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Increase In Strength But Not In Muscle Size? Motor Unit Recruitment and Neural Adaptation Explained

Have you just started working out but haven’t seen any physical changes in your body despite increasing strength?

What the heck is going on?

At the beginning of your fitness journey or when you first start a new training phase or start working on new exercises you send a novel response to your nervous system. Your nervous system will adapt to this but NOT by increasing muscle size - at least not at first!

Do you ever notice that movement start to feel less choppy and awkward than when you first started? This is due to the first changes that occur from the nervous system at the motor unit.

What is a Motor Unit?

A motor unit is a motor neuron and all the muscle fibres it innervates.

A muscle is made up of many motor units and thus many muscle fibres.

There are different types of motor units. For the sake of simplicity, I will call them “slow twitch” and “fast twitch”.

Slow twitch motor units are smaller and less powerful than the fast twitch BUT have more endurance. The slow twitch units are stimulated with even the lightest intensity of exercises (require less of a stimulus from the nervous system) and thus contract and get “recruited” for movement first. Fast twitch units require a greater stimulus to be activated AND they tire quite easily.

Many people do not know this, but you can do exercises and activities without recruiting all the motor units and thus muscle fibres which = untapped potential increases in muscle strength and size!

With new exercise/stimulus and practice the nervous system becomes better at activating more motor units and all the muscle fibres attached to it. The movement become more fluid and coordinated. You also become stronger because more muscle fibres are being activated than before – particularly those fast twitch fibres! The body will also increase the rate of firing for those motor units so that you can really capitalize on tension generated for LONGER (this means both endurance of the muscle AND the ability to maintain tension in a greater range of motion. HOWEVER, YOU REALLY ONLY HAVE THE ABILITY TO INCREASE FIRING RATE ONCE NEARLY ALL THE MOTOR UNITS ARE RECRUITED! This means if you do not take the time to focus on tension, and proper range of motion and activation you are missing out huge potential for strength and muscle size adaptation!!! This is also true for proper rehabilitation of injuries and even some musculoskeletal chronic conditions!

The above adaptation process of increasing the number of recruited motor units and the increased firing rate and synchronicity of the firing rate is why you see an increase in the amount of weight you can lift, improve end range of motion strength and control of the movement BUT do not see an actual change in muscle size.

**A little side note here is that you also tend to get more tired from increasing neural activity as well. This is why you get sorer when you try new exercises/activities.

Once we pass this period of neural adaptation, we move into the hypertrophy stage. This is where the muscle fibres start to grow. BUT THE ONLY MUSCLE FIBRES THAT WILL GROW ARE THE ONES WE CAN NEURALLY CONNECT TO. This is why you will see limit results in muscle growth unless you have adequately done the work to increase range of motion and tension!

Neural adaptation isn’t “sexy” but it is necessary if you want to see true increases in strength, muscle size, joint health, proper movement, and maximize your results to reach your goals whatever they maybe.

Stephanie Fusnik

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