Crossfit, Weightlifting and the Neuroendocrine Response – Part 2

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In part I of ‘Crossfit, Weightlifting and the ‘Neuroendocrine Response’ I went into the ‘endocrine’ component of the NE response and touched on some of the key hormones…

    • Testosterone
    • Growth Hormone
    • Insulin Like Human Growth Factor
    • Insulin & Glucagon
    • Cortisol & Catecholamines

…that play a major role in the development of muscle and strength.

In part II I am going to address the neurological aspect of our response to exercise and look at how the two systems (endocrine and neurological) work together to create the ‘Neuroendocrine Response’.

Neurons 101

Neurons (the ‘messengers’ of our nervous system) are responsible for taking information to and from various organs and tissues throughout the body, including our muscles. When we ‘decide’ to do a certain movement electrical impulses are sent from the brain and carried through the spinal cord to the associated muscles via hundreds of of tiny motor neurons. When our muscles sense/feel something (e.g. pain), the reverse happens and a signal is relayed via sensory neurons from the muscles to the brain and we react accordingly.

The site where the neurons and muscles meet is called a ‘Neuromuscular (NM) Junction’, and a single neuron is responsible for simultaneously activating multiple muscle motor units (the exact number varies depending the type of action in question – e.g. facial expressions vs flexion of the biceps brachii)

When the motor-neuron at a NM Junction ‘fires’ so do all of the muscle fibers it controls, and they contract at full force.… wait, so every time its a maximal effort? but what about when I do my 50, 60 or 70% squats? that’s not full effort?

Yes and no, each unit will contract at full force but the total force of a muscle action will vary based on

    • the number of motor units recruited
    • the frequency that the motor units fire

and both of these factors can be influenced by our ‘training status’. (i.e. how in shape, or out of shape we are)

Nervous System


The more muscle groups (and NM junctions) involved, the greater the force generated which means that functional movements (which engage multiple muscles and require us to use the whole body) are capable of creating much more force than isolation exercises. And the amount of force produced can be increased over time by repeating the movements  (aka practice) because this results in more efficient & coordinated communication between the muscular and nervous systems.


NM Junctions that fire frequently produces more ‘forceful’ movements and the capacity to do so is largely based on genetic design (i.e. does the junction stimulate muscles in the fingers or the quads?) but prolonged stimulation of any junction (“powerful or not”) can lead to NM fatigue. When the junction becomes fatigued is when we see a decrease in our performance  because of

    • reduced glycogen stores (not enough fuel in the tank)
    • increased blood & muscle lactate levels
    • and other neuromuscular chemical alterations

All of which contribute to a reduced ability to generate force . This breakdown is inevitable; however, the rate at which we reach the NM fatigue can be slowed down by increasing our fitness levels.

So essentially  the more neurons/motor units that we stimulate, the greater a force we can generate and our ability to maintain this force depends on how quickly we fatigue. What this means is that by training the CNS (central nervous system) and muscular system to ‘communicate properly’ we can see significant increases in strength even if there isn’t any increase in size.

Scott Panchik

The ‘Neuroendocrine Response’….duhn duhn duhhhh

Ok, so now you (hopefully) have a brief understanding of how the endocrine system and nervous system work so lets tie them together and address the concept of the ‘Neuroendocrine Response’ and what it is that makes Crossfit and weightlifting is so damn effective.

In order to  see strength increases we need to stimulate/train our the neuromuscular system AND produce sufficient amounts of those important ‘muscle/strength building’ hormones (e.g. testosterone, GH, IGHF-1).

Keeping in mind what I mentioned in Part I and earlier in Part II it begins to make sense why movements which….

“recruit large muscles and/or multiple muscle groups”

and are

“performed high volume & intensity with moderate-to-heavy weights and short rest periods”

can be way more effective because they stimulate hormone production (muscle growth) and when we practice them regularly we become more efficient at executing them (increased strength)

….hmmm sounds pretty much like Crossfit, or a fast-paced lifting session…

Katie Hogan Power Clean

So essentially doing full-body movements, lifting weights and upping the intensity  is the most effective way to train and while Crossfit definitely meets the requirements this doesn’t mean everyone has to do Crossfit to see results. Greg Glassman didn’t invent the Neuroendocrine Response, he started a movement which based its training style on getting that Neuroendocrine response, but that doesn’t mean other types of exercise can be just as effective.

So where does Insulin/Glucagon and the ‘Stress Hormones’ come in?

Insulin and glucagon control blood sugar levels, and blood sugar = fuel to exercise, i.e. its important to have ‘normal’ levels of blood glucose otherwise it becomes difficult to perform optimally. Blood sugar levels go up and down naturally, but drastic spikes (in either direction) are a good indicator that ‘something is wrong’, and usually this is related to elevated levels of cortisol (stress hormone).


Regular exercise can help regulate our cortisol and blood sugar, BUT only if we are eating/sleeping properly this AND taking sufficient rest days (and I don’t mean ‘easy WOD days’ I mean rest days); otherwise we risk ‘overstressing’ the body and when this happens we stuck with elevated cortisol levels and risk of

    • an increased appetite,
    • weight gain,
    • reduced metabolism,
    • muscle breakdown,
    • diabetes etc etc.

So takeaway message(s)

The ‘neuroendocrine response’ refers to an interaction between our hormones & neurons (endocrine system and nervous system) and how that contributes to the development of muscle and strength development. Because of the nature of ‘Functional Movements’ (recruitment of multiple NM junctions & muscle groups) by integrating them into our training sessions we can see greater increases in strength and lean muscle mass while simultaneously decreasing fat stores. However, it isn’t just the movements that matter, it is how we do them and it has been shown that intense exercise (high resistance, moderate to high volume and minimal rest periods) enhances the effectiveness of functional movements even more. But more importantly, theres the fact that without proper nutrition, adequate sleep and a (relatively) balanced lifestyle the body will release stress hormones and if we can’t get things under control these hormones can do serious damage to our training and our health. 

So go out there, lift heavy weights for time (with good technique), kick ass and don’t forget to eat, sleep and smile.

Taryn Haggerstone