HIIT: Why High-Intensity Interval Training Packs a Punch

High-Intensity Interval Training (HIIT) has redefined what it means to work out efficiently. It’s fast, adaptable, and evidence-backed. Whether you’re aiming for fat loss, cardiovascular health, metabolic efficiency, or mental resilience, HIIT delivers powerful outcomes; often in less than 30 minutes per session. But how exactly does it work, and why can it outperform traditional exercise?

What Is HIIT?

HIIT involves brief, intense bursts of activity (typically performed at 80–95% of one’s maximal heart rate), interspersed with periods of low-intensity recovery or complete rest. These cycles are repeated for 10 to 30 minutes depending on the workout structure.

Common sessions include:

Tabata: 20 sec max effort, 10 sec rest (8 rounds = 4 minutes)
4x4 Method: Four minutes of work at 90% max HR, 3-minute active recovery (4 sets)
10-20-30: 30 sec low, 20 sec moderate, 10 sec sprint, repeated

This structure taps into both aerobic and anaerobic energy systems, offering metabolic and cardiovascular benefits in a compressed timeframe.

1. Cardiovascular and Respiratory Benefits

HIIT vs. Traditional Cardio

One of HIIT’s most powerful effects is on cardiovascular health. Several randomized controlled trials have demonstrated that HIIT can lead to greater increases in VO₂ max (a measure of aerobic capacity) than moderate-intensity continuous training (MICT).

A 2014 meta-analysis showed that HIIT increased VO₂ max by an average of 6.2 mL/kg/min, compared to 4.9 mL/kg/min for MICT (Weston et al., 2014).
An increase in VO₂ max of just 3.5 mL/kg/min can reduce the risk of cardiovascular disease mortality by 13%(Kodama et al., 2009).

Improves Vascular Function

HIIT enhances endothelial function by increasing nitric oxide bioavailability, improving vasodilation and reducing blood pressure. In hypertensive individuals, HIIT can reduce systolic blood pressure by up to 9 mmHg, comparable to pharmaceutical interventions (Ramos et al., 2015).

2. Fat Loss and Body Composition

HIIT’s reputation for fat loss isn’t a myth. It activates excess post-exercise oxygen consumption (EPOC), meaning your body continues to burn calories post-workout to restore homeostasis.

A systematic review by Wewege et al. (2017) showed that HIIT reduced body fat by 1.6 kg, abdominal fat by 1.5 cm, and waist circumference by 2.5 cm, even when training duration was significantly shorter than steady-state cardio.

Preferential Fat Loss

Not all fat is equal. HIIT is particularly effective at targeting visceral adipose tissue (VAT), the metabolically active fat surrounding internal organs that contributes to insulin resistance and inflammation (Maillard et al., 2018).

3. Metabolic and Mitochondrial Health

Increases Insulin Sensitivity

HIIT enhances glucose uptake by promoting GLUT4 transporter activity in skeletal muscle. Even a two week programme involving just three sessions per week improved insulin sensitivity by 23% in sedentary adults (Little et al., 2011).

Stimulates Mitochondrial Biogenesis

High-intensity efforts trigger PGC-1α expression, which governs mitochondrial growth and energy efficiency. This translates into better muscular endurance and resistance to fatigue, an adaption that is vital for both athletes and individuals with metabolic conditions.

4. Muscle Retention and Strength Gains

Unlike steady-state cardio, which may promote catabolism when performed in excess, HIIT often preserves or increases lean mass, particularly when resistance movements are incorporate, such as squats, lunges or push-ups.

Robinson et al. (2017) found that obese adults who engaged in bodyweight HIIT for 12 weeks maintained or increased muscle mass while significantly reducing fat mass.
Combined with eccentric loading or plyometric moves, HIIT can stimulate myofibrillar hypertrophy, enhancing strength and power.

5. Cognitive and Mental Health Improvements

Mood, Anxiety, and Depression

HIIT stimulates endorphin release, improves neuroplasticity via brain-derived neurotrophic factor (BDNF), and enhances self-efficacy. A 2020 review across 13 studies reported that HIIT reduced symptoms of depression more effectively than moderate-intensity exercise, with effect sizes ranging from 0.35 to 0.70 (Martland et al., 2020).

Cognitive Performance

Regular HIIT has been linked to improvements in executive function, memory, and reaction time, especially in older adults. These changes are due to improved cerebral blood flow and mitochondrial function in brain tissue (Tsukamoto et al., 2016).

6. Time-Efficiency: The Ultimate Advantage

A defining feature of HIIT is how little time it requires to yield results.

15–20 minute sessions, 3–4 times per week, has been shown to:
- Reduce fat mass
- Increase VO₂ max
- Improve insulin sensitivity
- Enhance mood and energy

This makes HIIT especially appealing for:

Professionals with limited time
Travellers with limited equipment
Parents juggling responsibilities
Athletes in need of quick conditioning

Sample HIIT Workouts

Beginner: Low-Impact Total Body (20 min)

30 sec: Air squats
30 sec: March in place
30 sec: Modified push-ups
30 sec: Step-backs
Rest 60 seconds
Repeat 4 rounds

Intermediate: Cardio Burnout (25 min)

40 sec: Jump lunges
20 sec: Rest
40 sec: Push-ups
20 sec: Rest
40 sec: Mountain climbers
20 sec: Rest
40 sec: High knees
20 sec: Rest
Repeat 3–4 rounds

Advanced: Strength and Plyometrics (30 min)

30 sec: Kettlebell swings
30 sec: Box jumps
30 sec: Dumbbell thrusters
30 sec: Rest
30 sec: Burpees
30 sec: Pull-ups
30 sec: Jump squats
30 sec: Rest
Repeat 3–5 rounds

Safety Considerations

While HIIT is adaptable, it places high stress on the cardiovascular and musculoskeletal systems. Those with underlying health conditions, joint issues, or long periods of inactivity should consider:

Starting with lower-intensity intervals
Focusing on form and recovery
Consultation with a healthcare provider before beginning

Final Thoughts

HIIT is a scientifically validated training method that can help you build endurance, lose fat, preserve muscle, boost insulin sensitivity, and even improve your mental health, all in less time than traditional workouts. Whether you’re training for performance, health, or lifestyle efficiency, HIIT earns its place in your routine.

What do we do at Five?

Harrison became an avid user of HIIT when covid struck. With limited gym access and a busy work schedule, it made for a convenient, efficient means of training. Joe Wicks had an array of HIIT workouts to follow which Harrison used and adapted for his own goals. Movements focusing on explosvieness and agility, such as skater jumps and jumping lunges, were especially useful for maintaining match readiness for football. Whilst HIIT is not an as frequent inclusion in Harrison’s training schedule now, it is still utilised in the form of hill sprint sessions, a personal favourite.

References

Gillen, J. B., & Gibala, M. J. (2014). Is high-intensity interval training a time-efficient exercise strategy to improve health and fitness? Applied Physiology, Nutrition, and Metabolism, 39(3), 409–412.
Kodama, S., et al. (2009). Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events: a meta-analysis. JAMA, 301(19), 2024–2035.
Little, J. P., et al. (2011). A practical model of low-volume high-intensity interval training induces mitochondrial biogenesis. Journal of Physiology, 588(6), 1011–1022.
Maillard, F., et al. (2018). High-intensity interval training reduces abdominal fat mass in postmenopausal women with type 2 diabetes. Journal of Clinical Endocrinology & Metabolism, 103(3), 1113–1122.
Martland, R., et al. (2020). Can high-intensity interval training improve mental health outcomes? Psychology of Sport and Exercise, 48, 101811.
Ramos, J. S., et al. (2015). HIIT vs. moderate-intensity continuous training on vascular function. Sports Medicine, 45(5), 679–692.
Robinson, E., et al. (2017). The effects of high-intensity interval training on health outcomes in adults with obesity. Obesity Reviews, 18(6), 635–646.
Tsukamoto, H., et al. (2016). Greater impact of acute high-intensity interval exercise on post-exercise executive function. British Journal of Sports Medicine, 50(7), 552–557.
Wewege, M. A., et al. (2017). The effects of HIIT vs. moderate-intensity training on body composition. Obesity Reviews, 18(6), 635–646.
Weston, K. S., et al. (2014). HIIT in patients with cardiometabolic disease: a systematic review. British Journal of Sports Medicine, 48(16), 1227–1234.