Why High-Intensity Training Makes You a Faster Runner

Many runners share the same quiet hope: get faster without constantly pushing into the red. If you train alone — like I do — sessions that leave you gasping are hard to sustain day after day.

My early improvements came from following Jack Daniels’ Running Formula, and my times improved quickly. In late 2021, I shifted toward the double-threshold model used by Jakob Ingebrigtsen. Since 2022, though, my progress has stalled.

Looking back, the most obvious change is a sharp drop in high-intensity training — replaced by threshold work. That realization prompted me to revisit the question: why does high-intensity training matter, and how much do you actually need?

This article explains the physiology behind high-intensity training and how to incorporate it into your program effectively.

High-Intensity Training Is Non-Negotiable for Getting Faster

The conclusion first: consistent high-intensity training is essential if you want to get faster. No runner has ever reached the world’s elite by jogging alone.

That said, high-intensity training does not mean gasping for air every session. And more high-intensity training is not always better.

The sections below explain why high-intensity training works, what it does to your body, and how to use it effectively.

What Is High-Intensity Training?

Training intensity is commonly divided into five zones. Zone 4 and above corresponds to high-intensity training.

Rep workouts and sprint-based sessions, which exceed 100% of VO2max, are also classified as high-intensity.

Why High-Intensity Training Makes You Faster

Here is a summary of the key adaptations high-intensity training produces:

Each of these is explained in detail below.

Mitochondrial Function Depends on Training Intensity

The first benefit is improved mitochondrial function.

It’s often said that jogging increases mitochondrial volume — and that’s true. But when it comes to mitochondrial function, research shows a strong correlation with training intensity.

Four key signaling molecules drive mitochondrial biogenesis: AMPK (AMP-activated protein kinase), CaMK II (calmodulin-dependent kinase II), p38 MAPK (p38 mitogen-activated protein kinase), and lactate.

AMPK and CaMK II in particular are more readily activated at higher training intensities. That said, research on intensity-dependence is not yet fully consistent — and the case for high-intensity training may be clearer when viewed through the lens of fast-twitch fiber recruitment ※1.

Adaptations occur only in the muscle fibers that are actually used. Fast-twitch fibers that go unrecruited receive no mitochondrial stimulus. Low-intensity jogging alone cannot fully recruit your fast-twitch fibers — so those fibers miss out on the adaptation.

To adapt the fast-twitch fibers you need for faster running, you must recruit as many fibers as possible — and that requires high-intensity training.

Enhanced Lactate Transport from Fast-Twitch Fibers

High-intensity training also improves the capacity to export lactate from fast-twitch fibers into the bloodstream.

Fast-twitch fibers are low in mitochondria and cannot process lactate internally. As a result, lactate accumulates around the fibers. The hydrogen ions produced alongside lactate acidify the blood, making it progressively harder to keep fast-twitch fibers firing.

Excess lactate is released from fast-twitch fibers, taken up by slow-twitch fibers, and metabolized there. This process is called the lactate shuttle.

Releasing lactate from the cell requires a transporter called MCT4 (monocarboxylate transporter 4).

Fransson et al. (2018) ※2 reported that elite male soccer players who performed high-intensity sprint training for four weeks showed a 30–61% increase in MCT4 protein expression. High-intensity training that recruits fast-twitch fibers appears to be essential for upregulating MCT4. Low-intensity endurance training, by contrast, produces no comparable change.

Fast-Twitch Fibers Gain Endurance

The principle applies here too: only the muscles you actually use will adapt. To run fast with sustained endurance, your fast-twitch fibers need to develop their own endurance capacity.

Human muscle has three fiber types: slow-twitch muscle fibers (Type I), type IIa muscle fibers (intermediate), and Type IIx muscle fibers (fast-twitch). At low intensities, only slow-twitch fibers are recruited. As intensity rises, type IIa fibers join in — and at very high intensities, Type IIx fibers are engaged as well. Figure 1 illustrates this relationship.

To convert fast-twitch fibers into endurance-capable type IIa muscle fibers, you need to train them repeatedly and accumulate volume.

Fast-twitch fibers that once relied entirely on anaerobic metabolism can develop aerobic capacity after converting to type IIa fibers — and the body’s overall oxygen uptake increases as a result.

Achieving this conversion requires repeated high-intensity training. By consistently recruiting fast-twitch fibers, they gradually gain endurance — higher mitochondrial content, better lactate export — allowing you to sustain faster paces for longer.

Improved Muscle Buffering Capacity

High-intensity exercise generates large amounts of hydrogen ions — produced alongside CO2, lactate formation, and ATP breakdown.

Rising hydrogen ion concentrations limit performance in two main ways:

In short: high-intensity exercise produces hydrogen ions that work against your ability to keep moving. The body counters this through buffering action — preventing muscles and blood from becoming too acidic. Buffering in muscle occurs through two main mechanisms:

When hydrogen ions accumulate during intense exercise, the body reduces their concentration by ① binding them to intracellular buffers and ② transporting them out of the muscle fiber via MCTs.

Carnosine — one of the key intracellular buffers — is found in high concentrations in fast-twitch fibers. Low-intensity training barely recruits these fibers, leaving their muscle buffering capacity untapped. High-intensity training actively engages fast-twitch fibers, allowing carnosine’s full buffering potential to be realized.

Neural Adaptation: More Motor Units Recruited

High-intensity training also develops the nervous system. To illustrate: when resistance training begins, strength gains in the early weeks come largely from neural adaptation rather than muscle hypertrophy. The figure below shows this relationship.

Muscle strength improves through two mechanisms: muscle hypertrophy — where fibers grow larger and stronger — and neural adaptation. Neural adaptation dominates in the early phase of training.

Muscle hypertrophy is straightforward to picture: bigger muscles lift more weight. Neural adaptation is subtler.

Put simply: “repeating forceful movements trains your body to activate more motor units simultaneously — so each effort recruits more muscle fibers.”

Many muscle fibers lie dormant in everyday movement. Intense exercise gradually wakes them up and brings them into action.

The same applies to running. Short sprints and other high-intensity efforts trigger neural adaptation, increasing the muscle force you can produce in a single stride.

Neural adaptation is one more benefit that only high-intensity training can deliver.

Evidence: What High-Intensity Training Did to Thoroughbreds

To examine how high-intensity training reshapes energy metabolism, researchers had thoroughbred horses perform 3-minute bouts at 110% VO2max for 9 weeks.

The result: glycolytic enzyme activity did not increase, but mitochondrial enzyme activity and fat oxidation enzyme activity both increased significantly.

This means that training at a carbohydrate-heavy intensity zone actually improved fat utilization capacity and carbohydrate/lactate oxidation capacity — in other words, mitochondrial activity.

Enhanced carbohydrate, lactate, and fat oxidation in fast-twitch fibers is precisely what defines FOG (fast oxidative glycolytic) fiber conversion — the shift toward type IIa muscle fibers. High-intensity training drove that conversion.

How to Add High-Intensity Training Without Breaking Down

High-intensity training is essential for improvement — but more is not always better. Here is what to keep in mind when incorporating it.

Recovery Must Keep Pace with Effort

The most critical consideration is this: only do as much high-intensity training as you can fully recover from.

The body adapts through a cycle of stress, recovery, and adaptation. When load exceeds recovery capacity — and the next stress arrives before adaptation is complete — performance stagnates rather than improves.

Research on elite athletes consistently shows a training intensity split of roughly 80% low-intensity and 20% high-intensity.

In a randomized controlled trial by Stöggl & Sperlich (2014) ※3, this polarized training approach (80% low-intensity + 20% high-intensity) produced the greatest improvement in VO2peak (+11.7%), outperforming threshold-dominant, high-intensity-dominant, and high-volume training.

Multiple systematic reviews on endurance sports also confirm that keeping high-intensity volume at 15–20% of total training is the most effective distribution ※4.

Most recreational runners balance training with work and daily life — recovery quality is often compromised. In that context, overdoing high-intensity sessions will quickly outpace the body’s ability to absorb the training stimulus.

In Jack Daniels’ Running Formula, the recommended upper volume limits per session by pace are as follows:

Pace	Volume Limit
M pace	Up to 20% of weekly mileage
T pace	Up to 10% of weekly mileage
I pace	Up to 8% of weekly mileage
R pace	Up to 5% of weekly mileage

These are guidelines, not rules. The right intensity and volume will vary for every runner depending on their fitness base and recovery capacity.

Match Your Intensity Level to Your Training Goal

Even within the high-intensity zone, intensity control matters — because different zones target different adaptations.

Using Jack Daniels’ Running Formula pace designations as a reference:

Intensity Zone	Daniels’ Pace Name	Primary Training Goal
Zone 4	T pace	Improve lactate threshold
Zone 5	I pace	Improve VO2max
Zone 6	R pace	Improve top-end speed

This is a simplified mapping — the actual book describes slightly different objectives. R pace (rep pace) is classified here as “zone 6” for convenience.

In practice, I-pace training (zone 5) also produces lactate threshold improvements. The effects don’t cut off sharply at zone boundaries — think of them as a gradient, with the dominant adaptation shifting as intensity rises.

Because each intensity zone targets a different primary adaptation, it’s important to match your effort level to the goal you’re training for.

How Much High-Intensity Training Do You Need?

For practical guidance on incorporating high-intensity training, the 80/20 rule is a useful starting point.

An 80% low-intensity, 20% high-intensity split appears to maximize training adaptations — validated both by research and decades of elite training practice.

The right balance for you depends on your target race distance, training history, and recovery capacity. What works for a marathon runner may differ from what suits someone targeting a 5K.

Start by finding the amount of high-intensity training from which you can consistently recover and adapt. That is your effective dose.

References

※1 Gurd BJ, Menezes ES, Arhen BB, Islam H (2023) “Impacts of altered exercise volume, intensity, and duration on the activation of AMPK and CaMKII and increases in PGC-1α mRNA.” Seminars in Cell & Developmental Biology 143:17-27

※2 Fransson D et al. (2018) “Skeletal muscle and performance adaptations to high-intensity training in elite male soccer players: speed endurance runs versus small-sided game training.” European Journal of Applied Physiology 118:111-121

※3 Stöggl T, Sperlich B (2014) “Polarized Training Has Greater Impact on Key Endurance Variables Than Threshold, High Intensity, or High Volume Training.” Frontiers in Physiology

※4 Nøst HL, Aune MA, van den Tillaar R (2024) “The Effect of Polarized Training Intensity Distribution on Maximal Oxygen Uptake and Work Economy Among Endurance Athletes: A Systematic Review.” Sports (Basel)