Man and machine: endurance and the durability factor

The human body is an amazing machine capable of incredible physical feats. And when it comes to endurance performance, many researchers believe that the human endurance machine is superior to that of all other mammals. This is likely due to a multitude of reasons, including the evolutionary need to develop endurance to hunt down other faster mammals with long chases until they succumbed to exhaustion⁽¹⁾, and also a superior ability to shed excess heat during long-duration exercise⁽²⁾.

Machine but not machine

While it’s tempting to think of the human body as a simple machine when it comes to endurance performance, this analogy is not strictly true. Let’s use a car engine/human body comparison to illustrate this fact. In the human body, maximum sustainable power output is determined largely by aerobic capacity (VO2max). The analogy for a car engine is maximum brake horsepower (bhp) output, and here the analogy is appropriate. The efficiency of car is generally given as miles per gallon (mpg) at a given speed; here again, there’s a good analogy with the human body, where the economy of motion (eg running economy) is given as litres of oxygen per minute needed to sustain that speed.

However, the machine analogy starts to break down a little when we consider another key human endurance determinant - lactate threshold – which is the percentage of aerobic capacity that can be sustained before fatigue sets in and forces you to slow down. For athletes, this fatigue-inducing threshold is reached at around 80-90% of VO2max. In a well built car engine, operation at peak power is theoretically possible indefinitely (if it’s not well built, it will blow up of course!) and you don’t need to back off or stop the car in order for it to recover. But probably the biggest flaw in the ‘machine’ analogy is that the human body is affected by something called ‘durability’.

What is durability in endurance?

One of the most recent discoveries about human endurance is that aerobic capacity, threshold and efficiency can and do change as fatigue sets in⁽³⁾. In other words, they are not static. Therefore, unlike your car engine, which turns in the same maximum mph, mpg and ability to work flat out regardless of whether it’s been running for five minutes, five hours or 50 hours, the VO2max, lactate threshold and economy of your body tend to decline as exercise progresses. This quality is known as ‘durability’; when durability is high, the changes VO2max, lactate threshold and economy are relatively minimal and only set in after an extended period of exercise. Where durability is low, larger changes in these key performance markers are observed in a shorter period of time.

The importance of durability

It turns out (unsurprisingly) that a high level of durability is another key metric in how endurance athletes perform, with elite endurance athletes showing higher durability levels than amateur or recreational athletes^(3,4). High durability - ie ability to resist physiological changes – has been shown to have clear performance advantages. For example, a marathoner with high durability can resist the inevitable performance deteriorating changes and maintain performance capacity for prolonged periods of time. In line with this, lower heart rate drift (where it creeps upwards over time even though speed remains constant) in marathon running is associated with longer onset time to the start of drift and a faster relative marathon speed⁽⁵⁾. Also, the greater heart drift commonly seen in amateur and recreational marathoners is associated with a decrease in running speed at the later stage of the race, as well as worse total time⁽⁶⁾.

Recent evidence on durability

In a recent article on durability published in the December 2024 issue of SIB, Andrew Sheaff looked at Finnish research carried out by researchers from the University of Jyväskylä, in Finland investigating the impact of durability on lactate threshold in recreational athletes⁽⁷⁾. Over the period of a 90-minute run, the researchers found clear durability effects, with the lactate threshold decreasing 5.8% in females and 5.3% in males – see figure 1. In other words, over the period of this test, the workload at which first lactate threshold occurred in the athletes was reduced. This meant that while these athletes started their exercise at an intensity just below that required to produce a rise in blood lactate, the lack of durability meant they drifted into that threshold, causing a rise in lactate, even though the exercise intensity remained unchanged. This clearly demonstrates that your lactate threshold is not fixed in time, and can be significantly influenced by fatigue, even if you are exercising at relatively low intensities away from your lactate threshold!

Figure 1: Durability of first lactate threshold over time