When we think of fatigue, we generally think of burning muscles, lactic acid building up, and several other descriptors that have rightly or wrongly entered the lingo of endurance athletes and coaches over the years. In essence though, fatigue is all about slowing down, or preventing that from happening.

From a coaching standpoint we often think of the physiologic items that either cause this slow down or prevent it. Traditional coaches might think of increases in VO2max, HR, or acidic conditions and think of ways to influence these physiologic changes that are going on. However, what we seldom think of or address is how fatigue manifests itself.

We have all seen athletes start to change their form when the proverbial bear has jumped on someone’s back in the final portion of the race. Or as my college kids like to call, when someone “hits bricks.” You might see the athletes back arching, the turnover slowing, and the arms swing getting increasingly. But even before that point, there are subtle changes going on mechanically.

These changes occur as the body tries to navigate the ever changing environment. Why do we start to swing our arms more forcefully? Simply because our stride is shortening under fatigue and we are trying to compensate by increasing our arm swing, hoping that it causes us to maintain stride length and/or speed. It’s all compensation for trying to keep things together.

What I’m interested in is these subtle variations. We can use biomechanical changes as surrogate markers for what is going on in the body. If we see shifts in ground contact time, despite speed being the same, it might mean we can’t produce the force as quickly as we need to. If we start seeing a decrease in stride rate despite maintaining speed, we might infer that our ability to efficiently maintain quick recruitment of fibers is changing. Essentially, we are seeing changes in how we manage this speed of movement vs. force of movement problem that results in speed.

As an example of looking at these changes, we ran a quick experiment, where I had my assistant coach, Dillon Gracey, run a 5min mile on the treadmill. During one attempt he warmed up normally, while during the second one he warmed up using a weight vest. We think attached a RunScribe to his shoe and made him breathe through a mask so we could measure VO2 during the whole endeavor.

(green=weight vest…VO2/kg on Y-axis)

The weight vest induced a small degree of pre-fatigue which led to clear changes in the VO2 kinetics, causing him to use Oxygen at higher rates and to increase throughout the test instead of seeing a moderate plateauing effect.  From a biomechanical standpoint, although subtle, you also saw changes in how this pre-fatigue manifested itself.

 WEIGHT VEST/Pre-Fatigue

You can see clear trends in how he attacked the 5-minute mile. With pre-fatigue occurring, you saw very short ground contact times get progressively longer throughout, as compared to the normal situation where contact times were relatively stable across the run. Similarly, in stride rate we saw a subtle downward trend after pre-fatigue as his stride slowed, despite running at the same pace.

While the changes were subtle, when we throw in other variables like stride length and the range of motion, the trends remain the same. Keep in mind that this wasn’t a max test but rather a comfortably hard mile. So we would expect during a real max test to see larger differences as fatigue continued t grow.

So what?

The point is that we can see fatigue manifests itself before we hit a point of slow down. These subtle changes give us clues to how we are adapting to the demands of the activity. As we are able to test people in more performance like situations, the hope is that we’ll be able to find patterns of fatigue and understand how people compensate and cope with fatigue. For example, someone might be reliant on changes in ground contact while others on stride rate.

Each runner has their own “stride signature.” Meaning each runner has a particular combination of force into the ground, time on the ground, stride rate, and length that they prefer. We engrain these patterns over miles of training at various paces. Similarly, each person has their preference for how they increase the speed. Some may do it via increasing turnover, some with length, some with a combination of it. Lastly, we all have a way of dealing with fatigue. If we can learn how people tend to compensate, hopefully we can see if our training delays or combats these changes. If we can train to be able to utilize a combination of our stride length, frequency, GC, etc. we have more ways to go when fatigue occurs. So figuring out peoples “signatures” and then figuring out ways to enhance and combat the negative effects, we can improve performance.

Additionally, we can look at the optimal dose of warm-ups without causing pre-fatigue, and other things like how different kinds of fatigue may impact parameters.


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Understanding the Mechanics of Fatigue
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