Nervous System Fatigue and its role in Sport
Vince Lombardi once said ‘Fatigue makes cowards of us all’ and the one universal agreement in sport is that the struggle to avoid and delay this fatigue is unsolved.
In order to address this question though we need a greater understanding of the nature of fatigue and in particular the nature of the phenomenon known as nervous system fatigue. Fatigue in general can be suggested as the result of an interaction of central and peripheral physiological systems. This fatigue is clearly experienced and observed during prolonged exercise and as a result there is much research into the traditional version of fatigue where muscle glycogen and plasma glucose concentrations are depleted stress hormones are elevated and sex hormones lowered. But in relation to other types of sport we have had much debate on what exactly the role, nature and effect of the Central Nervous System is with regard to sports performance. This is what I would regard as a practical structure and guide on observance of the CNS with regard to athletic training.
What follows are more my ‘observations and guidelines’ rather than rules, as I will admit that I too have not a full, or near, complete understanding of it …
First of all my proposed definition of Central Nervous System Fatigue (CNSF) -
Central Nervous System Fatigue is the negative systemic response to imposed stresses (physical and emotional) interrupting homeostasis leading to a gradual but severe reduction in the efficiency of the elements of the Nervous System (such as the PNS, spinal cord, cerebellum, parasympathetic system, midbrain and diencephalons), which in turn affect the optimal functioning of bodily systems and the organism as a whole.
This disturbance in homeostasis occurs among the structural and non-structural body systems and it occurs in varying degrees and varying ratios. CNSF is certainly holistic in that it comprehensively encompasses the complete range of human systems. This is important, as it has a systemic effect that can have emotional impacts far beyond the physical sense. Another important aspect of this is that the athlete may experience the effects on the various systems at different stages meaning there is a staggered effect on the athlete’s body. This also in turn means the athlete may not experience the CNS in certain systems until too late.
CNSF develops gradually and as the fatigue progresses the reduction in systemic and motor control progresses. For example - The degree of fatigue will affect the athlete gradually - fine motor control my start to diminish and then gross motor control. With extreme CNS fatigue taste, hearing and visual systems could fail. However, since we deal mostly with athletes we never experience that degree of fatigue since we would expect a breakdown (injury) before reaching that stage.
There are a number of factors that appear to influence CNSF negatively during exercise such as excessive accumulation of heat in the brain impairing the ability to sustain maximal motor activation, exercise resulting in hypoglycaemia where cerebral glucose uptake is reducing the availability of substrates for the brain, changes in levels of ammonia, serotonin or dopamine. CNS stress is concurrent - various components of the CNS seem to be affected simultaneously - but from an athletic perspective to varying degrees. Different elements of the CNS diminish slowly, but some at greater degrees than others. In other words all elements will be affected slowly and gradually.
CNS Response
The response of the CNS and the nature of CNS fatigue is also interesting in that different stresses have different effects on the CNS. The CNS responds to all stresses not just high intensity stresses. There may be two, broadly speaking, CNS responses to physical stresses, Reaction and Protection. Low volume stress has a CNS effect in much the same way as high intensity stressors, but the nature of the CNS fatigue is different. In my opinion low volume CNS fatigue elicits a different systemic response from the body. It also places a different physiological demand on the organism, thus requiring a different response. Low volume CNS stress is gradual insidious and expectant as a result the organism moves to protect itself using many means such as stress hormones, limiting energy uneceesary expenditure etc. and has a greater period to do this. However with high stress CNS loading the priority is on reaction to the stress, heightening awareness of the organism rather than protection. Both CNS responses are overruled differently also in that the PNS has a greater role in Low Volume stress and in high intensity stress the CNS manages the response.
The PNS
Is there a role for the PNS in CNSF? The role of the PNS is very much two-way with the CNS. Impairments in the local nervous systems do have an effect on the CNS, but are relative and impairment of the CNS leads in turn to reduced PNS efficiencies. The CNS is the main central control mechanism of the subordinate systems, however it is not a one-way street, the nervous system is based on two-communication, meaning that the physiological and physical impact of muscular stresses impact directly on the CNS too. The CNS acts as a central mechanism for system integration of performance. The role of the PNS is to provide input from peripheral structures and organs and to react to stress imposed on the CNS.
Estimating and Monitoring CNSF
In athletics or sposrt of any kind there is an unhealthy race to place a measurement on anything and the CNS is no different. However since it is not even fully understood it cannot honestly be measured. What can be estimated however is the perceived CNS status of the athlete. To measure or estimate athletic CNSF we consider 3 key factors …
CNSF = (Engagement x Intensity x Density x ‘Factor X’)
Engagement - the degree of neural recruitment of the organism as a whole in an activity
Intensity - the degree of force expressed
Density - the degree of force divided by the total time of expression and rest
Factor X - A % factor representing the original state of the organism.
With regard to Engagement it is important to consider that the whole body most be considered as the concept of CNF is based on a comprehensive effect, not local. This does not refer to the musculature but rather the neuromusculature of the organism. It is also here that the CNS efficiency of the athlete must be considered, whether beginner, novice or elite. Intensity is a reference to the degree of force expressed through the whole body. With certain stresses the neural recruitment is much more limited than with other stresses, but the whole body effect is critical. It is not a measurement of the 1 repetition maximum as this figure can vary from day to day. The complete measurement of the CNSF is then a function of the Density of the force. This is not the same as Intensity, nor is volume a relevant consideration rather the force expression over the whole period needs to be considered.
It is also foolish to think that a simple formula such as above can be used to assess such a complex issue as CNSF accurately; rather it can be used as a guide. One other element to be factored is the ‘Factor X’. This is not a random figure but a method used to put a value on and assess the base status of the organism. This is included to take account of nutritional status, personal mindset, (for amateurs - work stress), personal life demands etc., adrenal efficiency, emotional state, of course perceived fatigue/energy status or CNS fatigue and can include or be measured/estimated using a simple subjective questionnaire test. The important point about the above is that while many more complex and perhaps more accurate formulations can be used there are few other as practically useful.
CNS Nutritional Manipulation
Certain substances can permit both an extension of CNS efficiency and a heightening of CNS control, but not directly. The CNS effect occurs through improved performance of organism systems such as the hormonal or endocrine systems. But with use of such substances there will always be a debt or price to be paid as the body recovers. Supplements themselves cannot directly restore CNS either. While many supplements claim to regenerate the CNS this not strictly possible. No supplement alone can directly regenerate the CNS as the systemic nature of it means more than one system must be regenerated or supported. Some can support elements of the system, but not regenerate it. Often misconstrued and confused with CNSF is adrenal fatigue with itself can decrease or increase the rate of and speed of CNS fatigue. The functioning of the adrenal system is very important to the functioning of the CNS as the release of hormones can have significant effects on the Nervous System.
Neurotransmitters
The role and status of neurotransmitters has also been proposed as the controlling variable in Nervous System Fatigue. However, CNSF is not as a result of the depletion of neurotransmitters such as achetylcholine, which are responsible for neuron signalling. It has been suggested that CNSF is as a result of an increase in the concentration of brain serotonin or 5-Hydroxytryptamine, but studies have proven this to be inaccurate. While this could produce a delay in signalling this is not the same as the CNSF phenomenon that is a comprehensive interruption of homeostasis affecting multiple human systems. Many people combine supplements and nootropics for CNS stimulation and regeneration. For example caffeine is a stimulant as are nootropics such as green tea extract, tyrosine, piracetam, chocomine etc. DMAE, Choline, Policosanol and Lecithin are actcelycholine precursors, which may assist in brain health rather than regeneration of the neural synapse.
Regeneration of the CNS
Rest is the single best method of restoring the CNS. There is no proven method of direct CNS regeneration, as a series of systems must be regenerated. Contrast bathing and cryotherapy appear to assist in CNS regeneration, but is this a direct CNS effect or through regeneration of the PNS? Supplements can only ever assist in clearing the way for neurotransmitter regeneration and speeding neurotransmitter delivery. Post physical exertion the organism (including CNS) must recover; massage, lymphatic drainage, PWO recovery foods etc will speed muscular recovery (i.e. physical recovery) and therefore reduce the overall stress on the organism permitting CNS regeneration.
Long Term CNS Fatigue
Stresses that affect bodily systems directly have greater effects on the CNS than those that affect it indirectly. In some cases people who suffer ailments to the digestive system or reparatory system tend to experience faster and greater CNS fatigue than those who do not. Another misnomer is that CNSF does not have a direct muscular component or effect. This is not the case. In some athletes there have been reports of consistent overstraining and under-recovery leading to long-term loss of muscular control in certain areas of the body most notably digestive system and for example facial muscles. These effects are manifest themselves in a gradual failure of various body systems to function properly or optimally. The degree of failure is relative to the degree of stress.
Elite Athletes and CNS
The CNS has a limited reserve or pool, in other words the body must redirect CNS control to the body systems that need it most at certain times and constant repeated exposure without recovery will lead to malfunctioning and fatigue of the CNS. CNS efficiency is specific to the task at hand and not transferable. The greatest issues facing coaches are in managing CNSF and muscular fatigue so as to train the elements they aim to train optimally not to the detriment of the other. The CNS can be trained to be more efficient for certain tasks and this is observed with elite athletes who can express greater forces with arguably the same or less strength. The greater the forces expressed the greater the use of the CNS and the greater the overall demand on the CNS. This in turn means the greater the performance the greater the debt to be paid post performance and the nearer the athlete is to systemic failure. It also worth considering that the organism responds directly to the nature stresses placed on it. For example the body responds directly to a specific stress with a specific adaptation, but a general stress has a general response, longer regeneration and adaptation. This may lead to coaches assessing their training to place specific stresses that can be overcome when time limits are an issue.
Arousal and the CNS
The autonomic division of the PNS has an element called the sympathetic system or “fight or flight” effect. This leads us to another interesting misconception but contributory element to the CNS functioning - the concept of arousal. This arousal has a definite hormonal factor that can assist in generating greater performance, but this hormonal response not the same as increasing or heightening CNS function. In cases of fight or flight there may be a capability to express greater CNS recruitment however this is due to actions of the hormonal, adrenal and endocrine systems, which are parts of the CNS and contributory factors in CNS performance.
Local Muscular Fatigue and DOMS
There has been some debate on DOMS and other forms of local post-exercise recovery. In my opinion DOMS is a signalling that there has been a stress imposed on the local system to which it is not accustomed and performance is potentially restricted and injury may be likely. Is there a CNS effect from DOMS? Yes, of course, there is a CNS effect from every imposed stress, but the degree of fatigue is small.
Programming and Organisation
Why any of this is relevant to us is that as coaches or athletes we need to be able to determine when the optimal time is to begin the next training session and what the nature of that session should be. The nature of the response to this question in my opinion is determined by the result you wish to aim for. By aiming for increased performance super-compensation between sessions is necessary so the session must be begun with maximal recovery and minimal fatigue. Some aim for increased performance over a meso-cycle or longer training block compensation between sessions is not necessary so the next session can begin with insufficient recovery.
Neural Athletic Efficiency
As I mentioned earlier the more neurally efficient the athlete the better they can recruit Motor Units to perform their tasks, regardless of muscle mass. While beginners may only be able to recruit 50% of their motor units the more neurally efficient athlete maybe able to recruit up twice as many motor units or up to 90% of their available motor units. The more efficient an athletes nervous system is, the greater the CNS pool available to the athlete, the greater percentage of motor units they can recruit the greater the display of absolute strength or power. Where it can be confusing is that the greater the display the greater the fatigue, the less volume of performance available or permissible, so an athlete who is more neurally efficient experiences a greater CNSF post performance. For less efficient athletes their nervous system recruits fewer motor units the greater the volume of the available CNS pool volume.
Testing Neural Athletic Efficiency
There are many methods of suggested testing for CNS efficiency such as performing submaximal reps in certain exercises for reps to see the number possible, the higher the number or the faster the concentric phase the greater the efficiency. The same for cleans or vertical jumps. While all are valid options they are open to abuse or misinterpretation due to varying training history, experience, training system etc. A better option is to use a combination of basic tests.
Mechanical Stimulation of the CNS
As outlined, any multi-joint exercise can be used to excite and stimulate the CNS. CNS stimulation and motor unit recruitment is quite high for maximum effort bench pressing. The bench press is used quite often pre-event in many sports for CNS stimulation as it re-stimulates the system but is an upper body multi-joint movement involving the wrists elbows and shoulders and recruits only approximately 35% of available motor units. The additional advantage is that the organism is neurally re-primed for competition but no systemic fatigue is experienced in the muscles concerned with sprinting – the key element in many sports. The squat could be used but the disadvantage of using it is that it demands 65% motor unit recruitment approximately and may create a local fatigue in the muscles of the leg. Of course one of the key elements here is the nature of the athlete. The more efficient the athlete the greater the demand on the CNS, but the longer they can maintain a state of CNS preparedness.
Multiple Daily Sessions
Fatiguing of the CNS with multiple sessions daily raises the conflict of an increased available CNS pool per session versus increased CNS recovery period between sessions. There are also hormonal factors, but leaving these aside there are other considerations. Firstly the action of CNS arousal places a demand on the CNS pool itself, therefore twice daily sessions mean reaching a state of heightened CNS arousal twice. However, it can be argued that using at least 4 [ref POOMA] hour windows between sessions permit CNS recovery albeit incomplete between sessions and increased CNS freshness for each session. If the workload is partitioned in a structured manner [the 72 hours rule] that observes muscular stress and consideration is given to local and PNS fatigue it is possible to train at a higher total CNS level for that day using multiple daily sessions.
Maximisation of CNS Loading
One of the key aims of respecting the limitations of the CNS on sprint training appears to be to aim to elicit maximum neural response, using the near-maximal stress, but with minimal necessary volume. In a practical sense this implies we train at maximal or near maximal to permit greater potential performance. If we train significantly beyond our maximal we increase the chance of injury. However, most importantly if we train at less than maximal or less than near maximal we are in danger of under-training the athlete and stressing the CNS for no-benefit. This is also possible inter-session, not just between sessions. But prolonged exposure to CNS stressors will in time lead to debilitating fatigue, possibly illness and injury.
The aim of training, with respect to the CNS, is for the complete organism to experience a systemic stress, through a neural and musculature pathway, during a given training cycle, and in response to this, reach a new level of preparedness after a period of regeneration.
There are two risks with training in such a respect
- If the organism is not permitted the appropriate recovery the systemic compensation is halted mid-recovery and the organism is not at a new level or preparedness and unable to handle imposed stresses near the level immediately previous.
- Also should the imposed stresses be beyond a reasonable ‘reach’ for the organism the likelihood of injury is increased. This is also in respect of an accumulation of stress within a certain training density.
It appears that the optimal method of developing neural efficiency is frequent CNS loading at maximal stress with very low volume.
Summary
§ The traditional version of typical fatigue is viewed as the result of an interaction of central and peripheral physiological systems where muscle glycogen and plasma glucose concentrations are depleted, stress hormones are elevated and sex hormones lowered.
§ Proposed definition of Central Nervous System Fatigue - Central Nervous System Fatigue is the systemic response to imposed stresses (physical and emotional) interrupting homeostasis leading to a gradual but severe reduction in the efficiency of the elements of the Nervous System such as the PNS, spinal cord, cerebellum, parasympathetic system, midbrain and diencephalons, which in turn affect the optimal functioning of bodily systems and the organism as a whole.
§ Central Nervous System Fatigue is holistic, comprehensively encompassing the complete range of human systems far beyond the physical
§ Factors that appear to influence Central Nervous System Fatigue negatively are - excessive accumulation of heat in the brain, reduced cerebral glucose uptake, changes in levels of ammonia, serotonin or dopamine.
§ The CNS responds to all stresses not just high intensity stress
§ There may be two CNS responses to physical stresses, Reaction and Protection.
§ The role of the PNS is to provide input from peripheral structures and organs and to react to stress imposed on the CNS.
§ The formula - Engagement x Intensity x Density x FactorX may help estimate athletic CNS status
§ No substance heightens CNS efficiency directly
§ Certain substances can permit both an extension of CNS efficiency and a heightening of CNS control through improved performance of organism systems such as the hormonal or endocrine systems.
§ With use of such substances there will always be a debt or price to be paid as the body recovers.
§ No supplement alone can directly regenerate the CNS as the systemic nature of it means more than one system must be regenerated or supported.
§ CNSF is not as a result of the depletion of neurotransmitters
§ Rest is the single best method of restoring the CNS.
§ There is no proven method of direct CNS regeneration, as a series of systems must be regenerated.
§ Contrast bathing and cryotherapy appear to assist in CNS regeneration
§ Stresses that affect bodily systems directly have greater effects on the CNS than those that affect it indirectly.
§ The CNS has a limited reserve or pool
§ The CNS can be trained to be more efficient for certain tasks
§ The greater the forces expressed the greater the use of the CNS and the greater the overall demand on the CNS.
§ This in turn means the greater the performance the greater the debt to be paid post performance and the nearer the athlete is to systemic failure.
§ The more neurally efficient the athlete the better they can recruit Motor Units to perform their tasks, regardless of muscle mass.
§ There are many methods of suggested testing for CNS efficiency e.g. submaximal reps in bench press, cleans or vertical jumps.
§ Best option is to use a combination of basic tests.
§ CNS stimulation to re-prime for competition and preserve motor unit recruitment is best obtained using maximum effort bench pressing
§ The more efficient the athlete the greater the demand on the CNS, but the longer they can maintain a state of CNS preparedness.
§ It is suggested using at least 4 hour windows between multiple daily sessions to permit CNS recovery
§ Other wise leave 72 hours between CNS demanding sessions
§ The aim of CNS training with regard to sprint training is to elicit maximum neural response, using the near-maximal stress, but with minimal necessary volume.
§ And oh yeh - Fatigue does make cowards of us all
(Something for the weekend, Work in progress, lets debate it!!)
