Base Training with the Fick Equation

Distance running is often discussed in terms of kilometres, splits and personal bests, yet beneath every performance lies a physiological equation that determines what is sustainably possible. The Fick Equation is not simply an academic concept reserved for laboratory settings. It is a practical framework that explains how oxygen travels from the air you breathe to the muscle fibres that ultimately generate forward motion. For competitive runners and coaches, it provides structure. It clarifies what limits performance, what can be improved and how base training should be organised to develop the aerobic system with intention.

The Fick Equation describes the factors that determine maximal oxygen uptake, or VO2max. It links the central cardiovascular system, which delivers oxygen, with the peripheral muscular system, which utilises it. We use the Fick model because it transforms the vague ambition of “getting fitter” into specific, trainable components. It benefits competitive middle-distance and long-distance runners, performance-focused recreational athletes, and coaches designing annual plans who want physiological alignment between training sessions and long-term outcomes.

VO2max represents the maximal rate at which oxygen can be taken up, transported and utilised during intense exercise. It remains one of the most widely recognised indicators of aerobic capacity. Although race performance also depends on running economy, lactate threshold and neuromuscular coordination, VO2max establishes the upper ceiling of aerobic metabolism. A higher ceiling allows an athlete to operate at faster absolute speeds before physiological limitation occurs.

However, VO2max does not improve through effort alone. It increases when training systematically enhances both oxygen delivery and oxygen utilisation. This is precisely what the Fick Equation explains.

The mathematical formula to determine a runner's VO2max is represented as follows:

VO2 max = Q(CaO2-CvO2)
Q is cardiac output
CaO2 = arterial oxygen content
CvO2 = venous oxygen content

In this equation, Q refers to cardiac output, CaO2 represents arterial oxygen content, and CvO2 represents venous oxygen content. The difference between arterial and venous oxygen content reflects the amount of oxygen extracted by the working muscles.

Put simply, performance improves when the heart pumps more blood, when that blood carries sufficient oxygen, and when the muscles become more efficient at extracting and using that oxygen. Each component of the equation is trainable to varying degrees, and base training provides the ideal window for developing them.

To continuously improve their performance, runners must seek ways to increase the supply of available energy to their muscles, improve the delivery of that available energy to the muscles, and increase the percentage of available energy that they can effectively use. In a runner's yearly training schedule, the base period is the ideal time to lay the groundwork for improvement in these areas.

Each breath initiates a cascade of events. Oxygen diffuses from the alveoli in the lungs into the bloodstream and binds to haemoglobin within red blood cells. Cardiac output determines how much oxygen-rich blood is transported to working muscles each minute. Once delivered, oxygen diffuses into muscle fibres and enters mitochondria, where it supports the production of adenosine triphosphate (ATP), the molecule responsible for sustained aerobic energy.

If any part of this pathway is underdeveloped, performance is limited. The Fick model separates this pathway into two key areas: central oxygen delivery and peripheral oxygen utilisation.

Oxygen is fundamental to aerobic metabolism. Increasing the volume of oxygen delivered to the muscular system increases the potential for sustained performance. Two primary factors influence how much oxygen is available in circulation: cardiac output and haemoglobin concentration.

Cardiac output is determined by heart rate multiplied by stroke volume and is defined as the total volume of blood that can be pumped by the heart. Maximum heart rate represents the total number of heartbeats an athlete can produce per minute. Unfortunately, it is largely genetically predetermined and does not significantly improve with training (~3-7%). Stroke volume, however, is highly adaptable. If an athlete has a higher stroke volume, then an increased amount of oxygen can be made available to their working muscles, which can then lead to a potential increase in the athlete's overall work capacity. Endurance training induces structural cardiac adaptations, including increased left ventricular chamber size, improved myocardial contractility and expanded plasma volume. These changes allow more blood to be ejected per beat, thereby increasing oxygen delivery at both sub-maximal and maximal intensities.

Controlled interval training performed at approximately 1500 metre to 3 kilometre effort has been shown to stimulate central cardiovascular adaptation, particularly improvements in stroke volume. During base training, these intervals should be executed at a hard but controlled intensity rather than maximal sprint effort. The objective is repeated quality exposure, not exhaustion.

When evaluating the level of oxygen available in the bloodstream, it is also important to look at the composition of the blood itself. More specifically, it is necessary to examine the amount of haemoglobin present within the red blood cells. Haemoglobin is an iron-rich protein that binds oxygen within red blood cells, that reduces the blood’s carrying capacity when levels are low.

Iron deficiency is relatively common among endurance runners, particularly those with high mileage or dietary restrictions. If an athlete has low haemoglobin levels, then less oxygen will be readily available to transport to their working muscles. Sadly, when haemoglobin levels are really low, an athlete can become anaemic, which will undoubtedly lead to decreased athletic performance, as well as a host of potential health complications if left unchecked. Even mild depletion can impair aerobic performance before overt anaemia develops.

So how does one raise their haemoglobin levels and avoid these potentially nasty complications? During base training, athletes should ensure adequate dietary iron intake and support absorption through appropriate nutritional strategies. Living in areas at higher altitudes for extended periods can help tremendously, but realistically, it is not an accessible option for most athletes. Maintaining healthy haemoglobin levels ensures that improvements in cardiac output translate into meaningful increases in oxygen delivery, which, in turn, will lead to improved performance.

While central delivery is critical, improvements in VO2max and performance are also determined by how effectively muscles utilise the oxygen they receive. The arteriovenous oxygen difference reflects this extraction capacity.

Endurance training increases mitochondrial density within muscle fibres and enhances oxidative enzyme activity. Mitochondria are the primary site of aerobic ATP production, and increased mitochondrial content expands the muscle’s capacity for sustained energy generation. Training also promotes capillary network expansion, improving oxygen diffusion and facilitating metabolic waste removal.

Modern endurance research consistently demonstrates that sustained aerobic training enhances mitochondrial pathways, improving both mitochondrial quantity and functional efficiency. These peripheral adaptations in turn allow runners to sustain a greater percentage of their VO2max during competition.

Two athletes with identical VO2max values may perform very differently if one has superior mitochondrial density and capillarisation.

Sustained running at approximately 70 to 90 per cent of current 5 kilometre effort provides a potent stimulus for mitochondrial and capillary development. This intensity range overlaps with steady-state and lactate threshold domains, which are strongly supported in contemporary literature as key drivers of endurance adaptation - increase mitochondria (size and quantity) and enzyme activity, and also increase the number and density of the capillaries surrounding the working muscles. These together increase the total volume of available oxygen in the body.

At the lower end, around 70 to 75 per cent effort, training promotes structural adaptation with manageable fatigue, and should make up the bulk of training during base phases. As fitness improves, selected sessions can progress toward 80 to 90 per cent effort, challenging lactate clearance and aerobic efficiency.

The following table illustrates approximate training paces corresponding to 70 to 90 per cent effort based on current 5 kilometre performance:

As the above paces indicate, the level of effort is neither easy or all-out race speed. Rather, training runs that focus on improving the efficiency of a runner's energy usage are run at a strong, controlled pace over relatively long durations. That being said, during base training, these runs should be performed in a more relaxed fashion, staying within the 70-75% effort range. Then, as athletes mature and are better able to tolerate more stress, runs should be performed in the 80-90% effort range. Again, the most important factor is that these runs are executed in a strong, controlled nature over an extended period.

Examples include:

Over the years, there have been countless athletes with extremely high VO2max capabilities, but come race day, their times were relatively lackluster. This is likely because their body's ability to utilise available oxygen was low, which was probably the result of not consistently running extended lengths of time at paces that fall within the 70-90% effort range of their current 5k race pace. Ultimately, these runs play an enormous role in determining how fast an athlete can race in distance events.

Running copious amounts of mileage over a multitude of seasons within 70-90% of the 5k pace range will result in an increased ability to utilise available energy, enabling athletes to complete runs at a greater percentage of their VO2max for longer durations. This will result in faster race times regardless of the event's distance, including even sprints.

For years, short, fast intervals have been a consistent element in nearly every top distance runner's training schedule. Short interval training complements sustained aerobic work by targeting central cardiovascular adaptation. Nearly all successful distance programmes incorporate controlled short interval runs at 200m, 300m, and 400m into their annual training schedule, even during base phases. With this in mind, an athlete that is serious about improving as a competitive distance runner, should be incorporating short-distance interval runs into their training schedule throughout the year.

It is also important to note that these intervals do not need to be all-out efforts, and in fact, all-out efforts are counter-productive during the base training phase. The key principle is control - A greater number of repetitions performed at consistent intensity is preferable to a small number of maximal efforts executed out of control. A 1500-3k hard effort will elicit the greatest benefit, stimulating stroke volume and cardiovascular responsiveness. Intervals performed at a 5k pace will also produce notable performance gains and are much more appropriate for those coming off an injury or extended breaks (i.e. Masters).

Examples include:

Every training day should have a purpose, and the chart above should help keep your paces within the intended range so that the system we are looking to train that day is being addressed. Running significantly faster or slower than the pace outlined for the day will work a different system, which is not ideal at this stage of training. Base training is about setting the groundwork for future success.

It is crucial to stick to the paces outlined above and base them on your CURRENT fitness, not your PR, and continue to train in this fashion until you have a recent race result to indicate your fitness level. By using race results, you ensure exactly where your body's fitness level is at and can plan your training paces accordingly. It is much better to train conservatively at this stage in terms of paces.

The Fick Equation provides a clear physiological blueprint for endurance development. Key components of high volume weekly mileage, regularly scheduled short intervals and several runs in the 70-90% of 5k pace range, perform essential functions in the continued improvement of race times. Short intervals increase the stroke volume of the athlete to circulate more oxygen toward working muscles, while 70-90% efforts improve the athlete's ability to utilize this added oxygen. The adaptations created by this training form the necessary groundwork for continued improvement in distance running performances over time.