Training Efficiency: HIIT Training

By Mark Christiani, O2X Integrated Programs Manager

High-Intensity Interval Training (HIIT) is an effective way to train with limited time. HIIT enhances cardiovascular fitness, increases endurance, and improves metabolic efficiency in a time-efficient manner. Additionally, it supports cognitive function and prepares the body for the high-intensity efforts required in tactical operations. However, HIIT is often misunderstood and confused with high-intensity training (HIT) or CrossFit. While HIIT may be incorporated into these training methodologies, they are not interchangeable.

HIIT vs. Functional Circuit Training at High Intensity

One of the most common misconceptions is equating HIIT with functional circuit training performed at high intensity, such as AMRAPs (As Many Rounds as Possible) or rounds-for-time workouts. While both demand high effort, their structure and physiological effects differ significantly. Intense Circuit Training typically involves performing a series of consecutive exercises with minimal rest, focusing on muscular endurance and metabolic conditioning. The intensity is often sustained at a moderate to high level without designated recovery periods. HIIT Requires short bursts of high effort followed by structured rest intervals (Atakan et al 2021). The key distinction is the deliberate inclusion of rest to allow for repeated high-intensity efforts. 

The Importance of Rest in HIIT

Rest periods in HIIT are not merely breaks. They are essential for maintaining high-performance quality throughout the workout. Proper rest intervals allow for the replenishment of energy and partial lactate clearance, enabling sustained high-intensity efforts across multiple intervals.

• Work-to-Rest Ratios:  Rest ratios vary in the literature but are commonly prescribed at 1:1 to 1:3 with sufficient recovery achieved in the rest time.  
• Rest Benefits: Sufficient recovery prevents excessive fatigue, preserves movement quality, and allows for higher power output across reps.

Heart Rate Ranges and VO2 Max in HIIT

To maximize the benefits of HIIT, it is important to train with appropriate intensity. With access to a heart rate strap judging this intensity can be made easier. HIIT intervals should be performed at 85-95% of maximum heart rate (HRmax), which corresponds to Zone 4 or 5 training. For reference HIIT sessions should target 80-90% of VO2 max, this is much harder to measure without extensive equipment but should give you an idea of 80-90% of your hardest effort. At the appropriate intensity, only a few words should be able to be spoken without pausing for a breath.

Training within these ranges ensures the desired adaptations, such as improved cardiovascular efficiency. Compared to moderate-intensity training, HIIT is particularly effective at improving VO2 max in a shorter time frame. While moderate-intensity exercise requires longer durations to achieve similar aerobic benefits, HIIT’s repeated high-intensity efforts drive rapid adaptations in oxygen utilization and aerobic capacity (Atakan et al 2021). The intensity of HIIT also stimulates greater recruitment of fast-twitch muscle fibers and enhances mitochondrial function, further contributing to VO2 max improvements in about half the total training time.

Lactate Buffering

HIIT demonstrates superior effectiveness in improving the body’s ability to buffer lactate compared to moderate-intensity exercise of a similar duration.  Tactical athletes often operate in environments where sustained high-intensity efforts are required, and HIIT enhances their ability to delay the onset of fatigue by Increasing the efficiency of lactate clearance, by enhancing mitochondrial density and function (Xie et al 2024). Some studies indicate that females may have a higher capacity for lactate clearance, which could require them to perform HIIT workouts with less rest time than males to illicit the same benefits (Schmitz et al 2020). While both moderate-intensity activity and HIIT will improve lactate buffering capacity, HIIT just does it in a more time-efficient manner.    

Brain benefits

Another benefit of HIIT is its ability to increase Brain-Derived Neurotrophic Factor (BDNF), a protein essential for brain function, learning, and neuroplasticity. BDNF plays a crucial role in cognitive resilience and neuroprotection, potentially reducing the risk of neurodegenerative diseases. Studies suggest that implementing HIIT protocols for at least 6-8 weeks, with sessions performed 2-3 times per week, can significantly enhance BDNF levels. This increase in BDNF can enhance memory, focus, and overall cognitive function, making HIIT a valuable tool for any population requiring sharp mental acuity and decision-making under stress (Mielniczek 2025).

Frequency and Duration for Cardiovascular Fitness

To see significant improvements in cardiovascular fitness and performance, HIIT should be performed:

• Frequency: 2-3 times per week, allowing for adequate recovery between sessions.
• Duration: Sessions should last 20-30 minutes, with a total of 6-8 weeks typically required to see measurable improvements in endurance and speed.
• Interval Structure: Optimal HIIT sessions for cardiovascular improvements should include a combination of short and long intervals:
  
  • Short Intervals: 30 seconds of very high effort followed by 30-90 seconds of recovery, repeated for 5-10 intervals
  • Long Intervals: 2-4 minutes at 85-95% HRmax, with equal recovery periods, performed for 4-6 repetitions.

Conclusion

HIIT stands out as one of the most time-efficient training methods for maximizing both physiological and cognitive benefits. By incorporating structured high-intensity intervals with appropriate recovery, tactical athletes can achieve significant improvements in endurance, VO2 max, lactate buffering, and cognitive function in a fraction of the time required by traditional moderate-intensity training.

Additionally, HIIT promotes heart health, enhances metabolic efficiency, and increases BDNF levels, which supports mental resilience under stress. Whether the goal is improving a 2-mile run, increasing overall fitness, or optimizing operational readiness, a well-structured HIIT program offers unparalleled benefits in a highly efficient manner.

References: 

1. Atakan, M. M., Li, Y., Koşar, Ş. N., Turnagöl, H. H., & Yan, X. (2021). Evidence-based effects of high-intensity interval training on exercise capacity and health: A review with historical perspective. International Journal of Environmental Research and Public Health, 18(13), 7201. https://doi.org/10.3390/ijerph18137201
2. Wen, D., Utesch, T., Wu, J., Robertson, S., Liu, J., Hu, G., & Chen, H. (2019). Effects of different protocols of high-intensity interval training for VO2max improvements in adults: A meta-analysis of randomized controlled trials. Journal of Science and Medicine in Sport, 22(8), 941–947. https://doi.org/10.1016/j.jsams.2019.01.013
3. Xie, H., Mao, X., & Wang, Z. (2024). Effect of high-intensity interval training and moderate-intensity continuous training on blood lactate clearance after high-intensity test in adult men. Frontiers in Physiology, 15, 1505723. https://doi.org/10.3389/fphys.2024.1505723
4. Schmitz, B., et al. (2020). Sex differences in high-intensity interval training—Are HIIT protocols interchangeable between females and males? Frontiers in Physiology, 11, Article 38. https://doi.org/10.3389/fphys.2020.00038
5. Mielniczek, M., & Aune, T. K. (2025). The effect of high-intensity interval training (HIIT) on brain-derived neurotrophic factor levels (BDNF): A systematic review. Brain Sciences, 15(1), 34. https://doi.org/10.3390/brainsci15010034

About O2X Integrated Programs Manager Mark Christiani:

Mark Christiani serves as the East Coast Human Performance Integration (HPI) Manager for O2X Human Performance, where he supports and develops On-Site Specialists across multiple locations while helping integrate strength and conditioning, injury prevention, and holistic human performance programs for tactical and industrial organizations.

Prior to this role, Mark worked as an On-Site Human Performance Specialist supporting the United States Army Reserve 81st Readiness Division, where he delivered strength and conditioning support aimed at improving Soldier readiness and reducing injury risk. Before joining O2X, he served as a Brigade Lead Strength and Conditioning Coach at Fort Bragg and held collegiate strength and conditioning positions at Savannah State University and University of South Carolina.

Mark is an Army veteran with over a decade of experience in tactical strength and conditioning. He holds a Master of Science in Sports Medicine from Georgia Southern University and is a Certified Strength and Conditioning Specialist (CSCS) through the National Strength and Conditioning Association.