Toolbox: Muscle Fatigue

If cycling is a sport of suffering, most of the time that suffering comes down to our muscles feeling like anchors on a climb or exploding in agony in a sprint. But fatigue, like love, is a many-splendored thing. Does the type and location of fatigue occur at different parts of our nerves and muscles depending on the type of exercise that we do?

House of Pain
Cycling is all about fatigue and pain tolerance. There are a thousand and one ways to hurt while on the bike. You can have your tongue dragging on the front wheel while hanging on for dear life in a fast-moving paceline. Or you can be Phillipe Gilbert at Kuurne-Brussels-Kuurne on a glory-or-bust attack with 4 km to go and dying a thousand deaths into a stiff headwind. You can be in a slow motion drag race up the Muur de Huy feeling as if your arms and legs are undergoing medieval quartering torture. Or you can make that last sprint off the wheel and hope the finish line comes before your legs give out.

4 km to go for Philippe Gilbert

Then there’s the fatigue from a long day in the saddle where you feel your energy slowly sapping away. Or that all-over fatigue at the moment you cross the line (if you’ve paced properly) at the end of a time trial. Basically, cycling hurts, so get used to it.

But is all the fatigue from cycling the same? Do we get tired the same way after different types of efforts? Some of it is obviously not the same. For example, riding a century without eating or drinking will likely mean you fatigue from metabolic reasons, notably a depletion of glycogen and hypoglycemia meaning that you are forced to reduce your intensity. In contrast, an all-out 200 m flying sprint at the velodrome has nothing to do with glycogen depletion and everything to do with neuromuscular fatigue.

Neuromuscular Fatigue
So let’s take the idea of neuromuscular fatigue. The neuromuscular system, as implied by the name, consists of your nervous system and the muscles themselves. The actual process of stimulating an individual muscle and getting it to contract seems simple to those remembering high school or college biology class with a frog muscle hooked up to a strain gauge (just like the ones now in our power meters) and an electrical stimulator. Just turn a dial to adjust the voltage and the muscle contracts. Pretty simple.

However, in the body itself, the actual neuromuscular system is incredibly complex. Muscle contraction depends on the brain activating the correct and appropriate neurons, that signal being sent in a coordinated fashion down your spinal cord to the right muscles, and the muscles themselves being ready to receive. At any of these points, a bottleneck can occur due to fatigue. So the question becomes: do different types of cycling effort cause different kinds of neuromuscular fatigue? Do we fatigue more at the brain or at the muscle, and does this change depending on the effort?

Far from an esoteric question for the lab coat types, we shall see that this study opens up a huge Pandora’s Box of possibilities for sport science application…

Thomas et al. 2015
In this month’s issue of the journal Medicine and Science in Sports & Exercise, a British and South African collaboration (Thomas et al. 2015) tested this exact question.

• Thirteen well-trained male competitive cyclists familiar with time trials of the tested distances. Fitness (58.4 mL/kg/min VO2peak) and peak power output (382 W) were good but it doesn’t appear that these were elite or professional cyclists.

• 4, 20, and 40 km time trials were tested on a Velotron ergometer. Subjects were generally used to 20 and 40 km time trials, but less so with 4 km TTs. Therefore, 4 km TTs were used as familiarization trials to get subjects used to this distance and reduce variability.

• Before and after each TT, a neuromuscular test battery was performed. The specific test was isometric knee extension, where the subject is seated with the knee at 90° angle and attempts to kick the leg as hard as possible. This is also called a maximal voluntary contraction (MVC)

• The neuromuscular tests were done within 2.5 min of finishing the TT to minimize recovery. The tests themselves are quite complex quite familiar to me, as I’ve used them quite a bit in my own work. I’ll simplify how they are done and what they test.

• To test central fatigue, transcranial magnetic stimulation (TMS) was used to stimulate the brain during the MVC, and also efforts at 75 and 50% of MVC. Basically, this gives us an idea of how strong the brain signals are going to the leg.

• To test peripheral fatigue, the local leg nerve (femoral nerve) is directly stimulated during the MVC. This tests how much the local muscle itself is responsive to brain stimulation.

The Shocking Facts
I felt that the study design was quite nice overall. What were the main findings?

• Not surprisingly, average power was highest with the 4 km TT (340 W), and progressively lower for the 20 km TT (279 W) and 40 km TT (255 W).

• Ratings of perceived exertion (RPE) was higher for the 4 km TT and similar for 20 and 40 km TTs, but the pattern of RPE increase were similar for all three TTs.

• Despite the different wattages and durations of effort, force at 100% MVC decreased equally after all three TT distances (-18%, -15%, and -16% for 4, 20, and 40 km TT, respectively). Remember, MVCs test the entire neuromuscular system (brain-muscle) but doesn’t tell us where the fatigue occurs.

• Peripheral (muscle) fatigue was higher after 4 km TT (-40%), and similar for 20 km (-31% and 40 km (-29%). So

• Central (brain) fatigue showed the opposite trend, with less cortical stimulation decrease after 4 km TT (-4%) compared to 20 km (-12%) and 40 km (-10%).

Remember that all three tests, despite different durations, had the subjects go as hard as possible. So the summary result is that, the shorter the maximal effort, the more localized the fatigue was to the actual muscles. In contrast, the longer the maximal effort, the more the fatigue was based within the brain or central nervous system.

What do I find interesting in terms of how this study can lead to new ideas to improve our performance, or to advance our understanding of training and recovery?

An uphill effort on the Mur de Huy for Gilbert

Individualized Recovery
The first implication is in recovery strategies and interventions. If neuromuscular fatigue differs depending on the type of effort, then it also implies that the optimal recovery strategy may differ depending on the effort. For example, popular recovery tools like compression socks or tights aim to reduce local inflammation of the muscle. Same with other interventions like cold water immersion. These data suggest that such strategies may be more useful after short efforts, but less so after longer races because the fatigue is more within the central nervous system.

Overtraining
Overtraining, specifically non-functional overreaching and overtraining syndrome, is a risk from a combination of both training and non-training stress. While the causes and risk factors are highly individualized and there is no standard template, one common theme is that there is a strong involvement of the central nervous system. So another potential implication from this study is that different types of training may be at higher risk of promoting overtraining. Specifically, the same overall quantity of work, done as long threshold efforts, may be greater actual training stress than the same workload but done as shorter, higher-intensity efforts. So monitoring training, even with a power meter, may not be as simple as comparing just overall kilojoules of work or advanced metrics such as Training Stress Score.

Summary
What this study demonstrates is that fatigue really is a multi-faceted phenomenon. Not just across systems like metabolism and cardiovascular, but also within a specific system like the nerves and muscles. This becomes important in the individualization of training and especially in the recovery from training.

Ride fast and have fun!

Reference:
Thomas K., Goodall S., Stone M., Howatson G., St Clair Gibson A., and Ansley L. 2015. Central and peripheral fatigue in male cyclists after 4-, 20-, and 40-km time trials. Med. Sci. Sports. Exerc. 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] .