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Diagnosis: How many base miles do you really need?

A young rider is looking to go professional, but he struggles with finding the right amount of zone 2 base training in the off-season.

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Editor’s Note: “Diagnosis” is a monthly column found in the print issue of VeloNews. It is a collaboration between the editors of VeloNews and the University of Colorado Sports Medicine and Performance Center. The anecdotes found in “Diagnosis” come from actual patients, and the names of these patients have been changed. 

A 25-year-old Cat. 1 cyclist — we’ll call him Max — visited the University of Colorado Sports Medicine and Performance Center in November 2015 in preparation for the 2016 season. The previous year he won one race, placed on the
podium twice, and had 20 top-10 finishes.

Max had been self-coached for several years but did not have a well-structured training program. Desiring more structure, he began working with a coach. He also aspired to turn professional.


Max performed a physiological test with the center’s director of sports performance, Iñigo San Millán. His maximal power output at the conclusion of their test protocol was 330 watts and 4.7 W/kg. His metabolic parameters suggested he had much room for improvement in his aerobic base, fat-burning capacity, and lactate-clearance capacity.

San Millán advised Max and his coach to develop a solid aerobic base with a large amount of Z2 training (~60-65 percent of his total training volume per week) during the winter, while including no more than five percent of higher intensity intervals. During the season he could dip into Z2 intensities at least twice a week for a total of ~20-25 percent of his volume.

Both Max and his coach believed the amount of advised training at Z2 was too much and the amount of high intensity was too little. Therefore, Max didn’t do much Z2 training (~20-25 percent), and increased high intensity to ~35-40 percent during the pre-season. Once the season began, Z2 accounted for only five percent of total training time.

In August 2016, Max returned to the Performance Center for another evaluation; he had been underperforming throughout the season.

San Millán observed that his physiological and metabolic response had deteriorated slightly compared to the off-season when they should have improved significantly. His maximal power output dropped to 320 Watts and 4.5 W/kg. His lactate clearance capacity had become slightly worse, and his fat utilization (oxidation) had also slightly deteriorated.

After evaluating his training regime, it became clear he hadn’t dedicated enough time to building a proper “aerobic base,” which would explain the poor results and drop in form.


San Millán educated Max and his coach on several key, complex concepts of the physiological and metabolic adaptations of aerobic base training.

First, an increase in base training volume improves mitochondrial size and function. This adaptation affects oxidative enzymes, which increase fat-oxidation capacity.

Second, aerobic training improves lactate-clearance capacity. As exercise intensity increases, glucose utilization increases, since fat cannot synthesize the energy one needs fast enough at higher intensities. The result of increased glucose utilization is an increase in lactate, as a byproduct. (Lactate is one of the best fuels, in fact, and should not be thought of as a waste product.) With lactate accumulation comes an accumulation of hydrogen ions associated with lactate production. These ions interfere with muscle contraction through different mechanisms and significantly decrease the force and velocity of muscle contraction and, therefore, power output. “[Max] felt empowered by these concepts, and changed his training regime for the next season,” San Millán says. Max also hired a new coach with a better understanding of physiology. Together they followed San Millán’s recommendations and developed a solid aerobic base that winter.


Max’s transformation was outstanding. During the 2017 season, he won eight races, placed on the podium nine times, and had 21 top-10 finishes. In August 2017 San Millán performed another physiological test. He documented a significant improvement in Max’s physiological and metabolic parameters. His maximal power output increased to 370 Watts and 5.4 W/ kg. His weight decreased from 70 to 68 kilograms. Both his lactate-clearance capacity as well as his fat-burning capacity improved significantly.

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For example, at an intensity of 4 W/kg in 2016 his lactate was 5.6 mmol/L, slightly above his so-called lactate threshold. In 2017, at the same intensity, his lactate was 2.1 mmol/L, which is slightly above what San Millán calls “resting levels.” Previously, he was not burning any fat; now he was burning plenty. Fat can only be “burned” in mitochondria, so the observed increase in Max’s fat-oxidation capacity corroborates an increase in mitochondrial function as a result of increased Z2 training.

“This significant improvement in metabolic efficiency was key to allow him to ‘travel’ through the competition with a much lower metabolic effort, and therefore save lots of energy for the last part of the competition, since he was able to burn a lot more fat,” San Millán says.

Max’s successful season helped him obtain his first professional contract.

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