That Leg Fatigue May Not Be From Your Legs

From time to time I will read an otherwise boring headline or article, and questions will pop into my head. Sometimes related and sometimes not. Not too long ago I saw an article headline flow through a news feed and triggered some questions. So I did some digging.  Time to back up just a bit…

In my never ending quest to be a better athlete and coach, I often dig around areas and topics that are otherwise overlooked. The popular topics on training tend to go to those who yell the loudest, whether they are right or wrong, and while occasionally it becomes interesting when someone challenges the current fads with actual facts, there’s usually not a lot to learn.

One of the overlooked areas in what we do is breathing. Not particularly exciting but extremely critical in what we do. If you choose not to do it, then any in all activities will cease in about 60 seconds. Now, a few years ago I challenged some pretty well informed people on this topic, as swimmers ALWAYS register a greater lung capacity in scientific tests versus other sports, and I was trying to get the “why”. The response to this was always “The respiratory system does not limit performance”, meaning that whatever you are doing, the lungs can deliver enough oxygen that the bottleneck will lie somewhere else. (So basically, they said swimming or any sport weren’t creating greater lung function.) Now I understood the concept, but it just wouldn’t register in the commonsense-o-meter. If it was never the limiter, then why do respiration rates vary so wildly during performance? If the lungs weren’t trying to keep up, then why would you breathe so hard? And why would swimmers always test out with a higher lung function and capacity?

Fast forward to this year. Across my news feed I saw two words that got me digging all over again. “Respiratory Fatigue”. My head had been so fixated on size and number of alveoli and oxygen delivery that I never even considered the work done to facilitate them (side note: Turns out swimmers do create greater lung function and more alveoli. One of the reasons swimmers are so barrel chested. See below.*)

The bigger problem, as it turns out, is respiratory fatigue, meaning that the diaphragm and the abdominal muscles that allow us to breathe start to actually get tired in endurance events. As those muscles tire, they are unable to expand and contract the lungs with the same force as they did at the beginning of the race/workout and therefore get us the same amount of oxygen.  Add to it a sport like running where the abdominal muscles also have to absorb the shock of each running stride and the fatigue can compound. It’s hard to pick up on though as our breathing rate and rhythm stay about the same in endurance sports.

The muscles of the respiratory system have always been thought to to be tireless, like the heart. Because breathing is so mission critical to living, the design and make up were not to tire. Turns out they do tire.

In some of the studies I’ve been reading, maximal efforts were the most commonly tested protocols and in some of those, ‘endurance’ was tested to the tune of an 8k bike time trial. So about 15 minutes was considered endurance. Not surprisingly, little or no change in performance was noticed in those ‘endurance’ tests. However, some of the tests actually involved endurance. Real endurance, like marathons and ultra’s. In those tests respiratory fatigue was noticeable.

So how much was performance effected? That one is tough to answer, as there is a combination of fatigue all contributing to the decline in performance. First, you have the working muscle that fatigues, like your quad. The muscle slowly reduces the length and force of contraction. Second you have what’s called central fatigue. This is the general body fatigue. The ‘tired’ feeling, the malaise, the bonking,  and related feelings you get when you’re tired. In other words, fatigue that cannot be blamed on muscle groups (like when you get sick and lay in bed all day). Finally you have respiratory fatigue. The lungs themselves take about 15% of our workload. And as intensity or duration increase, your body has to make some choices. So it redirects blood flow away and ‘steals’ blood from the other muscles (your quads in this instance) to keep you alive. So then you end up feeling the fatigue in your legs. A study of marathon runners showed a drop in respiratory (expiration) strength of 28%.

While it’s hard to assign absolute blame, studies have shown that strengthening the respiratory muscles show marked Improvement. In one test for example, some light inspiratory muscle training resulted in a  bump in performance after just a few weeks.

So what does all this mean? Well, I’m not entirely sure. It might not mean much, but it could mean quite a bit. It starts raising a lot of questions. Does this mean that interval training (and the vigorous breathing involved with that) has a lot to do with strengthening respiratory muscles as well as the performing muscles? And if that’s the case, you could potentially get a “lung workout” independent of a sport, thus saving the beating on your legs, but still getting a big chunk of the benefit. It also could speak to how you breathe. Deep versus Shallow? Is one rhythm better than another? Could focusing on something as simple as that made a large difference in your performance? What about altitude? Is the sleep high/train low theory part of this equation? We all know the red blood cell benefits, but the constant (while you sleep) conditioning of the lungs may also play a part in this equation. I could keep going but you get the idea.

There’s some good data that I’ve seen, but not great. So take a deep breath, and stay tuned.


***So why do swimmers always test better in the lung department? Two reasons. First, because your body, and more importantly your chest cavity is under water, you have to breathe with that additional pressure of the water pressing on it. Expanding the lungs with that added pressure forces them to be stronger in order to take an equivalent breath that you would take on dry land. Second, breathing freely isn’t an option in swimming as your face is in the water (even in backstroke there are turns and the underwater kick off the walls). So the respiratory muscles must work even harder to pull in enough air to maintain performance. The result is a broader chest, increased number of alveoli and stronger respiratory muscles.


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