Physiological Resilience: A New Pillar of Endurance Exercise Performance (Part II of II)

Being fast is great. But how’s your durability?

Monday, in part I of this post, I introduced the concept of physiological resilience — the idea that the factors that determine endurance performance (e.g., critical speed) aren’t fixed, but variable, and that their variation is determined by our ability to withstand fatigue during exercise.¹

Our ability to maintain performance under the stress of fatigue — physiological resilience — may be the fourth dimension of endurance performance.

And just like the other 3 pillars of performance — performance VO2, lactate threshold, and economy — there are several individual factors that determine one’s resilience.

https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/JP284205

What determines physiological resilience?

During exercise of 2 hours or more, efficiency tends to decline, such that the same speed/power output requires a greater oxygen consumption. This seems to coincide with an increase in fat oxidation and a reduction in carbohydrate oxidation.

Changes in muscle fiber recruitment (due to fatigue) and fuel utilization ultimately lead to higher oxygen consumption at the same intensity. We’re using more energy to run at the same speed. Furthermore, VO2 max during endurance exercise also declines, making our critical speed/power shift to a higher relative intensity of our VO2 max (i.e., exercise becomes more intense).

Metabolically, a decline in muscle glycogen levels also contributes to the drop in critical power during exercise. Less muscle glycogen and reduced carbohydrate availability reduce carbohydrate oxidation and increase fat oxidation during exercise, leading to greater oxygen consumption (for the same absolute ATP production, carbohydrate oxidation requires less oxygen than does fat).

Strengthening this are findings that providing athletes with carbohydrates during exercise helps preserve critical speed/power and mitigates the decline in muscle glycogen and blood glucose at the later stages of exercise. Physiological resilience (and its decline) parallels fuel availability.

Thus, maintaining critical power (enhancing physiological resilience) depends, in part,  on maintaining carbohydrate availability during high-intensity endurance exercise.

Studying resilience in the real world

What can field studies tell us about physiological resilience?

During a marathon, trained runners can sustain around 92% of their threshold pace (similar to critical speed) up until about 18 miles into the race, but they’re only able to sustain about 89% of their threshold pace in the final 3–4 miles.

The reduction in the ability to sustain threshold pace also coincides with an increase in heart rate from 88% of maximum up to 16 miles to 92% of maximum at the end of of the race — despite maintained or even decreased speed!

This heart rate-speed decoupling appears to happen about two-thirds of the way through a marathon and can involve an increase in heart rate at the same speed, a decrease in speed at the same heart rate, or an increase in heart rate and a decrease in speed at the same time. In all cases, we’ve moved above a sustainable intensity and the urge to slow down becomes tantalizing.

Returning to the idea that physiological resilience is highly individual, heart rate and speed decoupling also varies from person to person. Some people have a low decoupling (less than 10%) — these people run faster than people with a high decoupling (over 20%). The more resilient triumph.

The ability to prevent heart rate elevations while maintaining speed — or maintain speed in the face of an increasing heart rate — seems to be a major factor contributing to (or indicative of) physiological resilience. Interestingly, the study cited above observed that women experienced less decoupling than men, but the reasons for this weren’t fully explored.

https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/JP284205

The ability to withstand muscular fatigue also appears to contribute to physiological resilience.

You can think of resisting fatigue as one’s ability to run at a high speed or produce higher power output after prolonged exercise or a large workload has already been accumulated. Run or cycle for 2 hours and then test how much faster you can run or how many Watts you can sustain “all out” for a few minutes. How good is your finishing kick?

Fatigue resistance doesn’t appear to be related to traditional performance metrics like VO2 max, lactate/ventilatory threshold, or even peak power output. The “best” athletes on paper may not be the athletes with the highest physiological resilience and fatigue resistance.

But sometimes…they are.

Take Eliud Kipchoge, for example. He’s the epitome of “resilience.” Of course, his superior physiology puts him among the world's best distance runners. But what made Kipchoge the “chosen one” to break the 2-hour marathon barrier? Perhaps it’s this “fourth dimension” of performance. If you’ve ever heard Kipchoge talk about running — or if you’ve ever watched him run for that matter — it’s his composure that stands out. He looks just as fresh at mile 1 as he does at mile 26. It’s otherworldly.

 In fact, physiological resilience may explain the dominance of East African distance runners as a whole.

While the physiques of these athletes are certainly different than those of runners in, for example, the United States and Europe, East Africans don't appear to have better running economy or VO2 max numbers that are high enough to explain the chasm of performance separating them from runners from other nations.

Resilience may explain their dominance, and some have speculated that anatomical factors like leg length and tendon stiffness may even contribute to preventing performance deterioration (resilience) during running. Of course, there are also the factors of altitude, socioeconomic situations, and other unmeasurable variables that likely explain East African distance running superiority.

How to enhance resilience