Muscular endurance, what many in the endurance community now more accurately refer to as durability, is a cornerstone of ultrarunning performance. It reflects the ability to sustain movement, resist breakdown and continue producing force over long hours and rugged terrain. It’s not just about strength or aerobic fitness; it’s about holding form and function when fatigue sets in and you must continue climbing or descending.
And yet, despite broad agreement on its importance, how to develop muscular endurance is often misunderstood. Between anecdotes, legacy methods imported from mountaineering and tactical communities, and the rise of strength-forward endurance culture, we’ve seen an influx of interventions that feel tough but fail to deliver meaningful adaptations for ultrarunning.
The truth is both more boring and more liberating: muscular endurance is best developed through high-quality, terrain-specific running volume, not through clever workarounds, laboratory-esque fatigue protocols or strength circuits dressed up as specificity.
So what is muscular endurance and why does it matter in the context of ultrarunning? With insight from both research and expert coaches, we strip the noise away from the signal and return to what actually works.
What Is Muscular Endurance?
Muscular endurance is often tossed around as if everyone agrees on what it means, but definitions vary depending on the sport, coach or the training method being sold. In a general physiological sense, it refers to the ability of a muscle or muscle group to sustain force or repeated contractions over time without excessive fatigue. But in ultrarunning, that definition needs refinement. We’re not talking about holding a plank for 5 minutes or banging out 100 lunges in a hotel gym. We’re talking about the capacity of the legs to withstand tens of thousands of impact cycles up and down steep and technical terrain for 8, 12 or 24 hours (or more) without crumbling.
As Dr. Nick Tiller puts it, “[Muscular endurance] in ultrarunning is the ability of the muscles to sustain the rigors of the event without undue stress or injury.” In contrast to shorter endurance events where aerobic power or lactate clearance might be limiting factors, muscular endurance in ultrarunning is about resisting structural breakdown and neuromuscular fatigue. It’s less about producing maximum force and more about surviving repetitive force production over time. The longer the event, the more complex the fatigue mechanisms ranging from local muscle damage to central nervous system inhibition, and the more critical it becomes to understand muscular endurance as a multifactorial adaptation, not just a single trainable trait.
Why It Matters
Muscular endurance isn’t just a supporting player in ultrarunning performance; it’s central to how well an athlete holds up when the race moves beyond aerobic thresholds and into the territory of mechanical fatigue, muscle damage and psychological strain. Even among runners with similar VO2max or threshold pace, the one who slows down less in the final third of a race almost always comes out on top. That resistance to decline, or durability, is what muscular endurance ultimately buys you.
It shows up in the real world in all the ways we care about: maintaining form over technical terrain, avoiding catastrophic cramping or fatigue in the quads late in a 100k, or still being able to descend with control at mile 80. As Dr. Tiller notes, “One of the main determinants of ultramarathon performance is a high weekly mileage,” not clever substitutions. The act of repeatedly covering ground on tired legs, over time, is what teaches your body to withstand the physiological and biomechanical stresses unique to ultrarunning. And while strength training and downhill running can help support that development, they are supportive, not standalone solutions. The best predictor and builder of muscular endurance is, unsurprisingly, running (a lot).
Popular Approaches
With the rise of strength-forward training paradigms in ultrarunning, many influenced by mountaineering, military-style endurance and generalized “functional fitness,” we’ve seen a surge in athletes looking for new ways to build muscular endurance. While the intention is valid, many of these approaches either miss the mark on specificity, introduce unnecessary fatigue or simply fail to provide better returns than time spent running.
Weighted Vest Hiking
Weighted vest hiking has become a go-to method for athletes seeking to build muscular endurance without adding more running. The logic sounds solid: add external load, hike uphill, build leg strength and resilience. And in mountaineering or military contexts, that adaptation may be appropriate. But ultrarunning isn’t alpine climbing and the physiological demands differ in important ways.
Dr. Nick Tiller points out that “there are several studies showing benefits of weighted vest training on performance and running economy,” but “little data that it specifically enhances ultramarathon performance.” What’s more, “weighted backpacks increase the risk of injury, so caution is warranted.”
To understand the gap between theory and practice, it’s helpful to look at the actual data. A 2024 study published in Sports examined the effects of running with weighted vests loaded at 0%, 5% and 10% of body mass in trained trail runners. As the vest load increased, time to exhaustion decreased significantly, meaning athletes fatigued faster with heavier loads. The authors concluded that “increasing the load of the weighted vest produces an exponential loss of performance,” a result attributed more to neuromuscular disruption than to any meaningful metabolic adaptation (Martínez-Noguera et al., 2024). In other words, it made the effort feel harder, but didn’t enhance endurance in a way that transfers to improved performance.
Some might argue that added fatigue is the whole point of training; longer or harder efforts always carry performance costs that ultimately drive adaptation. And that’s true, to a point. But not all fatigue is created equal. The stress induced by prolonged, sport-specific running builds the mechanical and metabolic durability needed in ultrarunning. The fatigue from added external load, on the other hand, alters biomechanics and accumulates neuromuscular stress without the same endurance payoff, making it a less effective, and potentially counterproductive, tool for most trail and ultra athletes.
Nick DiMarco, MS, CSCS, TSAC-F, has coached tactical athletes within the US Army’s 10th Mountain Division and at the Air Force Special Warfare Academy. He now works as a professional ultrarunning coach, applying strength and conditioning principles to endurance performance. He puts it plainly: “Is it really that much better than loading your race kit with all the required gear and heading out for a run? I don’t think so.” He also cautions that vest hikes lack standardization and often introduce unnecessary axial fatigue without a reliable way to quantify or recover from the added load.
Weighted hiking isn’t inherently bad. It can condition the musculoskeletal system for multi-day or self-supported efforts. But in most ultrarunning contexts, it’s a solution in search of a problem. When done excessively or used as a shortcut, it replaces better training rather than supplementing it.
High-Rep Strength Circuits
Another popular method marketed as muscular endurance training is the high-rep strength circuit: post-run lunges, air squats, kettlebell swings, often for time, volume and until the athlete feels sufficiently wrecked. The goal is usually to simulate the grind of late-race fatigue and “build muscular endurance.”
But does it actually improve ultrarunning performance?
Not as well as you’d hope.
DiMarco sees this misunderstanding regularly. “If my athlete came to me and said, ‘I did 30 minutes of air squats and lunges after my run,’ I would tell them I’d rather they go run for another 30 minutes,” he says. “We’re just over-taxing those physiological systems.” These types of circuits often drive heart rates into aerobic zones similar to running but lack the mechanical loading, tendon behavior and movement specificity needed to meaningfully support performance. In essence, they feel hard, but they don’t add much.
The science echoes that. A 2023 systematic review published in Sports Medicine found that high-load (heavy) and plyometric strength training improved running economy in distance runners, while low-load, high-repetition training – the kind often used in fatigue-based circuits aimed at muscular endurance – offered little to no benefit (Balsalobre-Fernández et al., 2023). While the study didn’t assess muscular endurance directly, rather running economy, it reinforces the larger point: not all strength work transfers to the trail or road, and poorly designed gym sessions can waste energy rather than build durability.
Mistaking Specificity for Imitation
At the heart of this mistake is a flawed interpretation of the specificity principle. Just because ultrarunning requires enduring muscular effort doesn’t mean that copying fatigue in the gym improves performance. Athletes often assume that training in a similar format – long, repetitive, and tiring – will transfer. But this conflates imitation with specificity.
DiMarco explains this using an example from the NFL: “Jalen Hurts (QB for the Philadelphia Eagles) doesn’t sled push people in training to mimic game situations, he squats 600 pounds to exceed the demands of the sport in a measurable, controlled way.” The same logic applies to ultrarunners. You already log thousands of muscle contractions on the trail. Adding more of the same in the gym doesn’t improve your ability to endure; it compounds stress.
“Use the weight room to exceed those demands, to build strength, durability, bone density, tendon resilience and to address lagging stabilizers or injury-prone areas,” DiMarco says. That means heavy bilateral and unilateral lifts, explosive plyometrics and isolated rehab work, not fatigue-based mimics of running.
In short, if your gym work looks too much like your running, you’re missing the point. Specificity doesn’t mean copying movement patterns; it means supporting them with complementary stressors that enhance performance, not duplicate it.
What the Evidence Supports: How to Build Muscular Endurance the Right Way
If weighted vest hikes, fatigue-based strength circuits and gym imitation drills don’t reliably improve muscular endurance for ultrarunners, what does?
The answer is frustratingly simple and refreshingly clear: progressive, terrain-specific, high-volume running. As Dr. Nick Tiller said, “One of the main determinants of ultramarathon performance is a high weekly mileage. There are no shortcuts to accomplishing this, just effort, patience and strategic implementation to minimize the risk of injury.” This is supported by research from Millet et al. (2011), who found that neuromuscular fatigue in ultramarathon events is largely driven by cumulative mechanical and metabolic stress – adaptations best developed through sustained, sport-specific training.
Specificity still reigns. The most effective way to prepare the body for long-distance, elevation-rich trail running is to do exactly that: run long, run often and run with purpose. No combination of clever fatigue protocols or gym circuits can replicate the cumulative neuromuscular and biomechanical load of consistent, specific running.
Some may argue that for time-crunched athletes or elites already at their safe mileage ceiling, strength-based muscular endurance sessions offer a useful alternative. But that’s a misstep. As Tiller cautions, “Ultramarathon is a unique sport with unique and multifaceted demands. Any approach needs to be studied in the context of the sport’s distances and durations.” These non-specific interventions rarely produce better adaptations, just different ones, often with added fatigue and limited transfer.
If an athlete can’t increase volume, the solution isn’t to mimic fatigue in the gym; it’s to optimize their run training: smarter intensity distribution, strategic terrain specificity and supportive strength work that complements rather than duplicates running.
That’s not to say strength training isn’t valuable – far from it. DiMarco emphasizes that strength is a year-round asset when applied properly. “Strength training should augment either less intense work (rehab/prehab, neuromuscular) or more intense work (strength, power), and not too much in the realm that you’re already getting during your run training,” he explains.
His approach prioritizes bilateral and unilateral lifts, plyometrics and high-load, low-volume work. When run volume is low (off-season or during intensity blocks) strength training might be scheduled 2–3 times per week. In peak phases, it’s scaled down to once per week or every 8–9 days, using heavy but crisp sets to maintain strength without overloading recovery. The objective isn’t to add stress for its own sake, it’s to build qualities that running can’t.
For runners without access to steep terrain, incline treadmill workouts or stair climber offer a far better alternative to weighted hikes. As DiMarco points out, these tools replicate the biomechanics of uphill running without the unpredictable fatigue and spinal load of external vest weight.
And when it comes to fatigue resistance, it’s not just about the muscles. Tiller reminds us that “ultrarunning is one of the few sports where fatigue is so multifaceted,” with both peripheral fatigue (muscle damage, glycogen depletion) and central fatigue, a decline in motor output from the brain itself. Muscular endurance, then, isn’t just about legs – it’s about systems. And systems adapt best when trained through consistent, specific exposure – not workarounds.
Some coaches and athletes argue, based on their own experience, that local muscular fatigue, particularly in the legs, limits performance before aerobic capacity does. In hard intervals or sustained climbs, they claim it’s not the heart or lungs that give out first, but rather the legs. While this perception is common, it’s important to distinguish between subjective experience and objective adaptation. Muscular discomfort or localized fatigue may feel more immediate or intense, but endurance performance is primarily determined by systemic factors, especially when we’re talking about prolonged efforts over hours, not minutes. Fatigue is multifactorial, but the weight of evidence continues to point toward total training volume and aerobic development as the biggest drivers of long-term performance in ultrarunning (Millet et al., 2011)
You can’t hack your way to resilience in a sport this complex. You can only build it using one smart, specific session at a time.
Let Proven Principles Guide Programming
It’s tempting to chase novelty in training. Especially in a sport as complex and rugged as ultrarunning, where every athlete is a bit of a one-off and every finish line has its own story, it’s easy to get swept up in what “worked” for someone else, or what feels productive. But as Dr. Tiller reminds us, “The plural of anecdote is not evidence.”
Individual experiences, while meaningful, are shaped by expectations, beliefs and personal bias. What seems effective for one athlete may simply reflect their unique physiology or a misattribution of what actually drove improvement. That’s why Dr. Tiller emphasizes that “science is a process of systematic inquiry – the best tool we have for understanding the world.” And good coaching, he says, doesn’t blindly follow the science, but “integrates scientific findings with the individual needs of their athletes for the best of both worlds.”
Just because a workout feels hard doesn’t mean it’s driving a meaningful adaptation. Just because someone finished or even won a race after a block of weighted vest hikes or high-rep gym circuits doesn’t mean that’s what moved the needle. The best training interventions are grounded in timeless principles: specificity, progressive overload and adequate recovery. They can be scaled, replicated and justified across a broad athlete population, not just reverse-engineered from the success of one, two or 20 people.
Muscular endurance isn’t a puzzle to be solved in the weight room or a formula to crack with fatigue metrics. It’s not a math problem or a secret protocol reserved for elites. It’s a well-understood adaptation; one we’ve already seen develop reliably through strategic, terrain-specific running and appropriately loaded strength work. As tempting as it is to chase novelty or overcomplicate the process, the real work happens when we apply simple principles with consistency, patience and clarity.
In ultrarunning, as in most things, simplicity isn’t a shortcut – it’s the standard.
Reference List
- Balsalobre-Fernández, C., Santos-Concejero, J., & Muñoz-López, A. (2023). Effects of strength training on running economy in middle- and long-distance runners: A systematic review and meta-analysis. Sports Medicine. https://doi.org/10.1007/s40279-023-01978-y
- Martínez-Noguera, F., Martínez-Rodríguez, A., Alejo, L. B., Hernández-Davo, J. L., Cejudo, A., & Hernández-Davó, H. (2024). Effect of Weighted Vest at 0%, 5% and 10% of Body Mass on Gasometry Biomarkers and Performance during a Rectangular Test in Trained Trail Runners. Sports, 12(9), 229. https://doi.org/10.3390/sports12090229
- Millet, G.Y., Faes, C., Durey, A., & Ballay, Y. (2011). Neuromuscular consequences of an extreme mountain ultra‑marathon. PLoS ONE, 6(2), e17059. https://doi.org/10.1371/journal.pone.0017059