HIIT Protocols Boost VO2max in Four Weeks

Five HIIT Protocols Increased VO2max in Four Weeks

After four weeks of training, amateur soccer players across five different high-intensity interval training groups all improved their maximal oxygen uptake. The study from Ankara University and nine other institutions tested small-sided games, long intervals, short intervals, and sprint intervals. Every protocol worked. This finding from a 2026 paper in PeerJ confirms that HIIT is a reliable method for raising VO2max, a primary marker of cardiorespiratory fitness and health.

All HIIT Forms Boosted Aerobic Capacity

Researchers at Ankara University assigned 40 players to one of five HIIT groups. Each group trained for four weeks with equal total work volume. The groups were Small Sided Games Long Interval (SSG(LI)), Long Interval (LI), Small Sided Games Short Interval (SSG(SI)), Short Interval (SI), and Sprint Interval Training (SIT). The pre- and post-training assessments showed significant VO2max gains in all five. The work, led by Hasan Karabıyık, demonstrates that the specific HIIT format may be less important than the consistent application of high-intensity work periods.

VO2max: The Definitive Metric of Aerobic Fitness

Maximal oxygen uptake, or VO2max, measures the highest rate at which your body can consume oxygen during intense exercise. It is expressed in milliliters of oxygen per kilogram of body weight per minute (ml/kg/min). VO2max sets the upper limit for your aerobic energy system. A higher score means your heart can pump more blood, your muscles can extract more oxygen, and your mitochondria can produce more ATP. This capacity directly influences endurance performance and is a strong predictor of long-term health outcomes.

Why VO2max Matters for Health and Performance

VO2max is not just for athletes. It is a vital sign of metabolic and cardiovascular health. A 2026 study protocol for the BOOST-HEALTH Trial, published in PLoS One, explicitly links higher cardiorespiratory fitness to better management of type 2 diabetes. The research team, including A.M. Thomson and J.P. Little, notes that VO2max is a modifiable risk factor. Improving it can lower the risk of cardiovascular disease, improve insulin sensitivity, and enhance overall functional capacity. This makes it a primary target for exercise programs.

HIIT Protocols That Work: From Long Intervals to Sprints

The Ankara University study provides a clear menu of effective HIIT options. Each protocol was designed to match total work, allowing a direct comparison of their effects. The results show that the specific HIIT format is less important than the consistent application of high-intensity work periods.

Long Interval Training (LI)

Long intervals typically involve work periods of 2 to 4 minutes at an intensity close to your VO2max pace, followed by equal or slightly shorter recovery periods. In the study, this protocol produced clear improvements in aerobic capacity. This method stresses the cardiovascular system for sustained periods, forcing adaptations in cardiac output and peripheral oxygen extraction.

Short Interval Training (SI)

Short intervals use bursts of 30 seconds to 1 minute at very high intensity, often with longer or active recovery periods. The research found this approach equally effective for raising VO2max. The repeated, near-maximal efforts challenge both aerobic and anaerobic systems, leading to rapid physiological changes.

Sprint Interval Training (SIT)

SIT pushes intensity to its absolute limit. It uses all-out sprints of 20 to 30 seconds, separated by long recovery periods of 3 to 4 minutes. Even this extreme protocol, with its very short total time at high intensity, improved VO2max. This suggests that the stimulus of maximal effort, even briefly, can trigger aerobic adaptations.

Game-Based HIIT: Small Sided Games (SSG)

This study introduced a sport-specific twist. The SSG(LI) and SSG(SI) groups performed their intervals within a soccer-specific game context, rather than just running. They achieved the same VO2max gains as the running-based groups. This is important for athletes who need to maintain sport-specific skills while improving fitness, and it may improve adherence by making training more engaging.

Four Weeks of HIIT Improved Agility and Jumping Power

Beyond VO2max, the training produced specific physical gains. Reactive agility performance improved in four of the five groups: SSG(LI), SSG(SI), SI, and SIT. Vertical jump height increased significantly for both the right and left legs across participants. These changes occurred without significant improvements in sprint times or anaerobic power, suggesting the adaptations were specific to the training stimulus.

Heart rate variability (HRV) parameters also shifted. The low frequency (LF) component and the LF/HF ratio changed from pre- to post-training for the group as a whole. This indicates an effect on autonomic nervous system regulation, a marker of cardiovascular health and training adaptation.

How to Structure a HIIT Protocol for VO2max Gains

Based on the study’s design, an effective four-week program requires specific parameters. The researchers used a frequency of three sessions per week. Total session volume was matched across protocols, meaning the amount of high-intensity work performed was similar. This approach isolates the effect of interval structure.

Volume and Intensity Are Key

All five protocols used a work-to-rest ratio of 1:1, except for SIT which used a 1:8 ratio. The intensity for the LI and SI groups was set at 90-95% of maximum heart rate. SIT required all-out maximal sprints. The SSG groups maintained the same interval timing but within a game. This shows that a high relative intensity, applied consistently, is a common driver of improvement.

Progression Over Four Weeks

The four-week intervention followed a logical progression. Week 1 served as an adaptation phase. Weeks 2 and 3 increased the total number of intervals or the duration of work periods. Week 4 maintained the highest load. This structure allows the body to adjust before being challenged further.

Game-Based HIIT Matches Traditional Running for VO2max

Small sided games produced identical VO2max improvements to structured running intervals. This result from the Ankara University team has practical implications. For team sport athletes, integrating fitness work into skill-based drills can be efficient. For recreational exercisers, sport-based interval training offers a viable alternative to monotonous treadmill sessions.

The study’s design matched the total work volume between SSG and running groups. This means the physiological stress, and therefore the adaptation, was comparable. The game-based approach also improved agility, a benefit not measured in the pure running groups.

Heart Rate Variability Changes After HIIT

Autonomic nervous system balance, measured by HRV, responded to the training. The analysis revealed a significant main effect of Time for the LF component and the LF/HF ratio. This suggests a shift in the balance between sympathetic and parasympathetic nervous system activity following the four-week program.

These HRV changes are consistent with cardiovascular adaptations to exercise. They reflect the heart’s improved ability to respond to stress and recover efficiently.

Limitations of the Evidence

The Ankara University researchers note their study’s quasi-experimental design. This means participants were not randomly assigned to groups from a single pool, but rather existing teams were used. The authors state this limits the ability to definitively establish causation. The study also focused on amateur soccer players, a specific population. Results may not translate directly to sedentary individuals or elite athletes.

Another limitation is the short duration. Four weeks is enough to see initial adaptations, but longer-term effects on VO2max and health require further study.

Key Takeaways

• Five distinct HIIT protocols, including long intervals, short intervals, sprint intervals, and game-based versions, all significantly increased VO2max in amateur soccer players after four weeks of training.
• Small sided games, a sport-specific HIIT format, produced the same VO2max gains as traditional running intervals, while also improving reactive agility.
• All protocols used matched total work volume, indicating that consistent high-intensity effort, not the specific activity, drives aerobic improvement.
• Vertical jump performance improved for both legs, but sprint times and anaerobic power did not change significantly in this short-term study.
• Heart rate variability parameters shifted, suggesting HIIT influences autonomic nervous system regulation.
• Training frequency was three sessions per week, with intensity ranging from 90-95% of max heart rate to all-out sprints.
• Game-based HIIT offers a practical alternative for athletes, potentially increasing adherence by integrating skill work.

This article is for informational purposes only. Consult a qualified professional for personalised advice.

Sources:
https://pubmed.ncbi.nlm.nih