Become a Personal Trainer
Anatomy Feels Like A Lot To Remember Before You Qualify, And One Motor Unit Idea From The Level 3 Course Explains Why Hands Are Precise And Legs Are Strong
From the FASTER syllabus: this article is taught in full in the Level 2 and Level 3 personal training course from scratch →
In short: A motor unit is one motor neuron and every muscle fibre it controls. The number of fibres one neuron controls is the innervation ratio, and it runs from roughly five to nearly one thousand. A low ratio gives very fine movement, which is why the hands and face are precise. A high ratio gives large steps in force, which is why the big leg muscles are strong. The Level 3 personal training course teaches this as one idea a new trainer can see in their own body, and it turns anatomy from a list to memorise into something you can watch.
The short version
Anatomy looks like the part of a personal training course where everything has a long name and you have to keep all of it. Motor units are a good example of why it is gentler than it looks. A motor unit is one nerve and the muscle fibres it controls. When one nerve controls only a few fibres, you get fine, precise movement, like the fingers. When one nerve controls hundreds, you get big force, like the thighs. One of the things you learn on our CIMSPA-recognised personal training course is exactly this idea, and it is one picture, not a list.
Key Takeaways
- A motor unit is one motor neuron and every muscle fibre it controls.
- Few fibres per neuron gives fine control. Many fibres per neuron gives large force.
- You can feel the difference between your own fingers and your own thighs today.
What a motor unit is
The course text puts it plainly. A motor neuron starts at the spinal cord and goes into a muscle. Inside the muscle it branches and connects to individual fibres. The neuron and all the fibres it connects to are one motor unit. A whole muscle is made of many of them.
That is the whole definition. Nobody needs to recite the chemistry to understand it.
The number that does the work
The course calls the number of fibres one neuron controls the innervation ratio. It varies a lot. At the low end it is roughly five fibres to one neuron. At the high end it can be close to a thousand.
You do not need to memorise either figure. They give you a feel for the range, and the range is the point.
Why your hands and your legs behave so differently
Where the ratio is low, each neuron adds only a small step in force. That allows very fine movement, which is why the hands and the face can do such delicate work.
Where the ratio is large, each neuron starts a very large step in force. The course gives the big leg muscles as the example.
Try it in your own body. Pick up a pen and write your name. Then stand up from a chair. Both are movements, and they use the same system in two very different ways. The hand is built for precision. The leg is built for force. The nervous system does not change its rules between them. It changes the ratio.
Why this matters when you are changing career
If you are leaving another job to retrain, the worry that usually sits behind anatomy is simple. You pass the unit, the names fade, and a client asks something in your first month. A single clear idea lasts longer than a page of terms. Once you can say "few fibres per neuron, fine control; many fibres per neuron, big force", you can work out most of the rest by looking at what the muscle has to do.
That is how we teach it. The course is visual and interactive, and it fits how you like to learn: reading, listening, writing answers or dictating them. Anatomy sits next to movement you can see, so the idea stays attached to something real.
Where PT Manager fits
PT Manager, our free software, opens the moment you sign up with FASTER, so you can explore and set it up in your name straight away. Adding real clients unlocks the day you pass a course. If you are new to the industry, the Level 2 and Level 3 course gives you PT Manager from the start of a career. Take a look at PT Manager.
If you want the earlier step, the sibling post on what a motor unit is covers the definition.
Frequently asked questions
What is a motor unit?
One motor neuron and all the muscle fibres it controls. A whole muscle is made of many motor units.
What is the innervation ratio?
The number of fibres controlled by one neuron. The course gives a range from roughly five to nearly one thousand.
Why are the hands so precise?
Their ratios are low, so each neuron adds a small step in force, which allows very fine movement.
Why are the big leg muscles strong?
Their ratios are large, so each neuron can start a very large step in force.
Do I need to memorise the numbers?
No. The idea matters more than the figures.
The FASTER Journal shares the next teaching point first. Sign up at /newsletter and it arrives the moment it is ready.
You just read one answer. The course is all of them.
the Level 2 and Level 3 personal training course from scratch — taught by the people who wrote it, with free return access for life. The price is on the page, payment plans at checkout, no sales calls.
Also worth a look: PT Manager, free with any FASTER course.
See what the course covers →Frequently asked
What is a motor unit?
One motor neuron and all the muscle fibres it controls. A whole muscle is made of many motor units.
What is the innervation ratio?
The number of muscle fibres controlled by one neuron. The course gives a range from roughly five to nearly one thousand.
Why are the hands so precise?
Their innervation ratios are low, so each neuron adds only a small step in force, which allows very fine movement.
Why are the big leg muscles strong?
Their ratios are large, so each neuron can start a very large step in force.
Do I need to memorise the numbers?
No. The idea matters more than the figures: few fibres per neuron gives fine control, many gives big force.
Reading this for your own training? Every trainer on the FASTER register has been assessed by us — search by town or speciality, free.
Find a FASTER trainer →
