The Art of Movement Edition 36: Training Principles 7 - The ‘All or None’ Law
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For the rest of this year, I will use the newsletter to summarise some important principles around exercise and training which I hope will help the average person understand the process better. These are the concepts we need to take into consideration when identifying the best way to put our next foot forward in pursuit of our exercise goals.
Edition 35 - Training Principles 7: The ‘All or None’ Law
This is probably one of the more sciency editions of The Art of Movement, and I hope I am able to convey the message of this law to you in an effective way - however if you finish reading with more questions than answers please get in touch and ‘fire away’ (hopefully you get the pun by the end). Before we discuss the ‘all or none’ law, you must first understand what a motor unit is. A motor unit is composed of a single motor neuron that innervates all of the muscle fibres connected to it. Don’t let the terminology scare you, essentially we are just talking about a pathway which sends messages to your muscles and tells them to switch on!
We have thousands of nerves and millions of muscle fibres connected to them. Let’s take the calf muscle for example - for every 1 nerve, it connects to around 1900 muscle fibres, and the calf has around 1000 nerves! Which means (1000x1900) roughly 1.9 million muscle fibres in the calf muscle alone.
Small motor units activate a small number of muscle fibres to produce a small amount of force (think about the action of writing, for example - we just need the little muscles in our fingers), while large motor units activate thousands of muscle fibres to produce a large amount of force in a task (such as deadlifting a heavy barbell). The number of muscle fibres activated in a motor unit may differ, but there is always only one neuron, and the ‘all or none’ law simply means:
You will activate ‘all’ of a neuron, or ‘none’ of it.
A motor unit consists of a single neuron and all of the muscle fibres it innervates. When an impulse is sent down a neuron, all the muscle fibres within the motor unit are activated (innervated). In other words, the motor unit activates all of its muscle fibres or none at all.
But what has this got to do with exercise and how can it help you?
This principle helps debunk the myth of isolating one part of a particular muscle. It is an attractive idea to somebody pumping weights and working on their physique that they can simply do an exercise for their ‘upper’ rectus abdominis (abs).
As we can see, the rectus abdominis spans the whole front of our torso from mid-ribcage down. When you carry out an exercise which requires the use of this muscle, you are going to be able to recruit a certain amount of motor units, and the extent to which you can (or need to) recruit is going to influence the resulting muscular activity. But you are not going to be able to choose which section of this muscle you use, and which sections you don’t!
Another useful problem we can address knowing about ‘all or none’ relates to the phrase everyone talks about after visiting their physio:
‘I’ve got inactive glutes!’
What does that mean? Well, it essentially means that the muscle fibres in your glutes are not being sufficiently stimulated by the motor unit and therefore simply not activating. A neuron's threshold causes it to either fire an action potential - or not - in response to a stimulus, and in this case - the threshold is not being met.
As soon as we sufficiently stimulate, every fibre in the motor unit will activate
But why was it not being sufficiently stimulated in the first place?
This can be for many reasons. As we well know by now, most people live sedentary lives, and this results in an excessive amount of hip flexion (ie. sitting down). Meanwhile glute activation largely involves the opposite action - hip extension, which is something a sedentary person isn’t doing enough of.
So what happens to these muscle fibres and the nerves that are connected to them? They become dormant, less sensitive to the action we require from them and essentially begin to switch off, hence - inactive glutes. This is why when I take many people through the glute bridge exercise, they can feel activity or cramping in their hamstrings. The glute has become so unaccustomed to firing, that the body has rehearsed a pattern whereby the hamstrings take over the action and allow the glute to not contribute.
This is why consistently targeting activity in a muscle group is important, the strength of the contraction is dependent on the state of the muscle fibres. Individual muscle fibres do not respond at all if the stimulus is too weak. However, when the stimulus reaches a certain threshold, the muscle fibre responds maximally. If the stimulus is made even more powerful, the muscle tension increases by bringing more muscle fibres into play. It is a snowball effect!
This is where strict constraints to isolate the glutes and ‘teach’ them to fire again are hugely important, and why when carrying over to larger movements, the emphasis on technique and ensuring the glutes are active and producing force correctly is crucial.
Understanding the ‘all or none’ law I hope will just give you an extra nugget of knowledge and wisdom that you can take with you into your fitness journey, and allow you to focus on an aspect of muscle activation you may be struggling with, and turn it to your advantage. By targeting and isolating dormant muscles (that’s the whole muscle, not half of it or the top third of it!), you can neutralise your weak points and improve the efficiency of your functional movements.
“Repetition of the same thought or physical action develops into a habit which, repeated frequently enough, becomes an automatic reflex” ― Norman Vincent Peale
Movement of the Week: Side Plank with Rotation
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