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The Science of Getting Stronger: How Your Body Builds Strength


Strength. It's a word we hear often, but what does it truly mean? Is it the ability to lift heavy weights, the endurance to run a marathon, or perhaps the agility to perform a perfect backflip?

In this post, we're diving into the science of strength. From the microscopic fibers in our muscles to the complex neural networks in our brains, strength is more than just a physical attribute.

So join us as we dissect the types of muscle fibers, uncover the role of genetics and training, explore the world of neural adaptations, and understand how our nervous system plays a crucial role in developing strength.

Understanding Muscle Fiber Types

Type I Fibers (Slow-Twitch): These fibers are more efficient at using oxygen to generate ATP for continuous, extended muscle contractions over a long time. They're more resistant to fatigue and are predominant in endurance athletes.

Type II Fibers (Fast-Twitch): These fibers are better at generating quick, forceful bursts of speed than Type I fibers but fatigue more quickly. They are further categorized into:

  • Type IIa: Also known as "fast oxidative fibers," they are somewhat fatigue-resistant and can generate high force.

  • Type IIx (or IIb): These are "fast glycolytic fibers" and are the least fatigue-resistant but are capable of producing the highest force outputs.

Factors Influencing Fiber Type

  • Genetics: Genetics play a significant role in determining an individual's proportion of different muscle fiber types.

  • Training: Specific training can encourage fibers to develop characteristics of another type, although the extent of this conversion is a topic of ongoing research.

Neural Adaptations

Increased Motor Unit Recruitment

  • Understanding Motor Units: A motor unit consists of a single motor neuron and all the muscle fibers it innervates. The size and type of a motor unit vary. Smaller units control fewer fibers and are involved in fine, precise movements, while larger units control more fibers and are responsible for powerful, forceful movements.

  • Recruitment during Strength Training: When you perform strength training, especially with heavy loads, your body is prompted to recruit more of these larger motor units. The recruitment of more motor units means more muscle fibers are engaged, leading to greater overall muscle contraction strength.

  • Progressive Overload: As you increase the demand on your muscles through progressive overload (gradually increasing weight, intensity, or volume), your nervous system adapts by recruiting more and larger motor units to meet the increased demand.

Improved Motor Unit Synchronization

  • Synchronization for Force Production: Effective synchronization means that more motor units are firing at the same moment. This simultaneous activation produces a more forceful and efficient contraction than if the motor units were firing asynchronously.

  • Skill Component: Improved synchronization is not just about raw strength but also involves a skill component. As you practice a specific movement, your body becomes more efficient at synchronizing the motor units involved in that movement.

Enhanced Neural Drive

  • Increased Signal Strength and Frequency: Enhanced neural drive involves both the strength of the signals sent from the brain to the muscles and the frequency of these signals. Stronger, more frequent signals result in more powerful contractions.

  • Central Nervous System Adaptation: This aspect of neural adaptation is attributed to changes in the central nervous system (CNS). As you train, your CNS becomes more effective in sending rapid, strong signals to your muscles.

Motor Unit Recruitment

Basics of Motor Unit Recruitment

  • Motor Unit Definition: A motor unit comprises a motor neuron (a nerve cell) and the skeletal muscle fibers it innervates. Each motor neuron communicates with muscle fibers via a neuromuscular junction.

  • Size Principle: Motor units are generally recruited in order according to the size principle. This principle states that smaller motor units (which are slower to fatigue and control fewer muscle fibers) are recruited first. As the demand for muscle tension increases, progressively larger motor units are recruited.

  • Recruitment for Strength vs. Endurance: In activities requiring strength (like lifting heavy weights), the body recruits larger motor units early in the contraction. These units have more muscle fibers and are capable of generating more force but fatigue faster. In contrast, endurance activities predominantly use smaller motor units.

Recruitment During Strength Training

  • Recruitment Threshold: Strength training, particularly with heavy loads, challenges the body to lower the recruitment threshold for larger motor units, allowing them to be activated more readily and contribute to force production.

  • Activation of High-Threshold Motor Units: High-threshold motor units, which control fast-twitch muscle fibers, are crucial for generating high levels of force. These units are more likely to be recruited during high-intensity activities like heavy lifting or explosive movements.

  • Increased Rate of Recruitment: Not only does strength training increase the number of motor units recruited, but it also speeds up the rate at which they are recruited, allowing for quicker and more powerful muscle contractions.

Adaptations with Continued Training

  • Efficiency in Recruitment: With regular strength training, the nervous system becomes more efficient at recruiting the necessary motor units. This efficiency is crucial for both improving performance in strength activities and reducing the risk of injury due to improper muscle activation.

  • Recruitment of Additional Motor Units: As muscles adapt to training stress, the nervous system may also recruit additional motor units that were previously not utilized, or it may improve the coordination among existing units for better force production.

Role of Fiber Type

  • Fiber Type Influence: The composition of muscle fibers within motor units influences their recruitment. Motor units containing predominantly fast-twitch fibers (Type II fibers) are more suited for strength and power tasks and are preferentially recruited during such activities.

  • Adaptation of Fiber Types: Over time, with consistent strength training, there may be adaptations within the muscle fibers themselves, making them more responsive and efficient during high-force productions.

Rate Coding

Definition and Basic Concept

  • Action Potentials: These are electrical signals that motor neurons use to communicate with muscle fibers. The frequency of these action potentials is known as rate coding.

  • Force Production: The force a muscle produces is not only dependent on how many motor units are activated (motor unit recruitment) but also on how fast these units are firing. The more rapid the firing rate, the greater the force produced by the muscle.

Rate Coding in Muscle Contraction

  • Twitch Summation and Tetanus: When multiple action potentials are fired in quick succession, individual muscle fiber twitches can summate, leading to a sustained and stronger muscle contraction. If the firing rate is high enough, it results in a smooth, sustained contraction known as tetanus, which is crucial for maximal force production.

  • Recruitment and Rate Coding Relationship: Initially, as more force is required, the body recruits more motor units. As the demand increases further, the rate coding (firing frequency) of these motor units also increases to maximize force output.

Adaptations with Strength Training

  • Enhanced Firing Rate: One of the adaptations to strength training is the ability of the nervous system to increase the firing rate of motor neurons. This adaptation allows for a quicker and more sustained force production, crucial for lifting heavier weights.

  • Efficiency in Force Production: As rate coding improves, muscles become more efficient at producing force. This means that the muscle can achieve a higher force output without a proportional increase in effort or energy expenditure.

Neurological Efficiency

  • Improved Neuromuscular Junction Performance: Strength training enhances the efficiency of the neuromuscular junction (the point where the nerve connects with the muscle fiber), which plays a role in how quickly and effectively the muscle can be stimulated.

  • Central Nervous System Adaptation: The CNS becomes more adept at sending rapid, repeated signals to the muscles, improving overall muscular performance.


Strength is a multifaceted attribute that goes beyond mere muscle size or endurance. It's the result of a complex interplay between muscle fiber types, genetic predispositions, and the adaptations of our nervous system. By understanding the different muscle fibers and how training influences their development, we gain insight into optimizing our strength. Neural adaptations, such as increased motor unit recruitment, improved synchronization, enhanced neural drive, and efficient rate coding, play crucial roles in our strength development. As we continue to train and challenge our muscles, these adaptations become more pronounced, leading to greater force production and improved performance. Embracing the science of strength allows us to train smarter and achieve our fitness goals more effectively.


Want help becoming the healthiest, fittest, strongest version of you?

Most of us know that eating well, regular activity, and managing our sleep and stress levels are important for a healthy life. Still, we struggle to apply that information into our already busy lives. That's why the Active Wave coaching programs help you create a strategy to lose fat, get stronger, and improve your health, all in the context of your own life. We know that's the only way to keep those changes for good, no matter what situation you're in. If you'd like to chat about how you can start to change your life and reach your health and fitness goals, book a free, 10 minute call with one of our coaches today!

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