Introduction

Trail running is an endurance discipline, but speed and the ability to produce intense efforts play a decisive role in tackling climbs, accelerating between obstacles, and gaining positions on descents. To progress, it’s not enough to string together long runs: incorporating interval training (VMA work) and threshold work (active endurance) is essential to increase your VO₂max, improve your stride, and recycle lactic acid more efficiently. These workouts allow you to combine power and endurance, and they fit into an overall approach based on your personal context (work, family life), your level, and your goals.

1. Understanding VMA, Threshold Work, and Intensity Zones

VMA and Uphill VMA

VMA (Maximal Aerobic Speed) corresponds to the speed at which you consume the maximum amount of oxygen. In trail running, we distinguish between standard VMA (on flat terrain) and uphill VMA, which refers to vertical speed in ascents. A high VMA increases your "engine capacity"—the amount of oxygen your muscles can use—and enables you to run faster on all types of terrain. Knowing your VMA helps you calibrate your sessions: a half‑Cooper test (6 minutes all‑out) provides an estimate, while the Vameval test or a 6‑minute uphill test determines your uphill VMA.

Threshold Work and Intensities

Threshold work is performed at the boundary between aerobic endurance and anaerobic effort. It corresponds to an intensity close to 80–85% of your maximal heart rate (MHR) or VMA. At threshold pace, breathing becomes slightly labored—you can still speak, but only in short sentences. This zone develops the ability to sustain a steady effort and improves lactic acid recycling. Regular threshold sessions strengthen both speed and endurance.

Intensity Zones I3, I4, and I5

In many training plans, intensities are classified into zones: I3 corresponds to a moderate tempo at 85–92% MHR; I4 (threshold) sits at 92–96%; I5 covers VO₂max efforts at 96–100%. Interval sessions are mainly performed in I4 and I5, while threshold work falls within I3/I4. Be sure to personalize these zones based on your VMA and maximal heart rate.

2. Why Include Interval Training and Threshold Work

Improved Cardiovascular Capacity and VO₂max

Interval sessions maximize the time spent at high intensity and increase VO₂max—the amount of oxygen your body can use. The higher your "engine capacity," the more you can sustain high intensity without fatigue. This reserve is essential in trail running, where pace variations are constant.

Improved Stride and Technique

Working at speeds faster than your race pace improves stride length and relaxation. Interval training forces you into a more dynamic stride, which has positive effects on technique both uphill and downhill. Fast‑downhill sessions teach you to commit to the slope and anticipate obstacles.

Natural Strength Development

Repeats on hills naturally strengthen muscles: positive elevation develops strength endurance, while descents work the quadriceps eccentrically. Interval training thus becomes a general physical preparation tool, complementing the targeted strength training sessions already detailed in your previous content.

Mental Adaptation and Lactic Acid Recycling

Threshold work trains your body to handle prolonged discomfort and improves lactic acid recycling. Mentally, completing 10‑ to 15‑minute threshold segments builds resilience and prepares you for tough race phases.

3. Different Types of Interval Training for Trail Running

Interval training includes a variety of workouts based on effort duration, intensity, and elevation. Here are the key formats to integrate into your weekly routine:

3.1 Short Intervals (Short VMA)

The 30/30 is the classic: 30 seconds at 100–105% VMA followed by 30 seconds of jogging recovery. This session can be done on a track or smooth trail. For example, perform 2 sets of 10 repetitions (20 × 30/30) with 3 minutes of recovery between sets.

Intensity: 95–105% VMA (Zone I5), RPE 9/10.
Recovery: equal to work time (30 s).
Goals: develop aerobic power, improve the ability to handle repeated accelerations, and prepare the cardiorespiratory system for pace changes.

3.2 Long Intervals (Long VMA)

Intervals of 2 to 6 minutes are run at 88–95% VMA with active recovery of equal or slightly shorter duration. For example, perform 5 × 3 minutes at 90% VMA with 2 minutes of jogging recovery.

Intensity: 90–95% VMA (Zone I4/I5), RPE 8–9/10.
Total volume: 4 to 7 km of work.
Goals: increase tolerance to prolonged efforts, develop VO₂max, and improve uphill endurance.

3.3 Hill Intervals (Strength Endurance)

To build specific muscular power, perform hill accelerations. Example: 6 to 8 repetitions of 200–300 m on a 10–15% incline. Run while keeping a long stride and upright posture. Recover by jogging downhill.

Intensity: 90–100% VMA (Zone I4/I5), RPE 8–9/10.
Goals: improve strength endurance, strengthen quads and calves, prepare for long ascents.
Frequency: once per week during the specific development phase; reduce to every other week during tapering.

3.4 Fartlek and Combined Intervals

To avoid monotony and improve adaptability, vary your sessions:

• Nature fartlek: alternate 1–3‑minute fast segments with relaxed recovery.

• Pyramid workouts: 1 min, 2 min, 3 min, then back down (2, 1).

• Combined VMA/threshold: start with short repeats (5 × 1 min at 100% VMA), follow with 2 × 10 min at threshold, and finish with a cool‑down. This simulates race‑like pace changes.

Limit interval training to two sessions per week to allow absorption and avoid overtraining.

4. Threshold Work: Developing Active Endurance

Definition and Intensity

Threshold work is performed at 80–85% MHR, or roughly 80–83% of VMA. At this pace, conversation becomes difficult but still possible in short sentences. The goal is to sustain this rhythm long enough to improve lactic acid recycling and delay fatigue.

Why Train at Threshold

Training at threshold accustoms the body to longer discomfort and simultaneously improves speed and endurance. Fifteen‑minute threshold segments also build mental resilience because they require maintaining steady effort without “breaking down.”

Specific Threshold Session

Here is an example of an 11‑week progression suitable for long trail races:

This progression, typical of trail training plans, gradually increases threshold block duration and includes deload weeks to allow recovery. Be sure to warm up for at least 15 minutes and include a cool‑down phase.

Integrating Threshold into Long Runs

You can also incorporate threshold segments into long runs: run 1h30 at an easy pace, then add 2 × 10 minutes at threshold uphill or on rolling terrain. This prepares you to maintain steady climbing pace after several hours of running.

5. Planning and Integrating Intervals and Threshold Work into a Season

Weekly Distribution and Basic Rules

• One short VMA or hill‑interval session per week during the general development phase, then every 10 days during the specific phase.

• One threshold session per week, generally longer or included within a long run.

• Easy endurance runs to make up 60–70% of your weekly volume.

• Strength training and cross‑training: 1–2 sessions per week to stabilize your stride and prevent injury (core work, squats, Myocross Max).

• Rest and recovery: at least one rest day per week, plus regular deload weeks.

• Follow the 80/20 rule: 80% low intensity, 20% moderate to high intensity to avoid overtraining.

Adaptation Based on Level and Context

• Beginners: start with short intervals (6–8 × 30/30) and one 10‑min threshold block per week. Avoid steep climbs and prioritize endurance.

• Intermediate runners: alternate one VMA session (short or long), one threshold session, and one long run, with added variety (fartlek, pyramid).

• Advanced runners: mix VMA sessions on flat and hills, perform 2 × 15‑min threshold blocks, and integrate threshold segments into long runs over 2 hours.

• Busy work or family schedules: choose short but intense sessions (30/30 intervals or 4 × 4‑min threshold), supplemented with cycling or home‑strength sessions to maintain progress without eating up personal time.

• Specific goals (short trail vs. ultra): for short trail (10–20 km), emphasize VMA and uphill speed. For ultras, focus more on strength endurance and longer threshold blocks while limiting the frequency of intense sessions.

6. Tips to Optimize Your Sessions and Recovery

• Warm‑up and technique: do 15 minutes of easy jogging plus mobility drills before each intense session. Work on uphill stride and downhill cadence.

• Active recovery: jog between reps, practice yoga or easy cycling on following days, and sleep enough. Good recovery is key to progress.

• Gradual progression: increase repetitions or threshold block duration progressively, with a deload week every three weeks.

• Hydration and nutrition: fuel properly before and after sessions (carbs for energy, protein for recovery) and hydrate regularly.

• Listen to your body: if you feel excessive fatigue, persistent pain, or declining performance, reduce intensity or take an extra rest day.

• Mental tools: use visualization and mantras to stay motivated during hard reps and remember your long‑term goals.

Conclusion

Interval training and threshold work are powerful tools to boost your speed in trail running. By combining short and long high‑intensity intervals, hill sessions, threshold blocks, and long endurance runs, you simultaneously develop your VO₂max, muscular strength, and ability to sustain demanding efforts. Success lies in balance: smart planning, respecting recovery, personalizing training to your strengths and constraints, and preserving the spirit of exploration and enjoyment that defines trail running. With consistency and patience, these sessions will propel you to new heights—whether you’re aiming for a 20‑km mountain race or an epic ultra.