By Matt Russ

Originally For Triathlete magazine

One of the hardest concepts for an endurance athlete to understand—and actually follow—is aerobic base training. It seems counterintuitive to run, ride, or swim slower than your normal training pace in order to become faster later in the season. It can be even harder to step away from the high-intensity training that has become routine during race season and deliberately reduce both speed and intensity.

But that is exactly why aerobic base training is so important.

If you have the discipline to spend several weeks developing your aerobic system while other athletes are still hammering themselves with hard workouts, the work you do during your base period can pay significant dividends later in the season.

What Is Aerobic Base Training?

Aerobic base training is a period of endurance training focused primarily on developing the body's ability to produce energy aerobically—that is, using oxygen efficiently during prolonged exercise.

For triathletes and other endurance athletes, a strong aerobic base provides the foundation on which higher-intensity training can eventually be built. The goal is not to prove how fast you can train during base season. The goal is to make your aerobic system more efficient so that you can eventually sustain faster speeds and higher power with less physiological stress.

Before beginning a base period, however, athletes need to recover.

Start With Recovery

At the end of a season or major training phase, I prescribe a three- to four-week transition period for my athletes. Transition is a time to recover both physically and mentally before beginning structured base training.

That does not necessarily mean taking the entire period completely off. A complete cessation of training can result in unnecessary fitness loss and make the return to structured training more difficult. Instead, I give athletes considerable flexibility, plenty of rest, and an opportunity to step away from the constant monitoring of training data and electronic devices.

The objective is to disconnect from structured training for a while, recover, and then return with a renewed focus.

It is important to understand that you cannot train hard and effectively year-round without periods of reduced volume and intensity. Attempting to do so can eventually lead to burnout, excessive fatigue, stagnation, and injury.

For most athletes, the fall and winter are ideal times to develop an aerobic base because there are fewer important races on the calendar.

If one of my athletes wants to race during the base period, I generally consider it a C-level training or fun race. There is no major performance goal attached to it. Alternatively, we may select an event that emphasizes endurance and can be completed at a controlled, sub-maximal intensity.

The Physiology of Aerobic Base Training

There are two broad energy systems involved in endurance exercise: the aerobic energy system and the anaerobic energy system. Both are important, but training them requires different approaches.

The purpose of an aerobic base period is to place a strong emphasis on the aerobic system.

Why does that matter?

Because repeated aerobic endurance training produces a variety of physiological adaptations that improve the body's ability to transport and utilize oxygen and produce energy during prolonged exercise.

Over time, aerobic training can contribute to:

• Improved oxygen delivery to working muscles
• Increased mitochondrial density and function
• Increased capillary density
• Improved stroke volume
• Greater ability to utilize fat as a fuel source
• Improved ability to produce and clear lactate
• Improved endurance and exercise economy

These adaptations do not happen overnight. They develop gradually through consistent training.

Why Fat Matters During Endurance Training

Fat is an important fuel source during aerobic exercise. As endurance athletes develop their aerobic systems, their bodies become better at accessing and oxidizing fat during prolonged, lower-intensity exercise.

This is particularly important for long-distance athletes.

The human body has a substantial supply of stored fat, while carbohydrate stores in the form of muscle and liver glycogen are comparatively limited. During prolonged endurance events, preserving carbohydrate stores can therefore become extremely important.

The more effectively an athlete can use fat to support aerobic energy production at an appropriate intensity, the less dependent that athlete is on rapidly depleting carbohydrate stores.

This does not mean that endurance athletes should eliminate carbohydrates from their diets or attempt to race entirely on fat. Carbohydrate remains an important fuel for endurance performance, particularly as intensity increases.

The objective of aerobic base training is to improve the body's overall metabolic flexibility and its ability to produce energy efficiently at lower intensities.

Mitochondria, Capillaries, and the Aerobic Engine

Some of the most important adaptations from endurance training occur at the cellular level.

Mitochondria are structures inside muscle cells where much of aerobic energy production takes place. Think of them as the engines—or energy-producing units—inside your muscle cells. Consistent endurance training can increase both mitochondrial density and function.

Aerobic training also promotes adaptations in the cardiovascular system and the network of small blood vessels that supply working muscles.

Capillary density refers to the number of tiny blood vessels supplying the muscle. Greater capillary density can improve the delivery of oxygen and nutrients to working muscles and the removal of metabolic byproducts.

The heart adapts as well. An increase in stroke volume means the heart can pump more blood with each beat.

Together, these adaptations help create the aerobic engine that endurance athletes depend upon.

And this is precisely why base training takes time.

You cannot rush physiological adaptations simply because you are impatient to start racing fast again.

How to Progress Your Aerobic Base

Aerobic base training should have progression just like any other phase of a well-designed training program.

I normally prescribe 10 to 16 weeks of aerobic base training, although the appropriate duration depends on the athlete's experience, current fitness, training history, and the demands of the event they are preparing for.

The goal is not to spend 16 weeks doing exactly the same workout at exactly the same intensity.

I generally begin at the lower end of the aerobic intensity range and gradually progress toward the upper end as the athlete adapts.

For athletes using heart-rate-based training, the zones I have traditionally used fall approximately within the 71–90% of lactate-threshold heart rate or 61–80% of maximum heart rate, depending on the athlete and the particular zone system being used.

However, athletes should understand that heart-rate zones are not universal. Different testing protocols and zone models can produce different numbers. The important thing is that your training zones accurately reflect your physiology rather than simply copying someone else's numbers.

Strength and Technique During Base Training

Aerobic base training is also an excellent time to develop sport-specific strength.

For cycling, this can include controlled low-cadence work designed to develop muscular endurance.

For running, it can include slow hill running or hiking.

These workouts can gradually increase in duration and volume as the base period progresses.

Base training is also an excellent time to work on technique, form, and economy.

When intensity becomes high later in the season, it becomes more difficult to consciously control technique. Establishing good movement patterns when training intensity is relatively low allows athletes to develop habits that can carry into harder workouts and races.

Technique work also helps keep base training mentally engaging. Spending months doing nothing but slow endurance sessions can become tedious.

You Still Need Some Speed

Aerobic base training does not mean you should never move quickly.

Short running strides, foot-speed drills, fast pedal work, and other brief neuromuscular efforts can be incorporated into an aerobic training program to maintain coordination and neuromuscular recruitment.

Later in the base period, I may also introduce some power work, initially using very short efforts followed by generous recovery.

The key distinction is that these efforts are brief and controlled. They should complement the aerobic development rather than turn an aerobic workout into an anaerobic training session.

How Does Aerobic Base Training Improve Performance?

Consider a hypothetical athlete named Sam.

Suppose Sam's lactate-threshold running pace is 7:00 per mile and his fastest sustainable aerobic pace is approximately 8:00 per mile.

We begin his base training at the lower end of his aerobic zones, where he might be running approximately 9:00 per mile.

He isn't particularly impressed.

In fact, he may wonder why he is training so slowly when he knows he can run much faster.

But Sam stays disciplined.

Over the next 12 weeks, the accumulated training produces adaptations in his cardiovascular system, muscles, mitochondria, and ability to utilize fuel aerobically.

At the end of the base period, suppose Sam can now run 7:30 per mile aerobically.

That is a significant change.

The important point isn't that Sam suddenly became a faster runner because he spent three months running slowly. The point is that he has raised the ceiling of his aerobic system.

Now he has a stronger aerobic foundation on which to build the higher-intensity training required later in the season.

For an athlete preparing for an Ironman or another long-distance endurance event, this type of improvement can be particularly valuable because a large portion of race-day energy production needs to be aerobic.

This is where Sam will ultimately "cut his groove" on race day.

The Hardest Part of Base Training

The hardest part of aerobic base training is not understanding it.

It is having the discipline to actually do it.

Base training may require you to run much slower than your training partners. For a relatively new athlete, it may even mean alternating running and walking.

It may mean separating yourself from your normal training group because your individual goals require a different intensity.

And it means resisting the inevitable contest of egos that can develop during group workouts.

If you can find a training partner with similar goals and a similar fitness level, group training can work extremely well. But too often I see aerobic base workouts go awry because someone starts pushing the pace.

One athlete speeds up.

Another follows.

Pretty soon the entire group is training above the intended aerobic intensity.

The workout may feel productive because everyone is working hard, but that does not necessarily mean it is accomplishing the physiological objective of the workout.

The "No Man's Land" of Endurance Training

One of the most common mistakes I see is athletes spending too much of their training time in the intensity range between their aerobic and anaerobic thresholds.

This middle intensity can feel productive because it is considerably harder than easy aerobic training. However, it may not provide the same level of stimulus as a properly structured aerobic workout or a deliberately designed high-intensity session.

There is absolutely a place for this intensity during an athlete's overall training program.

The problem occurs when it becomes the default intensity for nearly every workout.

Athletes end up training too hard to maximize aerobic development but not hard enough to generate the specific adaptations associated with higher-intensity training.

I often see this pattern in athletes who train primarily by feel or who allow group rides and runs to dictate their intensity.

The heart rate climbs, the effort increases, but the athlete isn't necessarily getting the most productive training stimulus for the amount of fatigue being generated.

Base training is the time to be especially disciplined about avoiding this trap.

Make Sure Your Training Zones Are Accurate

Another critical component of aerobic base training is having accurate training zones.

I regularly performance-test my athletes to make sure their training zones remain appropriate and to validate whether the training is producing the expected results.

After performing many of these tests and comparing them with race data, I can develop a much clearer picture of an athlete's lactate-threshold heart rate and establish individualized training zones.

I also recommend validation through appropriate clinical or performance testing when available.

One problem I frequently encounter is athletes using training zones that are several years old.

If your fitness has changed substantially since those zones were established, your prescribed intensities may no longer accurately represent your physiology.

You could spend an entire base period training at the wrong intensity without realizing it.

Heart-rate zones, power zones, and pace zones should be periodically reassessed as an athlete's fitness changes.

Don't Be Afraid to Lose Some Speed

There is another psychological hurdle athletes have to overcome during base training.

You may temporarily lose some top-end speed.

That's okay.

When you deliberately emphasize aerobic development and reduce high-intensity work, you should expect some decline in your ability to sustain very high intensities.

This isn't a failure of the training program.

It is part of the process.

The high-end speed and anaerobic capacity you temporarily sacrifice during the base period can be developed later when the training program shifts toward race-specific intensity.

Think of base training as building the foundation before constructing the rest of the house.

If the foundation is weak, adding more intensity doesn't necessarily solve the problem.

Be Patient With Aerobic Base Training

For novice athletes, it can take several training seasons before the full benefits of a well-structured aerobic base become obvious.

That can be frustrating.

Endurance athletes are often attracted to training because they enjoy pushing themselves. Being told to slow down, reduce intensity, and wait for physiological adaptations can feel like the opposite of progress.

But endurance performance is built over time.

A well-developed aerobic system provides the foundation for everything that follows: higher training volumes, race-specific intensity, improved endurance, and ultimately better performance.

Aerobic base training is not wasted time. It is an investment in future performance.

The athlete who has the discipline to slow down when necessary may ultimately be the athlete who is capable of going much faster when it matters.

Holloszy, J. Biochemical adaptations in muscle. Journal of Biological Chemistry 242: 2278-2282, 167

Matt Russ enjoyed a professional coaching career spanning over two decades of working with professional endurance athletes. His athletes have achieved numerous regional, national, international and championship titles under his direction. Matt achieved the highest level of licensing by both USA Triathlon and USA Cycling, and was a licensed USA Track and Field Coach. His accomplishments include being named "Team USA" Coach for the 2011 Beijing World Championship. Head Coach and founder of The Sport Factory, a USA Triathlon Certified Performance Center, he has authored many articles on a wide variety of endurance related topics.