The cardiovascular system has a profound ability to adapt and change when it is repeatedly exposed to moderate to high intensity exercise loads for long durations. In simple terms, it undergoes conditioning. Traditionally VO2max measurements have been used to quantify the effects of conditioning on the cardiovascular system. Before discussing how VO2max increases, it's important to form a distinction between improvements in functional criteria and performance criteria. While the Fick Equation (which states that VO2max is the product of cardiac output and the arteriovenous oxygen concentration difference) tells us where to look for causes in VO2max, it doesn't tell us where to looks for improvements in performance. VO2max is a functional criteria , not a performance criteria, and we need to be careful not to conflate the two. The following example, applied to cars, makes the distinction:
The functional criteria of a car establish how much power its' engine can produce. Period. It says nothing about how far the car can go. On the flip side, the performance criteria of the car tells us how far it can go on a full tank of fuel.
Returning to athletes, a 10% improvement in VO2max tells us that function has improved. But, that doesn't warrant a performance improvement. With that 10% improvement in VO2max an individual may have a 200% improvement in muscular endurance, for example, or they may see zero improvement in performance whatsoever. That does not mean that VO2max isn't a useful measurement though. It only means that it is not a sole determinant of performance.
The best way to frame this is that a high VO2max is necessary but not sufficient, for high level performance, which is a sentiment i’ve previously expressed in an article titled, Sport Specific Limitations in Crossfit. Without a high level of function, we can't have a high level of performance. But, just having a high level of function (a huge engine), doesn't mean we'll have a high level of performance (great gas mileage). In fact, it seems there is a tradeoff at some point where additional increases in VO2max (function) actually impair efficiency (and thus performance).
How Does VO2max Increase?
Physical conditioning increases the functional capacity of the cardiovascular system in two distinct ways:
Physical conditioning increases maximal cardiac output (by means of increasing heart rate or stroke volume); and
Physical conditioning increases fractional oxygen extraction.
In normal young adults who are previously untrained VO2max can increase by upwards of 20% after three months of training and approximately 50% of these changes are attributed to increases in maximal cardiac output and oxygen extraction respectively. Additionally, the changes inc cardiac output are almost entirely due to changes in stroke volume versus heart rate. However, in more advanced athletes who have performed consistent training over years ~30-32% of improvements in VO2max are attributed to increases in stroke volume, ~8-10% are attributed to increases in maximal oxygen extraction, and the remaining ~60% of improvements are due to other factors including improved movement economy and increased pulmonary diffusion. Based on the greater body of literature, it is apparent that peripheral adjustments that lead to increases in oxygen extraction take place quite rapidly (you can observe improvements in oxygen extraction in as little as 2-3 weeks) whereas changes to the cardiovascular and circulatory system occur much more slowly — this, among other factors, explains why oxygen extraction limitations are rate in elite athletes.
How Does Stroke Volume increase?
There are three proposed mechanisms for how stroke volume increases, which are summed up in the following points:
Changes in the myocardial contractile state - while this mechanism has been proposed to account for the observed increases in stroke volume after training, I believe that it can be dispensed with rather quickly. Although the myocardial contractile state increases with exercise intensity, further increases from training are very small. The reason for this is that left ventricular ejection fraction is already high at ~85%. Additionally, end-systolic volume is low at peak exercise. As a result, it would be incredibly difficult to see further improvements with exercise training.
Changes in ventricular afterload - again, while it is often claimed that changes in after load account for the observed increases in stroke volume from physical conditioning, I find this implausible. For starters, there is little evidence showing significant effects of training on after load. Additionally, in cross sectional studies between highly trained athletes and sedentary people we see that both populations have roughly the same mean arterial pressure at their respective maximal cardiac outputs. This would suggest that physical training is accompanied by some adjustments that match total vascular conductance to maximal cardiac output.
Changes in ventricular preload - Based on cross sectional studies of ventricular preload (also known as end-diastolic volume, or EDV) in untrained and well training people, it appears that EDV is greatly increased in athletes. Additionally, in a longitudinal study by Rerych and colleagues they showed a substantial increase in EDV in a group of eighteen college athletes in response to six months of condoning — in total, EDV role from 133ml to 166ml at rest, and 166ml to 204ml during maximal effort exercise. It has also been suggested that structural changes to the heart allow for increased preload as well. In fact, medical imaging and autopsy have shown that chronic physical training increased ventricular volume and ventricular wall thickness and there are also significant correlations between heart size, stroke volume, cardiac output, and VO2max. Additionally, long-term training will results in increases in blood volume, which can have a small, but noticeable, impact on ventricular preload as well.
While this article was intended to cover the big-picture view of how physical conditioning increased stroke volume, and subsequently VO2max, I understand that being able to apply this information is key. Below i’ve posted a click through to an article titled, Training The Delivery Limited Athlete, which is a comprehensive to the different training interventions and tools I use for athletes who need to improve their maximal cardiac output.
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