HRV, or heart rate variability, is not a score that tells ultrarunners whether they are fit or unfit on a given day. It is a signal of how well the body is handling overall stress. That stress comes not only from training, but also from sleep, work, travel, fuelling, illness, and life outside running.

The key idea in the episode is that HRV is most useful when viewed as a trend, not as a single daily number. One low reading does not mean much on its own, but a drop over several days, especially alongside tired legs, poor mood, or bad sleep, can be an early warning sign that recovery is slipping.

The episode also explains that runners often misuse HRV by treating it like a green-or-red traffic light. Instead, HRV should be combined with how you feel, your sleep, resting heart rate, and how training is going. It can be especially useful for deciding whether it is the right day for a hard session or whether recovery should come first.

The main takeaway is this: don’t chase a perfect HRV score, use HRV to understand whether your body is absorbing training or just accumulating stress.Key references:

• Buchheit, M. (2014). Monitoring training status with HR measures: do all roads lead to Rome? Frontiers in Physiology.
• Esco, M. R., Flatt, A. A., Nakamura, F. Y., et al. (2025). Monitoring Training Adaptation and Recovery Status in Athletic Populations Using Heart Rate Variability. Sports Medicine / PMC review.
• Düking, P., Zinner, C., Reed, J. L., et al. (2021). Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies: A systematic review with meta-analysis. Journal of Science and Medicine in Sport.
• Manresa-Rocamora, A., Sarabia, J. M., Javaloyes, A., et al. (2021). Heart Rate Variability-Guided Training for Enhancing Cardiac-Vagal Modulation, Aerobic Fitness, and Endurance Performance: A Systematic Review with Meta-Analysis. International Journal of Environmental Research and Public Health.
• Schmitt, L., Regnard, J., Parmentier, A.-L., et al. (2015). Monitoring Fatigue Status with HRV Measures in Elite Athletes: An Avenue Beyond RMSSD? Frontiers in Physiology.
• Sammito, S., Böckelmann, I., et al. (2024). Update: factors influencing heart rate variability – a narrative review. Frontiers in Physiology.
• Herzig, D., Eser, P., Omlin, X., et al. (2018). The Association Between Endurance Training and Heart Rate Variability: The Confounding Role of Heart Rate. Frontiers in Physiology.

Training fatigue is normal. Overtraining is not. And very often, what runners call “overtraining” is actually a mix of excessive load, poor recovery, and under-fuelling.

This episode explains the difference between normal fatigue, non-functional overreaching, true overtraining syndrome, and Relative Energy Deficiency in Sport, or RED-S. The key message is that not all fatigue is the same, and the solution depends on the cause.

For ultrarunners, the biggest risks often come from stacking hard training, life stress, poor sleep, and low energy availability for too long. Warning signs include persistent tiredness, loss of performance, poor recovery, low mood, repeated illness, hormonal disruption, and recurring injuries.

The practical takeaway is simple: monitor warning signs early, fuel properly for the work you are doing, and make recovery as deliberate as training. The goal is not to avoid fatigue, but to make sure it is recoverable.

Main takeaway: Your body does not adapt to training you survive. It adapts to training you can recover from.

Key references

• Meeusen et al. ECSS-ACSM consensus on overtraining syndrome.
• Mountjoy et al. 2023 IOC consensus statement on RED-S.
• Stellingwerff et al. Overtraining Syndrome and RED-S: shared pathways, symptoms and complexities.
• Saw et al. Subjective self-reported measures for monitoring athlete fatigue.
• IOC consensus on load in sport and risk of injury.

Training hard has its place, but harder is not always better. In this episode, we explore why the real value of a session is not just in the stimulus it provides, but in the recovery cost it creates.

For trail and ultra runners, going too hard too often can quietly reduce consistency, compromise quality, increase injury risk, and leave you too fatigued to absorb the training that actually matters.

The key message is simple: the best training is not the hardest training, but the training you can recover from and repeat.

Key references:

• Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? 2010.
• Stöggl TL, Sperlich B. The training intensity distribution among well-trained and elite endurance athletes. 2015.
• Sperlich B, et al. The proportional distribution of training by elite endurance athletes. 2023.
• Casado A, et al. Training Periodization, Methods, Intensity Distribution, and Volume in Highly Trained Endurance Athletes. 2022.
• Jones CM, et al. Training Load and Fatigue Marker Associations with Injury and Illness. 2016.
• Drew MK, Finch CF. The Relationship Between Training Load and Injury, Illness and Soreness. 2016.
• Gabbett TJ. The training-injury prevention paradox. 2016.
• Kreher JB, Schwartz JB. Overtraining Syndrome: A Practical Guide. 2012.

Downhill running is one of the most damaging parts of trail racing, not because it is aerobicly hard, but because it places heavy eccentric load on the muscles, especially the quads. In this episode, we explain why downhill running creates so much soreness and fatigue, and why the solution is not to avoid it, but to train it progressively.

The key idea is that downhill training should be treated like strength training for runners: small doses create adaptation, but too much too soon can disrupt the rest of your training. We cover the repeated bout effect, common mistakes runners make, and how to build downhill durability safely through controlled exposure, good technique, and eccentric strength work.

The main takeaway is simple: train downhill like strength work, not like free speed.

Key References:

• Bontemps et al. (2020), review on downhill running, muscle damage, fatigue, and adaptation.

• Coratella et al. (2024), downhill running increases muscle damage markers and impairs force production, with recovery taking several days.

• Tallis et al. (2024), repeated bout effect in downhill running in trained runners.

• McHugh (2003), classic review on the repeated bout effect after eccentric exercise.

• Calvo-Rubio et al. (2024), review of mechanisms behind the repeated bout effect.

• Baxter et al. (2024), once-weekly submaximal eccentric resistance training can still improve neuromuscular function.

In this episode of The Trail Running Briefing, we explore one of the most important but misunderstood ideas in endurance training: you often run faster by slowing down more often. Many runners make the mistake of pushing too hard on easy days, turning most of their training into moderate effort and limiting recovery, consistency, and performance. This episode explains why truly easy running is essential for building aerobic fitness, supporting recovery, and preparing you to perform better in key sessions and races. The message is simple: easy runs should feel easy, and that discipline is often what leads to long-term progress.

Key references:

• Seiler S. (2010). What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes?
• Esteve-Lanao J, San Juan AF, Earnest CP, Foster C, Lucia A. (2007). How Do Endurance Runners Actually Train? Relationship with Competition Performance.
• Stöggl T, Sperlich B. (2014). Polarized Training Has Greater Impact on Key Endurance Variables than Threshold, High Intensity, or High Volume Training.
• Neal CM, Hunter AM, Galloway SDR. (2013). Six Weeks of a Polarized Training-Intensity Distribution Leads to Greater Physiological and Performance Adaptations than a Threshold Model in Trained Cyclists.
• Rosenblat MA, Perrotta AS, Vicenzino B. (2019). Polarized vs. Threshold Training Intensity Distribution on Endurance Sport Performance: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.
• Casado A, González-Mohíno F, González-Ravé JM, Foster C. (2022). Training Periodization, Methods, Intensity Distribution, and Volume in Highly Trained and Elite Distance Runners: A Systematic Review.
• Haugen T, Sandbakk Ø, Enoksen E, Tønnessen E, Seiler S. (2022). The Training Characteristics of World-Class Distance Runners: An Integration of Scientific Literature and Results-Proven Practice.

In this episode of The Trail Running Briefing, we unpack why Zone 3 is one of the most misunderstood training intensities in endurance sport. Often dismissed as “junk miles” or the “grey zone,” Zone 3 is frequently criticised simply because many runners use it by accident rather than with a clear purpose.

This episode explains why that view is too simplistic, especially for trail runners and masters athletes. We explore how well-structured Zone 3 work can help build strong, sustainable endurance, improve climbing-specific fitness, and develop the ability to manage lactate efficiently during harder efforts.

We also look at why combining brief periods of Zone 4 with sustained Zone 3 work can be so effective. Instead of seeing lactate as just a problem, this approach helps runners understand how the body can reuse lactate as a fuel source, while avoiding the excessive mechanical stress that often comes with faster, more aggressive sessions.

The key message is simple:

Zone 3 is not junk when it is used deliberately. The real mistake is drifting into it too often without intent.

This episode gives trail runners a practical framework for using Zone 3 wisely within a balanced training week.

Key references:

• Brooks GA. The Science and Translation of Lactate Shuttle Theory. Cell Metabolism. 2018.
• Faude O, Kindermann W, Meyer T. Lactate Threshold Concepts: How Valid Are They? Sports Medicine. 2009.
• Seiler S. What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes? International Journal of Sports Physiology and Performance. 2010.
• Billat LV. Interval Training for Performance: A Scientific and Empirical Practice. Sports Medicine. 2001.

This episode explains fuel utilisation in simple terms: your body is always using a mix of fat and carbohydrate, but the harder you run, the more you rely on carbs.

The key message is that many runners don’t “blow up” because they forgot to eat, they blow up because their pace created a higher carbohydrate demand than their fueling plan could support.

The episode uses a simple mental model of two fuel tanks:

• Fat tank = large, slower energy, supports easier efforts

• Carb tank = smaller, faster energy, increasingly important as intensity rises

It then shows how this appears in training and racing:

• Easy long runs often feel manageable

• Harder sessions, climbs, and surges can quickly increase carb demand and lead to fatigue if under-fueled

Common mistakes covered:

• Fueling by habit (same grams/hour for every run)

• Under-fueling key sessions to “train fat burning”

• Confusing training adaptations with race-day strategy

Practical advice:

• Match fueling to the session goal

• Practice race fueling in training

• Use pacing as part of your fueling strategy (surging early makes fueling harder)

Pace and fueling must work together: the harder the effort, the more carbohydrate you need to support it.

Main takeaway

Pace and fueling must work together: the harder the effort, the more carbohydrate you need to support it.

Key references:

• Jeukendrup (2014), Sports Medicine – carbohydrate intake during exercise, dose-response, multiple transportable carbohydrates, oxidation limits and practical recommendations.

• Wallis & Podlogar (2022), GSSI Sports Science Exchange – contemporary carbohydrate guidance for endurance athletes (before, during, after exercise; periodized carbohydrate intake).

• ISSN Position Stand on Ketogenic Diets (2024) – increased fat oxidation does not necessarily translate to improved endurance performance.

In this episode, we unpack why lactate threshold is one of the most useful predictors of ultra performance, not because you race at threshold, but because it sets the ceiling for what “sustainable” feels like for hours. A higher threshold means your steady effort costs less energy, surges hurt less, and you drift into the red less often on climbs, headwinds, heat, and technical sections. You’ll learn a simple mental model (threshold = your red line), the three most common training mistakes (turning threshold into a weekly race, using wrong zones, living in the grey zone), and a practical weekly approach: one controlled threshold session, lots of truly easy running, plus durability work to hold effort late in long runs. The key takeaway: raise your threshold, and your “easy” pace gets faster, that’s real ultra performance.

Key references:

• Joyner, M. J., & Coyle, E. F. (2008). Endurance exercise performance: the physiology of champions. Journal of Physiology.
• Bassett, D. R., & Howley, E. T. (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine & Science in Sports & Exercise.
• Seiler, S. (2010). What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance.
• Stöggl, T., & Sperlich, B. (2015). Polarized training has greater impact on key endurance variables than threshold or high-intensity training. Frontiers in Physiology.
• Brooks, G. A. (2020). The lactate shuttle theory. Cell Metabolism.
• San Millán, I., & Brooks, G. A. (2018). Assessment of metabolic flexibility and its role in performance. Sports Medicine.
• Vanhatalo, A., Jones, A. M., & Burnley, M. (2011). Application of critical power in endurance sport.
• Training for the Uphill Athlete – Johnston, S., House, S., & Jornet, K.
• Endure – Alex Hutchinson
• Science of Ultra – multiple episodes on durability, fueling, and pacing.