Dr. Martin Gibala is a muscle physiologist and professor at McMaster University in Hamilton, Ontario, Canada. He is best known for pioneering research on the health benefits of high-intensity interval training (HIIT) and his profound understanding of HIIT's physiological mechanisms. He is a co-author of the book "The One-Minute Workout."

In this episode, Dr. Gibala and I discuss:

• 00:03:22
  What is high-intensity training?
• 00:04:15
  Zone 2 vs. HIIT for VO2 max — which is better?
• 00:05:44
  The vital role of vigorous exercise
• 00:07:02
  Why VO2 max matters for longevity
• 00:10:07
  Why athletes vs. exercisers benefit from different intensity distributions
• 00:14:31
  Measuring maximum heart rate and VO2 max
• 00:22:53
  How the heart adapts to HIIT to increase VO2 max
• 00:28:09
  Why vigorous exercise accelerates mitochondrial adaptation
• 00:32:28
  Enhancing fat oxidation and mitochondrial growth with vigorous exercise
• 00:36:44
  How intensive exercise boosts fat breakdown
• 00:38:18
  Is high-intensity exercise better for autophagy than fasting?
• 00:47:36
  Exercise snacks
• 00:50:15
  Why 'choosing the stairs' reduces early death (VILPA study)
• 00:53:01
  Protocol for VO2 max (see also 1:33:58)
• 00:58:12
  The effect of HIIT on muscle fiber types
• 01:02:40
  How aging effects muscle fibers
• 01:06:31
  Does high-intensity training produce an "afterburn effect"?
• 01:08:35
  Why vigorous workouts are better for BDNF and cognition
• 01:15:37
  Anti-metastatic cancer effects
• 01:42:45
  Wingate training vs. reHIIT — a comparison of protocols
• 01:48:00
  Perceived exertion vs. HRmax
• 01:51:45
  Interval walking for people with type 2 diabetes
• 01:53:28
  Contraindications of HIIT
• 01:57:28
  Why preconditioning reduces risks from exercise
• 02:03:06
  Can resistance training be a type of aerobic exercise?
• 02:08:46
  Does cardio and strength training interfere with each other?
• 02:11:07
  How many minutes per week of high-intensity training?
• 02:19:20
  Are there sex differences and misconceptions in high-intensity training, for women?
• 02:20:04
  Should post-menopausal women do H.I.I.T.?
• 02:20:09
  Does intense exercise raise cortisol?
• 02:26:47
  Bone density and osteoarthritis
• 02:29:02
  Atrial fibrillation risk
• 02:31:35
  Hypoxic training and blood flow restriction
• 02:33:07
  Tips for training with joint issues

HIIT: A paradigm of efficiency to improve VO2 max

"VO2 max is the best objective measure of cardiorespiratory fitness and why fitness is essential… It protects you. It reduces your risk of dying and developing diseases. The best way to measure that is through a VO2 max test." Click To Tweet

High-intensity interval training (better known as HIIT) is an exercise protocol that revolves around alternating bursts of vigorous exercise and brief recovery periods. Pioneering studies, including those by Dr. Martin Gibala, underscore the transformative power of HIIT, revealing that its beneficial effects can rival, and at times even surpass, those derived from prolonged moderate-intensity endurance activities.

HIIT capitalizes on the principle of time efficiency, allowing you to achieve substantial fitness gains in considerably less time than traditional continuous exercise routines. Integrating HIIT into your workout routine optimizes time while triggering profound metabolic improvements that enhance your mitochondrial capacity and boost cardiovascular and cognitive health.

Driving these effects are what Dr. Gibala describes as the "intensity-duration trade-off" – a concept that reflects the non-linear relationship between exercise intensity (how hard you exercise) and duration (how long you exercise). As exercise intensity increases, the required duration can decrease without compromising cardiorespiratory benefits. In other words, you can achieve considerable fitness and health improvements in less time by engaging in high-intensity workouts compared to longer, lower-intensity workouts.

That's important because even a committed exerciser may run the risk of non-response – if you're not doing high-intensity training. That's because a surprising 40 percent of people might not see improvements in their VO2 max – even if they're meeting the guidelines for moderate-intensity exercise, roughly 2.5 hours per week.

Elevating VO2 max is a hallmark of HIIT protocols.

VO2 max is a complex, multifaceted parameter that reflects the intricate coordination between the cardiovascular and musculoskeletal systems during exercise, based on various factors ranging from cardiac output (which can differ considerably between trained athletes and weekend runners) to muscle oxygen extraction. Put simply, it's the maximum oxygen uptake by the body and a definitive marker of cardiorespiratory fitness.

A person can approximate their VO2 max through equations and online calculators, but the most precise determination requires controlled laboratory testing involving incremental exercise protocols. However, accessible tools like the World Fitness Level calculator track changes in VO2 max over time, enabling anyone to monitor their fitness journey. And fitness trackers like the Apple Watch also offer tools to measure aerobic capacity by estimating VO2 max.

Vigorous exercise improves cardiorespiratory fitness and reduces premature death.

Some evidence suggests there is no upper benefit to cardiorespiratory fitness, with the most elite athlete having a nearly 80 percent lower risk of premature death compared to a less fit person of the same age. Given that up to 40 percent of people might not experience VO2 max improvements when adhering to standard exercise guidelines focusing on low-to-moderate intensities begs the question: Should we pivot toward the explosive benefits of HIIT, harnessing the power of vigorous intensities to unlock a new world of physiological adaptation?

Fat oxidation isn't exclusive to zone 2 training.

"The most crucial adaptation in muscle is increasing mitochondrial content, which will allow a greater fat and carbohydrate oxidation capacity." Click To Tweet

Central to the physiological benefits observed with HIIT is its profound effects on the mitochondria – the cellular powerhouses that drive energy production. HIIT promotes mitochondrial biogenesis, contributing to the body's increased capacity for fat oxidation during exercise. A critical player in this process is carnitine palmitoyl transferase (CPT), an enzyme responsible for transporting fatty acids into the mitochondria for oxidation.

Dr. Gibala explains how exercise-induced adaptations, both through training and certain supplements like carnitine, enhance CPT levels, facilitating more efficient fat utilization and the preservation of carbohydrate stores. Exercise also promotes mitophagy (the turnover of dysfunctional mitochondria, fostering cellular resilience) and autophagy (the removal of cellular debris and damaged components).

Learn more about mitophagy in this clip featuring Dr. Guido Kroemer.

HIIT exerts multifaceted effects on human physiology.

"Exercise is a stress. It temporarily disrupts or damages things. And then its all about the recovery that makes the cellular processes better. We continually do that, and over time, things get better." Click To Tweet

But HIIT's effects extend beyond mitochondrial dynamics, influencing multiple aspects of human physiology. For example, HIIT promotes the release of catecholamines, a group of signaling hormones released from the kidneys or brain during stress or physical activity to increase heart rate and prepare the body for action, facilitating the "fight or flight" response. Catecholamines serve dual roles as neurotransmitters and hormones and preserve homeostasis via the autonomic nervous system. They also signal adipose tissue to break down triglycerides and release them into the bloodstream, subsequently facilitating the uptake and oxidation of fatty acids within the muscle tissue.

Vigorous exercise promotes the production of lactate, bolstering cognitive function.

Beyond its physical implications, HIIT has demonstrated potential cognitive benefits. Lactate, often misconstrued as a cause of muscle fatigue, is a significant player in these cognitive effects. Lactate is a weak base produced via the breakdown of glucose during exercise.

In zone 2 training, there's a deliberate effort to cap lactate accumulation. However, high-intensity workouts often produce more lactate than our muscles can handle. This surplus lactate can be "shuttled" to various tissues, including the muscles, heart, and brain, which use it as an energy source. Evidence suggests that lactate is the preferred fuel of the brain – pointing to its importance in maintaining cognitive function. Interestingly, HIIT-induced lactate promotes the release of brain-derived neurotrophic factor, a protein associated with cognitive function, pointing to the intricate interplay between exercise (particularly vigorous exercise), lactate metabolism, and cognitive health.

• Learn more about the health benefits associated with BDNF in our comprehensive overview article.

Higher exercise intensities may thwart cancer metastasis.

Exercise intensity also affects blood flow, creating shear force and playing a critical role in metastatic cancer resistance. One quality of cancer cells is that they sometimes have vulnerabilities that healthy cells do not, such as susceptibility to mechanical stress. In other words, when we exercise, we introduce cancer cells to a new type of stress. The turbulence created in our bloodstream can reduce the malignant cells' viability as they attempt to metastasize. Exercise may reduce the risk of cancer metastasis, and vigorousness may play a role, amplifying the effect.

Defining HIIT

"When we say high-intensity [effort], were talking [about] a relative heart rate of about 80 percent of your maximum. […] Youre working pretty hard, but its not an all-out sprint." Click To Tweet

An alphabet soup of HIIT protocols exists – HIIT, SIT, and REHIT, among them – with each form offering distinct advantages and outcomes. A common feature of all HIIT protocols is achieving an elevated heart rate of roughly 60 to 85 percent of one's maximum heart rate. You can determine your maximum heart rate using the Fox equation: 220 – your age in years.

HIIT involves intense but submaximal efforts, challenging a person to perform at an intensity below their maximum capacity. In contrast, SIT focuses on sprint-like training, replicating the intensity of sprinting from a perceived danger. REHIT takes this further, employing high power outputs surpassing VO2 max pace but for shorter durations, typically 10 to 20 seconds.

HIIT doesn't always translate to an all-out pace, however. In fact, the concept of resistance-style interval training, often referred to as functional training, underscores HIIT's versatility. Body-weight exercises like air squats, burpees, and push-ups performed in an interval manner can efficiently blend strength and aerobic conditioning. Although not a substitute for heavy weight lifting or endurance training, these body-weight exercises offer a time-efficient route to marked gains in both realms.

Finding a HIIT protocol that works for you

Dr. Gibala describes some of his favorite HIIT routines: "VO2 Max training interval," three five-minute intervals at maximum effort, including warmup, cool-down, and recovery in a 20-minute session. The minute-on/minute-off approach, repeated five or ten times, plus warmup and cool down. The "One-Minute Workout," featuring three sets of 20 seconds of intense exercise and two minutes of recovery, totaling 10 minutes – three times a week for optimal results. Tabata, Dr. Rhonda Patrick's favorite HIIT routine, consisting of a four-minute cycle with 20 seconds of maximum effort followed by 10 seconds of rest, repeated eight times. Dr. Patrick often opts for a 10-minute workout – doing two Tabatas back-to-back – a format that's also popular in Peloton programming.

Advice for novices

"So, if youre just starting out getting into exercise, maybe you just want to start out by walking fast for a few light posts and then backing it off. Thats a form of interval training. Dont worry at all about whether youre at 80 percent." Click To Tweet

Novices might find starting an interval training regimen intimidating. However, Dr. Gibala explains that beginning with an alternating pattern can help, gradually progressing based on individual fitness levels and goals. Whether you're an athlete, relatively sedentary, or striving for time-efficient VO2 max optimization, interval training offers adaptable strategies to suit diverse objectives.

Incorporating HIIT into your workout routine

"People who engaged in even three to four minutes… of VILPA-like activity [daily] had substantial reductions in all-cause mortality risk, were talking 25, 30 percent. So that would suggest that even brief, non-exercise, vigorous, intermittent physical activity can move the needle in terms of health outcomes." Click To Tweet

One strategy for incorporating vigorous exercise into your day involves "exercise snacks." These short bursts of vigorous-intensity exercise, lasting up to a minute and strategically dispersed throughout the day, include hopping on a stationary bike, performing a series of air squats, or engaging in bodyweight-style exercises at least four or five times daily.^[1]

Another strategy Dr. Gibala describes for engaging in more vigorous activity is VILPA, short for vigorous intermittent lifestyle physical activity. VILPA is non-structured exercise that involves short, vigorous bursts of activity that capitalize on ordinary, routine activities. Climbing uphill, briskly walking upstairs, and even vigorous housework all count toward your VILPA. Engaging in three to six minutes of VILPA-like activity daily may reduce the risk of early death by nearly half.

Charting the trajectory of HIIT-related research

As research in the field of exercise physiology advances, the future of HIIT appears promising. Dr. Gibala and his colleagues advocate for rigorous trial designs and more extensive studies to provide robust conclusions about HIIT's effectiveness and the mechanisms that drive them. Additionally, ongoing investigations aim to decipher individual responses to HIIT, considering factors such as sex, age, and pre-existing medical conditions, acknowledging the nuanced nature of physiological adaptations, and taking steps toward personalized exercise recommendations.

Scientists mentioned

• Ulrik Wisløff – WorldFitnessLevel.org calculator and pioneering work on high-intensity interval training and cardiovascular disease.
• Justin Brown, Ph.D. – Researches how exercise-induced increases in vascular shear stress reduce the risk of cancer recurrence and metastasis.
• Richard S. Metcalfe, Ph.D. and Niels B. J. Vollaard, Ph.D. – Collaborated on the reduced-exertion HIT (REHIT) protocol to promote physical activity and fitness in sedentary people.
• Edward F. Coyle, Ph.D. – Researches how acute, high-intensity exercise counteracts the negative cardiometabolic consequences of daily sedentary behavior.
• Paul Thompson, M.D. – A pioneer within the field of sports cardiology.
• Stuart Phillips, Ph.D. – Researches how nutrition and exercise influence human skeletal muscle protein turnover. Created the online free course "Hacking Exercise for Health." See FoundMyFitness episode with Dr. Phillips.
• I-Min Lee, Sc.D. – Serves on national and international expert panels involved in developing physical activity guidelines. Dr. Lee is a Professor of Medicine at Harvard Medical School and a Professor of Epidemiology at the Harvard T.H. Chan School of Public Health, USA.
• Jonathan P. Little, Ph.D. – Researches how type 2 diabetes-related metabolic issues affect cellular inflammation and how exercise and nutrition can mitigate inflammation for better cardiometabolic health.
• Emmanuel Stamatakis, Ph.D. – Researches the health-enhancing potential of vigorous intermittent lifestyle physical activity (VILPA).
• Bent Rønnestad, Ph.D. – Researches the effects of interspersing repeated sprints between bouts of endurance training.
• Carsten Lundby, Ph.D. – Researches the effects of HIIT on endurance capacity in elite cyclists.
• George Brooks, Ph.D. – Coined the term "lactate shuttle" to describe the metabolic pathway by which lactate is produced in one tissue and transported to another, serving as an energy source.

Notable and relevant publications:

• Twelve Weeks of Sprint Interval Training Improves Indices of Cardiometabolic Health Similar to Traditional Endurance Training Despite a Five-Fold Lower Exercise Volume and Time Commitment
• Short-term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance
• Physiological adaptations to low‐volume, high‐intensity interval training in health and disease
• Association of wearable device-measured vigorous intermittent lifestyle physical activity with mortality (VILPA)

Related FoundMyFitness resources

Articles:

• FoundMyFitness Topic - Aerobic exercise – A brief overview of aerobic exercise's health effects.
• FoundMyFitness Topic - Brain-derived neurotrophic factor (BDNF) – Vigorous intensity exercise robustly increases BDNF concentrations in the body, providing a possible link between exercise and improved mental and cognitive health.
• FoundMyFitness Topic - Autophagy – High-intensity exercise induces autophagy in multiple organs involved in cardiovascular and metabolic regulation, including the heart, muscle, liver, and adipose tissue.

Episodes & Clips:

• Stuart Phillips, PhD, on Building Muscle with Resistance Exercise and Reassessing Protein Intake
• Chris McGlory, PhD, on the Anabolic Potential of Omega-3 Fatty Acids
• Brad Schoenfeld, PhD: Resistance Training for Time Efficiency, Body Composition & Maximum Hypertrophy
• Dr. George Brooks on Lactate Shuttle Theory, Relevance for Traumatic Brain Injury, & More
• Exercise causes aging molecules to shift | Dr. Michael Snyder
• Mitophagy: A signal for mitochondrial biogenesis, metabolic adaptation, and cellular differentiation

Other related links

• World Fitness Calculator – Provides an estimate of your VO2 max.