How do muscles grow?The science behind Hypertrophy and Training
- Francesco Sidoti
- 23 gen 2022
- Tempo di lettura: 6 min
Aggiornamento: 5 ott 2024
Table of content
Have you ever wondered why resistance training—working out with weights—promotes muscle growth?
In this article, we will explore the physiological reasons behind this process and provide practical examples to help you design the most efficient and effective workouts for building muscle mass.
THE STRUCTURE OF MUSCLES
Muscle growth, scientifically known as muscle hypertrophy, refers to the increase in the cross-sectional area of muscle fibers, which causes muscles to appear larger.
Cross-sectional area is simply the size of the muscle when looked at across its width.
In simple terms, skeletal muscles (i.e., muscles) are organs attached to bones. Their primary function is contraction, which allows us to carry out various movements in our daily activities.
The structure of muscles is quite complex; the image below shows how skeletal muscles are structured from the large visible muscle to its smallest parts.
Fascia is a thin layer of connective tissue that surrounds the entire muscle, holding it together and providing structure.
Muscle fibers are composed of thousands of myofibrils, which in turn consist of multiple sarcomeres—the basic units of muscle contraction.
Inside each myofibril, there are two key protein filaments, actin and myosin, that are responsible for generating muscle contractions.
Muscles are connected to the nervous system through nerves called motor neurons.
When we decide to move, a nerve impulse travels from the motor neurons to the muscles, triggering contraction. This allows us to perform movements like running, lifting weights, or walking.
Muscle hypertrophy—or muscle growth—occurs when there is an increase in the size or number of muscle fibers within the muscle itself.
MUSCLE FIBERS TYPE
Muscle fibers are categorized into two main types:
Type I (slow-twitch fiber)
Type II (fast-twitch fiber)
The difference between them lies in their endurance and power: Type I fibers are fatigue-resistant, making them ideal for endurance activities, while Type II fibers are designed for strength and power, capable of generating a higher force output.
On average, most people have a roughly 50/50 split between Type I and Type II fibers, though individual variability exists.
Studies show that this variability can be influenced by the type of exercise performed over time.
For example, individuals who have trained in endurance-based activities from a young age will likely have a predominance of Type I fibers.
Have you ever noticed the stark contrast between a marathon runner and a sprinter on the track?
You can visibly see the difference in muscle size and definition, illustrating the impact of fiber composition on performance and physique.
Additionally, muscle fiber composition can vary between different muscles. The quadriceps, for example, are made up of approximately 60% fast-twitch fibers, while the soleus muscle in the calf consists of about 80% slow-twitch fibers.
When it comes to muscle growth, research indicates that Type II fibers tend to experience more significant hypertrophy (growth) than Type I fibers.
HOW MUSCLE GROWTH WORKS
The primary driver of muscle growth is an external stimulus strong enough to force the muscles to adapt. In simpler terms, muscles need a reason to grow, and resistance training with weights provides that stimulus.
Resistance training signals to the body that the muscles must become stronger to handle the load we are lifting. This principle ties into evolutionary adaptation theory, which suggests that organisms adapt to their environment to increase their chances of survival.
In the animal kingdom, species evolve and adapt to challenges like finding food, avoiding predators, and securing shelter. For humans, resistance training works in a similar way. The body perceives the external challenge of lifting weights as a potential threat, triggering internal mechanisms that lead to muscle growth as a way to "survive" future encounters with the same challenge.
However, as the body adapts to the initial challenge, to keep progressing and building muscle, we must continue to challenge the body by increasing the difficulty of your workout.
This is done through progressive overload, which involves either increasing the weight lifted over time or increasing the volume (reps/sets) of exercises. By continually pushing your muscles to work harder than before, you give them a reason to grow and adapt to the new stimulus.
MECHANISMS OF MUSCLE HYPERTROPHY
Specifically, the three main mechanisms that have been hypothesized as the main contributors of muscle hypertrophy are:
Mechanical Tension
This is the most important mechanism. It refers to the tension experienced by muscles when lifting weights; it is the force applied on a particular muscle.
For instance, when squatting with 80kg, the mechanical tension applied to the quadriceps is equivalent to that load.
The brain possesses mechanosensors that detect this tension and convert it into chemical signals within the muscle fibers. These signals tell the muscles, “We need to contract efficiently to lift this load!” and, over time, through progressively increasing the load, the body will adapt to the new (higher) load by increasing in muscle size.
To effectively stimulate mechanical tension, the load needs to be of sufficient magnitude. Typically, loads between heavier than 65% of your one-rep max (1RM) are optimal.
2. Metabolic Stress
This refers to the accumulation of metabolites (like hydrogen ions, lactate, and phosphate) in the muscle.
Metabolic stress is maximized during submaximal efforts lasting from 20 seconds to 2 minutes. For example, after completing a set of 20 repetitions of arm curls, you may experience a burning sensation in your muscles. This sensation occurs due to the accumulation of metabolites in the muscle.
To effectively stimulate metabolic stress, we therefore need to perform medium-high rep exercises.
3. Muscle damage
Muscle damage refers to the microtears that occur in muscles and connective tissues after resistance training. This often leads to symptoms like soreness and inflammation that can last for days.
While some muscle damage may accompany hypertrophy, current research suggests it plays a more marginal role compared to mechanical tension and metabolic stress.
TRAINING IMPLICATIONS
From a practical standpoint, the concepts of mechanical tension and metabolic stress are essential for structuring an effective training routine. These two elements work together to maximize muscle growth by targeting different aspects of muscle hypertrophy.
Mechanical Tension: involves using exercises that allow for heavy loads, typically in the 5-8 rep range. Examples of exercises that emphasize mechanical tension include compound movements such as squats, bench presses, and deadlifts.
Metabolic Stress: focuses on higher repetitions (typically 10-30) to increase time under tension and promote metabolite accumulation. This can be done using more isolated movements, which lead to a "pump" and the burn that accompanies higher-rep sets. Examples include leg presses, chest flyes, or bicep curls.
For instance, when training the chest, you might start with the bench press (a compound lift in the 5-8 rep range for mechanical tension) and follow it with an isolation movement like flyes (in the 12-15 rep range) to maximize metabolic stress.
The same principle applies to smaller muscle groups like the triceps. You could begin with a mechanically focused exercise like the French Press for low reps, then follow up with a lighter isolation exercise such as tricep pushdowns in a higher rep range (12-15).
To target the shoulders, I like to throw in some lateral raises as an isolation movement for the middle deltoid, which works well in the 12-15 rep range and promotes metabolic stress.
The same principles apply when designing a leg workout. Here's an example of how you can structure it:
Squats and Romanian Deadlifts: These compound movements should be performed with a heavier load in the 5-8 rep range to emphasize mechanical tension in the quadriceps and hamstrings.
Leg Extensions and Leg Curls: These exercises target the same muscle groups but use a higher rep range (12-15 reps) to stimulate muscle fibers via metabolic stress.
Finish off with Calf Raises in a moderate to high rep range to work the calf muscles and complete the lower body session.
AM I PROGRESSING?
Progression in the gym—known as progressive overload—is essential for muscle growth. If you can lift more weight, perform more reps, or complete more sets over time, you are progressing and building muscle (assuming your diet and recovery are on point).
However, for beginners, strength gains in the first few months of resistance training are unrelated to muscle mass.
Let me explain why.
Resistance training can induce two different adaptations:
Neural adaptations are improvements in strength related to factors such as better coordination, greater muscle recruitment, and more efficient motor patterns. The increase in strength during this phase is not attributed to muscle mass but rather to the individual’s ability to perform the exercise more effectively.
Muscular adaptations refer to strength improvements linked to increases in muscle mass.
Literature agrees that, for beginners, initial strength improvements are primarily due to neural adaptations. As the body learns to perform the exercise, further strength gains are attributed to increases in muscle mass.
As illustrated in the graph, overall strength increases alongside neural adaptations; muscle growth does not significantly contribute during the initial stages.
Over time, however, muscle growth becomes the most important factor explaining increases in strength. Once an individual becomes proficient at performing an exercise, monitoring overall strength levels can be a good indicator of muscle growth. For this reason, keeping track of the weights lifted and the volume performed is crucial to understanding if you’re progresing.
Enjoy your training, and thank you for reading!
INTERESTING READINGS:
Mechanisms of Muscle Hypertrophy and Their Application to Resistance Training – Schoenfeld (2010)
Mechanism of work-induced hypertrophy of skeletal muscle – Goldberg (1975)
Science and Development of muscle hypertrophy – B. Schoenfeld (2019)
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