Exercise Intensity: Progressive Abilities

Where do I go from here?

In the previous post, we covered the three foundational abilities: Low Aerobic (LA), Force Margin (FM), and Dynamic Balance (DB). These 3 are the bedrock of Heretic Trail base training and foundational fitness. They're developed first, maintained throughout your entire journey, and never fully leave your practice. They’re often the first things we come back to after a race or event, or in response to injury or irritations.

Once you've built a fitness base, you layer in three progressive abilities: Muscular Endurance (ME), Aerobic Capacity (AC), and High Aerobic (HA). These aren't "advanced" domains reserved for elite athletes. They're simply intensities you develop sequentially, building from and bridging to the foundational work.

Here's the key distinction: foundational abilities are always present in your training. Progressive abilities are emphasized strategically, based on your phase of training, your goals, and your individual needs. You don't train them equally all the time. You bring them in when they serve your progression. Every intensity has its place.

Here are three key things to remember about the progressive abilities and moderate and high-intensity training:

1.       All domains are useful. No intensity domain is "bad," “lesser,” or off-limits. Training works best when you use each intensity as needed. The domains are interconnected. Improvements at one intensity can support and enhance the others. They bridge to each other both up and down, and stepping through them progressively is key to long-term improvement. A well-rounded program touches all the domains over time, in the right proportions.

2.       Different domains adapt on different time scales. Each intensity domain improves and fades at its own rate. Your Low Aerobic base builds slowly over months and years. High Aerobic fitness develops at a moderate pace. Muscular Endurance and Aerobic Capacity can improve relatively quickly, often in a matter of weeks, but also fade faster if you don't maintain them.

3.       Individual needs vary. Your personal strengths and weaknesses determine how much focus you should give. If Aerobic Capacity is a limiting factor for you (short, fast efforts are a struggle, or your pace at AC isn't much faster than your pace at HA), then dedicating time to AC training can pay off greatly. But if it's not a major limiter, you don't need to overemphasize it. You might just do enough to raise the ceiling a bit, then spend more time on other domains. We use and train our abilities— foundational and progressive— strategically and based on your profile, not just because we’re trying to check boxes.

This post explores those three progressive abilities. How they're built. When they matter. And how they connect to everything you've already established.

Let's continue.

Muscular Endurance (ME): The Sustained Force Bridge

Muscular Endurance is the ability of your muscles, specifically those in the legs, hips, and core, to sustain prolonged or repeated contractions at moderate to high force. For trail and mountain athletes, this is vital for maintaining power and stability through uphill grinds, downhill descents, and technical terrain that demands continuous muscular effort.

Here's what distinguishes ME from Force Margin: FM is about explosive, maximum-intensity efforts lasting 10 to 30 seconds. ME is about sustained force production over minutes or even an hour. It's the difference between a single explosive hill sprint and a 20-minute uphill grind at a steady, demanding pace.

Muscular Endurance bridges two foundational abilities. It draws on the force capacity you've built through FM work, but applies that force over time in a way that requires the aerobic base you've developed through LA training. It's the connective tissue between strength and endurance.

What does Muscular Endurance training look like?

There are several effective modalities, each with a slightly different emphasis:

·         Low-cadence cycling: Riding in a harder gear at a slow cadence (40 to 60 revolutions per minute) either sitting or standing forces your leg muscles to produce sustained force with each pedal stroke.

·         Rucking: Moving over uneven ground with weight, forcing your legs to drive powerfully with each step.

·         Uphill weighted hiking or running repeats: Carrying a weighted vest or backpack during uphill hikes or runs adds resistance, forcing your muscles to work harder to maintain pace. This can be done on a treadmill or outside.

·         Low-cadence bounding uphill repeats: Longer bounding efforts (3 to 8 minutes) at a lower cadence (50 to 70 steps per minute) on a sustained hill, where the limiting factor is muscular fatigue rather than cardiovascular strain.

·         Running down stairs (or the Manitou Incline): improves downhill specific muscular endurance

The unifying principle: you're asking your muscles to produce force repeatedly over an extended period, creating a training stimulus that's neither purely strength-based nor purely aerobic.

What limits you at Muscular Endurance intensities?

The primary constraint is localized muscle fatigue. Your quads, glutes, hamstrings, and calves accumulate fatigue as they contract repeatedly under load. Glycogen depletion in those working muscles also plays a role, especially during longer ME efforts.

This is different from the central fatigue that limits Low Aerobic work or the explosive failure that limits Force Margin work. With ME, it's the muscles themselves that say "enough." They burn. They ache. They lose their ability to generate force efficiently.

What does Muscular Endurance feel like?

Muscular burn, not breathlessness. Yes, your breathing will be elevated, especially during harder ME efforts, but the dominant sensation is in your legs. They feel heavy, loaded, working hard. It's the feeling of sustained effort in the muscles, not the cardiovascular system.

After an ME session, your legs will be tired, possibly sore the next day. But you won't feel systemically drained in the way you might after a hard aerobic effort. The fatigue is localized.

Why is Muscular Endurance progressive?

Because it requires both a Force Margin base and a Low Aerobic base to train effectively. If your muscles aren't strong enough to produce force efficiently (that's FM), ME work will overwhelm them and lead to injury or excessive fatigue. If your aerobic system isn't developed enough to sustain effort over time (that's LA), you won't be able to execute ME workouts at the necessary duration or intensity.

ME is a bridge. You build it after establishing the foundation on both sides.

When do you emphasize Muscular Endurance in training?

Typically in the build phase, after you've established a solid aerobic base and maintained your Force Margin work. For mountain and ultra athletes and adventurers, ME becomes increasingly important as a bridge to race-specific preparation. Long climbs, technical descents, and sustained efforts on challenging terrain all demand muscular endurance.

But you don't train ME year-round at the same volume. Early in a training cycle, you might do very little. As you progress from base toward a goal, ME work becomes more prominent. Then, as you peak specifically for a race, ME volume decreases while race specific work takes over.

In practice, what does Muscular Endurance training look like?

·         A 4x8-minute session of low-cadence cycling, maintaining a steady, demanding effort

·         Uphill repeats of 5 minutes each on a steep trail or treadmill, carrying 5%+ (running) or 10%+ (hiking) body weight

·         A rucking (weighted vest) hike of 60+ minutes, carrying 15%+ body weight

·         Uphill low-cadence bounding drills for 3 to 5 minutes at a time, with long rest intervals

·         Running down stairs for 3+ minutes at a time in intervals, building to continuous over time

Muscular Endurance is especially important for athletes targeting mountain ultras, vertical kilometer races, or any event/adventure where sustained climbing and descending are major components. But even less extreme terrain benefits from ME work, whether that’s being able to walk around a hilly city all day or your neighborhood open space. Any time you're asking your legs to sustain effort over varied, challenging ground, muscular endurance matters.

Aerobic Capacity (AC): The Ceiling Raiser

Aerobic Capacity is the domain between your Critical Speed/Pace and your maximum ability to consume oxygen (VO2max). In practical terms, this is the range of intensities where you're working near your aerobic limit: short, hard intervals that push you toward the ceiling of your cardiovascular system.

VO2max, by definition, is the maximum volume of oxygen your body can use per minute. Training in the AC domain pushes you toward that ceiling, stimulating adaptations that raise your maximum aerobic capacity.

What limits you at Aerobic Capacity intensities?

Almost exclusively peripheral fatigue. Your muscles progressively fail to respond. The cardiovascular demand is maximal, but the breakdown happens locally in the working muscles. Lactate accumulates, pH drops, muscle fibers lose their ability to contract efficiently, and you're forced to slow or stop.

This is why progression and smart workout design are essential in the AC domain. The efforts are very intense, and the line between productive stimulus and excessive fatigue is thin. You need just enough volume to trigger adaptation without digging a hole you can't recover from.

What does Aerobic Capacity feel like?

Anyone who's done classic VO2max intervals will tell you: it's intense. Breathing is heavy and rapid. Legs burn. Heart rate is maximal. You're definitely in the red zone by the end of each interval.

These workouts are taxing, which is why you do them selectively and strategically. The silver lining is that a little bit of AC work goes a long way to stimulate fitness gains. It's like a strong spice in a recipe: very potent in small doses. You don't need much, but what you use matters.

Why is Aerobic Capacity progressive?

Because it's the ceiling of your aerobic system, and you can only raise the ceiling effectively once the foundation is solid. Without a strong Low Aerobic base, AC work becomes overly stressful and hard to recover from. Without Force Margin and Dynamic Balance to support efficient, powerful movement, AC intervals can lead to form breakdown and injury.

AC is the final layer. You build toward it, not from it.

When do you emphasize Aerobic Capacity in training?

In focused blocks, typically after establishing your aerobic base and doing some preliminary work in the High Aerobic domain. For many athletes, AC work appears in the later phases of a training cycle, often 4 to 8 weeks before a goal race, and then tapers off as you approach the event.

But again, individual needs vary. Some experienced athletes benefit from brief  and intense AC blocks earlier in the cycle to "raise the ceiling" before building fitness in other domains. Others who are speed limited need to first build it and then return periodically to AC work to continue the gains.

The key is matching the training to the athlete and the goal. Not everyone needs the same amount of AC work. Some need more. Some need less. That's the art of coaching and the beauty of your lab of one.

In practice, what does Aerobic Capacity training look like?

·         Medium length hill repeats at near-maximal effort: 1 to 3 minutes uphill, jog down, repeated several times

·         Flat running 30/30s, 40/20s, 1/1s, and similar fartlek style AC work

·         Track combos like 200/200s (when building) and eventually 300/100s, 400/200s, and 800/400s

·         Classic VO2max intervals: 2 to 4 minutes at a very hard effort, with equal or slightly shorter recovery, repeated 4 to 6 times (usually late in a block, and only once a foundation of AC work is a established)

After a solid AC workout, you'll be tired. These sessions demand respect. But they're also relatively short in duration, the training load is reasonably small, and with proper recovery, you bounce back quickly and easily.

High Aerobic (HA): The Sustainable Pace Bridge

High Aerobic is the intensity domain between your first ventilatory threshold (VT1, the upper boundary of Low Aerobic) and your Critical Speed or Critical Pace (CS/CP). This is the range where you're working noticeably harder than easy aerobic efforts, but not so hard that you're racing against an unsustainable physiological clock.

CS/CP is a crucial concept, so let's define it clearly. Critical Speed or Critical Pace represents the highest sustainable metabolic steady state: the fastest pace you can maintain without unsustainable metabolic drift toward your maximum aerobic capacity. It's the threshold separating metabolically sustainable from unsustainable intensities.

Here's what that means in practical terms: below CS/CP, your body keeps up with the energy demands. Oxygen consumption stays relatively stable. Lactate doesn't accumulate in a runaway fashion. You can continue until something else stops you (fatigue, glycogen depletion), but you're not in a race against your own physiology.

Above CS/CP, everything changes. Breathing ramps up exponentially. Perceived exertion climbs rapidly. Lactate accumulates faster than your body can clear it. You're on a countdown timer. The clock is ticking, and you will have to stop or slow down soon.

CS/CP marks the edge between cruising and crashing. It's the pace where fatigue starts to accumulate exponentially for the first time.

Here's the good news about CS/CP: Unlike VT1, which you have to feel subjectively through your breathing, CS/CP can be estimated objectively through testing and data analysis. Before you do High Aerobic work, we'll calculate your CS/CP based on recent performances or diagnostic efforts, and we'll refine it as your fitness changes.

What limits you at High Aerobic intensities?

A combination of peripheral fatigue (localized glycogen depletion in muscle fibers) and central fatigue (reduced neural drive) together demand increasing effort just to maintain the same speed. You're not quite at the point where physiological strain spirals out of control, but you're close. The effort is manageable, but only for a finite period.

That's why CS/CP defines the upper boundary of High Aerobic. Anything above it moves you into the Aerobic Capacity domain, where efforts become increasingly time-limited and recovery-intensive.

What does High Aerobic feel like?

In a word: work. Your breathing is definitely elevated, and you know you're pushing. It's not the comfortable rhythm of Low Aerobic. But it's also not a runaway train. You're in control, aware that you're working hard, but confident you can sustain it for a meaningful duration.

Think of a steady, tempo, or subthreshold run at a "comfortably hard" pace. Or a sustained climb where you're breathing steadily but not gasping. You could hold a brief conversation, but you wouldn't want to. That's High Aerobic.

Why is High Aerobic progressive?

Because it's the middle ground that benefits most from having both a strong aerobic base below it and some top-end work above it. Early in a training cycle, you might actually do very little in the HA domain. Instead, you build fitness by emphasizing Low Aerobic work and selectively including efforts above (Aerobic Capacity) and tangential (Muscular Endurance) to HA.

High Aerobic efforts become essential as you get close to a peak race, goal event, or adventure. By that point, you have the LA base to support sustained efforts and the top-end preparation to handle work near CS/CP. The HA domain is where you refine your ability to hold faster sustainable paces for extended periods.

Think of High Aerobic as the bridge between pure easy endurance and the really intense stuff. It's the middle gear you refine last as your training progresses.

When do you emphasize High Aerobic in training?

Typically we emphasize HA in the later phases of a training cycle, after establishing an aerobic base and doing some preliminary work at higher intensities. For athletes preparing for races or adventures with sustained hard efforts, HA work becomes increasingly prominent in the final 6 to 8 weeks before the event.

But the emphasis varies. Some athletes need more HA work than others, and different types, depending on their event demands and individual physiology.

In practice, what does High Aerobic training look like?

·         Tempo intervals at a steady, challenging pace for 60+ minutes of work time

·         Steady sustained climbs on trails where your effort is hard but controlled, not redlining

·         Cruise intervals: repeated efforts of 3 to 10 minutes at HA intensity with short running recovery

·         Subthreshold intervals: staying just below CS/CP

·         Alternating 800s: intervals done for 40-60 minutes alternating slightly faster for an 800 with slightly slower for an 800, never allowing the slow pace to drop

·         Progressive runs that start in Low Aerobic and gradually build into High Aerobic over the course of the session

High Aerobic work is challenging but not crushing. After a well-executed HA session, you should feel like you worked hard but could train again in a day or two. It's stimulating without being devastating.

Integration: The House of Fitness

Now that you understand all six abilities, let's see how they fit together. Think of your fitness like a house:

·         Low Aerobic is the wide base. It's the foundation of your metabolic capacity, the platform everything else rests on. You're always building or maintaining this base.

·         Force Margin is the structural support. It's the strength framework that prevents the house from collapsing under load. Like LA, it's always present in your training.

·         Dynamic Balance is the stability throughout. It ensures the structure moves efficiently and holds together under varied conditions. It's woven into the fabric of the house, not isolated to one floor.

·         Muscular Endurance is the connecting structure. It bridges the foundation (LA and FM) to the floors above. It's what allows sustained effort under load, linking strength and endurance.

·         High Aerobic is the middle floor. It's where you refine your ability to sustain faster paces for extended periods. You build this floor after the foundation and connecting structure are solid.

·         Aerobic Capacity is the ceiling. It's the top of your aerobic system, the maximum height the house can reach. You raise the ceiling strategically, not constantly.

Here's the critical insight: it's all one continuous spectrum. When we talk about distinct domains, we're drawing useful lines on a flowing gradient. In reality, your body shifts gradually from one energy system to the next as intensity rises. There's no on/off switch that flips at a precise pace. There's overlap and blending.

The domains are defined because they correspond to real physiological thresholds (VT1, CS/CP, VO2max), and they help us target training more precisely. But the transitions between domains are fuzzy, not sharp cliffs.

The bridges work both ways. Muscular Endurance bridges from FM and LA upward. High Aerobic bridges from LA below and AC above. Aerobic Capacity bridges from HA below. Each ability connects to the others, supporting improvements across the spectrum.

Progression through training means moving through the domains sequentially. You don't jump straight to High Aerobic work. You build the foundation first (LA, FM, DB), then layer in the structure (ME, AC), and finally renovate the living room to perfection (HA). Over a training cycle, your emphasis shifts. Over years of training, you cycle through these phases repeatedly, each time building from a higher baseline.

That's the art of training progression. Not random workouts. Not equal emphasis on everything all the time. Intelligent sequencing, based on where you are and where you're going.

Your Choice: Building from the Foundation

You now understand the full landscape of Exercise Intensity (EI). Six abilities: Three foundational abilities that you maintain throughout your training journey, and three progressive abilities that you build strategically, bridging from the foundation.

This knowledge transforms training from guesswork into purposeful practice. You're no longer blindly following a plan or hoping that random hard efforts will make you better. You understand what each intensity does, when to use it, and how it connects to everything else.

But understanding is just the first step. The real wisdom comes through practice. Through learning to feel these intensities in your body. Through noticing how your breathing changes at VT1, how your legs respond during Muscular Endurance work, what it feels like to sustain effort near CS/CP, or the intensity of pushing toward VO2max.

Start with the foundational abilities. Master Low Aerobic. Build your Force Margin. Train your Dynamic Balance. Those three are always present . . . they're the bedrock.

Then, as your training progresses, layer in the progressive abilities. Include Aerobic Capacity intervals strategically to raise the ceiling. Introduce Muscular Endurance when you're ready to bridge strength and endurance. Add High Aerobic work when you're peaking for sustained efforts at faster paces.

Every trail starts with a question. Every training cycle begins with a goal. Every session is an opportunity to practice. Over time, these practices accumulate into something larger: sustainable progress, enduring fitness, and the confidence that comes from knowing exactly what you're doing and why.

Devoted to reality. Your choice. Your trail.