When a cyclist is working at a very high intensity, say some power value they can only hold for 5 minutes, their body uses a combination of aerobic and anaerobic energy production to provide the muscle contractions necessary to produce the power needed. At lower intensities, the anaerobic (without oxygen) component falls away and the effort is entirely supplied by oxygen based energy processes. As the intensity rises, so does the respiratory rate and consumption of oxygen for aerobic utilization. Eventually, the amount of oxygen a rider consumes stops increasing. This point is considered the rider’s VO2max. It is generally measured in ml/min with values in the 4000s and 5000s. It can also be normalized by weight leading to units of ml/min/kg with top cyclists recording values in the 90s. Healthy adults are in the 40-50 range, good cyclists: 60 and the generally accepted threshold for going pro is around 70ml/min/kg.
VO2max is determined from a ramp test, where the intensity slowly increases over 5-10 minutes and oxygen consumption is measured. It can also be estimated from some standardized efforts such as a 5 minute all out effort. New riders love comparing their VO2max values, but in reality they don’t mean much. It has been shown that VO2max at a high level is not a great determinant of overall cycling success because rarely are races won or lost in situations where a rider’s VO2max is the limiting factor. Sprints are too high intensity, long climbs are too low intensity. Even races with a 5 minute climb at the end can be determined by fueling choice and fresh legs rather than lab VO2max values. With all this being said, VO2max is important. Having a ‘high enough’ VO2max allows cyclists to stay in the pack and respond to surges in pace.
Riders will generally train their VO2max for 6-8 weeks leading up to a big race. The human body’s maximal oxygen consumption is trainable in a relatively short amount of time, especially when compared to other factors such as stroke volume, muscle vascularization, etc. Let’s have a look at what the science says about the best way to improve VO2max so as to be ready for the demands of high intensity racing.
Oxygen Consumption Pathway
The pathway for oxygen through the body to the working muscle is as follows:
- First, air is transported through the mouth, into the lungs in a warming process to prepare it to be absorbed into the blood stream. The air is pushed into the alveoli (small holes/pockets in the lungs) through the force of inhalation.
- The oxygen is then diffused from the alveoli to the arterial blood – this action uses a gradient action. The diffusion rate is based on the relative concentrations of oxygen in the lungs and the blood; this is the main reason aerobic capacity drops at altitude.
- Next is transportation of the oxygen to the cells. The amount of oxygen (VO2) consumed is proportional to cardiac output (stroke volume times heart rate) and arteriovenous oxygen difference.
- The oxygen, upon arrival the muscles, is diffused into the mitochondria
- Mitochondria then use the oxygen to produce muscle contractions.
When a rider wants to improve their VO2max, well… the improvements have to come from one of these areas. The science seems to suggest that the most trainable part of the oxygen pathway is step three, the transportation of the oxygen to the cells. Step three is made up two parts: cardiac output and arteriovenous oxygen difference.
As mentioned, cardiac output is the product of stroke volume and heart rate. During base training, stroke volume can increase dramatically in response to the endurance riding. The rider will see that their heart rate drops for a given effort and this is due to their stroke volume increasing. Since their stroke volume goes up, their heart rate goes down for the cardiac output demand. Most studies on increasing VO2max show changes in stroke volume and cardiac output
Arteriovenous Oxygen Difference
Arteriovenous oxygen difference (a-v O2) is a measure of the difference in oxygen between arterial blood and venous blood. It quantifies the amount of oxygen used by the working muscles. As blood moves from a human’s heart into the arteries, it has a high concentration of oxygen. After passing through the working muscle, that value decreases and returns to the heart as venous blood. There is an active debate among researchers as to whether a-v O2 is trainable for an endurance athlete. Most studies do not see a change in a-v O2 after a specific protocol, but proponents of the trainability model believe stimulating changes in a-v O2 take months or years rather than the 6-8 weeks normally used in studies. There are a few studies to suggest sedentary individuals who train for a year see an increase in a-v O2, but due to the difficulty in measuring a-v O2, there are not many studies in well-trained athletes.
What Should You Do?
VO2max intervals have been studied extensively. Using a meta-analysis, there are a few key points to hit when training your VO2max. Firstly, the highest correlation with increasing VO2max is the amount of time spent at 90%+ of an individual’s VO2max. For cyclists, this means intervals should be done to spend as much time above 90% of their 5 minute maximal power.
Other important factors were the interval time and number of weeks training VO2max. The greatest stimulus came from longer intervals (2+ minutes) and moderate length protocols (4-12 weeks). It is likely that longer intervals showed better response because it is easier to maximize the amount of time spent at VO2max when long intervals are done. It also appears improvements in VO2max start to settle out after ~12 weeks although 12 weeks of VO2max training is very tough in a cyclist, both mentally and physically.
My recommendations for intervals are to one of two interval sets. For my athletes, I give 7x3min at VO2max or 6x4min also at VO2max. Both of these intervals keep rider’s time above 90% VO2max for at least 20 minutes. For riders who find it hard to finish 4 minute intervals, I give them the 7x3min ones, but ideally the 6x4min intervals are slightly better. It is ok for the athlete’s power to slip slightly. The goal is to hit VO2max, although usually unrealistic, drops in power up to 10% are allowable. If power continues to drop more than that, the rider should conclude their workout. Rest periods should be 1:1 or maybe slightly less if time crunched. One concern with excessive rest periods is the increase in total energy utilization, potentially decreasing the rider’s capacity to finish the interval set.
VO2max intervals make you fast, but be careful. Recover sufficiently, don’t do them two days in a row, and be willing to call it quits when it gets too tough.