Exercise Intensity: Foundational Abilities

How hard am I actually working?

This question matters more than you think, and may very well mean something different from your initial interpretation, because training isn't just about "hard" or "easy." It's about what's happening metabolically: how your body is producing energy at different efforts, and how that energy production shapes your adaptation over time.

This is where the concept of Exercise Intensity (EI) becomes helpful. EI refers to the metabolic domain of a given workout: not how difficult it feels (that's a different measure called Difficulty, which we covered in Pace and Difficulty), but which physiological system you're primarily utilizing to do the work underway. Understanding EI gives you clarity, precision, and a roadmap for progression.

If you're new to this concept, it might initially sound very technical. If you've been around training for a while, you've probably seen different labels: zones, thresholds, tempo, intervals. The terminology can be overwhelming, but beneath all that jargon, there are just a few core intensity domains and abilities that apply to every endurance athlete, regardless of pace or background.

At Heretic Trail, we organize our training around six of these domains. Three of them are foundational abilities you will develop first and maintain throughout your entire training journey: Low Aerobic (LA), Force Margin (FM), and Dynamic Balance (DB). The other three are progressive abilities you will build later on in training that will bridge your progress from the foundational work to your progressed work levels. We'll cover those in a separate post.

This post focuses on the three foundational abilities, which are the bedrock of everything else. Master these, and you've built a base that supports sustainable progress for years. And here's the best part: once you learn to feel these intensities in your body, the technical language fades away. The wisdom becomes felt, not just known.

Let's begin.

Low Aerobic (LA): The Endurance Foundation

Low Aerobic is the baseline intensity domain. It starts just above resting effort (essentially any exercise beyond sitting on the couch) and extends up to the point where your breathing first starts to noticeably shift.

That upper boundary (where you notice your breathing change) is often called VT1, or Ventilatory Threshold 1. It's the effort level where you transition from breathing comfortably to breathing more heavily. If you're talking while exercising, VT1 is roughly the point where continuous conversation starts to become difficult. You can still speak, but sentences get shorter. Words require more effort.

Here's what you need to know about finding your Low Aerobic ceiling on any given day:

·         You will use your breathing as the guide. VT1 isn't a fixed pace or heart rate. It's a metabolic shift you can feel. When your breathing changes from relaxed to noticeably working, that's your signal.

·         Your LA pace varies day to day. A 10-minute-per-mile pace might be Low Aerobic on a fresh, cool morning. But on a hot afternoon after poor sleep, that same pace could push you above VT1. Fatigue, heat, stress, and recovery all affect where your threshold sits on any given day.

·         Don't chase numbers. Forget about hitting a specific pace or heart rate zone. Instead, practice tuning into the sensation of your breathing. This is your lab of one.

One of the most important truths about Low Aerobic work is that it does not always feel easy, despite any meaning you might project onto “low”. The label "easy zone" is misleading and causes athletes unnecessary confusion. Yes, short recovery runs or gentle hikes will feel comfortable. But long-duration Low Aerobic efforts can become quite challenging by the end, especially as you accumulate fatigue over hours.

What limits you at Low Aerobic intensities?

The primary culprit is central fatigue: a reduction in your brain and spinal cord's ability to send neural signals to your muscles. After hours of steady movement, even at low intensity, your nervous system starts to tire. The signal weakens and the effort to maintain pace increases.

Muscle damage also plays a role. Rack up enough tissue stress over a long activity, and your ability to maintain pace deteriorates until you're forced to slow or stop. Context matters, too. If you completed a hard Diagnostic Time Trial or race yesterday, even a short LA session today might feel surprisingly tough despite the lower metabolic intensity.

What does Low Aerobic feel like?

The key is the breathing. Even if your legs are tired and your perceived effort is high, staying below that VT1 threshold means that metabolically you're in Low Aerobic territory. Your breathing remains steady and controlled, not labored or ragged.

The following is crucial to understand: executing workouts truly at LA intensity means holding back your effort to remain just under VT1, even when it's tempting to push harder. This restraint is what builds your low-aerobic base of fitness. In fact, training in the Low Aerobic domain forms the foundation of all endurance fitness beyond it. Consistent time spent at this intensity makes everything above it (harder) easier.

Why is Low Aerobic foundational?

Because it's the base layer of your metabolic fitness. Your heart's ability to pump blood. Your lungs' ability to take in oxygen. Your muscles' ability to use that oxygen efficiently over time. All of this develops through patient, consistent Low Aerobic work.

You'll train in this domain throughout your entire training journey. When you're building base fitness, LA work dominates your week. When you're sharpening for a race, you still include LA sessions for recovery and maintenance. When you're returning from injury, LA work is where you start. It's the one intensity you never fully leave behind.

In practice, what does Low Aerobic training look like?

·         Recovery runs or hikes the day after a hard workout

·         Long, steady trail runs or hikes where conversation remains possible

·         Easy-paced morning shakeouts

·         Active recovery sessions on non-running days (easy cycling, hiking, walking)

·         A major part of your weekly training volume, especially during base-building phases

Low Aerobic is the humble workhorse of endurance fitness. It doesn't feel heroic. It doesn't make for great training stories. But it's the foundation everything else is built on. Protect it. Respect it. Return to it again and again.

Force Margin (FM): The Strength Reserve

Force Margin is the highest intensity domain, but in a completely different way than you might expect. These are extremely intense, short efforts: all-out hill or stair sprints lasting 10 to 30 seconds, explosive strength movements, or dynamic power work. Think hills, stairs, sled pushes, explosive plyometric strength training, or traditional weight training.

We call it Force Margin because the goal is to expand the surplus of force and power you have available. By improving your top-end strength and, more specifically, your ability to recruit that strength dynamically, you create a bigger buffer (margin) that makes all the lower intensities feel easier by comparison.

Here's the concept: if your maximum dynamic force output is a 10, and you're running at an effort that requires a 7, you're working at 70% of your capacity. But if you increase your dynamic max to a 12 through Force Margin work, that same effort now only requires 58% of your capacity. The run feels easier, even though the pace hasn't changed. That's the power of building Force Margin.

What does Force Margin training look like?

The format varies significantly depending on the athlete, their current abilities, and the phase of training. Here are some common modalities:

·         Short hill sprints: 10 to 30 seconds all-out on a steep incline, with walking down for full recovery between reps

·         Short stair sprints: 10 to 30 seconds all-out, with walking down for full recovery between reps

·         Plyometric drills: Box jumps, bounding, single-leg hops, or other explosive movements

·         Strength training: Weighted squats, deadlifts, lunges, step-ups, or other resistance exercises targeting running-specific muscle groups

·         Sled work: Pushing or pulling a weighted sled for short distances

The key distinction: Force Margin work is always short and explosive, with complete recovery between efforts. You're not trying to accumulate fatigue. You're teaching your nervous system to recruit muscle fibers powerfully and efficiently.

What limits you at Force Margin intensities?

The limiting factor in an FM effort is usually your muscles or mechanics, not your breathing. If you sprint all-out for 20 seconds, your legs will cry out before your lungs do. You might feel breathless afterward, but the effort itself is constrained by how much force your muscles can generate and how well your nervous system can coordinate that force production.

This is a feature, not a bug. Force Margin work is about building muscular strength, neuromuscular recruitment, and movement economy. It's not about cardiovascular endurance. That's what the aerobic domains are for.

What does Force Margin feel like?

Brutal, but brief. These sessions demand high effort and focus, but they're over quickly. You give everything you have for a short burst, then recover fully before the next rep. There's no grinding through discomfort for minutes on end. It's more like turning on a light switch: maximum effort, then off.

Because of the short duration and full recovery, Force Margin workouts are less draining overall than they might sound. Yes, each rep is hard. But the total volume of hard work is low, and you're not accumulating the kind of deep fatigue that comes from sustained aerobic efforts.

Why is Force Margin foundational?

Because strength and power underpin everything else you do as an endurance athlete. Stronger muscles and tendons protect your joints from injury. Better neuromuscular coordination improves your running economy, meaning you use less energy at any given pace. Greater force production makes climbs more manageable and descents more controlled.

And here's the critical insight: like Low Aerobic work, Force Margin training is something you'll touch throughout your entire training journey. Even in the final weeks before a big race, you'll include some form of specific FM work to maintain that strength reserve. It's not something you build once and forget about. It's a foundational ability you return to consistently.

In practice, what does Force Margin training look like?

·         A set of 4 to 8 hill sprints, each 10-30 seconds long, at the end of a weekly run

·         A set of 4 to 8 stair sprints (can be weighted), each 10-30 seconds long, either indoors on a stair machine or outdoors (if you have a long unbroken set of stairs)

·         A 30-minute gym session with squats, deadlifts, and lower-leg work twice a week (they key isn’t maximum weight, but to focus on fast force production)

·         Explosive Plyometric Strength drills (squat jumps, bounding, box jumps) as part of a warm-up or standalone session

·         A set of 4 to 8 bicycle sprints using progressively bigger gears, done standing or sitting, 10-15 seconds long (for adventurers new to force work)

·         Weighted vest bounds on steep terrain (only for athletes with a deep base of existing force training already)

The beauty of Force Margin work is its flexibility. A new runner or adventurer might do big gear bike work and traditional gym work. An experienced mountain athlete might do weighted hill sprints and barbell training. A masters athlete returning from injury might do big gear bike-based Force Margin work and bodyweight gym work to minimize impact. The principle remains the same: build the top-end force capacity that supports everything below it.

Advanced is basic, done beautifully, for a long time. Force Margin is one of those basics.

Dynamic Balance (DB): The Movement Foundation

Dynamic Balance is the ability to maintain posture, rhythm, and foot placement while moving through space. It's about changing direction, rebounding off uneven surfaces, and landing with control. It's the skill of staying stable and coordinated while your body is in motion.

Here's what Dynamic Balance is not: static balance. Standing on one leg, balancing on a wobble board, or holding a yoga pose. Those circus tricks might look impressive, but they have surprisingly little carryover to the dynamic, ever-changing demands of trail running, mountain hiking, or any form of real-world endurance movement.

Static balance and dynamic balance are only weakly related. Improving one doesn't guarantee gains in the other. This is a critical distinction that many athletes and even some coaches miss. You can stand on one leg for two minutes without wobbling and still struggle to navigate a technical descent. That's because the neural pathways for static stability and dynamic movement control are different.

Dynamic Balance is about training the kind of stability you actually use on real ground, at real speeds, with real obstacles.

What does Dynamic Balance training look like?

There are several modalities, all high-cadence but focused on controlled, coordinated movement:

·         Strides: Short bursts (10 to 30 seconds) of faster running on flat or gently downhill smooth terrain with an emphasis on form and posture. Strides improve your ability to maintain good mechanics as speed increases.

·         Agility drills on trails: these are direction-change exercises using roots, rocks, uneven ground, and elevation changes that challenge your ability to shift weight and reposition quickly.

·         Agility drills on grass or turf: these are direction-change exercises using cones, ladders, short hurdles, or similar that challenge your ability to shift weight and reposition quickly.

·         Uphill technique work: Deliberate practice of efficient climbing mechanics, focusing on posture, foot placement, and rhythm on steep terrain.

·         Downhill technique work: Controlled descents where you practice reading terrain, adjusting stride length, and landing softly on varied surfaces.

The key is that Dynamic Balance work is done at easier intensities than Force Margin. You're not trying to generate maximum force. You're training neuromuscular coordination, proprioception (your sense of where your body is in space), and the ability to move efficiently through complex environments.

What limits you at Dynamic Balance intensities?

The primary constraint is neuromuscular coordination. Your brain's ability to receive sensory input from your feet, joints, and muscles, process that information rapidly, and send precise motor commands to keep you balanced and moving smoothly.

This isn't about strength (that's Force Margin) or aerobic capacity (that's Low Aerobic and beyond). It's about the wiring: how quickly and accurately your nervous system can adapt to changing conditions.

What does Dynamic Balance feel like?

Controlled. Rhythmic. Attentive. The cadence is very high, but you're moving with intention, paying attention to how your foot lands, how your weight shifts, how your posture adjusts. It's not the mindless shuffle of an easy recovery jog. It's engaged movement.

When you're training Dynamic Balance well, it often feels lighter and more fluid than other types of training. You're not grinding through fatigue or gasping for breath. You're simply moving with precision and awareness.

Why is Dynamic Balance foundational?

Because movement quality matters. If you can't control your body efficiently through space, you'll waste energy, increase injury risk, and struggle on any terrain that isn't perfectly smooth. Dynamic Balance is especially critical for trail and mountain athletes, where uneven ground is the norm, not the exception.

But even road runners benefit from DB work, where strides are a virtually universal and regular practice. Better coordination means better economy. Smoother movement means less wasted motion. And as you age, maintaining dynamic stability becomes even more important for injury prevention and confidence on varied terrain.

Like Low Aerobic and Force Margin, Dynamic Balance is a foundational ability you'll train throughout your journey. You might emphasize it more heavily during certain training phases (for example when base training, then again later when preparing for a technical mountain race), but you'll never abandon it entirely. It's woven into the fabric of good training.

In practice, what does Dynamic Balance training look like?

·         Four to eight strides at any time during an easy run, focusing on smooth acceleration and relaxed form

·         Agility surges on semi-technical or technical trails

·         Agility ladder drills, short hurdles, or cone work as part of a warm-up routine

·         Deliberate uphill hiking or running where you practice efficient weight transfer and foot placement

·         Technical trail runs where the terrain itself provides the balance challenge

·         Downhill repeats on varied surfaces, emphasizing control and rhythm rather than speed

Dynamic Balance work doesn't require special equipment or complicated setups. A slight hill. A section of trail. A few cones or markers. Done at a high cadence. That's often enough. What matters is the intention: you're training your nervous system to coordinate movement, not just accumulating miles or building strength.

Integration: How the Three Foundations Work Together

Low Aerobic, Force Margin, and Dynamic Balance aren't separate silos. They're interconnected abilities that support and reinforce each other.

Low Aerobic work builds your metabolic base. It develops the cardiovascular endurance that allows you to go long. It's the engine that powers everything else.

Force Margin work builds your strength reserve. It makes your muscles and tendons more resilient, improves your movement economy, and creates a bigger buffer that makes all lower intensities feel easier. It's the structural foundation that prevents injury and enhances performance.

Dynamic Balance work builds your movement quality. It ensures you can apply your aerobic fitness and strength effectively, on any terrain. It's the coordination layer that translates raw capacity into efficient, controlled movement.

Together, these three foundational abilities form the bedrock of your training. They're developed first because everything else builds from them. And they're maintained throughout your entire training journey because they never stop being relevant.

Here's the beauty of this approach: these aren't hierarchical. You don't "graduate" from Low Aerobic to Force Margin to Dynamic Balance. You train all three, consistently, in the right proportions for your current phase and goals. Some weeks you'll emphasize one more than the others. Some phases you'll spend more time developing a particular ability. But they all remain part of your practice.

In future posts, we'll cover the progressive abilities: Muscular Endurance, High Aerobic, and Aerobic Capacity. Those are intensities you develop later in a training cycle, building from and bridging to these foundational abilities. But without a solid foundation in LA, FM, and DB, the progressive abilities have nothing to rest on.

Your Choice: Learning to Feel the Differences

If this feels like a lot to absorb, that's normal. Training intensity domains and abilities are one of those concepts that sound more complicated than they actually are. The technical language can obscure a simple truth: your body already knows how to work at different intensities. You've been doing it your whole life.

What you're learning now is how to name those intensities, recognize them consciously, and train them with intention. That's the difference between random exercise and purposeful training.

Start by learning the edges. When does your breathing shift from easy to steady? That's VT1, the upper boundary of Low Aerobic. What does it feel like to give a short, explosive effort? That's Force Margin. Can you move with rhythm and control on uneven ground? That's Dynamic Balance.

Once you learn to feel these intensities in your body, the labels fade. The technical definitions become less important. What matters is the felt sense: you know when you're in Low Aerobic because your breathing tells you. You know when you're doing Force Margin work because your muscles, not your lungs, are the limiter. You know when you're training Dynamic Balance because your attention is on coordination, not just effort.

Practice the practice. Every run, every hike, every strength session is a chance to notice how your body responds. To calibrate your internal sense of intensity. To build wisdom through experience.

This is your lab of one. The training works from the inside out. And the more you practice tuning in, the clearer it all becomes.

In the next post, we'll explore the progressive abilities: the intensities you develop later, building from this foundation. But for now, focus here. Master Low Aerobic. Build your Force Margin. Train your Dynamic Balance. These three are the bedrock. Everything else rises from them.

Devoted to reality. Your choice. Your trail.