|11-30-2004, 09:10 AM||#1|
I've heard some intriguing, but somewhat cryptic, information on something called "CNS Fatigue". What literature I can find loosely associates CNSF with the concept of overtraining, but it also appears in the context of depression and chronic fatigue syndrome. Powerlifting threads suggest that CNS fatigue may precede muscular fatigue for complex or really heavy lifts. My own experience is that Crossfit-fatigue can feel different from day to day, and often feels very different from plain old muscular soreness. I'm guessing that there must be a relationship between CNSF and neuro-endocrine response. Can anyone educate me on the mechanism of action, recovery enhancements, and strategies for CNS conditioning ?
|11-30-2004, 04:14 PM||#3|
Although you may be referring to a different kind of fatigue (more motivational than physical), some current opinion regards the two as very closely linked. A proponent of the hypothesis that the brain is the "central governor" for muscular fatigue is T.D. Noakes. It's an interesting idea that he supports with data and analysis. A summary might be this:
Tim Noakes is a physician, an exercise scientist, and an ultramarathoner. He thus is uniquely qualified to write a book such as Lore of Running, an exhaustive (and exhausting) treatise that bills itself as "the runner's bible."
3. Noakes draws appropriate attention to the brain's ability to influence exercise performance. Perhaps the biggest difference between the third and fourth editions of Lore of Running is that the fourth edition features Noakes's newly developed "Central Governor" theory of exercise performance. According to this theory, exercise performance is determined by two key factors. "The first," he writes, "is a pacing strategy that is pre-programmed into the athlete's subconscious brain as a result of his or her previous training and racing experiences. The second are acute alterations to that pre-programmed strategy resulting from sensory input from a variety of organs -- heart, muscle, brain, blood, lungs, among others -- to the exercise controller or 'governor' in the brain."
Noakes argues that the brain limits muscular activity before actual exhaustion (going into anaerobosis, which would mean ATP production had failed to keep up with use and thus would lead to muscle rigor (seizing up) during exercies, which doesn't happen). This seems like an idea akin to the "allostasis" model.
A better summary of this is here:
Noakes and colleagues Associate Professors Alan St Clair Gibson and Vicki Lambert argue convincingly that the limitations model of fatigue, the hallmark of exercise physiology since it was first proposed by Nobel-winner AV Hill back in 1923, is all wrong. According to this model, fatigue originates in the muscles.
But Noakes and his group contend, instead, that it is the brain that "causes" fatigue. Hill's limitations model holds that fatigue kicks in either when the muscles run out of oxygen, glycogen (which fuel the muscles) or ATP (adenosine triphosphate, the molecule that muscles use to store energy), or when they are "poisoned" by certain by-products of exercise, such as lactic acid. Noakes put the first dents in Hill's model in an article that appeared in the Medicine and Science in Sports and Exercise journal - for now, the most cited text to come out of ESSM - way back in 1987. Here he showed that athletes could become fatigued even when they had enough oxygen left in their blood to keep going for some time.
You can go to PubMed, search for "noakes td", and see the journal articles his group has written about this. I've been wanting to post something about this but hadn't decided what.
PubMed is here:
|12-01-2004, 03:22 PM||#4|
Helpful info here. To expand, what is known about fatigue of the CNS as opposed to the role the CNS plays in the sensation of muscular fatigue ? Does the CNS become fatigued just like a muscle does? Certainly the neural output required to manage a bodyweight overhead squat is greater than a seated BW press on a machine. There's so much proprioceptive input and output going on it makes sense to me that the CNS experiences some sort of stress. I've heard some people talk about branched-chain amino acids as a recovery aid following work-outs.
|12-03-2004, 01:19 AM||#5|
Movements that involve a larger number of motor units present more opportunity for CNS fatigue. Olympic lifts versus a bench press, for example. However, compare a max overhead squat to a max back squat, and although the OHS involves more muscles, the back squat, due to the far greater load that can be handled, should create more neural fatigue.
Prilepin suggested in his chart 4-10 reps for loads in the 90%+ range.
# of Motor unit recruitment and load represent only a couple of considerations when it comes to CNS training and overtraining.
Consider Jogging, Running and Sprinting. They are all similar yet have vastly different effects on the CNS
|12-03-2004, 09:59 AM||#6|
Does it make sense that much of the efficacy derived from CrossFit is a function of the brain NOT being able to avoid exhaustion do to constant changes in loading, movements etc?
|12-06-2004, 01:01 PM||#7|
I have to admit to knowing hardly anything about the origins of fatigue but have been reading a couple of articles about it. Much work has been done on this subject, of course, and folks are dissecting the different contributors to human fatigue during performance of motor tasks. Two basic categories are peripheral fatigue (of the muscle and surrounding tissues) and central fatigue (of the brain and nerves that cause the muscle to fire and recruit the different motor units). (Motor units themselves are groups of individual muscle fibers that act as a single unit because they receive input from a single nerve’s terminals, so that when that nerve has an impulse sent down it, all the muscle fibers in that unit fire together. A motor unit can also consist of a single fiber in some cases. Small motor units are good when finely graded control is required, like in the hands or eyes.)
How are these things tested? A common model to test is locking a seated person’s arm at a 90 degree angle (with upper arm parallel to the floor and forearm perpendicular) in a force transducer (to measure force) and to hook up various muscles and nerves to electrodes. The subject is asked to perform a “maximal voluntary contraction”, or MVC, which means what it says--the subject tries to apply as much force as possible. Or so he (or she) thinks. Experimenters can then apply electric current to different places in the chain leading to muscle contraction and measure if there is any increased force production:
To the muscle, which will cause it to twitch or contract even more if there are fibers that the subject has not recruited, even though the subject is trying to activate the muscle maximally. This is called “twitch interpolation”, first described in 1954.
To the nerve serving the muscle, which shows whether there are nerve fibers that have not been fired from the spinal cord.
To the motor cortex itself in the brain, which shows whether the brain itself has not fully activated the brain area that is responsible for firing the muscle. This can be done from outside the skull using transcranial magnetic stimulation (TMS), which is similar to putting a couple of electrodes on the head and using them to cause current to pass across the brain area and activate the neurons by depolarizing them, if they can be.
So there are ways to try to find where the source of central fatigue might lie. A recent paper studied this (1). They wanted to measure the central contribution of fatigue and to see if the motoneurons in the primary motor cortex, the area of the brain that sends the signals out to the spinal cord for all voluntary muscle contractions, were getting fatigued, in the sense that they were losing the ability to fire maximally after repeated efforts.
After having subjects do repeated rounds of MVCs, they were able to show that the muscle, while somewhat fatigued, was still able to generate more force during the MVC. They could also apply a supramaximal stimulus to the nerve to the muscle and get more force, so they knew the nerve was capable of firing “harder”. Then they used TMS on the primary motor cortex and found “the fall in motoneurone activity is not because the motoneurones are unresponsive to extra input.”
They could also measure the proportion of fatigue due to central fatigue:
There is presumably some as yet untapped cortical drive that can increase motoneurone firing and produce additional force. When fatigue had reduced the maximal voluntary force by 40 %, voluntary activation (measured with responses to motor cortical stimulation) fell by 14 % (in absolute terms). If we calculate the force that could have been produced by the fatigued muscle if voluntary activation had not fallen, the difference between the calculated force and the measured force is about 10 % MVC. This indicates that central fatigue accounts for approximately one-quarter of the 40 % fall in maximal voluntary force produced by sustained maximal contractions. [my emphasis]
[W]hen central fatigue is present after sustained MVCs, it is not due to a lack of responsiveness of the motoneurones to input or to an inability of motor cortical neurones to produce additional output.
It is important to keep something in mind about these results. Remember that the experiment is set up so that no movement of the arm actually occurs; the contraction is isometric. Much feedback goes to the brain during movements, which in real life are typically dynamic. Other studies examine the role of central fatigue in the limits of force production during dynamic movements. It may be that the contribution of central fatigue to this type of movement is much different. This kind of difference is called “task dependency.” Like most everything else, it’s a complex subject. For example, you could imagine that the brain limits force production when the feedback it perhaps normally expects--that the muscle is shortening, the antagonist muscles are lengthening, and the joint is moving--is not there.
CrossFit is more about these dynamic movements. It would be interesting to know how much is “in reserve” centrally in terms of force production to individual muscles. In a complex lift, you’re only as strong as the weakest link. The brain seems to monitor these things so that one part does not fail catastrophically. (You’d hate to rip your forearm muscle in half just because your quads were capable of snatching some big weight after years of backsquats.)
I wonder if those stories about people performing feats of heroic strength during duress (the mother who lifts her car after an accident to get her kids out, those sorts of perhaps urban myths) are due to overriding these limits to perform maximally. I always imagined that after doing that, that hypothetical person’s body would be a wreck of torn ligaments, tendons, and muscles.
So Robb, uhh, I kind of lost track of the question :-) ! More study required.
Todd G, Taylor JL, Gandevia SC. Measurement of voluntary activation of fresh and fatigued human muscles using transcranial magnetic stimulation. J Physiol. 2003 Sep 1;551(Pt 2):661-71.
|12-06-2004, 02:23 PM||#8|
I begin to wonder if there mught be some correlation or measurement correspondance between CNS fatigue and the cognitive competance testing mentioned on Saturday's WOD post (041204). I'd love to see the outcomes of that testing.
Very cool post Rene. I was with NIH at the Baltimore Longitudinal Study on Aging, back in the '80s, and we did a bunch of MVC work with MRI. Pretty cool - we had a gripper inside a gigantic magnet (~6 foot diameter). You'd stick your arm inside the hole and squeeze the hell out of this gripper until failure. Like the studies you referenced, all our stuff was static. At that time we were measuring creatine phosphate levels in the muscle tissue and studying the decrease in muscular strength as a function of aging. Enrollees in the study would come back every few years for a 2 week round of measurements on a huge spectrum of tests.
|12-18-2004, 09:56 AM||#10|
Food for thought
"Here's a portion from one of the new chapters that will be added to the paperback version of my new book. I think that it might help you understand DB's principles (I do believe that I'm a wee bit better teacher... ).
Unfortunately I wasn't able to include the numerous charts and figures from the chapter...
The nature of the neural drive
To keep it simple, all motor actions first start by a neural action. Either a voluntary motor command or a reflexive one. This action, or command, is sent to the appropriate motor units. Upon reception of the impulse (potential of action) the motor unit is activated, producing force. This is obviously a gross oversimplification of the neuromuscular action, which would require a book on its own to fully explain. But for the scope of this book, it’s sufficient and will allow us to better understand how to manipulate the neural action/command processes.
The neural drive has three distinct characteristics which will vary in relative importance depending on the type of action needed. These three characteristics are:
A. Rate: How fast can the neural drive activate or deactivate motor units.
B. Duration: For how long the neural drive keeps the motor units activated.
C. Magnitude: The importance of the neural impulse. The larger is an impulse, the motor motor units it will activate.
Now, for any given muscle action there will be a different type of neural drive. A neural drive can be rate-dominant or duration-dominant. An important magnitude can occurs with both types of dominance as we will illustrate.
Rate-dominant drive of a high magnitude: In this first type of drive, we can see that the rate of the drive is very important. That is, it doesn’t take long for the neural drive to reach its peak. On the other hand, the duration of the drive is short. In real life we thus have a very rapid force production lasting for only a brief period of time. The relatively high magnitude indicates a high level of force production. This type of drive is characteristic of shock training methods such as plyometrics, depth landings and reactive strength exercises (catching a load and quickly reversing its motion).
Rate-dominant drive of a low magnitude: In this second example we still have an important rate and a short duration of action. But this time the magnitude is lower. Meaning that we are still seeing a rapid and brief neural drive, but the actual force production is not that high. Rapid unloaded limb movements and regular jumps and bounds are good examples.
Duration-dominant drive of a high magnitude: This type of neural drive occurs when we need to produce a high level of force for a relatively long period of time. We mean long compared to the rate-dominant drive. Generally speaking we are talking anywhere between 4 to 12 seconds when force production is concerned. This type of drive is characteristic of actions requiring a high level of force production that must be sustained. A good example is heavy lifting: lifting a near-maximal or maximal weight might take you 4-12 seconds. This requires that the nervous system sends a sustain drive for the duration of the effort.
Duration-dominant drive of a low magnitude: This type of drive is found in movements where you must produce a moderate amount of force for a longer period, when talking about strength training 20-70 seconds is a good approximation. In that case we can sustain the effort for longer than during a duration-dominant drive of a high magnitude, but the output is lower. This means that the neural drive is active for longer, but it is of lesser importance. A good example of such a drive would be found in sub-maximal lifting at a controlled tempo (sets of 8-20 reps).
Importance of the type of neural drive
Knowing the type of neural drive present in a given muscle action is crucial for several reasons. Among the most important we can name:
a. Reducing the risk of CNS overtraining
b. Higher rate of progress by avoiding opposite types of drive within a single session
c. Selecting training methods and means adapted to the needs of the individual
d. Selection training methods and means adapted to the needs of the sport
Reducing the risk of CNS overtraining
Neural drive magnitude, rate and duration all have an impact on CNS stress. A high magnitude is extremely demanding on the CNS by itself. In fact, the more important the magnitude of the neural drive is, the greater is the ensuing CNS fatigue. The duration of the drive can also have an impact in that cumulative CNS output can place a significant burden on the neuromuscular apparatus. A long duration by itself is not really stressful: if you maintain an extremely low magnitude for a long duration the actual CNS stress is virtually nil. However when a high magnitude occurs at the same time, the cumulative CNS fatigue effect is very important. A high rate of neural drive is also demanding on the CNS, especially when of a high magnitude. However since it’s almost impossible to have both a long duration and a high rate, the cumulative CNS fatigue effect from rate work is harder to accomplish. It’s still possible to do so, by using too many total repetitions.
The most CNS-demanding neural drive is thus duration-dominant and high magnitude. The second most demanding being a rate-dominant high magnitude drive. The third most demanding is a rate-dominant low magnitude drive while duration-dominant low magnitude work is the least demanding on the CNS, which is why it’s often used as a restorative method following a period of CNS demanding work.
Higher rate of progress by avoiding opposite types of drive within a single session
For maximum results you should not mix rate-dominant and duration-dominant exercises within the same training session. This would lead to sub-optimal neural adaptations, which would impair both short and long-term progress. I have myself been guilty of using a mixed approach; the Canadian Ascending-Descending program is such an example. It did produce good results, better than traditional strength training, so at first I did not question the validity of the approach. However as I improved my understanding of the neural processes involved in training I came to the conclusion that separating rate and duration work would bring the fastest results. And it did. It takes a big man to recognize his mistakes, and I fancy myself of being relatively big! So although a mixed approach will produce good results, separating rate and duration work into different sessions will lead to an even faster rate of improvement.
I find the following combination to work very well:
Two methods in one session
1. maximum effort concentric – repetitive effort concentric
2. maximum effort eccentric – maximum intensity isometric
3. submaximal eccentric – maximum duration isometric
4. high intensity absorption – dynamic effort concentric
Three methods in one session
1. maximum effort concentric – repetitive effort concentric – maximum duration isometric
2. maximum effort eccentric – maximum intensity isometric – submaximal eccentric
3. high intensity absorption – ballistic isometric – dynamic effort concentric
Four methods in one session
1. max effort concentric – repetitive effort concentric – max duration isometric – max intensity isometric
2. max effort eccentric – submaximal eccentric – max duration isometric – max intensity isometric
3. overspeed eccentric – high intensity absorption – ballistic isometric – dynamic effort concentric
Selecting training methods and means adapted to the needs of the individual
Each individual will have motor unit activation properties in which he’s more efficient. For example, you might be very efficient at producing a duration-dominant neural drive. This means that you can keep on producing the required level of force for a relatively long period. This is what I call “grinders”: when lifting a maximal load the speed will be extremely slow, almost static really, but it continues to move. Grinders can produce and sustain maximum force in 5-10 seconds, however they often have problems with explosive or reactive exercises requiring a rate-dominant neural drive.
On the opposite side of the coin you have rate-dominant individuals. I call them “hit or miss” because with them they either complete a lift with seemingly room to spare, or miss it at the first sign of slowing down. For example such an individual could bench press 315lbs in 2 seconds (rather easily), but miss 320lbs! These peoples can produce a very large amount of force in a brief period of time, but they cannot sustain it for long, hence the hit or miss phenomenon.
Then you have mixed individuals who are neither rate nor duration dominant. They are pretty much equal in both types of actions.
You can get a good idea of the dominance of an individual by timing the concentric portion of a maximum strength lift (bench press or squat for example). A duration-dominant individual will complete his maximal lift in 5-10 seconds; a rate-dominant individual will complete it in 1-3 seconds and a mixed individual in 4-5 seconds.
When you know somebody’s strength (dominance) you also know his weakness: if someone is obviously duration-dominant, more rate work should be included in the program and vice versa. Sometimes an individual might participate in a sport where one type of action is not needed. Still, you should work on individual needs (individual-specificity) first and sports related needs (sport-specificity) second.
Selection training methods and means adapted to the needs of the sport
Certain sports are rate-dominant (jumps, football, sprints, throws, etc.) others are duration-dominant (powerlifting, strongmen events, etc.) and many are mixed demands. Once that individual needs are filled out, you can start to maximize those capacities involved in the sport of choice of your athlete.
But remember, individual-specificity first, sport-specificity second!
Elastic versus Contractile force production
During any given movement/muscle action force is being produced via a combination of muscle contraction and elastic action. The muscle contraction aspect is also called voluntary muscle activation while the elastic action can be called reflexive action.
Generally speaking, the importance of the reflexive action increases when there is a rapid switch from stretching to contraction (or from eccentric to concentric). The faster the transition is, the more important will be the reflexive component. On the opposite, during single regimen actions (concentric-only, eccentric-only, isometric only) and during slow transition movements, it’s the voluntary muscle activation that plays the biggest role.
Some individuals have very good reflexive properties while have weak voluntary properties or vice versa. For maximum performance it is important to establish if an athlete is less efficient in one of these types of actions. In most cases, rate-dominant athletes have better reflexive properties than duration-dominant athletes while the later have stronger contractile properties" (Christian Thibaudeau)
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