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As a coach of young runners, one of the most common questions I get among aspiring young runners is “What can I do to improve my speed?” Many assume that means starting out with high intensity interval training repetitions on the track. However, the reality is that the ones who are saying this come in with better 400 and 800 meter times and weaker 3k and 5k times. They lack the aerobic foundation that lower intensity zone 2 aerobic training provides. Here’s why:

Aerobic Development Comes First

The aerobic system is the base of fitness that everything else builds upon. It strengthens the heart muscle, allowing it to pump more oxygenated blood to the working muscles. Without developing this foundational aerobic capacity first, runners won’t get the full benefit from more intense V.O2 max and speedwork. It’s like building a house with a weak foundation. It can be difficult for a young runner and even for some coaches to have their athletes run slow, while “speedwork” gives more in return performance-wise more quickly than putting in a lot of easy miles. However, the short-term success can come as a detriment to an athlete’s long-term success. The more runners spend developing their aerobic metabolism, the more they will get from speedwork. The more aerobically fit an athlete is, the quicker they can recover. Since V.O2 max is one of the key indicators, a more aerobically fit athlete will be able to spend more time training at their V.O2 max pace or heart rate compared to a person who is less aerobically fit. For example, an aerobically fit athlete who puts in a good summer can do 6 x 800s at their V.O2 max pace or heart rate, and a person who is less fit, they take longer to recover, may only be able to handle 3 or 4 reps at best.

In a study by Astrand in 1971, events from 400m to the marathon were once thought to have more of an anaerobic contribution. The 400m was once thought to have 18.5% being contributed from the aerobic system. A new model, a study by Gastrin (2011), was determined using the latest methodology in oxygen kinetics, and with a much more elite subject population than the “classic” model by Astrand. In the 400m, they found that the aerobic system contributes about 43.5% from the aerobic system. The 5k has 94% coming from the aerobic system and 6% from the anaerobic system. Therefore, you can see why having a solid aerobic foundation is important. This isn’t to say we ignore speed as distance runners; this is the furthest from the truth. We are still sprinting year-round. This includes examples like 30-60m sprints, hill sprints, and strides frequently in small doses with an endurance run. If we ignore speed, your sprint speed will get slower. However, if you don’t put the work in aerobically, your aerobic system will get weaker, and you will become slower at the longer races.

The Science of It

For coaches and athletes, it’s important to understand the basic science of why you’re doing something. Mitochondria play a key role in the aerobic energy system that distance runners rely on. You may remember from high school that the mitochondria are the “powerhouses” of the cell. As the powerhouses of the cell, mitochondria are responsible for producing the vast majority of ATP through aerobic respiration and metabolism. The greater the number of mitochondria in the muscles, the greater the capacity for aerobic energy over prolonged periods.

Consistent aerobic endurance training like the high mileage done by distance runners triggers a process called mitochondrial biogenesis – the growth and multiplication of mitochondria within muscle cells. Aerobic training, particularly long slow endurance training, enhances the ability to use oxygen and metabolize fats and carbohydrates as fuel efficiently.

Trained distance runners have higher concentrations of mitochondria compared to those non-trained individuals who skipped their summer training. This allows their muscles to resynthesize ATP aerobically at higher rates, delaying the build-up of lactic acid that correlates with hydrogen ions that cause fatigue, boosting your endurance performance. It takes many years to develop a high mitochondrial density through aerobic training. However, this will help set a young athlete up best for college and beyond to absorb a high training load and maximize their potential in their young adult years.

In addition to increasing mitochondrial density, consistent aerobic endurance training also expands the capillary network. Capillaries are tiny blood vessels that deliver oxygenated blood to the muscles and remove waste products during exercise. As your aerobic training load increases gradually over time, the muscles being worked release signaling molecules that stimulate angiogenesis – the growth of new capillary branches from existing vessels. Think of it as adding more and more roads of vessels around your muscles. This increase in capillaries allows for greater blood flow and oxygen delivery to the muscles at even faster rates.

The combination of mitochondria and capillaries multiplies the aerobic capacity and endurance potential of your muscles. It’s one of the key areas of physiological adaptation that allows elite distance runners to sustain such high exercise intensities over long distances. This is why it’s important to gradually build up your training volume through years of aerobic work.

Reduce Injury Risk

Another reason why aerobic training is important is that high school runners are still developing physically, and their bodies are going through a lot of changes. Loading up on too much and frequent high-intensity training puts a young runner at a serious risk for overuse injuries, such as stress fractures. The easy aerobic running allows their muscles and connective tissue to strengthen gradually with lower ground forces.

For example, one runner I worked with in the past year, her high school coach had her do 12-15 x 400s at a time that was faster than her current mile pace. I mentioned to the high school athlete that you need to go very light tomorrow; you will really feel this the next day. The next day came, and they were told to do (9) 200s, assuming at an even faster pace. The next day, this athlete was complaining of shin splints, and the next week, she couldn’t walk without pain and was limping. This was an unfortunate circumstance of doing too much and frequent high-intensity training. Many coaches (and athletes) want the quick fixes and quick results through high-intensity training, while this may work for someone who does track seasonally. But eventually, over time, this will plateau progress if they don’t get hurt.

Room For Growth

The aerobic system takes many years of consistent training and patience to fully maximize. Even elite professional runners are still seeing aerobic gains and efficiency improvements in their late 20s and 30s, while max speed tends to plateau in their late teens and early 20s. I believe we should train max speed and sprint all year round, but the focus for distance runners is on a steady build of the aerobic system. This doesn’t mean we are running easy all year round, but we are also balancing periodizing tempo sessions all the way down to V.O2 max sessions with correct work-to-rest ratios and progressions. However, 70-90% of our training is made up of easy aerobic volume.

Long-Term Development

The aerobic base built during the high school years provides a crucial foundation for further development as the athlete progresses through college, post-collegiate, and beyond. Putting the cart before the horse stunts an athlete’s long-term development. In other words, focusing too much on the anaerobic system before establishing a healthy aerobic base can undermine a runner’s long-term potential later down the road. As previously mentioned, this doesn’t mean we ignore speed; we still train speed all year-round.

A Typical Week

What should a typical week look like for a high school runner? This all depends on the athlete’s age, training age, and other factors. Some key points to consider are:

The majority of training (70-90%) should be at a conversational, easy aerobic pace that is 70-75% of one’s max heart rate, with only 1-2 quality workouts per week, depending on the time of year.

Gradually increase the overall training volume/mileage each year, but avoid drastic weekly mileage jumps.

Prioritize consistency over intensity, with no more than 1 hard day followed by an easy day.

Include aerobic cross-training options like cycling or aqua running 1-2 times per week.

Sprint at max velocity 1-2 times per week and include frequent strides before workouts and on easy runs.

Take a break from structured training post-season 1-2 times per year to recover.

On a typical 7-day cycle in mid to late summer, it may look something like this:

Monday: 4-6 x 30m Flying 30-meter sprints. Accelerate for 20 meters and run at max velocity for 30 meters. Rest 3 minutes between each. Followed by an easy aerobic run. 

Tuesday: Strides + Tempo run  

Wednesday: Easy aerobic run 

Thursday: Hill reps + Strides 

Friday: Easy run + Hill sprints 

Saturday: Long run progression 

Sunday: Rest or cross train 

While many inexperienced distance and track coaches may scream for high intensity for young high school runners Monday through Friday,  coaches and athletes need to resist the urge to ramp up the anaerobic work too soon. Focusing on a solid aerobic foundation through patient, conservative training is far wiser investment for long term development.