What is Cardiac Drift?
A primer for athletes
Cardiac drift refers to increasing heart rate despite constant intensity over prolonged exercise. What causes it? In a nutshell HR rises in order to maintain cardiac output despite a decline in how *hard* the heart is pumping (aka “stroke volume”). The lower the intensity, and more trained the athlete, then the lower the cardiac drift.
HR Drift Rises with high Intensity (fixed time)
It seems obvious that HR rises with intensity over a fixed time (eg 10mins). The reason is that cardiac output dramatically rises to meet the energy demands of muscle (and other tissues including the heart itself).
HR Drift Rises with long duration (fixed intensity)
It is a little less obvious (but actually not surprising) that HR rises with exercise duration even over a fixed intensity (eg 70% of VO2max). after all:
energy (kcal) = avg power (Watts) X duration (hours) X 3.6
Why 3.6? This is to adjust for time and the factor of 1000 reduction to get from cal to kcal (60 seconds/minute * 60 minutes/hour * 1/1000 cal/kcal = 3.6).
Three reasons contribute to rising HR with time: muscles become fatigued (and need more resources to keep going), the brain tells the body to go easiers (see central governor theory) and the main physiological reason is that HR rises essentially to compensate for lowered stroke volume as follows:
If HR rises in close correlation to the decline in SV then vital cardiac output can be maintained longer, and although it becomes increasingly uncomfortable, you can still escape the chasing dog. (You do remember the function of exercise is fundamentally to escape threat, and to pursue opportunities that help survival right?) 😂
Wait Why Does Stroke Volume Decline?
When the demands of cardiac output are very high, does it seem strange that SV declines? It would be great if it kept rising wouldn’t it?
Well, it would be dangerous because the heart can only stretch so far. The main reason why stroke volume declines is that with an increase in heart rate, the diastolic filling time becomes limiting, meaning there is not enough time to fill the chamber of the heart with blood before it has to be ejected.
In medicine this is called the Frank-Starling law :
the Frank-Starling law : the force or tension developed in a muscle fiber depends on the extent to which the fiber is stretched.
When increased quantities of blood flow into the heart (increasing preload), the walls of the heart stretch and force is high, but this cannot be maintained indefinitely.
What is the Effect of Training for Athletes?
As usual, when you train, you improve all aspects of the equation including improved:
longer ventricular ejection times
greater myocardial contractility
greater left ventricular diameter and mass
shorter diastolic filling times
shorter ejection times
The surprising bit is that in trained athletes, stroke volume can be maintained even at higher heart rates, ie diastolic filling is maintained despite very fast contraction times.
Gledhill (1994) suggested that the higher blood volumes in the trained subjects maintained an adequate ventricular filling pressure during exercise, thereby enhancing diastolic filling. This appears linked with increased plasma fraction in the blood in athletes (blood volume increases with training)/. There is also reduced afterload which enhances diastolic emptying (attributed to lower systolic and diastolic blood pressures in trained subjects)
Plasma volume may increase around 10% following a single exercise bout whereas increases in red blood cell volume usually takes 6-12 weeks
Should You Train to a Fixed HR?
If you cap/limit HR to say 90% of HRmax then your cardiac output would slowly reduce and hence performance would decline. What you would feel is increasing fatigue and you would have to reduce performance (ie effort) in order to balance HR. Wingo and Cureton showed that during submax cycling for 45 minutes maintaining a steady HR, reduced power output by 37%. Phil Maffetone showed the same in two runners (link) on a ten mile run trying to maintain a HR of 146, with each one-mile split recorded.
This is a good training session where you want to train to easy or moderate effort. It doesn't work so well at the hardest levels. It is an alternative to zone based training with a power meter.
CITATIONS
Gledhill N, Cox D, Jamnik R. Endurance athletes’ stroke volume does not plateau; major advantage is diastolic function. Med Sci Sports Exerc 1994;26:1116–21.