This article is a companion to our podcast episode on muscular endurance hosted by Steve House with UA coaches Ben Morley and Martin Zhor.
“You have put in the miles. You have built the aerobic base. You can go all day at a moderate pace. But somewhere above a certain intensity, on a steep climb with a heavy pack or at altitude on an 8,000-meter peak, your legs give out before your lungs do. You are not out of breath. Your legs are burning and your breathing is fine. You are just…done.”
Steve House
That gap between your cardiovascular capacity and what your legs can actually deliver is muscular endurance. It is the most undertrained quality in mountain sports, and when two of our coaches sat down to talk about it, the conversation kept returning to the same theme: athletes who look fit on paper but fall apart the moment the terrain gets steep and the loads get heavy.
Local Fatigue vs. Global Fatigue
Muscular endurance is the ability of specific muscle groups to resist fatigue and sustain submaximal force over thousands of repeated contractions while maintaining coordination and economy. It sits between strength and endurance, and it is where mountain athletes most frequently have a gap.
Martin Zhor frames the distinction this way. Global fatigue is cardiovascular: your breathing is maxed, your heart rate is at its ceiling, and the limitation is systemic. Local muscular fatigue is different. Your breathing is manageable and your heart rate is moderate, but the muscles doing the work have reached their limit. Each step requires conscious effort. The legs are heavy and no longer responding to what your aerobic system is trying to deliver. The diagnostic is simple: if your legs are failing before your lungs, muscular endurance is your limiter.
There is a counterintuitive wrinkle Martin emphasized. When local muscular fatigue sets in, heart rate actually drops, because the fatigued muscles can no longer demand oxygen at the rate the cardiovascular system is prepared to supply. You feel worse, but your heart rate goes down. Athletes accustomed to equating high heart rate with hard effort find this confusing. If your heart rate drifts lower during a steep, loaded climb while your perceived effort keeps climbing, that is not a sign of recovery. It is a sign that your legs have become the bottleneck.
How to Tell If Muscular Endurance Is Your Limiter
Ben Morley described a practical field test: run or hike uphill as hard as you can for 30 to 45 minutes on steep terrain. If muscular endurance is the gap, your heart rate will not reach the values you would expect from that level of perceived effort. The cardiovascular system is ready to deliver; the legs cannot absorb it.
A few questions worth sitting with: Do your legs feel heavy and lose their responsiveness on efforts under two hours, well before glycogen depletion is a factor? Do you slow down on steep terrain despite breathing comfortably? In races or hard workouts, do your legs give out before breathing becomes the problem? If yes to any of these, muscular endurance is likely where the work needs to go.
Martin added an important nuance for ultra athletes. At longer distances, the question shifts from whether you can sustain the effort to whether you can sustain the impact. After six or twenty hours on your feet, you are not limited by cardiovascular function but by whether your muscles can keep producing force and absorbing the eccentric loading of each downhill step. The athletes who have trained this quality are the ones still running the hills at mile 80 while everyone else is walking.
What Happens Inside the Muscle
Your muscles contain a spectrum of fiber types. Slow-twitch fibers have high endurance but low force output. Fast-twitch fibers produce high force but fatigue quickly. Between them sit the Type IIa fibers, and this is where the ME story gets interesting. ME training targets these intermediate fibers by loading the muscle enough that slow-twitch fibers alone cannot handle the work, forcing the IIa fibers to participate and adapt. With sustained training they develop more mitochondria, improve capillary networks, and become better at using oxygen to produce energy, which means movement quality stays higher for longer under fatigue.
One point that often gets overlooked is the role of the stabilizer muscles. When the small muscles that hold joints in position fatigue, the larger primary movers compensate, and that compensation degrades movement quality and opens the door to overuse injuries. ME training keeps the stabilizers functional deep into an effort, which is why it serves durability as much as performance.
Where ME Fits in the Strength Progression
This is where athletes most frequently go wrong. Both coaches were emphatic about the same principle: ME training is the third phase in a three-phase strength progression, and the sequence is non-negotiable. The first phase is general strength, lasting four to twelve weeks, focused on developing good movement patterns, building connective tissue resilience, and establishing baseline muscular capacity. The second phase is max strength, lasting eight to twelve weeks, using heavy compound lifting to raise the absolute force ceiling. The third phase is muscular endurance, lasting eight to sixteen weeks, training the muscles to sustain a high percentage of that raised ceiling over thousands of repetitions.
Ben put it simply: you raise the ceiling, then you train to sustain a high percentage of it. Skip max strength and go directly to ME and the ceiling is too low. Skip general strength and the connective tissue is not prepared for the loads that follow. The sequence exists for a reason, and compressing it reliably costs athletes more time than it saves.
Both coaches also referenced the same aerobic prerequisite. Your aerobic threshold should be within 10 percent of your anaerobic threshold, measured by heart rate, before beginning an ME block. If the gap is wider, your aerobic system still has significant gains available, and building that base first will make the eventual ME block substantially more productive. Once the ME phase is complete, athletes transition to high-intensity cardiovascular intervals. This is the payoff of the full sequence: fatigue-resistant legs that can finally push the cardiovascular system to its actual ceiling, rather than tapping out before it is maximally stressed.
What ME Training Actually Looks Like
The core outdoor ME workout is a steep weighted uphill carry. Martin uses a percentage of body weight to individualize the load, building progressively across the training block toward the specific weight an athlete will carry on their objective. The terrain needs to be genuinely steep, because low-angle trails and flat rucking do not provide enough muscular stress to drive meaningful adaptation. You need terrain where the incline forces your muscles, not your cardiovascular system, to be the limiting factor. A steep treadmill or stair machine works well for athletes without access to suitable outdoor terrain.
The right intensity has a clear feel: legs burning, but still able to carry on a conversation. If breathing is the limiting factor, the session is miscalibrated, either because the terrain is not steep enough or the pace is too high relative to the load.
Most athletes do at least some of their ME work in the gym, and many do all of it there. The key exercises are box step-ups for the concentric (uphill) demand and box step-downs for the eccentric (downhill) demand, while walking lunges are also effective. Ben described a progression where the primary variable is rest period duration: start with longer rest between sets and progressively shrink it, so that the accumulating fatigue drives the ME adaptation without needing to add load. For athletes doing step-offs, placing a smaller box on the landing side reduces the step-down distance to a manageable level under fatigue.
Upper-body muscular endurance is consistently underappreciated. Trail runners, ski mountaineers, and Nordic skiers who use poles for propulsion need triceps, lats, and shoulders that can sustain force output for hours, and SkiErg work and high-rep lat pull-downs are effective tools for building it.
Common Mistakes
The most frequent error both coaches see is carrying heavy packs on long aerobic workouts. The aerobic sessions serve a different purpose, and loading them shifts the training stimulus away from aerobic development without providing the concentrated muscular stress a proper ME session delivers. ME training supplements aerobic volume; it never replaces it. Related to this is rucking on flat terrain, which primarily trains the postural muscles of the upper body rather than providing enough leg stress to drive meaningful adaptation. The specificity that makes ME training effective comes from moving body mass plus load against gravity on steep terrain, not from walking on flat ground with a heavy pack.
Neglecting the downhill is another consistent gap, particularly for trail runners. The eccentric demands of sustained descending are a separate and trainable quality, and athletes who focus exclusively on uphill ME work may arrive at their goal event with strong uphill legs and quads that fall apart on the descents. Both the concentric and eccentric components belong in the program.
Finally, athletes who discover ME training and experience its rapid early gains often want to continue it indefinitely, but the gains plateau after roughly eight to sixteen weeks. Continuing beyond that without transitioning to the next phase represents a missed opportunity. ME training is a phase, not a lifestyle, and its effectiveness depends on being sequenced correctly within the larger plan.
Sport-Specific Applications
For mountaineers preparing for objectives like Denali, the ME block sits at the end of the strength progression as the last sport-specific phase before the expedition. Martin also builds in calf-specific isometric work for climbers who will spend extended periods front-pointing on steep ice, which places a distinct muscular demand compared to the repetitive contractions of hiking.
Martin shared an insight from his own experience on Manaslu. He arrived at the upper mountain with legs that were not the limiter. His muscles were strong and fatigue-resistant, and what limited him was acclimatization rather than muscular capacity. He wanted to go faster, but his body was struggling with the altitude. This is the ideal scenario: ME trained to the point where the muscles are no longer the bottleneck, and the remaining limitations are factors that can only be addressed through time at altitude.
For trail runners, Martin described watching the top three women at the Champex-Lac aid station during UTMB, roughly 120 kilometers and 15 hours into the race. All three were hiking the steep climb into the checkpoint, none of them breathing hard. The difference between them was visible in their leg mechanics: cadence, form, and the ability to maintain a purposeful stride under deep fatigue. You could see who had trained muscular endurance and who had not. Many trail runners train primarily by running and assume that hiking will take care of itself. It will not.
For ski athletes, double-poling for hours on end is a pure upper-body muscular endurance challenge: the aerobic system is ready to deliver, but the triceps and lats reach their limit well before the heart and lungs do. The same dynamic applies in ski mountaineering, where the transition from skinning to bootpacking on steep terrain exposes any ME deficit in the legs quickly. Ben was emphatic that athletes should resist the temptation to jump directly to on-snow work or ski-specific circuits before completing the full strength progression. The foundational phases are what make ME training productive, and compressing them undermines the whole block.Muscular endurance is the bridge between the aerobic engine you have built and the performance you need on steep, loaded terrain. Eight to sixteen weeks of focused work, once per week, placed in the late base or sport-specific preparation period. The intensity marker is local muscular fatigue at a conversational breathing rate. And when the block is complete and you move into high-intensity intervals, you will feel the payoff: legs that are finally fatigue-resistant enough to let you push your cardiovascular system to its actual ceiling.