Genes constrain the range of our potential, but not its expression. Luck limits where we start. Training determines where we end up. We can't control our genes, but we can control how much they are expressed.

An athlete spends six to eight years building his aerobic house.
~ Renato Canova, from a 2010 presentation at the International Festival of Athletics Coaching

How does the aerobic system evolve?

With the necessary commitment and expertise, how can the aerobic system improve?

First, threshold heart rates will rise with aerobic capacity.

Thresholds heart rates[1] start low relative to our maximum heart rate. And threshold speeds start low relative to our genetic potential.

With proper training, the first wave change is one of heart rate. Both our aerobic and anaerobic threshold heart rates will rise closer to our maximum.

Useless, generic formulas predict threshold heart rates of 70% and 80% of maximum, respectively. But proper training can well exceed those benchmarks. I have seen some athletes with threshold heart rates of 87% and 91% of maximum. Granted, it took decades of aerobic training to get to that level.

Second, threshold heart rates will converge.

A deficient aerobic system will have a larger gap between thresholds. The gap for a deficient aerobic system may be between 10 and 20%. The worst I've seen is in the neighborhood of 40%. In every case, a destructive addiction to high-intensity training created the largest gaps.

As a rough example, with an anaerobic threshold heart rate of 180 BPM, I've seen aerobic thresholds as low as 130 BPM. In contrast, an aerobic threshold heart rate in a well-developed aerobic system could be above 170.

For someone that is new to proper training, the first step in narrowing the gap is to build aerobic capacity. A good "white belt" rule of thumb is to achieve a gap of 10% or less between the anaerobic and aerobic thresholds.

In the example above, a good goal is to raise the aerobic threshold heart rate above 160. Only then would the athlete be ready to start adding some high-intensity training.

Like salt, high-intensity training is neither good nor bad; it depends on how much you use. Over-used, athletes can destroy their aerobic systems. They become worse fat burners[2] than a true beginner.

Third, threshold speeds start to rise.

After thresholds have converged, proper performance-driven training can begin. Aerobic capacity continues to increase so that the foundation can support a skyscraper.

The aerobic system is now strong enough to tolerate a small amount of high-intensity[3]. With the right dose, high-intensity will allow for an increase in speed at all heart rates. In my own training over a six-year period, my threshold speeds rose 28% and 21%, respectively.

Fourth, threshold speeds can converge.

Like heart rates, threshold speeds can converge. Also like heart rates, the aerobic speed tends to rise toward the anaerobic speed. This allows for faster paces at all intensities at a lower metabolic cost.

But not every event benefits from close threshold speeds.

For a long duration, steady-state event, fuel economy is the limiting factor. We have a limited amount of glycogen, and the brain will shut us down if its supplies get too low. The best way to marshall glycogen stores is to increase our aerobic threshold speed. Fuel efficiency increases as the aerobic threshold speed approaches the anaerobic.

So the higher the percentage that aerobic threshold speed is of anaerobic, the faster the athlete can go while burning less glycogen. With longer-lasting glycogen stores, they can go longer faster. That longer-faster speed comes from aerobic and anaerobic speeds being closer together.

For stochastic, shorter duration events, fuel efficiency is still important, but less so. Instead, it pays to have more anaerobic "gears" available. With a wider range of high-intensities to draw from, the athlete can attack and recover. The athlete can go faster more often. Those extra "gears" come from aerobic and anaerobic speeds being further--but not grossly--apart from each other.

But a high aerobic capacity is still important for two reasons:

  1. Even if fuel efficiency is less important in stochastic events, it's not unimportant. The higher our aerobic capacity, the less costly high-intensity will be.
  2. Aerobic capacity serves a second purpose. It's the primary method to "clean up" after a high-intensity effort. The higher our aerobic capacity, the quicker we can recover. The quicker we can recover, the more often we can attack.

Fifth, and most important, the duration at a threshold speed can lengthen.

This is where lifetime devotion to an endurance sport really pays off.

Speed depends on mechanical and neuromuscular qualities and can be considered basic, natural baggage. So the possibility of improvement is [proportionately] limited. [However,] the extension of the speed depends on the [metabolism]. So the possibility of improvement is [proportionately] high.
~ Renato Canova, from a 2010 presentation at the International Festival of Athletics Coaching

A lab test to determine thresholds increases intensity every three to five minutes. Holding a threshold speed for only five minutes informs assumptions about physiology. But that duration isn't much help in a goal event.

Five minutes isn't long enough to perform very well on race day. Extending the duration of a speed improves performance much more than increasing the speed. And unlike speed increases, that improvement can continue for many, many years.

Before I began training as a runner, I could only run one kilometer at 16 kph. With training, I eventually sustained that speed for 15 kilometers, a 1500% increase. Similarly, whereas I was able to run two to three kilometers at 13.4 kph, I eventually ran 90 kilometers at that pace, an improvement of 3000%. During this period, my best 1600m speed increased from about 6:00 to 5:00, an improvement of 16%. I suspect that this is the range of performance changes that most ordinary athletes can achieve with dedicated training over many years.

The point is simply to establish a rough idea of the magnitude of improvement that you can achieve by proper training alone--perhaps a 10% to 25% improvement in speed over a short distance and a 1500% to 3000% increase in the duration for which a specific running speed can be sustained. [Nothing other than training] will ever be able to increase your performance by 1500% to 3000%...not even the illicit use of anabolic steroids.

~ Tim Noakes, The Lore of Running, p.700



  1. There are two key thresholds that athletes need to be aware of: the ever-popular and gratifying anaerobic threshold, and the lesser-known, but more important, aerobic threshold. The anaerobic threshold is roughly defined as the top speed that we can maintain for 30-60 minutes (depending on experience). That speed is usually accompanied by a "maximum lactate steady state" (MLSS). The aerobic threshold is an easier intensity and a slower speed. It's usually associated with a lactate value of around 2 mM/L. Inexperienced athletes can maintain that pace for many hours (because the speed is slow) while professionals can for around two hours (because the speed is high). ↩︎

  2. By "fat burner", I do not mean someone who is all swole and beach-ready. Plenty of lean bodies house crappy aerobic systems. By fat burning, I mean burning fat when it's important; to fuel an effort, not to feed the ego. ↩︎

  3. How much high-intensity is too much? For most people, when the volume of high-intensity training exceeds 5% of total training time, their aerobic capacity will start to decline. A slight performance boost will deceive the athlete into thinking they've found a magic bullet. But if used too much, the negative side effects will overwhelm the positive. Performance will then plateau or decline. Suckered into a losing hand, the ignorant athlete then doubles down on even more high-intensity training. ↩︎