Last Updated on January 16, 2022 by Heather Hart, ACSM EP, CSCS
There’s a running joke (haha, get it?) among the ultramarathon world that those who cannot run fast, decide to run further. Of course, this isn’t entirely true – I know endless ultramarathon runners who can not only run 100 miles, but can also throw down a sub-20 minute 5K or a 5:00 (or faster) mile time. But it’s true that the majority of athletes who are attracted to the sport of ultrarunning truly do enjoy and prefer the endless hours of long, slow, aerobic pacing.
So it’s no surprise that I’m often met with with a little pushback when I assign my ultrarunning clients hard interval workouts.
But there are plenty of benefits of interval running workouts for ultrarunners, and for athletes truly looking to improve, these should should be incorporated into training. You don’t have to like them (believe me, they aren’t always my favorite either)…but you should do them.
In this post, we’ll discuss both why interval running workouts can be beneficial for runners, and how (and when) to add them into your ultramarathon training.
Understanding the Purpose of VO2 Max Interval Running Workouts:
As always, before we explain the “how”, we’ve got to understand the “why”. Yes, my friends…it’s time for some science.
Want to skip the science and get right to the practical application? Click HERE to jump right to the “How To Incorporate Intervals Into Ultramarathon Training“ section
What is VO2 Max?
In the simplest terms: VO2 max refers to the maximum amount of oxygen your body can take in and use during exercise.
More specifically, VO2 max is defined as the maximum rate of oxygen consumption measured during incremental exercise. This number, typically between 30-60 (though elite athletes often test between 70-80 or more), is measured in milliliters of oxygen per minute per kilogram of body weight (ml/min/kg).
The greater your VO2 max, the more oxygen your body can consume, allowing your body can use that oxygen to generate the maximum amount of ATP energy to keep your body working harder, longer.
For a more in-depth understanding of VO2 max and how it relates to running, check out this related article – VO2Max and Running: Does It Really Matter?
Is This the Same as My Lactate Threshold?
Not quite. VO2 max is a runners ability to maximally consume and use oxygen while lactate threshold is a determination of the point at which lactate (a chemical byproduct of anaerobic respiration – the process by which cells produce energy when oxygen isn’t available) cannot be cleared as fast as it is accumulated. Even though they are different, it’s important to note that lactate threshold occurs at a percentage of VO2 max.
What are Interval Running Workouts?
Interval running workouts are exactly what they sound like – workouts that alternate intervals of running at varying intensities or speeds. Typically a harder effort interval will be paired with a recovery interval, at varying durations.
The purpose of running intervals is to maximize the body’s exposure to stress at a given intensity. Because harder intensities can only be sustained for a short amount of time, repeating the intervals gives us the opportunity to log a larger combined total time at the desired intensity. And that accumulated stress, when done properly, results in physical adaptations.
While this is a more generic description of interval workouts, in reality, interval training gets much more specific:
Types of Interval Training:
High Intensity Interval Training (HIIT): This type of interval workout includes intervals performed at 90-100% of maximum heart rate or 75% or greater of maximal performance(in other words: HARD). The duration of the work intervals typically lasts up to 2-3 minutes, and includes a 1:2 or 1:3 work to recovery ratio (i.e. if your hard run interval is 2 minutes, you will rest 4-6 minutes in between). The purpose of the longer rest intervals is to allow appropriate recovery between hard intervals, in order to keep the harder intensity consistent throughout the workout.
Sprint Interval Training (SIT): This type of training includes intervals of much higher intensity (such as all-out sprints at near 100% effort). Because of this extreme intensity, these intervals are typically much shorter, and the rest intervals much longer compared to HIIT (one research study even proposed a 1:8 work to recovery ratio (4). In the running world, an example of an SIT workout would be 100 meter sprints, followed by 4 or 5 minutes of recovery.
High-Volume Interval Training (HVIT): This type of training includes intervals of a more moderate intensity (60-75% of maximum heart rate) over longer durations, with shorter rest intervals, or “rest” intervals that are more active recovery versus focused rest.
Variable-Intensity Interval Training (VIIT): This type of training purposefully rotates between intervals of near maximal effort, moderate effort, and low effort, with varying intervals of recovery.
How Do Runners Benefit From VO2 Max Interval Running Workouts?
VO2 max interval workouts help increase – you guessed it – a runner’s VO2 max, by increasing your heart’s stroke volume (the volume of blood pumped out of the left ventricle of the heart during each beat) and maximum cardiac output (the amount of blood your heart pumps each minute).
Studies show that athletes can increase their VO2 Max, anywhere from 5-30%. There are a number of limiting factors that affect the amount of improvement you can see, including genetics, gender, and even age. In general, people who are the least fit see the largest changes, and athletes who are highly fit see the smallest changes in VO2max due to training.
Why Should I Care About My VO2 Max?
Well frankly…you don’t necessarily need to care about the exact number of your VO2 max. In fact, without lab specific testing, you’ll never actually know the exact number anyway.
But, training specifically to improve your VO2 max will also help improve many other aspects of your running fitness along with it, such as your lactate threshold and aerobic, endurance pace.
In short, VO2 max interval running workouts are a highly effective approach to making your body better at running. And what runner doesn’t want that?
It’s Not Just Your VO2 Max That Improves:
Oxygen carrying capacity aside, VO2 max intervals can also help improve:
Muscular Strength: The long, slower, endurance effort of running an ultramarathon utilizes mainly type I (slow twitch) muscle fibers. However, VO2 max interval efforts rely primarily on type IIa (fast twitch) muscle fibers. While these fast twitch muscle fibers really aren’t relied upon during ultrarunning (unless, perhaps, you find yourself sprinting from a bear mid race), training BOTH muscle fiber types can increase your overall health and strength.
Running Speed: In order to hit your VO2 max pace, you are likely running much faster and harder than you are used to. As with any exercise the more we practice it, the better we become at that task (in this case, running faster). Further,VO2 max intervals increase your capacity to contract a large number of muscle fibers simultaneously, as the more muscle tissue is active at any given moment, the more oxygen the muscles demand.
Blood Plasma: Blood plasma is the liquid component of your blood, that makes up slightly more than half of your blood (the rest is red blood cells, white blood cells, and platelets). VO2 max intervals increase the adaptation of more plasma volume The more plasma you have, the greater your overall blood volume, which leads to an increased stroke volume with less cardiac effort.
Further, increased blood volume:
- helps reduce lactate accumulation because the extra oxygen leads to less anaerobic respiration.
- reduces the rate of glycogen depletion due to more blood flow to the skeletal muscle
- helps lower your core temperature and increase sweat rates due to higher blood flow to the skin (which will help you tolerate running in the heat better).
Which Type of Interval Training Is Best for Improving VO2 Max?
Let’s look back to the type of interval workouts, mentioned above. Which one best targets VO2 max improvements?
In a study published in the 2007 Medicine & Science in Sports & Exercise comparing the effects of aerobic endurance training at different intensities and interval durations, runners were split into four groups:
- Long slow distance running (LSD), performing a continuous run at 70% of max heart rate (137 ± 7 bpm) for 45 min.
- Lactate threshold running (LT): performing a continuous run at lactate threshold (85% of max heart rate 171 ± 10 bpm) for 24.25 min.
- 15/15 interval running (15/15): performing 47 repetitions of 15-s intervals at 90-95%of max heart rate (180 to 190 ± 6 bpm) with 15 s of active resting periods at warm-up velocity, corresponding to 70% of max heart rate (140 ± 6 bpm) between.
- 4 × 4-min interval running (4 × 4 min): performing 4 × 4-min interval training at 90-95% of max heart rate (180 to 190 ± 5 bpm) with 4 min of active resting periods at 70% of max heart rate(140 ± 6 bpm) between each interval.
Runners in the LT, 15/15, and 4×4 groups started with a 10-min warm-up and ended with a 3-min cool-down period at 70% of max heart rate. All training sessions were performed running on a treadmill at 5.3% inclination. All four training protocols resulted in similar total oxygen consumption and were performed 3 days a week, for 8 total weeks.
The results? Runners in both the 15/15 and 4×4 groups, running at 90-95% of maximum heart rate had a significantly greater improvement in VO2 max than the LSD and LT groups(1)
As we already discussed, by breaking the harder efforts up into smaller segments, you can perform a greater volume of high-intensity work. But how small is small enough, and can the intervals be too small?
In the study mentioned above, researchers concluded: “Although both the 15/15 training group and the 4 × 4 min training group improved VO2 max, the 47 repetitions of 15 × 15 s training at a velocity that eventually gives a heart rate at 90-95% max heart rate is difficult to administer. Interval training with longer intervals, like the 4 × 4 min training administered in this experiment, is thus recommended to improve VO2 max”.
Basically, researchers conclude that intervals in the 4 minute range of work are simply easier to keep track of than 47 repetitions of 15 seconds. In his recently published book, “Training Essentials for Ultrarunning: Second Edition” Jason Koop, head coach of CTS training and coach to elite ultrarunners agrees. Koop states:
“Shorter intervals are fine if you are looking for a different stimulus or just something novel to do. The benefit at the end of the day, as long as the time at intensity is equivalent, is likely negligible. Personally I prefer the longer style (one-to three-minute) intervals in trail running, as it’s more specific to the sport and the workout is easier to manage.”
Another study by Stephen Seiler and Jarl Espen Sjursen consisted of having runners perform self paced, high intensity intervals at either 24 x1 minute, 12 x 2 minutes, 6 x 4 minutes or 4x 6 m minute running bouts (all equaling 24 minutes of work) with a1:1 work-to-rest interval.
- Peak physiological responses were similar for interval durations between 2 and 6 minutes, and signiﬁcantly higher than the 1 minute intervals.
- One-minute intervals actually resulted in lower VO2 max during the work portion and higher VO2 max during recovery
- Runners were able to achieve their highest oxygen consumption during the 4-minute intervals
- A work duration of 4 min appears to be an optimal duration for achieving peak cardiovascular responses (4)
Specifics on recovery duration are limited. What seems to be the consensus among researchers and coaches alike is that recovery durations should be long enough to allow the athlete to physically recover to the extent that they are able to push hard enough during the next work interval to reach the desired intensity (90-95% of maximum heart rate.)
If recovery is TOO short, a runner might not be able to push hard enough on subsequent intervals to reach the desired intensity, thus, cutting the total time spent at that specific intensity too short.
But, if recovery is TOO long, then we are not keeping the stimulus level high enough throughout the duration of the workout to achieve the desired outcome.
Therefore, a 1:1 work to recovery ratio is most commonly recommended.
Incorporating VO2 Max Interval Running Workouts into Ultramarathon Training
So why exactly do ultrarunners – who seemingly spend 12, 24, 30 hours or more running long distances at very slow paces, want to improve their VO2 max?
Statistically speaking most ultrarunners pace most of their training focus on the long, slow, mid to lower intensity training. According to the United Endurance Sports Coaching Academy:
“This equates to ultrarunners developing a high level of fitness as it pertains to their aerobic conditioning and lactate threshold. However, their absolute threshold (VO2 max) is often low – or at least compared to that of runners that train for shorter running distances. For example, an elite ultrarunner might have lactate threshold that is within 95-97% of their VO2 Max. In comparison, an elite marathon runner’s lactate threshold might be between 75-85% of their VO2 Max. VO2 Max represents the absolute ceiling of one’s aerobic capacity, a runner cannot aerobically exceed this level. So in the case of an ultrarunner whose LT is around 95% of their VO2 Max, the area of focus should be on increasing their VO2 Max so that their LT can continue to increase” (5)
In simpler terms: when you improve your maximum aerobic capacity, everything underneath has the opportunity to improve as well, including your ability to utilize oxygen at “ultramarathon paces”. Incorporating VO2 Max interval workouts into your early season training can help you become a stronger, more efficient runner.
The Best VO2 Max Interval Running Workouts for Ultrarunners
Now, here’s where things get a little tricky. In diving through endless articles specifically dissecting the “how” of VO2 max intervals, there is a lot of conflicting information regarding whether shorter intervals (SIT) or longer intervals (upwards of 6 minutes) were most beneficial.
Consider The Conditions You Are Training For: in the sport of ultramarathon specifically, we rarely have the need for short, fast bursts of power and speed. Therefore, utilizing the longer intervals (2-4 minutes) is likely easier from a logistical point of view (less intervals to keep track of).
Remember Time at Intensity: Regardless of the interval duration, remember what matters most is the combined time at intensity for each workout. Too little time at intensity, and you will not stimulate the physical adaptations you are looking for. Attempting too much time at intensity is exhausting, and as such, your intervals will fall short of the actual intensity needed to elicit the physiological responses desired.
Jason Koop’s 3:3 Uphill Intervals:
In his book “Training Essentials for Ultrarunning: Second Edition” Jason Koop outlines an interval running workout that consists of 3 minutes of work paired with 3 minutes of recovery. The number of sets of 3×3’s begin at 4 for beginners (12 minutes at intensity), and increase to 6X3 for advanced athletes, or even 5×4:4 for elite athletes (20 minutes at intensity). Further, Koop recommends that these intervals be performed uphill (either outdoors or on a treadmill) versus on flat ground.
“It is preferable to perform them (intervals) uphill – especially when building fitness, as it enables a runner to increase their workload and reach their VO2 Max more consistently versus on a track or treadmill. Running (intervals) uphill also reduces the chance for injury as opposed to (intervals) on flat or downhill terrain. This is due to reduced ground reaction forces (i.e., impact). Lastly, since more time is spent running uphill than downhill during ultramarathons, improvements in uphill running will have a larger impact on race day.”
Note: Whether outside or on a treadmill, you’ll want to do these intervals at a % grade that is difficult, but sustainable for the entire interval. Too steep and you’ll end up having to walk, which will negate the purpose of the interval. And if outside, you’ll want to avoid a super technical trail that causes you to slow down while being mindful of foot placement. Again, these workouts should be a 9-10/10 effort.
When & How Should Ultrarunners Include Interval Running Workouts?
Understanding when to incorporate VO2 max intervals into your ultramarathon training is important.
First, remember VO2 max is your “ceiling”: As we’ve mentioned a few times, in order to raise your lactate threshold or even your endurance capabilities, you must first raise your aerobic capacity, in order to allow these other improvements to occur.
Second, remember the principle of specificity: As with any sport, training for the specificity of an ultramarathon should be the priority. And what’s the specificity of an ultramarathon? The ability to run a huge amount of miles (volume) at a much lower intensity.
When you perform VO2 max intervals, you are sacrificing volume for intensity. This is because these types of workouts are hard, and simply cannot be sustained for long.
Therefore, because the latter stages of your ultramarathon training should be focused on specificity, VO2 max intervals are best done during the early stages of a training cycle. Because the training adaptations to this type of workout occurs fairly quickly, it’s typically recommended that ultrarunners incorporate intervals for only one to two mesocycles (typically mesocycles are around 3-4 weeks each).
How Often Should I Perform VO2 Max Intervals?
The frequency of interval workouts absolutely depends on your fitness level and experience. For newer athletes, I recommend starting off with one interval workout per week. More experienced athletes can certainly incorporate two interval workouts per week, and advanced athletes can even attempt to have greater adaptations with back-to-back interval workouts.
Remember: higher intensity equals shorter adaptation process, but comes at the cost of longer recovery time. When incorporating VO2 max intervals into your training, you need to put an emphasis on recovery (both rest and nutrition), and monitor your body closely for fatigue. Ultimately, you do not ever want to incorporate interval workouts into your training plan in a way that will hinder your overall ultramarathon goals.
As an ultrarunner myself who is always significantly more comfortable running at my “all day” pace versus the highly uncomfortable “I might puke” pace, I get the hesitation and lack of enjoyment these types of workouts can bring.
BUT…when you consider that significant improvements in overall running fitness can occur in a short period of time with these types of workouts, it almost seems foolish to NOT incorporate them into your training.
- Helgerud, J., Høydal, K., Wang, E., Karlsen, T., Berg, P., Bjerkaas, M., Simonsen, T., Helgesen, C., Hjorth, N., Bach, R., & Hoff, J. (2007). Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine and science in sports and exercise, 39(4), 665–671. https://doi.org/10.1249/mss.0b013e3180304570
- Koop, J. (2021). Training Essentials for Ultrarunning (2nd Ed.). Koop Endurance Services, LLC
- Seiler, S., & Sjursen, J. E. (2004). Effect of work duration on physiological and rating scale of perceived exertion responses during self-paced interval training. Scandinavian journal of medicine & science in sports, 14(5), 318–325. https://doi.org/10.1046/j.1600-0838.2003.00353.x
- Townsend, L. K., Islam, H., Dunn, E., Eys, M., Robertson-Wilson, J., & Hazell, T. J. (2017). Modified sprint interval training protocols. Part II. Psychological responses. Applied physiology, nutrition, and metabolism = Physiologie appliquee, nutrition et metabolisme, 42(4), 347–353. https://doi.org/10.1139/apnm-2016-0479
- United Endurance Sports Coaching Academy (2021). Ultrarunning Coach Certification.
Heather Hart is an ACSM certified Exercise Physiologist, NSCA Certified Strength and Conditioning Specialist (CSCS), UESCA certified Ultrarunning Coach, RRCA certified Running Coach, co-founder of Hart Strength and Endurance Coaching, and creator of this site, Relentless Forward Commotion. She is a mom of two teen boys, and has been running and racing distances of 5K to 100+ miles for over a decade. Heather has been writing and encouraging others to find a love for fitness and movement since 2009.
Very interesting and well written article, Heather!
So, to simplify things, would you say that all workouts/types of runs an ultra unner needs are long, slow aerobic running (shorter and longer ones), longer lactate threshold (aka “tempo runs”) efforts and fast, all-out VO2Max intervals?
Have a nice day and thanks for taking your time!
Greetings from Austria,