Neuromuscular versus aerobic fatigue
by Dan Empfield 8.9.02

Most power measuring devices that mount on bicycles feature both a power meter and a heart rate monitor. This is very useful information, but it can also lead one to false conclusions.

It would be easy to adopt the view that the power meter on, say, a Computrainer, Power-tap, SRM or Polar S710 with power option measures the work you do, and the heart rate monitor measures the “cost” of that work. If this was the end of it, you could conduct your own personal scientific studies. I have. I’ve raised and lowered my seat, placed the seat fore and aft, raised and lowered my handlebars, thus altering my frontal profile. Know what I found? Almost none of these changes altered my heart rate to a point where I could say with any certainty that there was an aerobic cost or benefit to a particular change in rider position.

The easy conclusion—to which many have jumped, including (a few) scientists in their published studies and tech writers in their published columns—is that there is no real cost associated with varying one's seat angle, cadence, crank length, and so on. Where this view goes awry, in my opinion, is in the assumption that one’s heart rate—or even more precise measures like oxygen consumption and blood lactate—are the only arbiters of fatigue. This just isn’t so.

There is another sort of fatigue, and it is harder for the layman to measure. Because of the inability to easily quantify neuromuscular fatigue, most of the literature we read—on how to train, how not to overtrain, how to measure progress—relies almost exclusively on measuring the aerobic “load” you place on your body. You’ll read about your resting and work heart rates, heart rate zones, and the like. The only “muscle” you’ll be asked to measure is your heart. There is no way that I know of (yet) to easily measure the neuromuscular load on a triathlete’s most important skeletal muscles, specifically the anterior and posterior thigh muscles.

There is a neuromuscular fatigue separate from aerobic fatigue, and this is apparent to everyone. Step away from your computer, strap on your heart rate monitor, and “sit” without a chair underneath you, your back against the wall. After 30 or 45 seconds you’ll notice that your legs are in a good deal of pain. Yet your pulse won’t have risen very much. That’s neuromuscular fatigue.

I’ve written about this subject elsewhere on Slowtwitch, in articles on the science of seat angles, and on cadence. What’s new here is the juxtaposition of heart rate and muscular fatigue, and the insidious nature of how these two elements of fatigue don’t track.

I’ve previously put forward the hypothetical scenario in which a weight event (bench press) is contemplated, but to make this a little more sport specific, let’s choose the leg press. Keeping in mind that triathlon is a sport in which the ride is followed by the run, let us pretend that you’ve got a new multisport event available to you in which you must leg press a total of 5000 pounds, followed by a run up the stairwell of a 10-story building. In this event, you may press your 5000 pounds any way you wish: 50 reps of 100 pounds, or 10 reps of 500 pounds. Do so as quickly as you can, just remember that you’ve got 10 stories to climb afterward, and it’s the overall time that counts.

I suspect that you’ll actually achieve a higher degree of aerobic fatigue (you’ll have a higher heart rate during the leg press) if you choose to press the lower amount of weight more frequently. Should this be the case, you might think that you’re getting more fatigued because of your higher heart rate. But will that be your opinion after your first five flights of stairs? On the assumption—and it’s just an assumption—that the person leg-pressing 500 pounds 10 times finds when climbing the stairs he’s made a poor strategic decision, it’s neuromuscular fatigue that’s done him in.

Not only are aerobic and neuromuscular fatigue not linked, they sometimes, for the triathlete or bike racer, run counter to each other. A higher heart rate often follows a higher cadence but, in my view, the higher cadence is often necessary to achieve a higher resistance to muscular fatigue. In other words, one might be well advised to pay a slightly higher aerobic price during the bike ride in order to reap the benefit of less muscular fatigue during the ride and, importantly, the run. He’ll pay that higher aerobic price by virtue of his faster cadence.

Cycling cadence has been copiously studied by the scientific community, and several things are apparent. When subjects are tested throughout a range of cadences, and are free to choose their own cadence, the cadence they choose is invariably higher than the optimal “aerobic” cadence. Vercruyssen, et al, (“Effect of exercise duration on optimal pedaling rate choice in triathletes,” Canadian Journal of Applied Physiology, February, 2001) decided that for his test subjects (nine triathletes) the optimal neuromuscular cadence was just shy of 90bpm, yet the “energetically optimal cadence” was 78bpm. His athletes chose, when given the option, to ride a cadence of 90bpm—the neuromuscular optimal—though they slowed to 82 near the end of a one-hour bike ride. Vercruyssen guessed, by the way, that the cadence of his subjects slowed because near the end of the ride it was more important to ride at a more energetically advantageous cadence.

Perhaps this is true. Or perhaps the cadence slows because there has been a neuromuscular degredation that renders the higher cadence no longer achievable. This is where the power meter comes in. I suspect that when cadence declines, power declines as well. Only if a person were able to keep the power output from degrading, and freely choose to do so at a lower cadence, could one make the statement that a subject was choosing to pay a neuromuscular cost near the end of an exercise.

Just what is the difference between aerobic and neuromuscular fatigue? My best explanation as a lay writer is that aerobic fatigue is more systemic, and is more reliant on the oxygen end of the equation. Neuromuscular fatigue is local—that is, it occurs in very specific muscle groups, not throughout your body. Furthermore, it can be specific to a part of a muscle. It’s not related to oxygen debt, the problem is the hyper-utilization of electrolytes and/or glycogen.

This was well demonstrated, I think, by Ahlquist, et al, in “The effect of pedaling frequency on glycogen depletion rates in type I and type II quadriceps muscle fibers during submaximal cycling exercise.” (European Journal of Applied Physiology, 1992:65). He demonstrated that a trained group of cyclists would put out the same power with the same aerobic cost—same oxygen consumption—whether pedaling at 50rpm or 100rpm. Blood lactate levels remained the same. You’d think there was no difference in economy between these two cadences. What was found, though, was that the quadracep glycogen depletion was significantly greater at the lower cadence, and furthermore that this depletion occurred only in the fast twitch muscle fibers. In other words, even though we, as endurance athletes, might be predisposed to having a greater degree of slow twitch muscle fibers, we will still recruit and utilize our fast twitch muscles when the power requirement increases.

That’s exactly what happens when the cadence decreases. The peak torque during each revolution of the pedal stroke increases, and this causes increased recruitment of our fast twitch fibers, which are less able to burn fat, and which utilize glycogen at a higher rate. So, while your heart rate will not increase with the use of a lower cadence, you’ll get more tired in the long run, and I mean that literally: you’ll suffer during the long run after you’ve depleted your muscles during the long ride.

I spoke for about an hour yesterday to a fellow who was bedeviled by his inability to run a fast half-marathon in his half—Ironman races. He rides well, and he runs well during a standalone half-marathon. He just can’t run well at the end of a 56-mile ride. I suspect it might be the result of neuromuscular fatigue accumulated during the bike. How would one determine or measure this?

Power output is a great measure, if considered over a long enough distance. Certainly, if one is incapable of putting out close to the same sort of power at the end of a ride as in the middle, fatigue is setting in to a greater rate than might be smart for a triathlete. The question remains, what is the heart doing? If one’s heart keeps beating faster and faster as the power decreases, then it seems to me an aerobic price is being paid. This is what commonly happens during a 40k time trial. You reach your anaerobic threshold at (say) 160bpm, but your heart rate doesn’t stop there. It gradually increases, to the point where you might have reached 175bpm or 178bpm by the time you’re finished.

More common in a prolongued effort—say, a half-Ironman bike ride or longer—is for the opposite to happen. Your average pulse during the last 20-percent of the event is likely to be lower than for the first 80-percent of the ride. Why is that? Is your heart getting tired? Perhaps, but more likely your cardiac output is “governed” by the neuromuscular fatigue in your major skeletal muscles. In other words, your legs are so fatigued your heart isn’t able too be used to its maximal potential. Consequently your pulse, which might have been a steady 155 (let us say) throughout most of your 56-mile ride, is in the last several miles of the ride hovering at 150 to 153. Perhaps it is lower yet, and if my hypothesis is right, the rate of decrease in your heart rate is proportional to the amount of neuromuscular fatigue.

If that is the case, one way to measure the rate of neuromuscular fatigue is to consider your heart rate curve over a longer distance exercise (say three hours). As your ride progresses you‘ll be able to see whether, and at what rate, you’ll be able to retain a chosen level of power. The point is, realize that during an exercise such as this, a lower heart rate is not necessarily indicative of a more efficient seat angle, cadence (whatever). In fact, it might be the opposite.

A higher cadence isn’t necessarily the best cadence for every sort of ride. There is the aerobic cost to be considered. It is customary for those performing short timed races on the velodrome to ride way in excess of 100rpms. RAAM riders, on the other hand, might ride at 60rpm. The aerobic cost of riding a high cadence is too high for ultradistance riders, one assumes, and they’re apparently not putting out enough watts for the neuromuscular fatigue to be the overriding factor.

If I understand the literature, then, it seems intuitive that if you’re riding fairly easily, and not putting out enough power to generate a great degree of neuromuscular fatigue, you’ll ride with a slower cadence so as to guard against aerobic fatigue. The harder you work, however, the more neuromuscular fatigue becomes a limiting factor, and the faster you ought to pedal, so as to keep from recruiting too many fast twitch fibers and in so doing generate a high usage of glycogen.

My point in writing the above is to bring to light the fact that there is another sort of fatigue to which you must be aware, and using heart rate to adjudge the value of changes in (for example) bike position or cadence might be ill-advised.