
There is no singularly ideal intensity that can optimize everything and that holds here. VO2 max intervals will help to maximize an athlete’s top end in this regard but will do little to improve their aerobic endurance per se. That will require different training. At least some aerobic training will need to be done.
As an interesting aside, a very early paper by Hickson found linear improvements in both VO2 max and endurance with one of the most intense training programs I’ve seen. It took beginners and trained them 6 days per week. Three days per week were VO2 max intervals on the bike (6X5’/2′) and the other three was a 40 minute run done at the fastest speed possible (essentially tempo/sweet spot/threshold rate). Essentially it combined a method to improve top end power with the most time efficient method of improving actual endurance. And everybody quit training when it was over.
But there are other issues. Like anaerobic training, VO2 max is only so trainable. There is also a huge genetic component here. Top athletes are invariably born with a high VO2 max but also a genetic propensity to improve it by large amounts. For most, this isn’t the case. Short-term studies may show impressive improvement in VO2 max with this type of training but that doesn’t mean it will continue forever.
Also like anaerobic training, the improvements in VO2 max from this type of training tend to occur quickly but taper off equally quickly. Roughly three weeks, or perhaps 6 total workouts, tends to generate the maximum adaptation and that’s it. Like anaerobic work, an athlete can end up working their brains out for very little return.
I’d note that in some populations, notably untrained beginners, you may see the results from VO2 max training occurring for longer periods such as 8 weeks. At that point, no further gains are generally made. So it works fast but stops working equally fast. I’d note that, like anaerobic capacity training, VO2 max training hurts. So we might question the potential adherence for a beginner to most types of VO2 max training.
This raises the seemingly obvious practical question: if interval training of any sort, whether in athletes or the general public stops working in 3-8 weeks, what should that person do the rest of the year. Coaches and trainers who became enamored of HIIT methods seemed to think that incorporating them year-round was ideal. And this just isn’t the case.
There are three primary “types” of intervals that can be done. The first is neuromuscular power, short bouts of near maximal activity meant to improve top speed and neurological effeciency. The second are anaerobic capacity, lasting 30-90 seconds at a near maximum effort to improve the body’s ability to buffer acidosis. The third are VO2 max intervals, lasting 2-5′ at an intensity that generates VO2 max, improving that characteristic. I’ve summarized the basic loading parameters for each below.
Next I want to start looking at how all of the different components of endurance training might fit together into a training program for different goals.