The Benefits of HIIT and Other Forms of Interval Training

By Maya Shetty, BS

In an era where time is a precious commodity, individuals are constantly seeking efficient ways to achieve their fitness goals. Amidst this quest, high-intensity interval training (HIIT), which is a type of interval training (IT), has gained popularity among fitness enthusiasts due to its ability to yield significant health benefits in a short amount of time.

“I recommend interval training for people who are busy but still want to get fit, because it gets great results in a fraction of the amount of time compared to more traditional cardio workouts. Interval training can be a powerful tool in a well-rounded fitness plan,” says Anne Friedlander, PhD, exercise physiologist and assistant director of Stanford Lifestyle Medicine.

What is Interval Training?

Interval training (IT) was originally defined in 1973 as “intermittent periods of intense exercise separated by periods of recovery.” HIIT stands for high-intensity interval training, which has become the most popular form of IT in recent years. This back-and-forth between physical effort and rest is effective because it challenges the body without causing total exhaustion. IT can encompass various exercise modalities, including cardio, explosive movements with weights, and bodyweight exercises, as long as the period of intense exercise strenuously stimulates the body.

Despite IT’s recent surge in popularity in the US, European runners in the 1950s commonly used this training method, including long-distance runner Emil Zatopekl from the Czech Republic. Zatopekl’s success in the 1952 Olympics is believed to have ignited the widespread adoption of IT across various sports disciplines.

In 2006, a study conducted by Canadian researcher Martin Gibala sparked enthusiasm for IT to improve fitness, not only among athletes, but for the general population as well. The Gibala paper found that even though study participants trained eight hours less over a two-week period, their IT protocol (four to six, 30-second cycling intervals at supramaximal effort with a four-minute recovery) yielded similar fitness and muscle improvements compared to their moderate intensity continuous training (MICT) protocol (90 to 120 minutes of continuous cycling). Adaptations included better time-trial performance (time to complete a set amount of work on a bike), increased oxidative capacity (ability of muscles to use oxygen), buffering capacity (ability to handle acidity during exercise), and glycogen content (more energy stores in the muscles).

Since this pivotal 2006 study, researchers have studied IT protocols extensively and found, when compared to MICT, participants can elicit the same or greater benefits in mitochondrial density, aerobic capacity, metabolic health, insulin sensitivity, body composition, heart disease progression, and markers of genetic age.

In the last two decades, IT has evolved into various types, such as HIIT and sprint interval training (SIT), which have differences in physiological and psychological responses depending on the duration and intensity of the exercise protocol. Many use the classifications outlined in a 2014 review paper, which defines HIIT as a training protocol that requires “near maximal” effort, eliciting 80 to 100 percent of maximal heart rate (HRmax) or aerobic capacity (VO2max). (A very rough estimate of your max heart rate can be obtained using the formula: 220 – your age.) SIT, on the other hand, is classified as a training protocol that requires “supramaximal” or all-out effort, using at least 100 percent of one’s maximal aerobic capacity.

“No matter what your starting fitness level, adding intensity to your workouts can help you achieve your health and fitness goals,” says Dr. Friedlander, adjunct professor in the Stanford Program in Human Biology. “There are many different protocols out there, but the key element is pushing yourself hard for short bursts of time separated by recovery periods. Those bursts of hard work will jump start beneficial adaptations in your physiology and metabolism.”

What are the Benefits of Interval Training?

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Muscle Health

The 2006 Gibala paper was the first to reveal how SIT has nearly the same effects on skeletal muscle adaptations as MICT despite significantly less training time and workload. Additional research has shown that a single session of HIIT or SIT increases mitochondrial biogenesis (the process through which muscle cells increase the number and functional capacity of mitochondria), and repeated sessions lead to overall increases in mitochondrial density. Mitochondrial biogenesis plays a crucial role in energy production, metabolic regulation, and overall cellular health. It is important to have healthy and functioning mitochondria to maintain muscle health and decrease the risk of sarcopenia (age-related muscle loss).

Research suggests, in the short-term, that the intensity and interval nature of HIIT and SIT increases mitochondrial biogenesis more than MICT, even when performed for less time and at an equal or less total amount of work. Therefore, incorporating interval training into your lifelong exercise routine (such as adding short bouts of running to your daily walks) will help improve your short-term, and possibly long-term, muscle function.

Body Composition

A meta-analysis that included individuals of all ages and health statuses found SIT protocols were more time efficient than MICT and HIIT in decreasing body fat.

Another meta-analysis, looking at studies including only overweight individuals, found HIIT protocols were more time efficient than MICT in reducing whole-body fat mass and waist circumference. Here, the researchers found that running protocols were more effective than cycling protocols for decreasing body fat, potentially because running uses more muscles throughout the body, which leads to greater energy expenditure.

Cardiovascular Health

A wealth of research since 2006 has consistently demonstrated that IT provides superior improvements in markers of cardiovascular health, cardiometabolic health, and mitigates the risk of heart disease progression more efficiently compared to MICT.

Cardiorespiratory health: VO2max (the body’s maximum rate of oxygen consumption) is a common indicator of cardiorespiratory health. Individuals with a higher VO2max tend to have better overall physical fitness, improved lung function, stronger heart muscles, and enhanced oxygen delivery to the body’s tissues.

A meta-analysis determined that both HIIT and SIT protocols are more efficient than MICT regarding the improvement of VO2max (cardiorespiratory fitness). One study found that SIT increased VO2max to the same extent as MICT, but in one-fifth of the time.

The duration of intervals may also have an impact on VO2max. A comprehensive meta-analysis revealed that IT protocols with longer intervals of intensity (three-to-five minutes) yielded greater improvements in VO2max compared to shorter interval protocols performed for the same total amount of time.

Cardiometabolic health: Interval training is also a powerful tool for improving markers of cardiometabolic health, such as insulin sensitivity and insulin resistance.

A meta-analysis found a variety of IT protocols were superior to MICT in improving insulin resistance, lowering blood glucose levels, and decreasing body weight. The analysis examined various IT protocols, including HIIT and SIT, with a range of two-to-sixty intervals, durations from four seconds to five minutes, and both maximal and supramaximal intensities. Some studies were conducted in lab settings with stationary bikes or treadmills, while others were outdoors on tracks or trails.

“Although research has confirmed that IT is a more time-efficient method than MICT for improved muscle, cardiorespiratory, and cardiometabolic health in the short-term, more research is needed to confirm that those superior benefits continue in the long-term,” says Dr. Friedlander. “As with many aspects of life, balance is key.  Therefore, integrating IT into a well-rounded fitness plan is probably healthier and more advantageous in the long-term than doing IT as your only training method.”

Brain Health

Research suggests that regular physical activity can mitigate age-related volume loss in brain regions associated with memory by improving blood flow to the brain and increasing the maintenance and production of neurons.

One study suggests that intense exercise, like HIIT and SIT, can enhance memory in older adults more effectively than MICT protocols. Participants in this study underwent a HIIT protocol, which included four-minute intervals of running on a five percent incline, interspersed with three-minute recovery periods of walking. They performed this protocol three times per week for a total of 12 weeks, resulting in improved memory performance significantly more than MICT or stretching protocols.

Another study found that young adults had more memory improvements following HIIT sessions (two-minute intense intervals interspersed with two-minute recovery periods) than MICT training sessions. The underlying mechanisms behind these improvements in memory are unknown, but the study did find a correlation between memory improvements and the release of blood lactate following exercise.

During exercise, the chemical lactate travels via the bloodstream to the brain and it is hypothesized that lactate promotes the creation of new cells and blood vessels, thus improving brain function. Higher exercise intensity leads to higher lactate production, resulting in increased levels of lactate in the bloodstream.

It has also been hypothesized that exercise induces the release of brain derived neurotrophic factor (BDNF), a protein that plays an essential role in the health and function of our brain cells. BDNF can be thought of as a “brain fertilizer” that helps neurons grow, survive, and communicate with each other.

Research studies have determined the majority of BDNF is produced in the brain, but other parts of the body, such as skeletal muscle and blood vessels, can also produce BDNF when stimulated by exercise. The previous studies in older and younger adults found that both HIIT and MICT protocols increased BDNF in the bloodstream immediately following exercise.

“Whether your goals relate to health, performance, or body composition changes, research has shown time and time again that IT protocols provide a time efficient way to target those goals,” says Dr. Friedlander. “However, you shouldn’t do IT every day as the intense nature of the activity requires sufficient recovery time, so as not to wear yourself down.”

Types of Interval Training Workouts

Since it is not recommended to do IT every day, one can integrate one of the following IT workouts a couple of times into their weekly exercise regimen.

Here are a few IT training methods that can be modified to any fitness level:

Martin Gibala General Recommendations: Gibala, one of the most notable IT researchers, recommends three sessions of HIIT or SIT exercise per week. Intervals should be between one and four minutes and the entire workout, including rest, should be between 20 to 30 minutes.

Fartlek: Fartlek is a running training method that involves alternating between faster-paced running and slower recovery periods, which can be at an easy or moderate pace. Fartlek training is flexible and can be adjusted based on the terrain, fitness level, and desired outcomes.

Tabata: Tabata involves 20 seconds of all-out intense exercise followed by 10 seconds of rest, repeated for a total of eight rounds (four minutes total). Tabata workouts can be customized for various training modalities that include stationary bikes, sprints, or body weight exercises.

4×4 Interval Training: In 4×4 IT, one performs four sets of intense intervals, with each set consisting of a four-minute, high-intensity exercise followed by a three-minute recovery period. This protocol should last a total of 28 minutes with each set lasting seven minutes (four minutes of activity and three minutes of recovery).

10-20-30: 10-20-30 training involves intervals of an aerobic exercise that alternates interval times. The workout consists of repeating cycles of 30 seconds of low-intensity exercise, followed by 20 seconds of moderate-intensity exercise, and finally, 10 seconds of high-intensity exercise. This pattern is repeated for several intervals, typically for a total of four to five minutes. 

“Whatever your fitness level, incorporating safe and balanced interval training into your exercise routine can improve several aspects of your health and fitness.  This is even true as we get older,” says Dr. Friedlander. “As we age, we tend to stay in our comfort zones, but sometimes it is good to push ourselves to keep our maximum capacity higher. This allows all of our other activities of daily life to feel easier. It can help us live life to the fullest into our later years.”

 

“If you’re new to interval training, start by adding a few bursts of running or hill climbing into your daily walk or find the stairs in your building at work and do three rounds of vigorously climbing while slowing descending them,” says Dr. Friedlander. “Regardless of your age or fitness level, we encourage you to embrace the challenge of IT and experience its whole-body health benefits!”

 

By Maya Shetty, BS


Enhancing Health and Speed in Female Distance Runners Through Improved Diet

A groundbreaking nutrition study for female collegiate runners was recently published by Professor  Michael Fredericson, MD, Stanford Lifestyle Medicine (SLM) Director, and Megan Roche, MD, member of the SLM Movement & Exercise pillar. The study’s goals were to improve the health of these runners through a diet intervention, decrease the incidence of bone stress injuries (aka stress fractures), and ultimately improve their performance.

“A runner’s diet is extremely important for maintaining overall health,” says Dr. Fredericson, who has served as the Stanford University track team head physician for decades. “Runners who are excessively lean are prone to injuries, infectious diseases, mental health problems, and loss in bone density.”

The study followed 78 female long-distance runners at both Stanford and UCLA. Over a four year period, the athletes were educated on caloric and balanced nutrient intake, with regular presentations by dieticians and individualized consultations. Athletes with nutrition deficiencies, irregular menstruation, or injuries had more frequent meetings with dieticians. Throughout the study, incidence of bone stress injuries were tracked in the participating athletes.

The nutrition counseling resulted in a remarkable 50 percent reduction in bone stress injuries and doubled the occurrence of regular menstruation among athletes, indicating improved overall health. Even though bone stress injuries affect runners of both genders, there is a higher prevalence among females, which is why the study focused specifically on female distance runners. While the reasons for these findings remain unclear, factors such as male bone density and potential protective effects of testosterone might contribute to the observed gender discrepancy.

In addition to these improvements in health, the nutrition counseling intervention also enhanced athletic performance. Athletes who regained their menstrual cycles and achieved higher bone density also demonstrated enhanced athletic capabilities.

“Your strongest self is your fastest self,” says the second researcher of the study, Dr. Roche, who is an ultrarunner and running coach. “The idea that a leaner body makes for a faster stride is common among distance runners. But it’s inaccurate and sets a dangerous ideal.”

Crucial to the success of the study was shifting the team’s mindset on nutrition and weight. At Stanford University, the coaches and dieticians for the  women’s track and cross-country teams, who were actively involved in the study, played a significant role in fostering a positive perspective on nutrition among the athletes.

Dr. Fredericson and Dr. Roche hope that athletes from every sport, age, and gender can learn from this study. Here is their advice for all athletes:

  1. Eat enough to maintain a healthy weight, which varies from athlete to athlete.
  2. Replenish within 30 minutes after a workout, ideally with carbohydrates and protein.
  3. Consume four to five frequent smaller meals throughout day, rather than three larger meals.
  4. Don’t shy away from healthy, plant-derived fats, like nuts and avocados.
  5. Remember to get enough calcium and Vitamin D for bone strength.

 

By Maya Shetty, BS


Practice of the Month: HIIT Workouts for All Levels

High-Intensity Interval Training (HIIT) is an intense workout that effectively boosts aerobic capacity and overall health in a shorter amount of time than traditional workouts. 

Former pro-athlete, celebrity trainer, and founder of Platinum Fitness, Peter Park, recommends HIIT workouts for his clients at every fitness level. He suggests one-to-two HIIT workouts per week, in addition to three-to-four days of low-intensity cardio exercise, such as walking or biking for 20 minutes to an hour. 

“Cardio workouts build endurance while HIIT builds muscle strength and higher-end aerobic capacity,” says Park, author of Foundation Training: Redefine Your Core, Conquer Back Pain, Move With Confidence and Rebound: Regain Strength, Move Effortlessly, Live Without Limits.

HIIT Routine

Park has 23 years of experience training elite athletes and has designed the following HIIT routine:

Warm up: Perform five reps of the exercises in the graphic at a slow and controlled pace, modifying as needed, and repeat.

Main set: Perform eight reps of the exercises at a faster pace with minimal rest between each exercise, then take a 45-second rest. Repeat with nine reps, then 10 reps for a total of three cycles.

Exercise Details:

  1. Body weight squats: Perform squats to parallel while extending your arms fully as you descend. Ensure you engage your core and maintain proper posture throughout the movement. Aim for a complete range of motion, focusing on squeezing your quads, core, and glutes at the top of each repetition. Maintain a steady tempo while staying in control.
    • Modification: Sit in a chair and stand back up.
  2. Push-ups: Keep your core engaged and maintain proper posture while performing push-ups. Maintain a controlled tempo and form.
    • Modification: Perform the push-ups on your knees, or against a wall or table.
  3. Jumping jacks: Stay smooth and controlled throughout the movement.
    • Modification: Stand with your feet hip-width apart and perform the jumping jack movement with your arms only.
  4. Stand-ups (aka Burpees): Begin by lying on your stomach on the floor with your hands at chest level. Jump your feet forward, contracting your abdominals, and stand up quickly. Return to the lying position and repeat the process.
    • Modification: Press hands into floor and step back into a high plank position, then step forward and return to a standing position.

Tips for HIIT at Every Fitness Level

Beginner: For individuals just starting out, focus on incorporating intense periods of exercise lasting at least one minute into one’s regular training routine. For example, one could speed up sections of a morning walk or finish a workout with a burst of all-out effort. Don’t worry if you become breathless and have trouble speaking – this is how you’re building your aerobic capacity!

Intermediate: Individuals who already participate in regular exercise but are relatively new to HIIT should focus on incorporating two-to-three HIIT workouts per week into their regular exercise regime. Any exercise modality can be turned into an IT protocol as long as one can push themselves into near maximal or maximal exertion. To optimize benefits, research suggests higher intensities and longer intervals are best. 

Advanced: In well-trained individuals, intense exercise is necessary to generate additional improvements in exercise performance and overall health. To achieve these benefits, extremely high-intensity sprint interval training (SIT) or long-interval HIIT at least three times per week may be necessary. 

Important Considerations

Prepare your body before engaging in HIIT: The intense nature of HIIT can be problematic for individuals with weaker musculoskeletal systems or chronic health issues. Additionally, healthy individuals unfamiliar with this workout type also risk injury. So, remember to build up your capacity for this workout by gradually incorporating periods of higher intensity into your regular exercise routine.

“Pushing your body to its physiological limits without proper training can lead to injuries, compromising your overall health and performance,” says Park. 

Don’t perform interval training every day: While it may be enticing to engage in HIIT daily due to its wide-ranging benefits, this workout can strain the body and hinder physical adaptations when performed too frequently. Instead, opt for one-to-two HIIT sessions per week and mix them with low-intensity activities such as walking or yoga for active recovery. 

Have fun: While incorporating HIIT into your exercise regimen, remember to prioritize your safety and comfort, but also do your best to make it enjoyable so that you stick with it. “HIIT workouts are much more fun with a training partner, says Park. “So, grab your spouse, friend, or co-worker and help each other get out of your comfort zone!”

 

By Peter Park, CSCS, BS & Maya Shetty, BS


Welcoming Our New Program Manager, Sharon Brock

Stanford Lifestyle Medicine (SLM) is pleased to welcome Sharon Brock, MEd, MS, as the new program manager and lead writer/editor. With a master’s degree from Columbia University journalism school and 17 years of experience as a science journalist, Brock brings a wealth of editorial knowledge to the position and is committed to the overall goal of growing the program with enhanced content across all platforms. 

“I am excited to be at Stanford Lifestyle Medicine,” says Brock. “I’m incredibly inspired by the researchers of SLM. I believe their work is essential for the future of medicine and I’m honored to be part of this movement.”

In addition to improving content quality, Brock works closely with the team’s research fellows. As a team under Brock’s leadership, they produce social media posts, blogs, and newsletters that translate complex scientific research into language that is understandable and practical for the general public.

“Lifestyle medicine has traditionally been neglected in modern medicine, but with the help of Stanford researchers and this great team of writers, we can make it more mainstream and accessible for everyone,” says Brock.

Brock grew up in Los Angeles and graduated from University of California, Santa Barbara in 1999 with a bachelor’s degree in the biological sciences and a master’s degree in education. After teaching high school biology in the Bay Area for five years, she moved to New York to earn a second master’s degree in journalism from Columbia University.  

For the past 17 years, Brock has been working as a health and medical editorial professional, including a writing position at UCSF and as the editor-in-chief of the USC Keck Medicine magazine KeckMD, which highlights the work of USC physicians and researchers. She hopes to similarly collaborate with Stanford’s team to share their research findings and innovations in the field of Lifestyle Medicine. 

In addition to being an editorial professional, Brock is also a UCLA-certified mindfulness teacher and bestselling author of the book The LOVEE Method: Mindfulness Meditation for Breast Cancer. Following a breast cancer diagnosis in 2018, her training in mindfulness helped her navigate the year-long journey of chemotherapy treatments and surgeries with greater strength. 

When the mindfulness practices not only served her emotionally, but also physically–she had minimal side-effects from chemotherapy–she saw it as her duty to publish a book providing mindfulness tools for other women facing this diagnosis. In 2021, her book became a bestseller and was featured on Good Morning America, and it continues to serve as a mental health resource for women going through breast cancer treatment across the country.

“It was a very challenging time, but I asked myself, ‘How can I turn this pain into purpose? How can I use this experience to serve others?’” says Brock. “Although writing this book was a lot of work, when I receive emails from women I’ve never met saying this book was their greatest source of support while going through cancer, it makes it all worth it.” 

Brock was thrilled to find this position at Stanford Lifestyle Medicine, which is a great fit with her education and editorial experience. Also, as evidenced by her book, Brock is aligned with the mission of the program to educate and empower the general public with actionable lifestyle choices to enhance health and wellbeing. 

“The other day, a researcher on the team spoke about a study where meditation and gratitude practices supported women going through cancer treatment, as shown by blood test results,” says Brock. “As the researcher spoke, I smiled and thought to myself, ‘I’m in the right place.’”

 

By Carly Smith, BS, MPH (c)