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One of the biggest misconceptions in endurance training is that we should train one physiological capacity at a time. The traditional model usually follows the same progression. First build an aerobic base, lots of easy endurance training. Then add “threshold” training in the middle of the summer. Then, as we move into the pre-competition period, shift the emphasis toward VO2max and race specific intensity.
The emphasis certainly should change throughout the season. But this approach often leads coaches and athletes to unintentionally neglect important physiological systems for weeks or even months. The problem is that the human body doesn’t work that way.
Aerobic performance is not determined by one physiological system. It emerges from the interaction of multiple systems, each supporting the development of the others. When one begins to decline, it limits the effectiveness of the entire system.
The goal of periodization is not to sequentially build individual physiological systems. It is to continually maintain every important system while changing the emphasis toward the adaptations that offer the greatest opportunity for improvement.
Every Workout Sends a Different Signal
Training isn’t simply stress. Every workout sends a unique physiological signal for adaptation.
Long aerobic training stimulates mitochondrial biogenesis, capillary development, fat oxidation, plasma volume expansion, and cardiac remodeling.
“Threshold” training improves the ability to sustain a high fraction of aerobic power by increasing oxidative capacity, metabolic stability, and lactate transport. It should be noted, in spite of the current buzz, that “threshold” training is not a magic solution. Some athletes may need a heavier focus on VO2max training. Usually “threshold” type training is best for those doing high volumes and with an already high fractional utilization.
(Note: while I use the common term “threshold” here, there is ample evidence that no threshold actually exists and that it is a vastly misleading term. There is also no consensus on what this so-called threshold is. Depending on which of the many methods you use to define it, the speed or intensity at these various definitions can differ by 40 to 50 percent.)
VO2max intervals place the greatest demand on oxygen delivery, stimulating central cardiovascular adaptations while also improving mitochondrial function, the oxidative capacity of Type II fibers, lactate transport, and the speed or power that can be sustained at VO2max.
Sprint training recruits high-threshold motor units, preserves neuromuscular coordination, maintains the oxidative capacity of fast-twitch fibers, and develops the ability to rapidly produce force.
Strength training increases maximal force production, improves movement economy, enhances tendon stiffness, and helps reduce injury risk.
These are not interchangeable adaptations. Each contributes something different to developing aerobic fitness and performance.
The Systems Support Each Other
These systems work synergistically. The mistake is assuming the adaptations are simply additive. They are not. They interact, and the combined effect on aerobic fitness is larger than the sum of the parts.
A larger aerobic system allows the athlete to recover more quickly between hard sessions and to produce an increasing proportion of energy aerobically.
Better “threshold” capacity allows more quality work to be performed with less fatigue. Greater maximal aerobic power raises the ceiling from which “threshold” speed or power can improve.
Improved strength reduces the relative force required for every stride.
Sprint training preserves the neuromuscular qualities that improve efficiency even during submaximal exercise. At every intensity, we can move faster and more efficiently.
What Happens When One System Is Ignored?
Every physiological adaptation has a maintenance requirement. Remove the stimulus long enough and that adaptation begins to decline. An athlete who abandons speed training gradually loses neuromuscular recruitment and maximal power. An athlete who eliminates heavy strength training slowly loses force production and movement economy.
When an athlete reduces aerobic endurance training, they don’t simply lose endurance. They lose the physiological foundation that supports every other adaptation.
An athlete who removes VO2max work for extended periods may maintain excellent endurance while gradually losing the central cardiovascular adaptations that support maximal aerobic performance.
These changes are rarely dramatic at first. What we see instead are the symptoms of overtraining. Performance in training declines. Interval sessions become harder. Race pace plateaus or drops.
Many coaches respond by adding more volume or more intensity. Neither is the likely answer. The solution is to restore the balance between the physiological systems that support performance.
Why High-Intensity Training Belongs in the Base Period
One of the most common statements heard during base training is: “We shouldn’t do any hard training yet.”
Current evidence suggests otherwise. High-intensity training during the base period is not about maximizing anaerobic fitness. It is about maintaining the physiological systems that complement the full development of aerobic capacity.
Appropriately dosed high-intensity training helps preserve central cardiovascular function, maintains high-threshold motor unit recruitment, supports the oxidative capacity of fast-twitch muscle fibers, reinforces movement economy at higher speeds, and prepares the athlete for the demands of future training. Just as importantly, these adaptations support aerobic development rather than detract from it.
A stronger cardiovascular system allows greater oxygen delivery during long aerobic sessions. Maintaining fast oxidative fibers improves the ability to perform “threshold” work later in the season. Preserving movement economy reduces the energy cost of every stride. Maintaining maximal aerobic capacity raises the ceiling from which “threshold” performance can continue to improve.
High intensity doesn’t compete with aerobic development when it is properly dosed. It enhances it. This is why elite endurance athletes rarely eliminate intensity completely. They change the dose and the density. Not the presence.
I often hear athletes say their first interval session or time trial in late July felt terribly slow, or metabolically challenged. Of course it did. That should come as no surprise. If high-intensity work has been absent for weeks or months, the physiological systems required to perform it have detrained. The athlete now has to rebuild those systems before meaningful race-specific adaptations can occur.
Emphasis Is Not Elimination
This doesn’t mean every quality deserves equal attention every week. Training emphasis should absolutely change throughout the year. An aerobic development phase should contain more aerobic volume. The pre-competition period should emphasize “threshold” and race-specific intensity. The competition season should prioritize race performance and recovery.
But changing emphasis is very different from abandoning a physiological system. Even during an aerobic block, athletes should continue performing some high-intensity work, sprint training, and strength training.
During race season, aerobic volume remains essential. While emphasizing VO2max development, “threshold” training still supports the ability to perform quality work.
The dose and density change. The physiological adaptation remains.
There Is No Optimal Intensity Distribution
One of the most common questions in endurance sports is: “What is the optimal training intensity distribution?”
The question assumes there is one correct percentage of easy, “threshold”, and hard training. There isn’t. The optimal distribution depends on the athlete’s training background and on which physiological system currently limits performance. Those limiting factors change throughout an athlete’s development.
Early in a career, building aerobic capacity may deserve the greatest emphasis. Later, improving sustainable race pace may become the priority. Still later, improving neuromuscular power, force production, or movement economy may produce the greatest gains.
As those priorities change, training distribution changes with them. Not because the principles change. Because the athlete changes.
The coach’s responsibility is to continually adjust aerobic volume, “threshold” training, VO2max work, sprint training, and strength training so that each enhances the adaptations produced by the others while minimizing interference.
There is no universal training formula. There is only the continual process of identifying the athlete’s current limiter, maintaining every important physiological system, and emphasizing the adaptations that will produce the greatest improvement.
Great coaching is not about prescribing the perfect intensity distribution. It is about continually identifying the athlete’s current physiological limiter while maintaining every system that supports performance. As the athlete changes, the emphasis changes, but the physiology never disappears.
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