The hardest stretch of any long endurance event is rarely the start. The work that determines the result is almost always the work that happens late, when fatigue has accumulated, fueling has started to matter, and the gap between athletes who held together and athletes who fell apart becomes visible. The last 10K of a marathon, the final climb of a long bike ride, the summit push at the end of a long day in the mountains: these are where the result actually gets settled.
Endurance science has a name for this now. The current generation of researchers calls it durability, and the concept has moved from a niche discussion among physiologists to something most serious athletes have heard about. In our recent podcast conversation, Joe Friel and I spent a long stretch on it, because the durability question is particularly important for older athletes, who tend to lose late-event capacity faster than they lose any other dimension of fitness.
What is durability? A capacity distinct from VO2max and threshold
Durability is the ability to sustain meaningful output deep into a long event, after fatigue has accumulated. It is a different capacity from VO2max (peak aerobic power) or lactate threshold (the intensity ceiling you can hold). VO2max and threshold describe what an athlete can do fresh. Durability describes what is left in the third and fourth quarters.
For mountain and endurance athletes, durability is often the more decisive capacity. A marathoner with elite VO2max who fades in the last 10K loses the race. A mountaineer with high threshold who is empty at the start of the summit push misses the summit. The athletes who hold their pacing and their decision-making in the late hours of a long event are usually the athletes who trained durability, not just peak capacity.
When durability training belongs: the build phase, after the aerobic base
Joe was careful in our conversation to point out that durability training only makes sense after the aerobic base is built. He has seen too many athletes try to skip ahead.
“If you don’t have good endurance, there’s really no reason to be doing durability workouts. That’s a waste of time. You’ve got to have good endurance first.”
The build period is where durability work belongs. The aerobic base needs to be in place first because durability work piles fatigue onto fatigue, and an athlete without the base capacity to absorb it ends up overtrained rather than more durable.
The structure of a durability workout: pre-fatigue, then race-specific intensity
A durability workout has a specific structure. The first one to two hours create fatigue at low intensity, often a target heart rate zone or caloric expenditure. The second portion is then race-specific: threshold work, VO2max intervals, race-pace efforts, or technical terrain, performed on top of the accumulated fatigue. This trains the body to produce its hardest work in a tired state, which is what races and long days actually demand.
For a marathoner, that might mean an hour of zone three pace work after fatigue is established. For a long-course cyclist, it might be zone four and zone five intervals in the final hour, replicating the climbs late in the race where the legs are already heavy. For a mountain athlete preparing for a long alpine day, the analog is fatigue accumulated through hours of pack-loaded climbing followed by the technical or steep terrain that arrives near the summit. For a Denali expedition or a multi-day alpine route, the durability requirement extends across days, not hours, and the training signal is whether the legs and the head still work on day three.
What gets trained here goes beyond physiology. The integrated system of fueling, hydration, mental focus, and pacing under accumulated fatigue is what gets rehearsed. A durability workout is one of the few places in training where all of those variables get tested together at race-relevant intensity.
How to fuel durability work: 60g versus 90g of carbs per hour, plus caffeine in the final hour
Carbohydrate intake during durability training should match event duration. Sixty grams per hour is appropriate for two- to three-hour efforts. Ninety grams per hour, using a glucose-fructose blend, is appropriate for ultra-distance efforts. Hydration follows from sweat rate, and a ten-percent under-hydration is enough to compromise the workout. Strategic caffeine in the final hour is one of the better-supported levers for late-event performance.
Joe pointed to recent research on caffeine intake in the final hour of an event as one of the factors that influences durability, alongside the more obvious carbohydrate and fluid considerations. Whether the athlete drinks consistently or under-hydrates by ten percent changes the conversation again. Durability is the integrated outcome of all of these decisions, trained together over a season.
The two numbers Joe watches: Efficiency Factor and aerobic decoupling
The training data problem most athletes have today is the opposite of the data problem athletes had a decade ago. There is now too much information, not too little. Open a workout in TrainingPeaks and there are twenty data points from a ninety-minute session. Heart rate variability, time in zone, normalized power, intensity factor, training stress score, chronic training load, acute training load, and on. Joe’s approach is to pick one or two metrics per training period and treat the rest as background.
Efficiency Factor (EF): the base-period metric
Efficiency Factor (EF) is pace or power divided by heart rate, in a steady aerobic effort. Across a base period, the same effort should produce a slowly rising EF as aerobic fitness deepens. To track the trend, compare a current zone-two session to a similar session run eight weeks earlier. If EF is climbing, the base is working. If it is flat across two months of consistent volume, the training is not producing what it should.
EF is the most reliable signal that the base phase is doing its job. It also tells you when the base phase is complete: when EF has stopped improving across two or three eight-week comparisons, the aerobic system has produced what it is going to produce in this cycle and the build phase is the right next move. The reason eight-week-spaced tests work better than weekly ones is that day-to-day EF is noisy. The longer baseline strips out the noise and shows the trend.
Aerobic decoupling: the build-period metric and a proxy for durability
Aerobic decoupling (also written Pa:Hr for run pace, or Pw:Hr for cycling power) is the relationship between heart rate and pace or power across the duration of a workout. In a fresh, fit athlete, the ratio stays roughly constant from the first half of a long aerobic session to the second half. As fatigue accumulates, heart rate drifts upward relative to the work being produced. That drift, expressed as a percentage, is decoupling, and it is one of the cleanest proxies for durability available in everyday training data.
The benchmark most coaches use for build-phase aerobic efforts is a decoupling figure under five percent. Higher drift means the athlete cannot yet hold the work in a fatigued state, which is the signal that more durability work is needed. Lower drift means the work is being absorbed and the athlete is closer to ready.
“What I want to see is that number going up gradually over time. That means your durability is improving. That’s the most important thing.”
Other metrics still get tracked, though none are the focus. Training stress and load numbers belong in the background as a sanity check on volume. Heart rate variability is useful as a recovery signal for some athletes. The two metrics that drive decisions are EF and decoupling, in that order across the season.
A coach’s hierarchy: why most data is noise
What struck me about this part of our conversation was less the metrics themselves and more Joe’s reason for narrowing them. The athletes who get into trouble with training data are the ones who track everything and assume everything is equally important.
“As a coach, I’m going to be looking at lots more data. But I’ve got a hierarchy in my mind. The problem with an athlete looking at lots of data is they don’t have this hierarchy. They tend to see everything as being important.”
A coach’s hierarchy is the ranked order of which metrics matter most in a given period. The hierarchy shifts as the athlete moves from base to build to peak, and as the athlete’s strengths and weaknesses change across years. EF leads in the base phase, decoupling in the build phase. Wearable readiness scores, HRV trends, training stress, and chronic training load all sit underneath as context, not as primary signals.
For a masters athlete training without a coach, the practical move is to choose. Pick the one or two metrics that match the period you are in. EF in base, decoupling in build, time-in-zone-two as the daily check, and let everything else live in the background. The discipline here is acting on less, even when the wearables and the dashboards are showing more.
Durability for masters athletes, mountain athletes, and across the menopausal transition
For masters athletes, durability is arguably the highest-leverage capacity to train. VO2max declines roughly ten to fifteen percent per decade after fifty, and threshold tends to follow, though more slowly. Durability is the dimension where an older athlete can still compete, because the body’s ability to defend a submaximal effort across hours is more trainable into and beyond the sixties than peak capacity is. For a fifty-five-year-old athlete competing against a thirty-year-old, the third hour is where the gap closes or opens.
Baseline EF and decoupling values may run lower in older athletes than in younger ones. The signal worth watching is the trend, not the absolute number. Improving EF over a base block and stable, low decoupling across a build block mean the training is working, regardless of where the values sit on a population chart.
For mountain and expeditionary athletes, durability is the capacity that actually allows the summit. Denali requires a body that can perform on day eighteen of an expedition. A long alpine route requires legs and decision-making that hold across a fourteen-hour day. The metrics that track that capacity in everyday training are EF in the base block and decoupling in the build block, and the workouts that build it are the pre-fatigue-plus-intensity sessions described above.
For female masters athletes, EF and decoupling are sex-neutral metrics. The framework holds without modification. The adjustments female athletes make are usually in load distribution and recovery timing across menstrual cycle phases or the perimenopausal and postmenopausal transition, rather than in the metrics themselves. The hierarchy stays the same; the dosage shifts. Our coaches who work with female mountain athletes integrate these adjustments into the same durability framework rather than building a separate one.
The whole athlete: what the data does not see
The last point worth pulling out of our conversation came when I asked Joe how he talks an athlete through an “I cannot keep up anymore” moment. His answer turned to the rest of the athlete’s life rather than to training load: sleep, nutrition, injury history, family support, whether the athlete is enjoying the work.
He told a story about an athlete he once coached who was trying to qualify for Boston and whose wife thought the running was silly. The athlete started getting up at four in the morning on Sundays to run before his wife woke up so he could spend the rest of the day in a “normal” pattern. He never qualified. Joe’s read on it was that the limiter was psychological rather than physiological, and that the absence of support was holding the training back in ways that no amount of intensity work could fix.
This is the part that does not show up in the data. Coaching the masters athlete is coaching the whole athlete, and the metrics are only one input. EF and decoupling tell you whether the training is working in physiological terms; they cannot tell you whether the rest of the athlete’s life is set up to support what the training is asking of it.
Frequently asked questions
What is the difference between durability and VO2max?
VO2max measures peak aerobic capacity in a fresh state. Durability measures the ability to sustain meaningful output deep into a fatigued state. An athlete with high VO2max and poor durability fades in the back half of long events. An athlete with moderate VO2max and good durability often outperforms them.
How do I track Efficiency Factor (EF)?
Run a steady zone-two effort of consistent duration, ideally on a familiar course or trainer setting. Calculate EF as average pace or power divided by average heart rate. Repeat the same workout eight weeks later and compare. Improving EF means the base phase is working. Flat EF across two cycles means the base phase has produced what it is going to produce in this cycle.
What is a good aerobic decoupling number for build-phase efforts?
Under five percent is the working benchmark for build-phase aerobic efforts. Higher drift indicates the athlete is not yet able to hold the work in a fatigued state. As decoupling drops over a build block, durability is improving.
When should I start doing durability workouts?
Durability work belongs in the build period, after the aerobic base is established. Athletes who attempt durability workouts before the base is in place tend to overtrain rather than build durability.
How much carbohydrate should I take during a durability workout?
Sixty grams per hour for efforts of two to three hours. Ninety grams per hour for ultra-distance efforts, using a glucose-fructose blend so absorption keeps up with intake.
Are EF and decoupling different for women?
The metrics themselves are sex-neutral. Female athletes may need adjustments in training load timing across menstrual cycle phases or the menopausal transition, but the framework and the hierarchy do not change.
How do wearable readiness scores fit in?
Readiness scores are useful context but secondary to EF and decoupling for training decisions. Use them to flag a recovery problem worth investigating, rather than letting them override the training signal coming from EF and decoupling.
If you want help building that hierarchy for your own training, including the parts that do not show up on a power file, our coaching team works with masters and mountain athletes through 1:1 coaching and structured programs. The framework underneath is the same one Joe described. What changes is that someone else is holding the hierarchy for you while you do the work.