Combining HIIT and Steady State for VO2 Max Gains

Combining high-intensity interval training (HIIT) with low-intensity steady-state endurance exercise produces robust improvements in maximal oxygen uptake (${\text{VO}}_2\text{ max}$), but the ideal combination depends heavily on baseline training status [1, 14]. For untrained to recreationally active individuals, almost any combination of continuous aerobic exercise and interval training increases ${\text{VO}}_2\text{ max}$ similarly, with intensity offering a time-efficient stimulus [11, 14]. For well-trained and elite endurance athletes, structured combinations that assign roughly 75% to 80% of training time to low-intensity exercise and 15% to 20% to high-intensity intervals—known as polarized training—elicit greater improvements in ${\text{VO}}_2\text{ max}$ than threshold-heavy models [1, 2, 3]. However, this polarized advantage in trained cohorts is primarily evident in short-term blocks of under 12 weeks and does not consistently translate into superior improvements in time-trial performance or lactate threshold power [1].

Training Intensity Zones and Distribution Models

To compare exercise combinations systematically, sports scientists use a three-zone intensity framework defined by physiological markers [8]:

• Zone 1 (Low Intensity): Below the first ventilatory or lactate threshold (${\text{VT}}_1/{\text{LT}}_1$), where ${\text{LT}}_1$ typically occurs around 52% of maximal aerobic power or 65% to 70% of functional threshold power (FTP) [8].
• Zone 2 (Moderate/Threshold Intensity): Between ${\text{LT}}_1/{\text{VT}}_1$ and the second threshold (${\text{LT}}_2/{\text{VT}}_2$ or maximal lactate steady state), terminating around a blood lactate concentration of 4 mmol/L [8].
• Zone 3 (High Intensity): Above ${\text{LT}}_2$, encompassing intensities near or above ${\text{VO}}_2\text{ max}$ [8].

Interval modalities in Zone 3 are further classified into traditional HIIT (bouts near 100% ${\text{VO}}_2\text{ max}$), sprint interval training (SIT; supramaximal efforts >100% ${\text{VO}}_2\text{ max}$ lasting 10–30 seconds with 4–4.5 minutes recovery), and repeated sprint training (RST; sprints $\le$10 seconds with recoveries $\le$60 seconds) [18].

How athletes combine these zones forms distinct training intensity distributions (TIDs) [3, 8]:

• Polarized (POL): Allocates roughly 75–80% (and up to 80–90%) of training volume to Zone 1, less than 10% to Zone 2, and 15–20% to Zone 3 [2, 8].
• Pyramidal (PYR): Allocates 60–70% (or 75–85%) to Zone 1, 15–25% to Zone 2, and 5–10% to Zone 3 [4, 8].
• Threshold (THR): Allocates roughly 45–50% to Zone 1, 45–50% to Zone 2, and 5–10% to Zone 3 [3].

Adaptations in Untrained and Moderately Active Adults

In young, healthy adults, ${\text{VO}}_2\text{ max}$ responds favorably across a broad range of exercise intensities and combinations [14]. In an analysis of 28 short- to moderate-term training studies (4–8 weeks), gains in ${\text{VO}}_2\text{ max}$ showed no statistically significant relationship with training intensity, session dose, baseline aerobic fitness, or total volume [14].

Dividing training into three intensity tertiles (~60–70%, ~80–92.5%, and ~100–250% ${\text{VO}}_2\text{ max}$) yielded comparable absolute increases in maximal oxygen uptake (+0.29 L/min, +0.26 L/min, and +0.35 L/min, respectively) [14]. Higher-intensity intervals accomplished these gains with significantly lower session doses and total training volume, establishing high-intensity exercise primarily as a time-efficient strategy rather than an exclusively superior one in less trained populations [14]. Improving baseline aerobic fitness is clinically meaningful, as each 1-MET increase ($3.5\text{ mL}\cdot {\text{kg}}^{-1}\cdot {\text{min}}^{-1}$) in ${\text{VO}}_2\text{ max}$ is associated with a 10% to 25% reduction in cardiovascular disease and all-cause mortality [14].

Direct comparisons between isolated modalities also show that while 8 weeks of HIIT can elicit greater relative increases in ${\text{VO}}_2\text{ max}$ than continuous moderate exercise (15% vs. 9%) [11], both continuous steady-state training and sprint interval protocols generate similar increases in muscle oxidative enzymes over 6 weeks [11].

Combinations for Well-Trained and Elite Athletes

As baseline training status advances toward national and international competitive levels, training intensity distribution becomes a decisive factor [1, 3, 18]. Highly trained individuals already possess well-developed cardiovascular and muscular systems, making further improvements in ${\text{VO}}_2\text{ max}$ harder to stimulate through moderate continuous exercise alone [3, 13].

A meta-analysis of 17 studies ($n=437$) demonstrated that a polarized distribution is superior to other training distributions for improving peak oxygen uptake ($\text{SMD}=0.24,p=0.040$) [1]. This superiority was particularly pronounced in highly trained athletes ($\text{SMD}=0.46,p=0.01$) and in interventions lasting fewer than 12 weeks ($\text{SMD}=0.40,p=0.01$) [1]. In a 9-week randomized trial with well-trained endurance athletes (baseline ${\text{VO}}_2\text{peak}$ of $62.6\text{ mL}\cdot {\text{kg}}^{-1}\cdot {\text{min}}^{-1}$), polarized training produced an 11.7% gain in ${\text{VO}}_2\text{peak}$ ($+6.8\text{ mL}\cdot {\text{kg}}^{-1}\cdot {\text{min}}^{-1}$), outperforming high-volume low-intensity, threshold, and interval-only models [19]. Velocity and power at 4 mmol/L blood lactate also improved after polarized (+8.1%) and HIIT-concentrated (+5.6%) training, while threshold and high-volume training elicited no significant gains in key performance metrics [19].

Similarly, a 9-week study in competitive cyclists found that a polarized distribution improved ${\text{VO}}_2\text{ max}$ by 11.7% compared to 3.5% in a threshold-focused group, alongside greater gains in power at ${\text{VO}}_2\text{ max}$ (+8.1%) and time-trial performance (+5.1%) [5]. Reviews examining distributions with ~81% low-intensity, ~3% threshold, and ~15% high-intensity training have also reported consistent work economy gains between 1% and 8.1% [20].

When incorporating HIIT into an elite athlete's program, work-to-rest parameters influence the aerobic adaptation [13]. In national-level athletes, HIIT significantly enhanced ${\text{VO}}_2\text{ max}$ over conventional training when recovery intervals were $\ge$2 minutes and recovery intensity remained low ($\le$40%) [13]. For cardiovascular adaptations, intervals such as $4\times 4$ minutes at 90–95% maximal heart rate with 3-minute active recoveries produced a 10% greater improvement in stroke volume than long, slow continuous running [11].

Complementary Physiological Mechanisms

Combining low-intensity steady-state exercise with high-intensity intervals works because each modality stimulates aerobic adaptation through distinct molecular and physiological pathways [11]:

• Upstream Signaling: High-volume continuous endurance exercise activates the master regulator of mitochondrial biogenesis, PGC-1$\alpha$, predominantly through the calcium/calmodulin-dependent protein kinase (CaMK) pathway, whereas high-intensity interval training signals primarily via the 5'-AMP-activated protein kinase (AMPK) pathway [11].
• Enzymatic Capacity: Zone 3 sprint intervals markedly upregulate key mitochondrial enzymes, including citrate synthase (+36%), malate dehydrogenase (+29%), and succinate dehydrogenase (+65%) [11].
• Aerobic Base Requirements: Both polarized and pyramidal distributions rely on a large volume of exercise below ${\text{LT}}_1$ (>75% of total time) to develop mitochondrial density, fat oxidation capacity, and movement economy without accumulating excessive autonomic fatigue [4, 6].

Periodization Across the Season

While polarized blocks drive short-term ${\text{VO}}_2\text{ max}$ peaks, athletes do not necessarily remain in a polarized model year-round [6, 8]. Sequential periodization studies show that transitioning from a pyramidal distribution during the general base phase to a polarized distribution during the pre-competition phase produces greater improvements in ${\text{VO}}_2\text{ max}$ and 5 km running performance (+1.5%) than maintaining a static distribution throughout [8]. Longitudinal monitoring in elite rowers confirms that athletes naturally adopt this shift, building high base volume in preparation and concentrating intensity into polarized distributions before competition [6].

Additionally, threshold-dominant training retains specific utility. In ultra-endurance runners, while polarized training improved ${\text{VO}}_2\text{ max}$ and fat metabolism, threshold training was superior for preserving neuromuscular function, maximal voluntary contraction, and rate of force development [6]. Over extended multi-month macrocycles, pyramidal training has also proven highly effective, producing greater 10K running performance gains than threshold-heavy training (2.7% vs. 0.4%) [5].

Limitations and Caveats

Although polarized training shows a statistically significant advantage for increasing ${\text{VO}}_2\text{ max}$ and ${\text{VO}}_2\text{peak}$ in well-trained cohorts, this benefit comes with notable caveats [1]:

• No General Superiority for Other Performance Metrics: In meta-analytic pooled results, polarized training showed no statistically significant advantage over other distributions for time-trial performance, time to exhaustion, or speed/power at the second lactate/ventilatory threshold [1].
• Timeframe Constraints: The observed superiority of polarized distributions for peak oxygen uptake is concentrated in short interventions under 12 weeks, with less evidence demonstrating sustained divergence over longer multi-year blocks [1].
• Population Specificity: In untrained and recreationally active groups, the exact ratio of interval to continuous training is far less critical than consistent adherence, as total aerobic adaptation occurs readily across a wide spectrum of intensity combinations [14].