High intensity interval training (HIIT) is one of today’s most talked about forms of exercise. While the approach has been floating around gyms for a long time it has only been in the last few years that our colleagues in the research community have really begun to increase the amount of research being done in this area. In 2012 when I attended the American College of Sports Medicine annual conference there was a session on HIIT research and the general attitude was that they had suddenly stumbled across an amazing new technique and were just starting to learn how it works. This is the general flow of almost everything in the world of fitness and training. A few coaches and trainers begin utilizing an approach, it theoretically makes sense and people seem to be making improvements and before you know it practitioner after practitioner are jumping on board and creating their own versions of it (many trying to sell their formula). Once a certain critical mass is reached the research community begins to take notice and starts looking into the effectiveness of the approach. They either quickly debunk it and move on or they find there is something to it and continue to do more and more research. In the case of HIIT their early research was strongly in support of the approach and as the years have gone by the body of literature on the subject has grow quickly. A scan of published studies on PubMed using the search term “high intensity interval training” comes up with 1,235 citations. Just in the first three months of 2017 there are 94 new studies and for 2016 there were 263. I can promise you that if I tweaked the search terms a little or looked on another site for citations that we could increase those numbers. Clearly there are far too many studies to do one simple deep dive into the research and draw a full understanding of it so with that thought in mind, I present to you the first installment of High Intensity Interval Training research. I will do my best to curate the literature and pull out the pieces that are most relevant to front line coaches and trainers as well as everyday exercisers looking to maximize their health and fitness improvements.
The most recent HIIT study that has been getting considerable coverage from the mainstream media has the very digestible title “Enhanced Protein Translation Underlies Improved Metabolic and Physical Adaptations to Different Exercise Training Modes in Young and Old Humans”. Sounds very exciting doesn’t it? Well that is exactly the type of attention grabbing headline you expect from that newsstand favorite, Cell Metabolism. Now the headlines that most popular media are using for this article tend to be along the lines of “HIIT Training, The Fountain of Youth”. While that may be a bit of hyperbole, that title is far more along the lines of what this study actually suggests is happening at the cellular level.
Robinson et al.(2017) examined and compared a variety of responses to high intensity interval training, more traditional strength training and a combination of the two (both performed at a lower intensity and volume then the individual test groups). The authors went one step farther in that they used both a young group of subjects (18-30 years) and an older group (65-80).
Subjects in the HIIT group trained 3 days per week on a cycling ergometer for 4 x 4 min at greater than 90% of peak oxygen consumption (VO2 peak) with 3 minutes pedaling at no load in between sets. They then walked on a treadmill 2 days a week for 45 min at 70% of VO2 peak. The resistance training (RT) group per formed 4 sets of 8-12 repetitions of a series of both upper and lower body exercises twice a week. The combined training (CT) group underwent a 12-week sedentary period followed by 12 weeks of cycling 5 days a week for 30 minutes at 70% of VO2 peak and resistance training 4 days per week with less repetitions then the RT group used.
Cardio Respiratory Fitness, Muscle Mass and Insulin
Let’s begin reviewing the results with the easy and to be expected outcomes. The youngsters saw significant increases in absolute VO2 peak for all three training approaches, the greatest results with HITT leading the way followed by CT and then RT. For relative VO2 peak the young group saw an approximate increase of 28% with HITT, 17% with CT and no statistically significant change with RT. The older subjects saw absolute VO2 peak increases for both the HITT and CT group. They did see improvements with RT but the results were not statistically significant. Absolute VO2 peak increased 17% for HITT and 21% with CT. There were no significant changes for RT. For the non-science geeks, remember VO2 peak refers to the top rate at which your body utilizes oxygen and a bigger number is better, so big props to the older group for such significant changes.
All three training groups for both age cohorts saw increases in fat free mass (that means added muscle mass). As expected resistance training had the greatest impact and more so for the younger group but the results were positive for everyone in all conditions. Leg strength increased for the RT and CT groups in both age categories but not the HITT group. This could be to the specificity of training for the cycling group.
It is well known that insulin sensitivity is improved with exercise. This means the body does a better job of responding to the presence of insulin and moving glucose from the blood stream into storage as glycogen. The younger group of subjects saw improvements in insulin sensitivity for all three training categories. The older subjects saw improvements for both HIIT and RT but not the combined training.
Mitochondrial changes
Let’s start by remembering that mitochondria are the power factories in your cells. The more you have and the better they work the more ATP (ATP = energy molecules) you can produce and the more work you can do. Also keep in mind that mitochondria are primarily associated with aerobic metabolism (with oxygen = cardiorespiratory fitness). It is common for people to have reduced mitochondrial content and function with age and while this study is not looking at the causes for these declines with age, it’s fairly safe to say it is both a simple function of aging and also a result of a function of detraining for most individuals. And while we are not touching on this subject today, do note that mitochondria are exceptionally complex structures that do far more than just produce ATP.
The authors found that with HITT the younger subjects had a 49% increase in mitochondrial respiration and the older subjects had an outstanding 69% increase. Yes you read that correct, a 69% increase in their ability to produce energy, pretty amazing. With the CT group the younger subjects saw a 38% increase but the older subjects did not. Neither group saw improvements with RT suggesting that people need some form of cardiorespiratory stimulus different from resistance training to achieve these benefits. Both HITT and RT resulted in increased in mitochondrial protein content for older adults explaining some of the improvements that they experienced. It would be interesting to see a similar study conducted that used high intensity intervals done with various resistance training exercises to determine if those movements could generate similar mitochondrial changes as well as how different types and volumes of HITT influenced results.
Exercise effects on skeletal muscle gene expression
A gene is a set of instructions encoded on our DNA. On its own it doesn’t do anything but if it is “expressed” it is turned on in such a way that a copy of those instructions are made and sent to other parts of the cell for its function to be carried out, usually the creation of a protein or other molecule that has a particular job in the cell. Remember that not every gene expression and function they impact is positive and some can be involved in issues like cellular death or uncontrolled reproduction (cancer) so in some instances we want to see certain genes down-regulated while would like an increase in activity of others resulting in positive impacts on the cell and general body.
The authors refer to particular gene sequences that are turned on or off as gene transcripts. Looking at transcription in skeletal muscle related to mitochondria, muscle hypertrophy (growth) and insulin sensitivity before training, the older subjects had 267 gene transcripts lower and 166 higher than the younger group. That represents quite a significant difference in gene activity with age. Following training the largest numbers of genes were turned on with HITT in both young and old subjects. Compared to HITT, RT and CT increased 35% and 28% fewer genes in the younger subjects. In the older subjects 70% and 84% fewer genes were expressed compared to HITT. Score another one for older adults performing HITT.
In young subjects 274 unique genes were increased by HITT, 74 by RT and 170 by CT. Older exercisers had 396 genes increased by HITT, 33 by RT and 19 by CT. Both age groups clearly show the most gene activity as the result of HITT and this does not even count genes that were affected by more than one training mode.
The next question was how many of the genes affected are impacted in both the young and older group and how many are specific to age. Of the total 553 genes affected by HITT in older subjects (157 had some overlap with RT/CT so 157 + 396 unique HIIT genes = 553), one –third (181) were also shared with the younger HITT group and the young RT and CT groups shared 114 of these genes. A full third of the older HITT genes (186 of 553) were unique to the older group suggesting that a large number of the genes impacted by HITT are age specific.
Impact of training on skeletal muscle proteins
When someone says protein most of us conjure up images of steaks and chicken breasts and post workout shakes, thinking of the macronutrient we all eat to survive. Yes protein is something we eat and we all know that our bodies use it to produce muscle but at the cellular level a variety of proteins are being constantly produced and they have an assortment of functions, acting as catalysts, signal receptors, switches, motors and more. As we previously mentioned, many of the gene transcripts that the researchers examined are turned on by different types of exercise and when they are expressed, many of them instruct the cell to build a particular type of protein depending on the gene.
The younger subjects in this study saw increases in protein abundance for 265 proteins following resistance training. They saw 114 proteins increase following HIIT. Only 35 of the proteins that increased were affected by both the RT and HIIT. In the older subjects 147 proteins increased in abundance following RT and 227 increased following HIIT. Only 38 were shared between the two types of training. This suggests both significant impacts of the different types of training as well as the very large impact that HIIT has on older adults.
Looking at only mitochondrial proteins, the younger subjects had 141 increases with RT and only 25 with HITT while the older subjects increased 53 mitochondrial proteins with RT and 169 with HIIT. Again very high rates for both types of training for older adults, especially with HIIT.
Study limitations
This was a very well conducted study but like all research, there are limitations. There were a limited number of subjects and after they were broken down into three different training protocols for each age group that resulted in a fairly small number of subjects in each group. The depth of cellular testing that was done was significant and produced meaningful results that do suggest these benefits can be achieved by the broader population but there is always a limit to how far we can extrapolate the results with a small study population. There was also a fairly strict exclusion criterion so the individuals who were ultimately selected were extremely healthy to begin with. That leads to the immediate question of how individuals with an assortment of medical conditions would respond to the same training protocols.
There are also issues regarding the training protocols used. For the resistance training group, they were given what seems to be a very traditional basic plan that isn’t necessarily reflective of the type of training most people are doing in gyms today. No low rep heavy work. No resistance training based intervals or combinations that are designed to raise heart rate and present a metabolic challenge. It would be interesting to see how more modern resistance training techniques fared. The current trend for “metabolic” training and short rest times could produce results more similar to what we consider pure cardio intervals like biking and running.
For the HIIT intervals, what affect do different time intervals have? Thirty seconds versus a minute versus the four minute bouts used in this study. What about different volumes? Surely there has to be differences when those variables are changed. How many intervals do we need to maximize benefits and what is the optimal time of those intervals? Also, what about different types of stimulus, running or rowing as opposed to biking? And what happens at lower levels of intensity? Certainly not everyone can get out and start working at 90% VO2 peak and many people will never be able to train at that level for a variety of reasons.
Obviously no one study can answer everything but this study clearly adds to the body of literature that says HIIT really does something different in the body as compared to other forms of exercise and is worth continuing to ask these questions and investigate these issues.
So there you have it, HIIT increases VO2 peak, insulin sensitivity, mitochondrial respiration, muscle mass and strength in both young and old subjects with really remarkable improvements in older adults. And while resistance training is great, it doesn’t impact the cardiovascular related elements in nearly as meaningful a way. Not only does HIIT have those affects, on the cellular level it greatly increases gene expression and positively impacts regulation of muscle growth, mitochondrial pathways and protein synthesis. Now get out there and start adding some HIIT training to your programs and keep thinking about all those amazing changes that are happening at the cellular level as a result.
Works Cited
Robinson, M., Dasari, S., Konopka, A., Johnson, M., Manjunatha, S., Esponda, R., Carter, R., Lanza, I., Nair, K.S., (2017). Enhanced Protein Translation Underlies Improved Metabolic and Physical Adaptations to Different Exercise Training Modes in Young and Old Humans. Cell Metabolism 25, 581-592.