Muscular Endurance for Ultrarunners: Does High-Rep Strength Training Work?

Written by:

Cliff Pittman

CTS Pro Ultrarunning Coach

Updated On

October 29, 2025

Strength training is an established and recommended component of trail and ultramarathon training, but athletes still have questions about whether they should lift heavy, with relatively low numbers of repetitions, or lift lighter weights for higher rep counts? High-load, low-rep training develops strength, durability, bone density, and tendon resilience. Proponents of low-load, high-rep strength circuits claim they improve muscular endurance. There’s been renewed interest in the idea of strength training for muscular endurance in 2025 because it was cited as a key training component behind elite victories at UTMB. So, if you’re headed to the gym or designing your at-home strength routine, which should you focus on?

What is muscular endurance in ultrarunning?

We can start with the basic definition of muscular endurance, which is the ability of a muscle or muscle group to sustain force or repeated contractions over time without excessive fatigue. Dr. Nick Tiller provides a more sport-specific version of this, stating, “[Muscular endurance] in ultra-running is the ability of the muscles to sustain the rigors of the event without undue stress or injury.” Because of the duration of ultramarathons, muscular endurance is more about surviving repetitive force production over time and resisting structural breakdown and neuromuscular fatigue.

Do Low-Load, High-Rep Strength Circuits Develop Muscular Endurance?

Low-Load, High-Rep (LLHR) strength circuits are an attempt to mimic the specificity of repetitive muscular contractions in a fatigued state, like an athlete would experience in the final third of an ultramarathon event. In practice, you’ll see LLHR circuits that include exercises like post-run lunges, air squats, and kettlebell swings. But in contrast to traditional strength training that focuses on a prescribed number of repetitions, LLHR circuits are often prescribed as AMRAP (as many reps as possible), either for an elapsed time (e.g., one minute) or until the athlete feels sufficiently wrecked.

A 2021 review study by respected strength and conditioning researcher Brad Schoenfeld and colleagues examined current evidence into the “repetition continuum”, which they explained as:

“The repetition continuum proposes that the number of repetitions performed at a given magnitude of load will result in specific adaptations as follows:

1. A low repetition scheme with heavy loads (from 1 to 5 repetitions per set with 80% to 100% of 1-repetition maximum (1RM)) optimizes strength increases.
2. A moderate repetition scheme with moderate loads (from 8 to 12 repetitions per set with 60% to 80% of 1RM) optimizes hypertrophic gains.
3. A high repetition scheme with light loads (15+ repetitions per set with loads below 60% of 1RM) optimizes local muscular endurance improvements.”

The concluded that, “Evidence for a load-specific effect on local muscular endurance remains equivocal.” They added, “Studies that have compared the effects of heavy- and moderate-load training show similar increases in muscular endurance between the conditions.” (Schoenfeld et al., 2021)

In other words, muscular endurance increases from strength training anywhere along the repetition continuum, not specifically from LLHR strength circuits. That means LLHR can be a component of strength training but are not necessarily the secret weapon to creating muscular endurance.

Do Low-Load, High-Rep Circuits Improve Ultrarunning Performance?

Here’s the more specific question. If strength training in general can improve muscular endurance, is it better for ultrarunners to focus specifically on LLHR circuits because there’s a sport-specific benefit? Not as much as you’d hope.

CTS Ultrarunning Coach Nick DiMarco fields this question 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.” LLHR 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 LLHR training 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.

Mistaking Specificity for Imitation

The specificity principle of training essentially states that your training must address the specific demands of your goal activity. The simplest application of the principle is that runners need to run. But to achieve overload and allow for adequate recovery (also training principles), you must find more creative ways than “just run more” to address the specific demands of ultramarathons. Hence, the interval workouts, intensity distributions, strength training, and other interventions that improve fitness and performance beyond what you can achieve by just running more.

One of the mistakes athletes make is conflating imitation with specificity. Athletes often assume that training in a similar format (e.g., long, repetitive, and tiring) will transfer. But, just because ultrarunning requires enduring muscular effort doesn’t mean that copying fatigue in the gym improves performance. 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 because it’s just more of the same stress. To be effective, the gym work needs to exceed the demands faced on the trail, but in a measurable and controlled way.

DiMarco advises athletes, “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.” That means heavy bilateral and unilateral lifts, explosive plyometrics, and isolated rehab work. You’re not necessarily trying to copy movement patterns from running, either. Your goal is to support those running-specific movement patterns with complementary stressors that enhance performance.

Evidence-Supported Ways to Build Muscular Endurance For Ultrarunning

If LLHR strength circuits don’t reliably improve muscular endurance for ultrarunners, what does?

1. High Run Volume

   The frustratingly simple answer is that muscular endurance develops alongside other key adaptations from progressive, terrain-specific, high-volume running. As Dr. Nick Tiller puts it, “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. These adaptations are best developed through sustained, sport-specific training

   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.

2. Strength Training

   A more integrated approach to strength training is important for running performance, including muscular endurance. DiMarco explains, “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.”

   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 running can’t.

3. Steep Inclines

   Steep uphills and downhills can be used to exceed the demands you’ll face during an ultramarathon by manipulating pace, time-at-intensity, and frequency. For runners without access to steep terrain, incline treadmill workouts or stairmills 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.

4. “All Of The Above” Approach

   What is perceived as local muscular fatigue isn’t always related to the muscle’s ability to continue contracting. Dr. Tiller reminds us that “ultra-running 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). Fatigue resistance isn’t all in the muscles.

   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)

References

Balsalobre-Fernández, C., Santos-Concejero, J., & Grivas, G. V. (2016). Effects of Strength Training on Running Economy in Highly Trained Runners: A Systematic Review With Meta-Analysis of Controlled Trials. Journal of strength and conditioning research, 30(8), 2361–2368. https://doi.org/10.1519/JSC.0000000000001316

Millet, G. Y., Tomazin, K., Verges, S., Vincent, C., Bonnefoy, R., Boisson, R. C., Gergelé, L., Féasson, L., & Martin, V. (2011). Neuromuscular consequences of an extreme mountain ultra-marathon. PloS one, 6(2), e17059. https://doi.org/10.1371/journal.pone.0017059

Schoenfeld, B. J., Grgic, J., Van Every, D. W., & Plotkin, D. L. (2021). Loading Recommendations for Muscle Strength, Hypertrophy, and Local Endurance: A Re-Examination of the Repetition Continuum. Sports (Basel, Switzerland), 9(2), 32. https://doi.org/10.3390/sports9020032.