By Rob Shaul
Polarized endurance training dominates modern endurance coaching. In its simplest form, polarized programming emphasizes very easy aerobic work—often called Zone 1 or Zone 2—paired with very hard, high-intensity intervals, while intentionally avoiding moderate-intensity efforts. The idea is that athletes accumulate large volumes of easy work they can recover from, sprinkle in short bouts of high intensity, and stay away from the “middle ground,” which is believed to impair recovery and stall progress.
MTI does not follow this model.
That decision isn’t rooted in ignorance of conventional endurance theory. I’m very familiar with traditional endurance programming and the polarized approach. Like many areas of MTI programming, our approach to endurance training is contrarian—not for the sake of being different, but because the foundational assumptions behind polarized training have not held true for the athletes we serve.
The central assumption behind polarized endurance programming is not that moderate-intensity work is ineffective. It’s that athletes supposedly can’t recover from it. I distinctly remember reading a passage in Joe Friel’s Total Heart Rate Training where he stated that moderate pacing is actually the most efficient way to increase fitness, but recovery is the limiting factor. Zone 1 and Zone 2 base training, in this framework, allows athletes to keep moving, steadily build mode-specific fitness, and tolerate high training volumes because the intensity is low enough to recover from.
I haven’t found this to be the case.
Across my own training, our lab rats, and thousands of military athletes who have used MTI programming, moderate-intensity endurance work has proven to be an effective and recoverable way to build fitness. This is especially true when endurance training is not the sole fitness focus. MTI does not work with year-round endurance specialists. The endurance work we program for military athletes is typically event-based—peaking speed over ground for a selection or fitness test—while concurrently training strength, work capacity, tactical agility, and chassis integrity.
Even on the mountain side, the athletes who find and use MTI are usually multi-sport mountain athletes. There is always a baseline level of mountain endurance required, but we’re generally not working with full-time, year-round ultra runners. We do offer ultra running plans, but these are most often used by multi-sport mountain athletes preparing for a specific 50k or 100-mile event, after which they return to a broader training focus.
It may be true that elite, year-round endurance athletes struggle to recover from sustained moderate-intensity work. To my knowledge, that assumption hasn’t been rigorously tested, but even if it’s valid, it doesn’t apply cleanly to the population MTI serves. When athletes train at moderate intensity, they also don’t need to go as long or as far. The training is more efficient. The prevailing response in endurance coaching has been to push polar training even further—more hours, more low intensity, more volume—rather than questioning whether the model fits the athlete.
That brings up a second problem: the aerobic base itself.
One of my major issues with polarized endurance programming is how poorly “adequate aerobic base” is defined and assessed. Most conventional endurance programming treats base building as a time-based phase rather than an assessment-based requirement. Joe Friel has suggested that an athlete has a sufficient aerobic base when they can sustain four hours at a Zone 2 heart rate. Johnston and House at Uphill Athlete use a heart rate drift test, which, while thoughtful, strikes me as overly complicated for what it attempts to measure.
What’s missing is clarity. If an athlete performs well on a heart rate drift test, do they still need twelve weeks of base training before progressing to more intense work? If they complete those twelve weeks but test poorly afterward, are they still unprepared? And does every endurance event—regardless of duration—require the same base-building timeline? In ultra running programming especially, it’s common to see identical base phases prescribed for a 30k event and a 100-mile event. That simply doesn’t make sense.
For more on this issue, we’ve written extensively about it in Defining the Nebulous Aerobic Base. The lack of clear assessment and event-specific logic makes conventional base training feel more like dogma than craftsmanship.
Training time is another constraint that polarized programming often ignores. Many of the tactical athletes and mountain athletes we work with don’t have the luxury of dedicating thirty to forty hours per week to endurance training for months on end. Most people have jobs, families, and real-world responsibilities. Even if the polarized model were marginally superior from a pure performance standpoint—and I’m not convinced it is—it’s often not deployable. Training that looks great on paper but can’t be executed in the real world fails the athlete.
MTI programming for long events—tactical selections, ultras, extended mountain objectives—does require commitment. But it’s also comparatively short in duration. These are peaking plans. We assume athletes can temporarily accept higher training volumes in the lead-up to an event, then pull back afterward. That’s very different from prescribing endless base hours logging zone 2 miles with no clear endpoint.
Another area where MTI diverges sharply from traditional endurance programming is our use of distance rather than time to prescribe training. Many polarized plans rely almost exclusively on time-based prescriptions—“90 minutes at Zone 2,” for example—without ever addressing the actual demands of the event. As a self-taught strength and conditioning coach, this has never made sense to me. If an athlete is training for a marathon, they should know what it feels like to run twenty-six miles across multiple days and at least eighteen miles in a single effort.
Endurance performance isn’t just about aerobic capacity. It’s also about joint, muscle, and ligament durability, as well as mental fitness. These qualities are mode-specific and must be trained. I’ve failed as a coach if an athlete reaches their event and encounters physical or mental stress they’ve never experienced before. This is why MTI selection plans include long, multi-hour weekend mini-events, and why our triathlon programming includes actual practice triathlons. Stress inoculation matters.
Ultimately, we’ve found that assessment-based threshold intervals and moderate-intensity endurance work are the most effective and efficient way to train speed over ground for events up to roughly twelve miles. Most MTI endurance programming would be labeled “peaking” by conventional standards. The difference is that we don’t require months of poorly defined aerobic base work beforehand.
Traditional polarized endurance programming has clearly worked for elite, highly committed endurance athletes, which is why it dominates that space. But it has not worked well for MTI or the athletes we serve. The required training time is excessive, the assumptions about recovery haven’t matched our experience, and the lack of assessment-driven clarity around aerobic base development doesn’t meet our standard for programming craftsmanship.
Accommodation is universal. Endurance athletes who have trained polar for years will eventually see diminishing returns from endless hours of Zone 2 work. The common response from endurance coaches seems to be more of the same—more volume, more low intensity. MTI has taken a different path, not because we’re dismissive of endurance science, but because our athletes demand solutions that are effective, recoverable, event-specific, and executable in the real world.