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Neurological Fatigue: The Science

Fatigue related to a diminished ability to make muscular effort as a result of impairment of the central nervous system and/or peripheral nervous system has commonly been referred to as 'neurological fatigue' (NF). El Paso, TX. Chiropractor Dr. Alexander Jimenez examines the data.

NF will not only affect performance in the form of lowered muscle contraction force (see poorer and slower and less spring), but in addition it will lead to longer recovery times, bad sleep patterns as well as altered mood states. What's more, the inability to create voluntary muscle contraction appropriate for the demands of their physical requirement of this sport/activity can then cause injury.

It is apparent that the cause of fatigue is complex, influenced by both occasions happening in the central nervous system (CNS) and the peripheral nervous system (PNS).


Central fatigue is neural fatigue originating in the central areas of the nervous system such as the higher cortical areas in the brain, brain stem, spinal cord, or cranial nerves. The exact mechanism for CNS fatigue remains largely unknown but it appears that two primary kinds of central fatigue exist:

1. acute CNS fatigue may occur as a result of decreased reflex sensitivity and or less than optimal output from the motor cortex;

2. chronic CNS fatigue, on the other hand, is likely caused by increased inhibitory drive to the alpha motor neurons.

Motivation and psychological states also have been linked to both acute and chronic CNS fatigue, but it is not known if that is a cause or effect relationship. It's generally accepted that acute bouts of exercise causing central fatigue require an average of 48 hours to completely recover from. Chronic central tiredness, however, might take much longer. The common practice amongst coaches is to use a 10-day recovery protocol of low-intensity training to reunite the nervous system into a state of balance and homeostasis.

The need that's placed on the CNS is a product of both the volume and intensity of training. CNS fatigue can be brought on by performing a higher quantity of low-moderate intensity training or when compared to a low-moderate volume of high-intensity coaching. Insufficient reconciliation of work and rest intervals coupled with over-ambitious training will probably result in CNS fatigue. However, high-intensity training asserts more for CNS activity. The best way to think of the impact various actions have on the CNS possibly to put the mona continuum. The more intense something is (as a percentage of maximal work or speed), the lower the quantity which can be achieved until the CNS gets fatigued.

Central fatigue is a tiredness different from peripheral fatigue (in the muscles themselves). Researchers have discovered that central fatigue is different in the so-called fatigue feeling brought on by physical (muscular) fatigue, and in fact is generated in a state that is not accompanied by physical exhaustion. This is something that is relatively new. External factors like lack of sleep, personal stress, sickness, bad diet, inadequate hydration, and genetics might also give rise to CNS fatigue, especially chronic fatigue.

Serotonin levels in brain regions has been indicated as a potential causative factor in the growth of central nervous system exhaustion. It is not likely, however, that single neuro- transmitter levels are responsible for CNS fatigue. It's more probable that a combination of neurotransmitters and the boosters which directly controls CNS fatigue, like dopamine and noradrenaline.

Meeuson et al (2006) propose that this revised central fatigue hypothesis implies that an increase in fundamental proportion of serotonin to dopamine is related to feelings of tiredness and lethargy, accelerating the onset of fatigue, whereas a low ratio favors improved performance through the upkeep of motivation and arousal. Possible manipulation of these neurotransmitter levels may then potentially enhance CNS recovery.

Serotonin levels have been demonstrated to increase during intense exercise in conducting rats and also to remain high in the point of exhaustion (Meeuson 2007). Dopamine release is also elevated during exercise but appears to fall at tiredness, a response that may be important from the fatigue process. The rate neurotransmitter synthesis chiefly depend on the peripheral access to the amino acids tryptophan and tyrosine, with increased brain delivery raising dopamine and dopamine/ noradrenalin activity, respectively. It's been demonstrated that BCAA ingestion can limit the serotonin levels and tyrosine can increase dopamine/noradrenaline levels in the mind. Although tryptophan levels stay reasonably steady, the intake of large carbohydrate meals, immobilization and stress might cause transient increases in tryptophan levels (Davis 2000).

Nybo (2010) states that exercise in hot surroundings challenges not just the cardiorespiratory and fluid foundation balance of their human, but in addition the brain is affected by heat. Exercise-induced hyperthermia is associated with CNS fatigue. Improving dopamine action has been demonstrated to counteract heat-mediated CNS fatigue and improve performance whereas preventing noradrenaline uptake has been shown to aggravate CNS fatigue.

Peripheral (or Localized Fatigue)

Although occasionally called muscular fatigue, peripheral nervous system fatigue (PNS) is still a sort of CNS fatigue since the CNS controls skeletal muscle function. Unlike central fatigue, however, PNS fatigue is localized into a given body site and tends to be found at the peripheral nerves, autonomic nerves (sympathetic and parasympathetic).

There are several potential mechanisms for PNS fatigue, which range from the accumulation of inorganic Phosphate and H+, to the failure of the sarcoplasmic reticulum to release adequate Ca++ because of signaling problems from the T-tubules, to inadequate manufacture and release of Achetylcholine at the neuromuscular junction. Unlike in the case of CNS fatigue, each of these mechanisms are severe and should not produce long-term exhaustion. The truth is it is generally accepted that a span of 24 hours is enough to return the body to homeostasis after PNS fatigue.

Factors Contributing To CNS Fatigue

Training to collapse accelerates CNS fatigue (peripheral and central) negatively impacting muscular co-ordination. When training it's important to understand that your mind will probably recall the previous set or drill over any other. Therefore, the conventional burnout method, as an example in resistance training, leaves your nervous system remembering a light load that mostly taxed the slow-twitch fibers. This is bad news if it's done week in and week out since you'll lose your maximal strength levels in no time.

Complicated Loading Parameters

Within a training program there are many different training variables and external factors that struggle for CNS activity. Using complicated pyramid sets, for instance, may be overly complicated for the CNS and as a result strength development won't be optimized.

Excessive Amounts Of Speed training

Speed work may encourage CNS fatigue (peripheral and central) several hours after the session has ended. In an exercise-intensity continuum, speed training is the hardest on the CNS. Anything that involves maximal velocity and elevated levels of co-ordinated force (sprinters can use force into the floor up to four times their body weight) compete for CNS activity. It's important, therefore, to employ sufficient rest periods during a rate session for CNS recovery and restoration of high-energy phosphates.

Non-Training Factors

It's well known that external factors like lifestyle stress, work pressure, family stress, poor sleep, alcohol and poor diet all contribute to exhaustion in the athlete. These hormones operate mostly at the system level -- muscle, skin, bone, tendon, heart, lung etc.. However, it is also likely that these hormones affect the integrity of the nervous system, both peripheral and central.

Signs Of CNS Fatigue

There are some obvious and recognizable signs that may indicate that athlete is experiencing neural fatigue.
  1. Lack of motivation
  2. Poor memory
  3. Poor mood states
  4. Cognitive impairments
  5. High perceived exertion
  6. Impaired co-ordination
  7. Inhibition of central drive to muscles
  8. Heavy footsteps, a sign of central fatigue
  9. Impaired grip strength
  10. 10.Muscle twitches – particularly around the eyes and face

Objective Measures Of CNS Fatigue

The commonly used measures which are simple to execute and also provide objective comparable data are:

1. Standing long jump. With feet placed together on a 0cm marker, the athlete leaps as far as possible in a horizontal direction. The measure is then taken. Typically most athletes, depending on sport and the ratio of fast-twitch to slow- twitch fibers, will achieve something around 2m as a standard long jump. These can be measured routinely, usually at the start of the week following a weekend competition, and an objective measure can be obtained. This gives the clinician a global interpretation of neuromuscular function. It may be a depressed PNS that results in poor motor output and thus a poor jump, or it could be accumulative and unrecovered muscle metabolite depletion. These tests can be compared to baseline tests that are done in non-fatigued states.

2. Watt bike power tests. On a stationary bike that has the capacity to measure power output (wattage), perform a simple 3-5 second blast as hard as possible. The best power output is measured in watts is then recorded. Again, these can be compared to baseline results.

3. Force platform jumps. If the equipment is available, then a force platform that measures impulse on a drop and jump can also be used as a measure of motor system excitability. This is measured as a time in contact and force output that provides the impulse measure. If the athlete is fatigued they may spend too long on the platform then the ratio of force to time drops. Similarly, the time in contact may stay the same; however, they may not produce the same force profile, again dropping the ratio.

Preventing Or Fixing CNS Fatigue

First signs of central fatigue
When someone is over-trained, 10 days of recovery utilizing low-intensity training and therapy are recommended.

It is important during training (especially strength or speed training) to be aware that there is a huge difference between the 95th and 100th percentile of intensity. Athletes can still develop strength and speed significantly without training at 100%.

Athletes may get hurt the next session after they’ve run a personal best (PB) on the track or hit a PB in the weights room: not just because they’re psyched up and trying to beat their PB, more because their CNS hasn’t recovered from the previous session. After strength PB, for example, there should be no attempt at the same PB for at least 10-12 days.

Weekly planning
There is a common misconception during team sport programs that it is the tactical training sessions that contribute the most to fatigue. However, in relation to the rationale behind the cause of CNS fatigue, team training sessions may be classed as having a moderate effect on CNS fatigue. This is because most of the work done in a team training session will be at moderate velocity speeds and force outputs when compared to speed or Olympic lifting. It is recommended that trainers should first have a look at their own training prescription and decide if there is too much (or too little) high-intensity CNS-fatiguing exercises. Only then can we start to criticize team training sessions.

During the week, there has to be a balance between low and high-intensity training in relation to CNS fatigue. If two sessions are done in one day, try to make sure they are not both extremely taxing on the CNS. There also has to be a balance between high and low mental performances. High morale, disciplined sessions should be interchanged with more relaxed fun-type sessions.

Know your exercises
The higher the CNS demand of an exercise, the less volume or numbers should be done. For example, drop jumps are more CNS taxing than jumps up onto a box. Olympic lifts are more taxing than squats. Usually, any movement that involves more of a ‘shock’ will stress the CNS more.

Know your athletes
High-intensity training elements must compete for central nervous system energy. A novice sprinter can’t tax the CNS significantly no matter how hard he tries because he cannot output enough force, but as he improves the CNS demand rises exponentially, even if the volume of sprinting remains constant. This relays the importance of differentiating between advanced and novice athletes within a squad.

Hot and cold showers
To promote blood flow to the brain, hot and cold showers can accelerate CNS recovery. It is important, however, that the head is fully immersed under the shower during this treatment.

Replenishment of muscle glycogen
Carbohydrate feedings are usually taken immediately post-exercise in an attempt to re-fill depleted muscle glycogen stores. However, it may be more important to ingest carbohydrates at this time in an attempt to prevent CNS fatigue, as carbohydrates are the sole energy fuel for the brain.

Recovery of protein balance
After training, especially weight training or speed training, protein breakdown goes way up, thus creating a negative protein balance and a good potential for muscle loss. Although this eventually rebounds and the body goes into an anabolic state, in the time immediately following training, muscle can be lost. Since no athlete can afford muscle loss, this is an important focus for recovery and subsequent muscle gain. The protein can also accelerate entry of carbohydrates into the muscle cell.

Supplements to combat CNS fatigue
Some evidence exists showing that when neurotransmitters like acetylcholine, dopamine, and norepinephrine get depleted, physical and cognitive performance suffers. Since these neurotransmitters can be depleted from intense repeated bouts of strenuous exercise, this can be detrimental to the athlete. Since neurotransmitters can be depleted during exercise and this depletion can cause fatigue and over- training, nutritional strategies may offer some support. Decreased testosterone and increased cortisol is also an indicator of CNS fatigue and any dietary manipulation to increase testosterone levels is recommended. The following supplements are recommended:

Tyrosine: Tyrosine also crosses the blood/ brain barrier and competes for the same receptor site as tryptophan (the body’s first line of performance inhibition). Tryptophan is a precursor for the fatigue promoting neurotransmitter, serotonin. To block out the sedating effects of tryptophan, tyrosine has to get there first so it is wise to take it before competition. Tyrosine may also help with dopamine and noradrenaline depletion.

Branch Chain Amino Acids (BCAAs): BCAAs also suppress the uptake of tryptophan by the brain. They compete in a similar way as tryptophan for the same receptor site.

Lecithin: Lecithin is a compound containing two fatty acids and choline. It's by far the most frequent phospholipid in your system. Phospholipids are cells forming a protective sheath around cells and providing to their own framework. As a supplier of choline, lecithin is needed to maintain cell membrane integrity and to facilitate the movement of fats in and out of cells, in addition to ions, wastes, and nourishment. Also, the neurotransmitter acetylcholine includes lecithin as a component. Due to its choline make-up, lecithin has been touted as a memory booster by improving cognitive function. Supplementation with lecithin may prevent the depletion of acetylcholine found with instruction. Since acetylcholine is energetic in promoting muscular force, memory and consciousness, this would offer both cognitive and performance advantages.

Avena Sativa: Avena Sativa is a plant that has chemical properties that increase the levels of free testosterone in the body.

Neural Fatigue & Injury

If the neurological system is depressed (CNS or PNS) then the athlete may not be able to produce either a maximum muscle contraction that might result in poor torque generation around a joint, by way of instance, they might not have the ability to produce enough power to move the body from a standing start quickly enough. Not only will performance endure but also the joints that ought to stay secure and locked in the action of the movement (eg backbone) might also not have sufficient stiffness due to bad muscle recruitment to stabilize. The joints (back for instance) may subsequently suffer undesirable movement in the Shape of a shear force and this force might potentially harm the joint. What's more, if the system is still drained neurologically, the athlete may then suffer an accident (such as a pulled hamstring) if called upon to produce an explosive high-speed movement in training or competition.

Brasil-Neto et al (1993) Postexercise depression of motor evoked potentials: a measure of central nervous system fatigue. Experimental Brain Research. 93; 181-184
Davis et al(2000) Serotonin and central nervous system fatigue: nutritional considerations. Am J Clin Nutr 2000;72(suppl): 573S–8S.
Davis et al(1997) Possible mechanisms of central nervous system fatigue during exercise. Med Sci Sports Exercise. 29(1); 45-57.
Nybo L (2010) CNS fatigue provoked by exercise in the heat. 1(2); 779-92.
Meeusen R and Watson P (2007) Amino acids and the brain: do they play a role in ‘central fatigue’? Int J Sports Nutr Exercise Metab. 17: supps S37-46.
Meeusen et al (2006) Central fatigue: the serotonin hypothesis and beyond. Sports Med. 36(10); 881-909.

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The information herein is not intended to replace a one-on-one relationship with a qualified health care professional, licensed physician, and is not medical advice. We encourage you to make your own health care decisions based on your research and partnership with a qualified health care professional. Our information scope is limited to chiropractic, musculoskeletal, physical medicines, wellness, sensitive health issues, functional medicine articles, topics, and discussions. We provide and present clinical collaboration with specialists from a wide array of disciplines. Each specialist is governed by their professional scope of practice and their jurisdiction of licensure. We use functional health & wellness protocols to treat and support care for the injuries or disorders of the musculoskeletal system. Our videos, posts, topics, subjects, and insights cover clinical matters, issues, and topics that relate to and support, directly or indirectly, our clinical scope of practice.* Our office has made a reasonable attempt to provide supportive citations and has identified the relevant research study or studies supporting our posts. We provide copies of supporting research studies available to regulatory boards and the public upon request. We understand that we cover matters that require an additional explanation of how it may assist in a particular care plan or treatment protocol; therefore, to further discuss the subject matter above, please feel free to contact us. Dr. Alex Jimenez DC, MSACP, CCST, IFMCP*, CIFM*, ATN* email: phone: 915-850-0900 Licensed in: Texas & New Mexico*