What Is The Minimum Effective Dose Of HIIT?
In past articles, I’ve covered the minimum effective dose of Zone 2 training and strength training. Today I’ll dive into maybe my favorite type of workout: high-intensity interval training. I love it because it’s intense, it’s short, and, unlike those other types of workouts, I have more say into the type of exercise (sprints, cycling, kettlebells, etc.) and the duration of each set (10 seconds of 4 minutes?). The workouts are that sweet spot between cardio and strength training but intense enough that they’re typically shorter than both.
How short and how often are the two main questions. What I really want to know is what is the minimum effective dose of HIIT? How little do I need to train to get or stay fit?
What Is HIIT?
HIIT (High-Intensity Interval Training) is a form of exercise that involves alternating short bursts of intense physical activity with periods of active recovery or rest. The key characteristic of HIIT is the intensity of the exercise, which pushes you to work at or near your maximum effort during high-intensity intervals.
HIIT workouts include a series of repeated intervals, with each interval lasting anywhere from a few seconds to minutes. During these high-intensity intervals, you perform any physical activity that pushes your body to a point of physiological fatigue. The beauty of HIIT lies in its flexibility and adaptability to different types of exercise, such as running, cycling, bodyweight exercises, and more.
From a physiological standpoint, HIIT engages multiple energy systems within the body. During the high-intensity intervals, the anaerobic energy pathways, such as the phosphagen system and glycolysis, become the primary sources of energy production, leading to the rapid utilization of creatine phosphate and glucose. This results in an elevated oxygen deficit and a subsequent increase in post-exercise oxygen consumption (EPOC), commonly known as the "afterburn effect." EPOC leads to a higher metabolic rate and continued caloric expenditure during the recovery period, contributing to overall energy expenditure and potential fat oxidation.
The Physical Benefits Of HIIT
High-Intensity Interval Training (HIIT) is a single bout of exercise that provides a range of physiological benefits supported by scientific evidence.
The intensity of HIIT triggers significant cardiovascular adaptations, including improvements in maximal oxygen uptake (VO2 max), which reflects the body's ability to utilize oxygen during exercise. This increase in VO2 max is attributed to enhanced cardiac output, improved oxygen delivery to the working muscles, and an augmentation of the oxidative capacity of muscle fibers through mitochondrial biogenesis.
HIIT promotes glycogen depletion, triggering intracellular signaling pathways for glucose transport and insulin sensitivity. This results in improved glycemic control as muscles become more efficient at absorbing and utilizing glucose. Additionally, HIIT induces changes in skeletal muscle composition, favoring oxidative (Type I) muscle fibers. This shift enhances fat oxidation, allowing the body to utilize stored fat as an energy source. As a result, HIIT contributes to improved body composition and increased muscle tone.
HIIT has also been shown to generate post-exercise oxygen consumption (EPOC), often referred to as the "afterburn effect." EPOC encompasses the increased energy expenditure and elevated oxygen consumption that persists after exercise. This phenomenon is attributed to the body's efforts to restore homeostasis, such as replenishing oxygen stores, metabolizing lactate, and repairing damaged muscle tissue. The heightened metabolic rate during EPOC can lead to additional calorie burning, even during periods of rest.
HIIT also impacts various hormonal and molecular mechanisms in the body. HIIT has been found to stimulate the release of growth hormone (GH), which plays a crucial role in muscle development, fat metabolism, and overall body composition. The increased GH secretion observed during and after HIIT sessions can support muscle protein synthesis, enhance lipolysis (the breakdown of fat), and contribute to muscle growth and definition.
Furthermore, HIIT has been shown to activate molecular pathways involved in mitochondrial biogenesis and antioxidant defense systems. These adaptations result in an increase in the number and quality of mitochondria, the energy powerhouses of cells. With more efficient and abundant mitochondria, HIIT promotes greater energy production and utilization, leading to improved endurance and exercise performance.
The Principles Of HIIT
The principles of High-Intensity Interval Training (HIIT) are centered around the concept of alternating periods of high-intensity exercise with periods of active recovery. These principles guide the structure and effectiveness of HIIT workouts. Here are the key principles of HIIT:
Intensity: HIIT involves performing exercises at a high-intensity level, pushing your limits during the work intervals. This intensity should be challenging and elevate your heart rate significantly.
Duration: HIIT workouts are typically shorter in duration compared to traditional steady-state cardio exercises. The total duration of a HIIT session can range from as little as 4 minutes to around 30 minutes, depending on fitness level and workout design. Many people think that longer is better but if you’re doing it right then you shouldn’t be able to sustain a HIIT workout longer than 25 minutes or so.
Interval Ratios: HIIT workouts are structured using specific work-to-rest ratios. Common ratios include 1:1 (equal work and rest intervals), 2:1 (twice as long rest intervals as work intervals), or even higher ratios. The choice of interval ratio depends on factors such as fitness level, goals, and workout intensity.
Exercise Selection: HIIT workouts can incorporate various exercises targeting different muscle groups and energy systems. The exercises can include cardio exercises like sprinting or cycling, bodyweight exercises, or resistance exercises.
The Minimum Effective Dose of HIIT: The Science
The minimum effective dose of HIIT refers to the smallest amount of HIIT training required to elicit significant health and fitness benefits. When it comes to HIIT, there are many ways to skin a cat. And while there are many studies out there on interval training there aren’t many that focus on the minimum effective dose of interval training. After combing the literature, I pinpointed these as the ones that provide the greatest insights into the least amount of HIIT you need to reap the health and fitness benefits.
A 2014 study took semi-pro soccer players and broke them up into two training groups in the offseason. Each group did a HIIT session consisting of 5 sets of 4 minutes of work at 87-97% max heart rate but one group did it weekly and one did it every other week for 6 weeks. The results found that while the every-other-week group had a slight reduction in the 20-meter shuttle run there was no difference in VO2 max between the two groups.
Frequency: Every other week
Intervals: 5 sets of 4 minutes
Rest: Unknown (likely at least 3-4 minutes)
Research published in Medicine & Science in Sports & Exercise found that 120 seconds of work might be all it takes (not including rest time). Over the course of eight weeks, participants engaged in a training program three times per week. The program involved performing 30 sets of 4-second sprints at maximum effort. Each training session lasted a total of two minutes. Throughout the program, the cardiovascular demands were gradually intensified by reducing the recovery time between sprints. Initially, participants had 24-30 seconds of rest between sprints, but this was progressively decreased to 15 seconds. As a result, the total duration of each training session decreased from 17 minutes to less than 10 minutes. The participants showed a 13.2% VO2peak increase, a 7.6% total blood volume boost, and a 17.2% maximal anaerobic power improvement.
Frequency: 3 times a week
Intervals: 30 sets of 4-second sprints
Rest: 24-30 seconds initially, decreasing to 15 seconds by the end of the study
A review published in The Journal of Physiological Sciences found that Tabata training may be the way to go for efficient training. Their research has shown that VO2 max is increased by as much as 15% and maximal accumulated oxygen deficit (MAOD) by as much as 35% in a 6-12 week Tabata training program. Furthermore, as little as 2 days a week of Tabata has been shown to improve VO2 max. They concluded that “these improvements of both the aerobic and anaerobic energy-releasing systems after Tabata training are comparable to those provided by conventional aerobic and anaerobic training, including other types of HIIT.”
Frequency: 2 sessions a week
Intervals: 8 sets of 20 seconds
Rest: 10 seconds between sets
The Minimum Effective Dose Of HIIT
So where does all of that research leave us? Science has shown us that a variety of high-intensity/low-intensity ratios with a variety of rest can work. So, the minimum effective dose of HIIT comes down to what you’re looking for.
If you want to have the shortest possible workout….
Do 8x20 seconds of high intensity with 10-second of rest twice a week (Tabata)
If you want to do less frequent workouts….
Do 5x4 minutes of high-intensity work with 4 minutes of rest between sets every other week
If you want the least amount of high-intensity effort….
Do 30x4 seconds of high intensity with 15-30 seconds of rest between sets 3 times a week
These workouts are based on what is currently available in the literature today. While there aren’t many studies focused on identifying the minimum effective dose (MED) of high-intensity interval training (HIIT), I suspect that doing any truly high-intensity workout every other week will grant you the minimum amount of health and fitness benefits.
Important points:
When you are in your high-intensity phase it should be at or near “all out.”
When you are in your low-intensity or rest phase it should be minimal or no work.
Mix it up. Avoiding doing the same workout each time. Instead, vary the length of the “on” and “off” time of the intervals to make sure you’re stretching the benefits of the workout.
Most studies on HIIT use a stationary bike as the exercise because it’s an easy way to track heart rate and keep the intervals accurate but you can replace this with any type of exercise including running, swimming, bodyweight work, kettlebell exercises, etc.
The research has shown time and time again that HIIT is an incredibly powerful tool to improve our health so as long you follow these basic guidelines and do a HIIT workout with some regularity then you should see improved health.
What I Do
I love intervals. They’re difficult, they’re short and efficient, and I always feel great (read: exhausted) afterward. I am to get at least one HIIT workout a week but I usually hit 2-3. Here are my go-to’s for running (occasionally cycling):
A Tabata session on the stationary bike (good time-crunch option).
Hill sprints. It depends on the hill length but usually 6-12 sets of 20-60 seconds with a slow walk back down as the rest.
8x(1 minute high intensity/1 minute low intensity)
8x(2 minutes high intensity/2 minutes low intensity)
4x(4 minutes high intensity/4 minutes low intensity)
13x(30 seconds high intensity/15 seconds low intensity)
Here are some of my go-to’s for HIIT sessions where I typically use a jump rope, kettlebell (swings, hip hinges, squats, overhead press, rows), resistance bands (rows, shoulder pulls, lat pulldowns), and body weight work (push-ups, plyo jumps):
20-30x(30 seconds high intensity/20 seconds low intensity)
20-30x(45 seconds high intensity/15 seconds low intensity)
Occasionally I’ll also incorporate a HIIT workout into my morning or nighttime writing. As I’m typing away, I am sitting on a FlexiSpot Desk Bike. I jack the resistance up to 8, set an interval timer for 30 seconds every 3 minutes and then I just pause my writing for that time and go.
Takeaway
The minimum effective dose of High-Intensity Interval Training (HIIT) showcases the incredible potential of short, intense workouts to yield significant health and fitness benefits. Research studies have revealed that as little as two minutes of HIIT per session, spread across a few times a week or a longer bout of 5x4 minutes every other week can result in remarkable improvements in cardiovascular fitness, insulin sensitivity, muscle power, and endurance capacity.
What’s great about HIIT is that you can tailor the workouts to your preference (shorter sets or longer sets, shorter rest periods or longer ones) while using whatever exercise modality you like best (sprinting, cycling, bands, kettlebells) while still reaping all of the physical benefits. These personalized workouts are time-efficient and adaptable way to achieve your fitness goals.
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