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HIT and Running: An in-depth conversation with Skyler Tanner about Resistance Training and Cardio

Skyler Tanner explains what happens to the cardiovascular system during high intensity training (resistance training) and cardio (aerobic exercise), as well as things like Vo2Max, endocannabinoids and sprint interval training.

Skyler Tanner is an exercise physiologist based in Austin, Texas where he operates Smart Strength, a HIT-focused, evidence-based, strength training studio. Skyler also regularly brings a fascinating perspective to all things exercise, through insights on his blog, skylertanner.com.

I have been intrigued by his recent writing on combining resistance training or HIT with running, an interest that he has developed over the last 5 years. Before any die-hard HITers choke on their coffee at the mere mention of running, Skyler still communicates that by far the most important thing that you can do for your physiology exercise-wise, is to apply HIT. Some may feel they have little interest in running, so why care to read about this topic on a HIT-focused blog?

Here’s a handful of reasons;

  • Skyler has an active mind and a passion for exercise
  • This is an opportunity to review some of the ways the cardiovascular system responds to exercise; HIT and aerobic, are there any valuable differences? Recently discovered benefits?
  • As a trainer you will likely have clients who are runners, cyclists etc.
  • At some point you may even want to rev your own running engines for a little fun

A couple of weeks ago I connected with Skyler over Skype to discuss how he got interested in running. We talked about the impact that resistance training or HIT has on the heart and cardiovascular system, and if there are any unique benefits that running/aerobic exercise stimulates. We discussed in some detail the aerobic protocol that is most akin to HIT: sprint intervals. I also asked Skyler whether he actively encourages his clients to add some form of aerobic exercise between their HIT workouts.


What happens to the heart and cardiovascular system during resistance training?

We began our conversation by discussing the cardiovascular system’s response to HIT. Skyler explained that one of the most notable responses is an increase in stroke volume in the heart. This means a greater amount of blood is pumped out of the left ventricle with every cardiac beat, concurrently there is also an increase in heart rate or the number of times the heart beats per minute. An individual may start out their workout with a heart rate of 80 beats per minute (bpm) and by the end of the first exercise, let’s say a pulldown, this may have elevated to 120 bpm.

The combination of an increased stroke volume and greater bpm produces an increased cardiac output: more blood (measured in liters per minute) is pushed out of the heart.

Heart rate x stroke volume = cardiac output

Skyler went on to mention that at the same time as this increase in cardiac output, there is a decrease in systemic vascular resistance– the blood vessels dilate to allow for the increase in blood flow. All of which means that more oxygenated blood can reach the working musculature during your pulldown.

Although heart rate increases during sets taken to momentary muscular failure (MMF), it does not reach very high levels under normal resistance training workout conditions (no rush-factor and the use of proper breathing). Skyler suggests that heart rate typically settles at about 60% of an individual’s heart rate maximum (HRM) during this type of workout. As I happen to have my heart rate readings from recent workouts to hand, let’s do the math.

Keep in mind that my maximum heart rate, achieved during a field test, is 204 beats per minute. It turns out that my average heart rate over the course of a 20-minute HIT workout (8-9 exercises, full body) is 114 bpm or 56% of my maximum. My peak heart rate reached momentarily during a recent HIT workout is 136bpm or 67% of maximum. Those figures tally well with Skyler’s reference to 60%, he explained that this coincides with the levelling off, of stroke volume. Of course, if you do use the rush-factor (no rest between sets/exercises), then heart rate can increase significantly above this level.


VO2 Max

Our discussion then shifted to VO2 max, a measurement of the maximal amount of oxygen a person can use during intense exercise. It is widely considered a marker of cardiovascular fitness and aerobic endurance and is measured in milliliters of oxygen used in a minute, per kilogram of body weight (or the ever so slightly less of a mouthful: ml/kg/min). The higher the individual’s VO2 max, the fitter they are considered.

VO2 Max is widely considered a marker of cardiovascular fitness and aerobic endurance. The higher the individual’s VO2 max, the fitter they are considered.


In the general population, a VO2 max of 45 ml/kg/min in a 35-year-old man, would be considered good (VO2 max typically declines with age). This reading in the athletic population would also be normal for a Powerlifter or a shot-putter. Elite cyclists, distance runners and Nordic skiers however, can double that VO2 max with readings ranging from the low 80’s through to the high 90’s. The highest ever recording of 97.5 was measured in 18-year-old cyclist, Oskar Svendsen in 2012.

What does working at VO2 max effort feel like? For a runner it is the fastest possible pace sustainable for about 8 continuous minutes… or damn unpleasant.

We have seen above that it is possible for a professional athlete to have a reading of anywhere from the 40’s to the 90’s depending on the sport. Does that mean that the sport you engage in dictates your VO2 max reading? Perhaps a little, but that is certainly far less of a factor than genetics.

Exercise can have a positive impact on a person’s VO2 max, Skyler mentions that this is often in the region of a 10-15% improvement, but of course ultimately potential is dictated by genetics. This is as true amongst athletes as it is in the general population. Just as switching an elite marathon runner to a bodybuilding-only routine is not going to produce a Mr. Olympia, getting a shot-putter to exclusively train as a middle-distance runner is not going to produce a VO2 max of 90+. The sport selects out the best suited physiology, anything after that (exercise, nutrition etc.) is refinement.

Source: Kenney WL, Wilmore JH, Costill DL. Physiology of Sport and Exercise. Champaign: Human Kinetics; 2012.


What effect does HIT with sets taken to MMF have on VO2 max?

Skyler, who has been training HIT for decades and has only fairly recently introduced running into his exercise regime, says he has not seen any further improvement in VO2 max over that which he had already attained through HIT. As Steele et al. point out: “Research comparing resistance training conducted to failure… with aerobic training suggests that the two modalities do not differ in the degree of VO2 max adaptations produced.”(1)

This means that you can probably max out your VO2 max with HIT alone, like Skyler did. The reason for this is that HIT stimulates enhancements in the ability to extract oxygen from the blood at the tissue level, in the working muscle. In effect it doesn’t matter if you are sprinting or performing a leg press to MMF, your body needs to get as good as it can at getting the oxygen it needs.


What happens to the heart and cardiovascular system during aerobic exercise?

Next, I wanted Skyler’s view on the impact of traditional aerobic exercise on the cardiovascular system, both acutely and chronically. Skyler explained that just like with HIT, when an individual begins an aerobic activity such as a run, heart rate increases, as does stroke volume and overall cardiac output. However, whereas heart rate during HIT tends to plateau at around 60% of maximum, the heart responds to sustained aerobic activity by increasing heart rate beyond this level when needed to match intensity.

For example, during a field-test to find my own heart rate maximum, I ran for 2×3 minutes flat out with a 2-minute rest between the intervals. I reached a maximum heart rate of around 200 bpm during the second interval, having moved through the entire range of heart rate response.

When it comes to the chronic effect of regularly running at a moderate aerobic pace, Skyler mentioned the plasticity or trainability of heart rate response to the demands of aerobic exercise. The more you practice moving at a pace, the more efficient the heart gets at providing oxygenated blood for that level of effort. Over time the heart can support the sustaining of a certain pace with a lower heart rate (within limits), and heart rate at rest can decrease too.

This is certainly beneficial for the running enthusiast, but does this adaptation provide any physiological benefit to someone who doesn’t care about running? According to Skyler, the best evidence of any wider benefit of this is research connecting lower resting heart rate with long-term cardiac health and a reduction in all-cause mortality (2,3,4,5). It is worth mentioning that this adaptation can likely be attained through rush-factor HIT performed in a circuit manner too.

That said, unless you want to be a runner/cyclist or other endurance athlete then there isn’t really any hugely compelling evidence to suggest that you need to do a specific aerobic activity beyond HIT for your physiological wellbeing.

Why would you if there is no additional benefit? And how is it that more people regularly engage in running than resistance training, despite the fact the physiological benefits of HIT are greater?

There are myriad reasons why this may be the case; running is instinctive, as Skyler mentions, kids do it all the time without thought, and for adults there are almost zero barriers to entry. Resistance training on the other hand, is more complex to understand, to get right and to program effectively: to the beginner it can be comparatively confusing and intimidating. There is another reason people engage in aerobic exercise.


The Gift of Flight or Getting High

During our preamble to the Skype interview, Skyler and I were exchanging anecdotes about the feel-good factor we both get from running, Skyler described this as the gift of flight, which reminded me of Richard Bach’s book, Jonathan Livingston Seagull. There is a certain something about flying across terrain at a swift pace, entering a flow state, and enjoying the moving meditation offered by running.

Skyler explained that in running animals (think mouse, dog, horse and human) there can be a bliss-like state experienced after about 20 minutes of continuous moderate intensity activity that just isn’t produced during resistance training.

In the past, this so-called runner’s high was ascribed to the release of endorphins (peptides which activate the body’s opiate receptors). In the last five years or so, research has shown that this effect is likely at least as much due to the release of endocannabinoids as endorphins, if not more so. Endocannabinoids are exactly what they sound like, cannabinoids that are biosynthesized in the body. You literally can get high on your own supply… if you are prepared to go for a 20-minute jog. Endocannabinoids have the potential to reduce anxiety and sensations of pain and are crucial for the “main aspects of a runner’s high”(6).

Although circulating endocannabinoid levels are increased during all intensities of exercise from very low to very high (which would include HIT), the “bliss amide” anandamide (a type of endocannabinoid) is released to a far more significant degree at moderate exercise intensities(7).

The sweet spot of endocannabinoid release is moderate exercise.


The extent to which the individual experiences this side effect of endurance exercise, is likely somewhat genetically moderated, your mileage may vary (just make sure you do inhale… and exhale). For those who experience a significant runner’s high that may be reason enough to add the odd 20-minute jog to your week.


Sprint Interval Training

So far, significant endocannabinoid synthesis has been the first definitive additional benefit of adding any type of aerobic exercise to a solid foundation of HIT that Skyler and I discussed. What about sprint intervals though? It seems that today as many, or more people are likely to perform sprint intervals as those who go out for a measured jog.

Skyler’s perspective is that sprint intervals do not add anything that resistance training or HIT doesn’t provide, from a general fitness perspective. See also our commentary on research by Steele et al. comparing sprint intervals with resistance training to MMF.

However, Skyler elaborates that sprints are very useful, essential for someone who intends to run races to the best of their potential. He performs sprint intervals specifically to help improve his 1 mile, 5k and 10k times.

To run fast, Skyler explains, you need to practice the skill of running fast, you need to develop your running economy and aerobic performance. You also need to improve your ability to tolerate the sensations of suffering that are experienced both during maximal sprints and during best-pace races. Sprint intervals are a very efficient way to stimulate these adaptations and improvements(8).

Expanding on what it takes to become a good runner, Skyler points out that there are 4 key steps to running well:

  1. The general skills of running, coordination and technique refined through drills
  2. Strength, best attain through resistance training, something the HIT’er will already have
  3. Speed, developed through sprinting
  4. Marrying the ability to move at a faster pace to the demands of a particular (race) distance.

Sprint intervals are important if you need to move fast (on your feet, on a bike, on a boat, on skis etc). However, if you already do HIT and have no interest in developing your ability to run fast, or to move fast in any aerobic activity then you don’t really need to sprint.


Does Skyler encourage his clients to do any aerobic activity?

Skyler comes from a resistance training background, his Smart Strength facility in Austin focuses exclusively on HIT strength training. He believes in getting strong first and foremost because “it is safe and provides the most benefit.”

It is only in the past few years that Skyler has developed a personal interest in running, initially as an opportunity to spend a little more quality time with his wife (a cross-country runner) one morning each weekend. He maintains that once a week running frequency to this day, in tandem with his once a week HIT workout and most importantly he enjoys it.

As our conversation was drawing to its close, I wanted to find out if Skyler actively encourages his clients to do some form of aerobic activity, or even specifically to run like he does himself.

When his clients ask him if they need to do anything in between their 1-2x per week Smart Strength HIT workouts, Skyler replies: “What would you rather be doing physically?” Whatever activity the client responds with, Skyler flashes back: “Do more of that if you want to.” 😉


1.  Resistance training to momentary muscular failure improves cardiovascular fitness in humans: A review of acute physiological responses and chronic physiological Adaptations. Available here.
2. Resting heart rate and all-cause and cardiovascular mortality in the general population: a meta-analysis. Available here.
3. Association of Resting Heart Rate and Temporal Changes in Heart Rate With Outcomes in Participants of the Atherosclerosis Risk in Communities Study. Available here.
4. Heart rate is an independent predictor of all-cause mortality in individuals with type 2 diabetes: The diabetes heart study. Available here.
5. Effect of Resting Heart Rate on All-Cause Mortality and Cardiovascular Events According to Age. Available here.
6. A runner’s high depends on cannabinoid receptors in mice Johannes Fuss, Jörg Steinle, Laura Bindila, Matthias K. Auer, Hartmut Kirchherr, Beat Lutz and Peter Gass PNAS October 20, 2015. 112 (42) 13105-13108; published ahead of print October 5, 2015. Available here.
7. UNIVERSITY OF EASTERN FINLAND, Faculty of Health Sciences School of Medicine Exercise Medicine RAJALA, EVELIINA : Endocannabinoids and exercise Master’s thesis, May 2015
8. Extremely short duration high-intensity training substantially improves endurance performance in triathletes. Available here.


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