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Muscle fibre recruitment is a measure of how many motor neurons are activated in a particular muscle and, therefore, how many muscle fibres are activated. The more muscle fibres are recruited, the stronger the muscle contraction will be. Muscle fibre recruitment is important for strength training and for runners who want to improve their performance. The body recruits muscle fibres in a specific order, starting with slow-twitch fibres and moving on to fast-twitch fibres as the slow-twitch fibres fatigue. To target muscle growth, fast-twitch fibres should be the focus of training as they have the most potential for growth. There are several ways to stimulate fast-twitch muscle fibres, including lifting heavy weights, lifting light weights explosively, and lifting light to medium weights to failure.
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What You'll Learn
Fast-twitch vs slow-twitch muscle fibres
Muscle fibre recruitment is an important principle to understand in weight training. All muscle groups contain a mixture of slow-twitch and fast-twitch muscle fibres. Slow-twitch fibres are small and weak but have good endurance, whereas fast-twitch fibres are large and powerful but have poor endurance. Slow-twitch fibres are used for low-intensity activities and have a steady supply of energy, while fast-twitch fibres are used for big bursts of energy.
Slow-twitch fibres are also called "red" muscles because they have a rich blood supply and use oxygen for energy. They are considered "plugged in" to the heart. The muscles in the back of the lower legs and the back, for example, are mostly made up of slow-twitch fibres as they have to work tirelessly to help you stand, sit up, and maintain posture. People who excel at endurance sports tend to have a higher number of slow-twitch fibres.
Fast-twitch fibres, on the other hand, use an anaerobic energy system, which means they don't rely on oxygen. They are powered by their own quick source of energy, similar to a "battery". Muscles with more fast-twitch fibres look lighter due to reduced blood supply. These fibres are used for speed and powerful movements such as sprinting, powerlifting, and blinking. Training regimes that focus on heavy loading or high power output are required to recruit these fibres effectively.
To target and stimulate fast-twitch fibres in the gym, there are three main approaches: lifting a heavy weight, lifting a light weight explosively, or lifting a light to medium weight to failure or near failure. These approaches require significant effort, and it is important to incorporate rest and variation in training intensity to avoid overtraining and burnout.
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Heavy loading and high power output
To recruit muscle fibers, it's important to understand the principles of strength training and the different types of muscle fibers. There are two main types of muscle fibers: slow-twitch and fast-twitch. Slow-twitch fibers are small and weak but have good endurance, making them ideal for long-distance running or other endurance activities. On the other hand, fast-twitch fibers are large and powerful but have poor endurance, which is why they are recruited during high-intensity activities such as sprinting or weightlifting.
When it comes to heavy loading and high power output, the focus is on recruiting these fast-twitch muscle fibers. The key is to lift heavy weights or perform exercises that demand high power output. By lifting heavy weights, you will quickly fatigue the slow-twitch muscle fibers, and your body will be forced to recruit the fast-twitch fibers to handle the load. This is often referred to as the size principle, where the recruitment of motor units and muscle fibers occurs in order of size, from smaller slow-twitch to larger fast-twitch.
To effectively recruit fast-twitch muscle fibers, aim for lifting weights that are between 80% to 90% of your one-rep maximum (1RM). This will ensure that you are challenging your muscles sufficiently to activate the fast-twitch fibers. Additionally, you can incorporate exercises that involve explosive movements, such as box jumps or power cleans. These exercises require generating a lot of force in a short period, which recruits the fast-twitch fibers.
It's important to note that the recruitment of muscle fibers also depends on the order of exercises and body position. For example, performing a flat bench press will recruit different muscle fibers than an incline bench press. Varying your exercises and incorporating different movement angles can help ensure complete development of the muscles. Additionally, when designing your workout program, consider incorporating a combination of heavy lifts and lighter exercises to avoid fatigue and promote muscle growth effectively.
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Resistance exercises
To promote muscle growth, it is important to target all muscle fibre types. This can be achieved through a mix of moderate and heavy resistance exercises. For maximal strength gains, focusing on heavy weights and low repetitions recruits the high-threshold motor units (Type IIx fibres), leading to strength improvements. If you want to build strength, use the size principle to focus on your fast-twitch muscle fibres. Since they are the fibres recruited when your muscles are forced to lift a load heavier than what the slow-twitch fibres can handle, use a high resistance (80 to 90% of your one-rep max). The heavier the weight, the greater the number of fast-twitch fibres you will recruit during a lift.
If you are an advanced lifter, it is recommended to incorporate a variety of approaches in your workout routine. For example, you can start your workout with a heavy or explosive lift and then finish off with lighter exercises taken to near failure. You can also train a body part heavily one day and lightly the next time you train it. It is important to remember that to gain muscular size and/or strength, you will want to primarily target fast-twitch fibres because they have the most potential to grow.
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High-force activities
To recruit more muscle fibers, the focus should be on engaging in intense, high-force activities. For example, maximum effort running and heavy lifting recruit a higher number of muscle fibers due to their demanding nature. During a race, runners need to generate more force, leading to the recruitment of additional muscle cells, resulting in longer strides and higher cadence. Similarly, heavy weightlifting requires the activation of more muscle fibers to lift heavier loads.
Drills and plyometrics are excellent examples of high-force activities that not only recruit more muscle fibers but also train them to work in the proper sequence. These exercises help improve coordination, increase the range of motion, and enhance the ability to produce force rapidly. As a result, athletes can benefit from improved speed, performance, injury resilience, and overall strength.
Additionally, the concept of "graded recruitment" explains that as force demands increase, larger muscle fibers are recruited. For instance, performing a simple task like getting up from a chair may only require a partial recruitment of muscle fibers, while engaging in high-force activities will activate a larger percentage of muscle fibers, leading to greater muscle contractions and improved performance.
To effectively recruit muscle fibers during high-force activities, it is crucial to understand the different types of muscle fibers. Slow-twitch (Type I) fibers are small and weak but have greater endurance due to their higher mitochondrial content, ensuring a steady oxygen supply for aerobic performance. On the other hand, fast-twitch (Type II) fibers are larger and more powerful but have poor endurance, making them ideal for high-force and high-power activities like sprinting or powerlifting.
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Muscle activation
During muscle contractions, motor units are generally recruited in order of smallest to largest, known as Henneman's size principle. This means that slow-twitch fibres are recruited first, followed by fast-twitch fibres. Slow-twitch fibres are small and weak but have good endurance, while fast-twitch fibres are large and powerful but have poor endurance.
To increase muscle activation, it is important to recruit as many muscle fibres as possible. This can be achieved through intense, high-power, and high-force activities such as maximum effort running and heavy lifting. For example, runners who incorporate activities like hard runs, weight lifting, long runs, and drills into their training regimens are able to recruit more muscle fibres, leading to improved performance and reduced risk of injury.
In weight training, lifting a heavy weight of over 80% of your 1-rep max will require the recruitment of stronger fast-twitch fibres. Lifting a light weight explosively or lifting a light-to-medium weight to failure can also stimulate fast-twitch fibre recruitment. It is important to incorporate different training intensities and exercises to avoid overtraining and burnout, as well as to target different muscle fibres and promote overall muscle activation.
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Frequently asked questions
There are two types of muscle fibers: slow-twitch and fast-twitch. Slow-twitch fibers are small and weak but have good endurance, while fast-twitch fibers are large and powerful but have poor endurance.
There are three main ways to recruit fast-twitch muscle fibers: lifting a heavy weight, lifting a light weight explosively, or lifting a light to medium weight to failure.
Muscle fiber recruitment refers to the number of muscle fibers that are engaged within a muscle contraction. The more muscle fibers that are recruited, the stronger the muscle contraction will be.
