Above: Jackson-based alpinists complete the 15K run assessment final event of the MTI Alpinist Fitness Assessment.
By Rob Shaul
BLUF
A 2025 Journal of Strength & Conditioning Research scoping review analyzed over 600 studies of aerobic endurance programming and found that models emphasizing Zone 1 (low-intensity) training—specifically Polarized and Pyramidal approaches—produce the largest improvements in VO₂ max, threshold power/speed, and race performance.
Across 15 qualifying studies (412 endurance athletes; 6–26 week study duration), the consistent outcome was that the more time athletes spent below their aerobic threshold, the greater their total aerobic adaptation and competition performance—especially among trained and elite athletes. Threshold-heavy programs that focused on Zone 2 work produced smaller gains and greater fatigue.
Study Overview
The researches screened 623 peer-reviewed endurance-training studies and found 15 studies that met their criteria for inclusion in the analysis.
The included 15 studies covered 412 athletes, with study durations of 6-26 weeks.
Eight of the included studies looked at cyclists, three at runners, two at cross country skiers and one each at rowers and triathletes.
Three of the studies researched recretional athletes, 8 researched trained athletes, and four researched elite athletes.
This scoping review looked at VO2 Max improvment, power/speed at ventilatory thresholds, lactate thresholds, time-trail or race performance and movement economy as measures of programming success.
The review included direct comparisons among four aerobic programming models and how much each time the programming model had the athletes train in either Zone 1, Zone 2 or Zone 3.
1. Polarized (POL) – mostly easy (Zone 1) with a small amount of hard (Zone 3) work
2. Pyramidal (PYR) – large low-intensity base, moderate threshold, little high intensity
3. Threshold (THR) – frequent work around lactate threshold (Zone 2 dominance)
The study defines Zones 1, 2, and 3 using the traditional triphasic model of endurance physiology (Skinner & McLellan, 1980). These zones are anchored to two key physiological thresholds — the aerobic threshold (VT1/LT1) and the anaerobic threshold (VT2/LT2) — which can be measured using either ventilatory or blood lactate testing.
Zone 1 (Z1) – Low Intensity Training – Below the aerobic threshold (VT1/LT1)
Zone 2 (Z2) – Moderate Intensity Training, Between the aerobic and anaerobic thresholds.
Zone 3 (Z3) – High Intensity Training, Above Anaerobic threshold.
See below for training intensity distribution amongst the programming models.
| Training Time in Zone | Polarized Model | Pyramidal Model | Threshold Model |
| Zone 1 (Easy) | 75-80% | 70% | 45-50% |
| Zone 2 (Mocerate) | 0-6% | 20% | 45-50% |
| Zone 3 (High) | 15-20% | 10% | 5-10% |
| Mostly easy, some hard, almost no middle. | Tapering distribution — low > moderate > high. | Lives in the grind zone, |
Key Findings:
| Outcome | Findings |
| VO2 Max Improvement | POL and PYR produced the largest increases (+5–11 %), compared with <5 % for THR |
| Threshold Power/Speed (VT2 / LT2) Improvement | Improved most under POL (+5 %) and PYR (+4 %), indicating better aerobic durability. |
| Time-Trial / Race Performance | 3–6 % faster finish times with POL and PYR training vs. 1–2 % for THR. |
| Fatigue / Perceived Exertion | THR produced the highest RPE scores and smallest subsequent gains. |
| Movement Economy | POL and PYR improved running or cycling economy (~2–5 % reduction in O₂ cost) |
| Bottom Line | When ≥ 70 % of total training time occurs in Zone 1, aerobic and race-day outcomes improve most. |
However …
Concerning endurance performance, scientive evidence highlighted the POL and PYR models as most effective for enhancing athletic performance. The research further demonstrates no significant difference in effictivement between the POL and PYR models.
The reasearch also shows that a minimum percentage of training volume in Zone 3 is necessary for performance improvement. However, exactly how much volume an athlete should train in Zone 3 is unclear.
The superiority of the POL and PYR models has only been demonstrated in elite/intenational or world class athletes. “In conclusion,” the authors write, “POL and PYR models seem to be more effective in elite/international and world-class athletes, whereas in lower-level athletes the differences between TID models are negligible.”
Importantly, of the studies included in this analysis, only one included a competition greater than one hour. The assumption is that it’s possible that higher level athletes compete in longer endurance events than lower level athletes and longer event performance benefits from weighted training time in Zone 1.
The authors further caution that because the study covered several modes (cycling, running, rowing) a programming model’s effectiveness between modes wasn’t determines. For example, Polar endurance programming could really benefit runners, but less so for swimmers.
Application to MTI Endurance Programming
One issue not addressed by this anaylsis was the actual training time required in Zone 1 to have an effect. This analysis just looked at the overall percentage of training time in each zone.
This is a key piece of information. Training time is an issue MTI athletes and us coaches, need to manage. Standard, polar endurance programming for long endurance events (ultra marathons) dedicate up to 40 hours/week to training, most of which is easy movement in Zone 1. Few working athletes can dedicate this much time to training and we’ve found only one measure of adequate aerobic base and it requires a blood lactate test.
Also, ultra endurance athletes don’t also need to spend equal parts training time to training strength, work capacity, or chassis integrity. The tactical athletes and mountain professions we work with can’t afford to let fitness in these none-endurance attributes atrophy significantly. Some mountain athletes can – who are focused on one sport or season (Ultramarathons) for example.
So, even MTI’s endurance-event programming often includes strength, work capacity and chassis integrity, though most of the work is endurance-focused.
Because of the time constraints on our athletes, and need to train fitness attributes in addition to endurance, MTI endurance programming mostly deploys the Threshold model or even only threshold training. Our own research has shown that the most effective way to a PFT 2-mile run time, for example, is to run assessment-based threshold intervals and often PFT plans only include threshold intervals for endurance training.
Source
Rivera-Köfler, T., Varela-Sanz, A., Padrón-Cabo, A., Giráldez-García, M. A., & Muñoz-Pérez, I. (2025). Effects of Polarized Training vs. Other Training Intensity Distribution Models on Physiological Variables and Endurance Performance in Different-Level Endurance Athletes: A Scoping Review. Journal of Strength and Conditioning Research, 39(3), 373 – 385.*
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