HIIT Workout: The Cardio for Heart Health, VO2Max & Longevity
HIIT alternates high intensity with active recovery, effectively improving cardiovascular fitness, VO2max, and metabolic function while supporting weight loss. This high-intensity interval training creates physiological adaptations that strengthen heart health and longevity, making it a time-efficient approach to improving fitness and exercise capacity.
What is HIIT Cardio?
High Intensity Interval Training (HIIT) is a form of cardio that involves alternating between bursts of high-intensity exercise followed by a phase of active recovery at lower intensity. (1)
The high-intensity part is typically at ≥90% of maximal oxygen uptake or >75% of maximal power. Subjectively, that is an effort of ~ 8.5-10, on a scale from one to ten. The low intensity phase is an effort of ~ 4 out of 10, corresponding to 60-70% HRmax. (2)
HIIT is an essential training component for improving athletic performance as it targets the cardiorespiratory system and aerobic capacity. It is greatly used in both aerobic and anaerobic sports for improving speed, endurance, and stamina. (2) (3)
An example of an HIIT workout would be an interval training on a cycle ergometer comprised of four parts. Beyond the warm up and cool off phase lasting 1 minute, there’s the high-intensity all-out effort for 20 seconds, with 60 seconds active recovery repeated for four sets. This is one of the various HIIT methods, called short sprints.
Is HIIT cardio?
Yes, HIIT is a form of cardio that alternates between short bursts of high-intensity effort and active recovery. It is designed to improve cardiovascular fitness, VO2 max, and metabolic function in a time-efficient manner, utilizing exercises like running, cycling, or rowing performed at near-maximal intensity.
Is HIIT aerobic or anaerobic?
HIIT incorporates both aerobic and anaerobic systems. During intense intervals, the body primarily relies on anaerobic metabolism for quick energy, while recovery periods engage the aerobic system to clear lactate and replenish energy stores.
The Science Behind HIIT
During the intense phase of the HIIT workout, the body heavily relies on anaerobic energy systems, primarily breaking down glycogen for quick energy, increasing lactate levels. The heart works at maximum capacity, pumping blood rapidly to sustain the need for oxygen to working muscles. Breathing rate increases to meet oxygen demand, although oxygen supply lags behind needs.
During the active recovery phase, the body works to get back into balance, shifting to aerobic metabolism while paying off the debt created in the anaerobic (intense) phase. This way the body can clear lactate and replenish energy stores like ATP and creatine. Heart rate and breathing rate decline, allowing recovery.
Such physiological stressor causes numerous adaptations in the body, that can improve exercise capacity. (2) (1) (4) (5)
- increased stroke volume
- increased VO2 max
- enhanced aerobic capacity
- increased mitochondrial density
- enzyme activation (glycolysis)
Such adaptations lead to improvements in function. From a health perspective, this can mean lower blood pressure and decreased strain on the heart. From a performance perspective, it can mean greater output in terms of speed, endurance, and stamina.
What form of HIIT is most effective?
The most effective form of HIIT depends on your goal. For VO2 max improvement, longer intervals (e.g., 3 minutes at 8/10 effort with equal recovery) are effective. Short intervals (e.g., 30s at 9/10 effort with 30s recovery) are great for metabolic conditioning, while 10-second sprints with longer recovery focus on anaerobic capacity.
What is the 80/20 rule for HIIT training?
The 80/20 rule advises that 80% of your training should be low-intensity, steady-state exercise, while only 20% should be high-intensity like HIIT. This balance maximizes gains while minimizing overtraining and injury risk.
Increase VO2 Max Levels
Maximal oxygen uptake (VO2 max) is the measure of aerobic capacity. It is a reliable indicator of physical fitness and cardiorespiratory function, greatly correlated to success in endurance sports. (6)
VO2max is the maximal amount of oxygen the body can uptake, transport, and utilize to produce energy. (7) It is measured during an incremental test, with a mask that shows oxygen intake. As intensity increases, at some point the body achieves peak oxygen uptake. With further increases in demand, the body can’t supply more energy via aerobic metabolism using oxygen, which signifies the VO2max spot.
Elite athletes have greater VO2max values than non-athletes, primarily stemming from higher cardiac output. (4) Higher VO2max is also linked to longevity, as it’s a great measure of the cardiorespiratory system.
The data shows HIIT, or intense exercise to be one of the most effective ways to increase VO2max compared to low-intensity steady state training. (8)
The Benefits of HIIT Cardio
HIIT stimulates specific physiological adaptations, particularly in the cardiorespiratory system. The heart is forced to increase output, and RBC’s to transport it to tissue to sustain higher energy demands. This stressor can positively affect metabolic function, improving glucose uptake in the muscle and contribute to fat loss. Lastly, HIIT is an essential piece to an athlete’s conditioning plan as it effectively improves various aspects of exercise performance.
Heart Health
During high-intensity intervals, the heart works at near-maximal capacity, which is a strong enough stressor to strengthen the heart muscle (myocardium). As a result, stroke volume is increased, meaning the heart can output more blood per beat, lowering resting heart rate associated to better heart health. (4) (9)
HIIT also enhances endothelial function, which improves blood flow and may reduce the risk of atherosclerosis. (10) (11) (12) HIIT can also lower blood pressure and improve markers of cardiovascular health, making it a time-efficient alternative to low-intensity continuous exercise.
The data consistently shows HIIT’s effectiveness in improving lipid profile as well, a major factor contributing to CVD risk. After following high-intensity programs, changes like decreased total cholesterol, LDL, and TG occur. Additionally, HIIT improved the good cholesterol, HDL and HDL to cholesterol ratio. (13) (14) (15)
Metabolic Function
HIIT is a powerful tool for improving metabolic function through its effects on insulin sensitivity, glucose metabolism, and energy regulation.
The intense bursts of activity deplete glycogen stores in the muscle and activate glucose transporters like GLUT-4, which help the muscle absorb glucose more efficiently. Reducing glucose in the blood is impactful in various ways: it lowers insulin levels, improves HbA1C levels, and improves glycemic response, particularly in overweight or diabetic people. (16) (17) (18) Such adaptations can significantly improve metabolic function and glycemic control, leading to stable energy levels.
Beyond, HIIT may stimulate mitochondria biogenesis, assisting in creating more power cells in the body which create energy. Intense exercise has been shown to increase PGC-1α, a key regulator of mitochondria biogenesis in the muscle. (19)
Fat Loss
Fat loss occurs when the body is forced to tap into fats for fuel. This happens during prolonged periods of glucose deficits (keto, fasting, low-carb) or increased energy expenditure, causing a net energy loss.
In terms of practicality, it is much more efficient to manage weight via a light calorie deficit (~300kcal) and increased general movement, whether by neat activity (walking) or exercise.
Most exercise scientists focused on the EPOC effect of HIIT, proclaiming it to be the magic bullet for weight loss. However, the EPOC effect, while it is present, this increase in calorie burning after intense workout isn’t that substantial to significantly impact weight loss long term. EPOC burns 6-15% additional calories after a workout. Added to a 300 calories burned, that’s only 45 calories extra. (20)
The data shows HIIT can reduce total, abdominal, and visceral fat mass. Compared to lower intensities, HIIT was superior in reducing whole body adiposity, rather than visceral or abdominal fat. (21)
Although HIIT may not be superior to lower-intensity cardio, it can be more beneficial in improving lipid profile, glucose control, and cardiorespiratory fitness, which can indirectly impact weight loss positively. (22)
Athletic Performance
For athletes, HIIT cardio enhances performance by improving both aerobic and anaerobic capacities. The repeated cycles of intense effort train the body to tolerate and clear lactate more efficiently, delaying the onset of fatigue.
Enhanced mitochondrial density and enzyme activity allow for greater energy production during high-demand activities, while improved VO2 max translates to better endurance. HIIT also develops fast-twitch muscle fibers, which are crucial for explosive movements like sprinting or jumping.
By integrating HIIT into their training, athletes can achieve greater power output, quicker recovery between efforts, and improved overall stamina, providing a competitive edge in sports requiring speed, agility, and endurance. (23)(24) (3)
How to Get Started: HIIT Exercises
Since HIIT is one of the most effective ways to boost exercise performance, improve heart health, and metabolic function, we’ll showcase a couple of high-intensity interval training strategies. The fundamental is combining a burst of short and intense activity followed by an active recovery period of low intensity one to five x the duration.
Cycling is the most practical way to do intense cardio relatively safely. Metrics of intensity are more important than the activity itself. Depending on fitness level, goals, and personal preferences it can be swimming, cycling, sprinting, running, rowing, or other repetitive cardio exercises.
1. Longer Intervals
Longer intervals focus on sustained effort at the velocity of your VO2 max, building endurance and aerobic capacity. This style is ideal for runners looking to push their limits.
1:1 exercise to recovery ratio
Workout Example: After a 5-minute warm-up, run for 3 minutes at a speed that feels like 8/10 intensity, close to 85-90% HRmax (just below all-out effort). Follow this with 3 minutes of jogging or brisk walking at 50% of that speed. Repeat for a total of 4 sets.
2. Short Intervals
Short intervals use bursts of near-maximal intensity followed by brief recovery, ideal for cyclists or anyone with access to a stationary bike.
1:2 exercise to recovery ratio
Workout Example: Cycle for 30 seconds at 9/10 intensity (near-maximal effort) at 90-95%. Recover with 60 seconds of light pedaling at 40-50% of your top speed, or 65-70% of HRmax. Repeat for 7 rounds, depending on your fitness level.
3. Short Sprints
Short sprints emphasize explosive power and anaerobic capacity, making them perfect for individuals aiming to improve speed and quick recovery.
1:6 exercise to recovery ratio
Workout Example: Warm up for 3 minutes with light jogging or cycling. Perform 8 all-out sprints of 10 seconds each (speed 10/10, 100% HRmax), followed by 1 minute of light movement in zone 2 (at 4/10 speed). This type of HIIT is intense but effective, delivering maximum results in a short amount of time.
Regardless of the method chosen, HIIT cardio isn’t a daily thing. Although it takes less time, it’s quite intense, requiring higher recovery time. Overdoing it daily can disrupt hormones and lead to increased stress and burnout. Intense HIIT is best done once or twice a week, unless it’s a part of a structured conditioning program for athletes.
Is 20 minutes of HIIT a day enough?
20 minutes of HIIT daily is too much cardio for many. HIIT should be short and intense. Such high stressors require longer recovery for adaptation, making intense HIIT more of a weekly, rather than daily routine.
What are HIIT Exercises?
To achieve the metabolic demand of high-interval intensity training, it’s best to do a cardio exercise that’s controlled and repetitive. While weight lifting or functional trainings can be a form of HIIT, real HIIT that’s structured to challenge the cardiorespiratory system in a measurable way is better done via other cardio exercises like cycling, swimming, running, sprinting, jumping rope, or rowing.
What is the best form of HIIT cardio?
The best form of HIIT cardio is cycling. It’s the most measurable, consistent, and practical way to achieve the demands of HIIT. Cycling on a stationary bike is a relatively safe method, reducing impact on the joints.
Disadvantages of HIIT Training
As with any stressor, excessive HIIT training can tip the scale towards suboptimal recovery, fatigue, and burnout. Overtraining happens when the body can’t sufficiently recover from the stressor. Since HIIT requires longer rest intervals, the risk of overdoing it is high.
One of the most debated drawbacks is increasing cortisol levels. When this stress hormone runs over testosterone, the body is in an overtrained state. However, the data shows the opposite. Sensible HIIT training in most cases increases testosterone levels and decreases cortisol, having a positive effect on the body. Longer low-intensity steady state cardio is more suboptimal in that perspective, reducing testosterone levels. (25) (26)(27)
The other potential disadvantages of HIIT cardio include:
Increased Risk of Injury
HIIT’s intense movements can put a strain on the joints, muscles, and tissues, increasing the risk of injuries like sprains or strains. Beginners or those with conditions are especially vulnerable without proper form and recovery.
High Physical Demands
HIIT’s intensity can overwhelm those new to exercise or with low fitness levels. It may also strain the cardiovascular system, posing risks for individuals with heart conditions if not done cautiously.
Potential for Overtraining
Frequent HIIT can lead to fatigue, reduced performance, and overtraining, causing hormonal imbalances and inflammation. Balancing HIIT with rest and low-intensity exercise is vital.
Who should not do HIIT workouts?
HIIT is not recommended for beginners, pregnant women, individuals with joint or cardiovascular conditions, or those recovering from injuries, as the high intensity and impact can exacerbate existing issues. Always consult a healthcare professional if unsure.
What are the long term effects of HIIT training?
Long-term HIIT training enhances VO2 max, mitochondrial density, insulin sensitivity, and cardiovascular health. However, overdoing HIIT can lead to overtraining, burnout, or chronic fatigue if recovery is insufficient.
What are the disadvantages of intensity exercise?
HIIT can increase the risk of injury, particularly for those with poor form or inadequate conditioning. When performed too frequently it may cause excessive fatigue, hypoglycemia, or stress on the cardiovascular system, and hinder recovery.
