Building a Strong Base
Building a Strong Base
Why the slow, unglamorous work of developing your aerobic engine is the single most important investment an athlete can make.
In the previous post, we discussed the significance of individualization. Now we turn to the second principle on my list: the importance of building a comprehensive fitness foundation.
I have a straightforward perspective on performance physiology:
Performance is largely the composite of two types of fitness: base fitness + race fitness.
Base fitness refers to the size of the athlete's "engine." Race fitness refers to how well that engine is optimized for a particular event. Athletes can achieve the same level of performance through very different combinations: a small, finely tuned engine running at 100% capacity, or a larger, "untuned" engine loafing along at a comfortable percentage.
In physiological terms, an athlete's VO2max represents engine size, while their ability to sustain a given percentage of that VO2max represents the tuning. Consider an athlete running a 5K in 15 minutes (requiring approximately 70 ml/kg/min of oxygen). They could get there with a moderate engine of 75 ml/kg/min operating at 93% capacity, or with a larger engine of 80 ml/kg/min cruising at just 87%. In general, having more capacity is preferable because it leaves room for future growth.
Efficiency Factor: a practical measure
Beyond lab testing, one useful way to understand the base/race fitness distinction is through the Efficiency Factor (E.F.), calculated by dividing average normalized power by average heart rate. An athlete producing 300W at 150 bpm has an E.F. of 2.0. An athlete might sustain 260W during an Ironman bike split either by having a high E.F. of 2.0 at a low heart rate of 130 bpm (around 65% max), or by having a lower E.F. of 1.6 while maintaining a higher heart rate of 160 bpm (around 80% max). The implications for the subsequent run and overall recovery are enormous.
Lift or Pivot?
If an athlete wants to raise their 4-hour power from 240 watts to 260 watts, there are two basic strategies:
Lift the Curve
Raise all power numbers across the board through general preparation. Focus on base qualities (low-intensity aerobic fitness, basic strength/power). Slower gains, but sustainable and with a high ceiling for continued development.
Pivot the Curve
Bring up the endurance side of the curve at the expense of the power side through specific preparation. Target peripheral adaptations within a narrow fiber band. Quicker gains, but limited in total improvement potential.
The critical trade-off: you can't lift and pivot at the same time. Too much time spent pivoting risks the whole curve falling as the base erodes. For the athlete committed to long-term development, spending a large portion of each season devoted to lifting the curve is the better option.
So what's the drawback? Building a larger engine involves physiological changes that take a very long time to fully optimize. These are multi-year adaptations, accrued heartbeat by heartbeat, contraction by contraction. The massive engine is built by simply racking up the beats.
While the relative importance of base work varies by event, every athlete benefits from developing these base qualities. From sprinters to ultramarathoners. The primary reason: development of general work capacity directly determines the rate of recovery from the specific work we'll describe in the next post. Athletes with a poor base will simply not tolerate the same specific training load as athletes with a high work capacity.
Let's look at the six foundational qualities that determine engine size.
Cardiac Remodeling
Under conditions of progressive training volume, the heart changes significantly in structure and function over long time horizons. Data from Annelise Berbalk and colleagues at the IAT in Germany shows an almost doubling of total cardiac volume from novice to world-class athlete: from approximately 700ml to more than 1,200ml for elite male endurance athletes.
As heart chamber volume increases, more blood (and more oxygen) is delivered to the muscles with each beat, requiring fewer beats per minute to sustain a given power output. This shows up in a progressive rise in Efficiency Factor over time.
The improvement from "average fit young guy" to "world-class endurance athlete" over 12 years amounts to only about 0.05 EF per year: roughly 7 additional watts at the same heart rate. Within a single year, this change is almost imperceptible. Viewed over a decade, it's transformative.
"It takes a long time to get good!"
– Scott Molina, Ironman World Champion
An important discovery from physiologist Per Olaf Astrand: cardiac stroke volume reaches near-maximum levels at relatively mild exercise intensities of around 40-60% VO2max. This means that relatively light training is sufficient to fill the heart to near-maximum capacity and provide a stimulus for chamber expansion. You don't need to go hard to grow the heart.
Slow-Twitch Fiber Development
Type I (slow-twitch) muscle fibers are resistant to fatigue, efficient at using oxygen to produce energy, and rich in mitochondria. Over long periods of endurance training, the proportion and capability of these fibers increases significantly. Ed Coyle's research at the University of Texas has consistently demonstrated a strong relationship between years of training and Type I fiber development, with a nearly linear improvement continuing for 12+ years.
Since these fibers are lipolytic (fat-burning) in nature, their development shows up as a progressive rightward shift of the lactate curve: the point where lactate begins to rise moves to higher and higher power outputs over time.
The average rate of aerobic threshold improvement: only about 12.5 watts per year. This directly informs how training zones should be set. "True" increases in the most important physiological qualities for endurance are very slow, which means training zones and pace/power targets should stay the same for long periods.
If training zones seem to progress significantly faster than 10-15 watts per year, it usually means the athlete is using "top-down" zone setting based on tests with a significant anaerobic component. Anaerobic fitness improves faster than aerobic fitness, which tricks the athlete into training at higher intensities than intended. This eventually leads to serious problems.
Importantly, Harms and Hickson found that low levels of physical activity (approximately 50% of VO2max or less) are sufficient to induce most of the total possible increases in mitochondria of fast-twitch red and slow-twitch muscles. The positive changes in the aerobic portion of the lactate curve are driven by low-intensity work, and are not enhanced by higher intensity.
Fat Oxidation
Developing high levels of fat oxidation allows athletes to rely more on fat as a fuel source during exercise and recovery, sparing glycogen and delaying fatigue. Endurance training increases the expression of fat-burning proteins and the number of mitochondria in muscle fibers, and research by Orstenblad et al. found that fat oxidation is strongly linked to slow-twitch fiber development.
In my own database, there is a strong relationship between fat oxidation and years of training: roughly an additional 8 kcal/hr per year of energy generation from fat at the first ventilatory threshold. Over a decade, for an Ironman athlete, this amounts to an extra 800 kcal of essentially "free" energy during a race.
For well-trained athletes with many years under their belts, fat oxidation at low intensities can approach or exceed 10 kcal/min, with an additional 5 kcal/min from exogenous carbohydrate. This is sufficient to fuel up to 250W of output with no net cost to glycogen stores. This is the primary reason top endurance athletes can handle 30-40 hours per week of training. Without this adaptation, such volumes would be impossible.
Beyond race performance, high fat oxidation also accelerates recovery between hard sessions by significantly reducing (or eliminating) the carbohydrate cost of rest and low-intensity activity, freeing ingested carbs to replenish glycogen stores instead.
Muscular Economy
Economy measures how much oxygen the muscles need to produce a given amount of work. An athlete with high economy might require only 190 ml/kg of O2 to run one kilometer, while a less economical athlete might need 240 ml/kg. For an average-sized runner, that difference translates to roughly 20 extra kcal per kilometer. Over a marathon, these differences become a primary performance limiter.
Paula Radcliffe's economy improved approximately 13% over 11 years, from 205 to 175 ml/kg/km when she set the marathon World Record of 2:15. Remarkably, her VO2max stayed relatively constant at around 70 ml/kg/min throughout. It was the change in economy, not engine size, that accounted for the vast majority of her performance improvement.
A 13% reduction in energy cost also means 13% more work can be accumulated with the same energy stores during training. Combined with improved fat oxidation, the shift in total work capacity over many years is enormous.
Autonomic Function & Heart Rate Variability
Heart rate variability (HRV) measures the fluctuations in time between heartbeats. It reflects the autonomic nervous system's regulatory capacity and is influenced by stress, sleep quality, and aerobic fitness. HRV is the quintessential base quality: higher levels allow faster recovery between sessions and a markedly stronger response to high-intensity training.
Boutcher and Stein (1995) demonstrated this dramatically. After 24 sessions of moderate-intensity training, the high-HRV group improved their VO2peak by 20%, while the low-HRV group improved by a dismal 1%. This matches my own experience monitoring HRV across my athletes: those with very low HRV numbers generally fail to improve regardless of the training load.
In my athlete database, mean rMSSD increases clearly with each additional year of consistent training, averaging roughly 3ms per year. This increase is independent of age-related decline and fitness-related gains: it's purely the result of stacking years of aerobic work.
Preserving Basic Strength and Speed
"The anaerobic fibers of today are the aerobic fibers of tomorrow."
In keeping with the "lift both sides of the curve" objective, it's important to remember that the fast-twitch fibers currently used for short, explosive efforts will eventually develop greater aerobic capacity with training. You need to maintain these fibers so they're available to "grow into."
This matters even for ultra-endurance athletes. In an Ironman, it's common to need 500W or more for passing and surges. Maintaining high maximal power relative to aerobic fitness also improves the response to aerobic training, because developing greater aerobic capacity in intermediate fibers presupposes that you have those fibers available in the first place.
For developing athletes, I recommend maintaining maximal power at approximately 4x aerobic threshold power, and 30-second power at roughly 3x aerobic threshold power. For example, if AeT power is 200W, aim for ~800W peak power and ~600W for 30 seconds. This ensures there's room for continued growth on the aerobic side.
Base qualities like cardiac remodeling, slow-twitch fiber development, fat oxidation, economy, autonomic function, and basic speed take a very long time to fully develop. But their potential to improve performance is immense. For endurance athletes, prioritizing these qualities and allocating a large portion of training toward them is essential.
Even for non-endurance athletes, these qualities significantly improve the ability to handle and recover from specific training. The extra work capacity becomes energy that can be channeled into sport-specific preparation.
In the next post, we'll expand on the "race fitness" side of the equation, looking at the shorter-term physiological changes we can make to shape or "pivot" the curve toward a specific event.