CNS Fatigue and Overtraining

By Ozzy – Contributor to MuscleTalk

Before I begin, I would like to say that I do not think that training to failure is not a useful tool in bodybuilding, because we all know it is (see Training Past Failure).

For the purpose of this article we will assume that ‘failure’ is the point of momentary concentric failure – the inability to fully complete another concentric contraction.

Muscle fibres have 2 recruitment patterns:

  • Innervate units that recruit the same fibres but at different times so some rest while others work
  • More fatigue resistant fibres are recruited before fibres that are more rapidly fatigued

Type 2b fibres are the more resilient fibres so once these fibres have been fatigued you will no longer be able to lift, and it is also these type 2 fibres which have the most potential for growth. This is strong evidence that training to failure is obviously a requirement to achieve growth.

Time under tension is a well-known tool to encourage growth. To an extent; the longer the muscle is under tension the more microtrauma is incurred, causing a growth stimulus. This is strong evidence that training to failure is necessary to stimulate growth as training to failure means that the muscle is under tension for greater periods of time.

However, when it comes to the nervous system, both central and peripheral, training to failure is not the best stimuli for growth. As muscle fibres exhaust and failure becomes imminent the nervous system recruits all available motor units and fires them as much as possible, however as maximum contraction continues this frequency of firing decreases.

Quote from an article on the weight-trainer website: “We know that each neuron must release the neurotransmitter acetylcholine (ACh) every time that it fires (or ‘twitches’) a motor unit. We also know that the neurons transmit impulses down the length of their axons by way of Sodium/Potassium transport and the Sodium/Potassium ATPase Pump. The signal is carried across the membrane of the muscle cell in the same manner. The whole process also relies heavily on optimum calcium levels and enzymes that are involved in the synthesis and breakdown of acetylcholine and numerous other substances. The frequency of motor unit firing decreases, therefore, as these substrates are exhausted – yet as failure approaches we continue our maximal effort to lift the weight.”

Ever heard of the Isotron machine? Designed by Dr. John Ziegler, it is used to monitor overtraining via the electrical impulses passed through muscles. The machine supplies an electrical impulse to a muscle being tested. It found that overtrained or recently trained muscles require a much higher current than a rested muscle for strong contractions to be achieved.

What does this mean?

As your nervous system controls your muscles through electrical impulses, overtrained or recently trained muscles require a larger signal to actually complete a contraction of the same magnitude as a fresh, rested muscle (obviously good cause to think whether you are overtrained or not!). By training intuitively you can still cause sufficient microtrauma to your muscles fibres to encourage growth, without continually draining your nervous system. Training to just before failure will still create gains.

So far this relates to the peripheral nervous system, so what about the central nervous system (CNS)?

If you didn’t know then here is a little background for you: The central nervous system functions by sending electrical impulses through your nerves to the designated motor unit. This signal cannot be sustained for long periods of time with speed and power for the optimum frequency. Through continuous signals sodium, potassium and other substrate concentrations decrease to the point where contractions become slower and weaker. Eventually a state of inhibition is reached to prevent itself from further stimuli. Hence when you seem to lose all strength and drop the weight in pure exhaustion.

Although not understood, one fact in training is that mood and emotional state can affect the discharge characteristics of the CNS. ‘Psyching up’ for a big lift or thinking that one more rep is out of the question, yet you somehow manage it, are both examples of this. Both of these examples demonstrate CNS manipulation that in turn makes you physically stronger (you shouldn’t, however, always ‘psych up’ as you will over train rapidly). This means that training in a normal frame of mind on a regular basis, without the constant desire to thrash yourself, should be a regular feature of your training.

So again, we ask, what does this mean?

Well, by training to failure each time you train you are going set your nerve cells into a constant state of inhibition leading you to tax the CNS far to much through the increased out put of electrical impulses. This will lead to rapid overtraining. That leads to time off and bodily and mental states lacking motivation, appetite, etc. It also means that it is not always muscular failure which is occurring; more CNS failure, which means that your muscles are not being worked anyway so stimuli for growth is not being achieved every time you train.

Couple muscular and neuro failure together and what do you get? Poor form and therefore poor training. Poor form leads to injuries and injuries lead to more time off.

So, in conclusion to all this, muscular failure, be it concentric, eccentric or isometric, is not necessary to provide a growth stimulus. What is necessary are good form, continuous training, the build up of fatigue products and good diet and resting patterns. Fibres need sufficient training for microtrauma to be incurred causing the release of regenerative hormones to be released in the cells which leaches into the surrounding area as well as intracellular calcium levels to rise to trigger both growth and destructive processes (destructive to remove such substrates as lactic acid) without over taxation of the nervous system.

I hope this demonstrates that the CNS is a vital part of your training and that by training to failure time and time again you will offset the positive effects of it with the negative effects.

Once again I will iterate that I do believe that training to failure is a useful tool for growth stimuli, only not the only tool.

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Team MT

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