Toolbox: Different Types Of Fatigue

Toolbox: Cycling fatigue, like love, is a many-splendoured thing. We have all been drop-dead tired at the end of a race, hard climb, or interval workout. But are all these types of fatigue the same thing, or do they affect your body in different ways?

Dissecting Fatigue
Despite over a century of dedicated research amongst physiologists and exercise scientists, the concept of fatigue remains a huge topic of debate. Why exactly do I get dropped going up this steep hill? Or why was I not able to complete that century ride in under 5 h as I had planned? It seems ridiculous that something so apparently simple has become so difficult to isolate and pin down. But on the good side, it keeps scientists like me busy and employed.

A lot of the problems with understanding fatigue is our reductionist approach as scientists, where we want to control as many variables as possible in order to study just one single thing like glycogen level, temperature, cadence, etc. Drilling down to a single variable may answer that question directly, but it doesn’t allow us to see the forest for the trees, or the interconnections between different variables and systems.

For example, some of my own research has shown that high core temperature can reduce our ability to recruit and activate muscles. However, high temperature affects many other systems in our body, such as possible changes in brain blood flow, metabolism, breathing patterns, etc., all of which may both cause fatigue independently but also influence neuromuscular function.

Neuromuscular Fatigue
At the absolute most basic definition, fatigue for cyclists is the inability to generate the desired power output for the desired duration, and this ultimately comes down to the muscles not generating sufficient force.

The muscles are only the end of the entire chain from the brain through to the different nerves. And beyond the straight physiological view of this neuromuscular system, there is also the psychological component such as arousal, motivation, prior experience, etc. and how they interact with and affect our neuromuscular physiology.

But let’s stick with the physiology for now and ask this question: is the fatigue we feel following different durations of high intensity due to the same neuromuscular impairment? And if not, might this have implications on how we recover from different efforts or races?

Thomas et al. 2015
In the March 2015 issue of Medicine & Science in Sports & Exercise, a UK-South African collaboration asked the specific question:

What is the level of central (i.e., closer to the brain and central nervous system) versus peripheral (i.e., closer to the muscle) neuromuscular fatigue following self-paced time trials of 4, 20, and 40 km?

Here was the basic setup:

• 13 well-trained male cyclists (VO2max = 58.4 mL/kg/min, power at VO2max = 383 W) participated, all active TT riders with experience in events of these distances.

• After fitness testing and a practice 4 km TT, participants completed 3 TTs of 4, 20, or 40 km, with the order of these tests randomized and balanced across participants. Typically expected control and replication of things like time of day, restriction of caffeine and intense exercise prior to trial, hydration, etc. were performed.

• TTs were done on a Velotron cycle ergometer, with visual feedback of power, distance covered, and cadence visible. This was a good choice, as the goal of the TTs was to elicit fatigue and this feedback provided motivation to ride as hard as possible.

• Following the TTs, a battery of neuromuscular tests was performed using isometric contractions (i.e., the leg remained in a fixed position) during a knee extension (i.e., kicking forward of the knee). I won’t get into the technicalities of all the different tests employed, but they were designed to test the actual muscle itself along with other possible sites of limitation/fatigue up the neuromuscular chain up to the brain.

• Tests were done within 2.5 minutes of completing the TTs before recovery became a confounding factor.

The Long and Short of it
The study overall was quite carefully designed and performed. As you would expect, average power output was highest with the 4 km TT (340 ± 30 W), lower with 20 km (279 ± 22 W), and lower again with 40 km (255 ± 21 W). This was what the authors hoped for, causing different levels of stress on the body. This likely included different muscle recruitment strategies, with the 4 km TT making greater demands on type II (fast twitch) muscle fibres.

The 4 km TT would also likely have higher metabolic demands on the anaerobic pathways, with greater rate of glycogen use. This could be seen in the lactate values in 4 km (9.1 mM), 20 km (7.8 mM), and 40 km (5.1 mM). Of course, the duration of TT was shorter, so absolute glycogen use would be highest in the 40 km TT.

So as we go through the findings, keep in mind there’s really two things that are changing here, namely intensity AND duration. So when we’re saying things like “the 4 km TT did this…” what we’re meaning is actually “harder, shorter efforts did this…”

Some of the neuromuscular findings:

After all 3 TTs, maximal knee extension force decreased, with no difference in the magnitude of decrease between 4, 20, and 40 km TTs. So basically, the entire neuromuscular system as a whole was equally fatigued regardless of time trial distance.

Peripherally at the muscle itself, all three TTs caused a decrease in electrically stimulated force. However, this decrease was greatest in the 4 km TT, such that the muscle’s capacity to contract and generate force decreased more following 4 km TT compared to either 20 or 40 km TT.

Centrally, activation and excitability at the level of the brain was lower following all three TTs, but the magnitude of decrease was greater with 20 and 40 km TT compared to 4 km TT.

Implications
On the face of it, the findings match our expectations. At one extreme, doing maximal 200 m sprints certainly has you feeling a different kind of tired than at the end of a hilly century ride. The nice aspect of this study was that the type of race/effort was the same in being a TT, with the main difference the intensity and duration of effort.

What this study adds is clear data that fatigue IS situation-specific. In general, the higher the intensity, the more peripheral fatigue at the muscles dominates in fatigue. The longer the effort, the more the fatigue is situated in the central nervous system.

As I hinted in the introduction, where I think this knowledge really comes in handy is in seeing how this data might help us to optimize recovery strategies, both day-to-day recovery and also following single-day or stage races. For example, a track sprinter, pursuiter, or prologue specialist on the road might benefit more from recovery strategies focused on the muscle itself, such as cold water baths, compression garments, or muscle stimulation.

In contrast, following a road race or a longer time trial, alternate recovery strategies focused on recovery of the central nervous system might be more beneficial. These latter strategies are starting to emerge, and it will be interesting to see how research in these areas play out.

Ride fast and have fun!

Reference
Thomas K., S. Goodall, M. Stone, G. Howatson, A. St Clair Gibson, and L. Ansley. 2015. Central and peripheral fatigue in male cyclists after 4-, 20-, and 40-km time trials. Medicine and Science in Sports and Exercise 47:537-546

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About Stephen:
Stephen Cheung is a Canada Research Chair at Brock University, and has published over 80 scientific articles and book chapters dealing with the effects of thermal and hypoxic stress on human physiology and performance. Stephen’s Cutting-Edge Cycling, a book on the science of cycling, came out April 2012, and he is currently co-editing a followup book “Cycling Science” with Dr. Mikel Zabala from the Movistar Pro Cycling Team. Stephen can be reached for comments at [email protected] .