how muscles grow
What is hypertrophy?
Muscle hypertrophy refers to the process of increasing the size of skeletal muscles. When we talk about "building muscle," we are referring to hypertrophy, specifically an increase in the cross-sectional area of individual muscle fibers.
Hypertrophy doesn’t involve creating new muscle fibers (which is called hyperplasia and is still a controversial topic with little evidence in humans). Instead, hypertrophy is about enlarging the muscle fibers you already have, making them thicker and longer.
Hypertrophy Is a Slow Process
One thing to keep in mind is that hypertrophy takes time. Unlike fat loss, where you might notice visual changes in a few weeks, building muscle requires patience and consistency. It can take months of proper training and nutrition before you start seeing noticeable increases in muscle size.
This slow progress is one of the reasons many people give up on their goals prematurely. They expect rapid changes and get discouraged when the scale doesn’t move or their muscles don’t look significantly bigger after just a few weeks of training. Understanding the science behind hypertrophy can help set realistic expectations and remind you that consistency is key.
Why Does Hypertrophy Happen?
Your body is constantly adapting to the demands placed on it. When you lift weights or engage in resistance training, you're creating a stimulus that your muscles aren't used to. To meet these new demands, your body responds by repairing and reinforcing muscle fibers, leading to an increase in size.
But how does this happen? Scientists have identified several mechanisms that drive hypertrophy, and the three main theories are:
Mechanical Tension
Metabolic Stress
Muscle Damage
However, the relative importance of each theory is debated, and not all of them contribute equally to muscle growth. Let’s take a closer look at these theories.
Understanding the Theories of Hypertrophy
Now, let’s break down each of the three main theories that explain how hypertrophy occurs. These mechanisms don’t work in isolation; rather, they interact and complement each other to drive muscle growth. However, as you’ll see, some have stronger evidence than others:
Mechanical Tension: The most well-supported theory and the primary driver of muscle growth. This involves the force or load your muscles experience during resistance training.
Metabolic Stress: Known as the "pump" effect, this is the result of increased blood flow. This has not been shown to contribute to hypertrophy.
Muscle Damage: Once thought to be essential for growth, muscle damage happens when the neural mechanisms (CNS fatigue) and cellular mechanisms including calcium ion related fatigue or metabolite fatigue (PNS fatigue) result in a decrease in performance. This does not result in hypertrophy, and will even hinder the amount of resources that go towards hypertrophy if the damage is excessive.
In the next sections, we’ll take a closer look at all 3 and how to use your knowledge of them to your training advantage!
Mechanical Tension – The Main Driver of Muscle Growth
What is Mechanical Tension?
Mechanical tension refers to the force or load that is applied to muscles during resistance training. This tension occurs when muscles contract to move or control a weight, causing stress on the muscle fibers. Mechanical tension is the most well-supported and critical mechanism for muscle hypertrophy, as it signals your muscles to adapt and grow.
How Mechanical Tension Works
When you lift a weight, the tension created on your muscle fibers sends a signal to your body that it needs to adapt to handle this new load. This signal is what triggers hypertrophy at a cellular level, specifically through a process known as mechanotransduction.
Mechanotransduction is the process by which cells convert mechanical stimuli into biochemical signals. In simple terms, when your muscles experience tension, they initiate a chain reaction that leads to increased protein synthesis (the process of building muscle proteins) and muscle repair, ultimately making the muscle fibers thicker (myofibrils in parallel) and longer (sarcomerogenesis) over time.
Mechanical tension happens during the phases of muscle contraction:
Eccentric contraction – when the muscle lengthens under tension (e.g., lowering a weight during a bicep curl).
Concentric contraction – when the muscle shortens under tension (e.g., lifting the weight during a bicep curl).
Isometrics - can be thought of as extremely slow concentrics (Overcoming Isometrics) or extremely slow eccentrics (Yielding Isometrics)
All contraction types create mechanical tension. However, how the muscle grows will change depending upon contraction type.
Eccentric contractions result in muscle growth via sarcomerogenesis (the addition of sarcomeres in series). This results in an increase in length.
Concentric contractions result in muscle growth via the addition of myofibrils in parallel. This results in an increase in diameter.
Progressive Overload: The Key to Maximizing Mechanical Tension
What is progressive overload?
As you continue to train and get stronger, you will also need to increase the intensity of your training to continue to grow. This is the law of diminishing returns. The better you get at something, the more focused and dialed in you need to be to accomplish greater results. While 50 lbs may have been enough to stimulate hypertrophy when you first started, that same exercise may require you to use 100 lbs because you need more weight to recruit HTMUs (High Threshold Motor Units).
Intensity is defined as a percentage of your 1RM. Doing exercise that is difficult and leaves you out of breath may feel difficult, but will not necessarily contribute to hypertrophy.
Examples of progressive overload:
The best definition of progressive overload is from Paul Carter:
“Being able to improve upon the previous workout performance with all things being equalized, which shows that adaptations have occurred from that previous workout”
This means:
You get the same amount of reps but with more weight
OR
You use the same weight but get more repetitions.
Best Ways to Maximize Hypertrophy?
All exercises can and will create mechanical tension. However, the amount of stability you have during the exercise will dictate how many high threshold motor units (HTMUs) you are able to recruit due to coordination.
Higher required coordination = decrease in HTMU recruitment
This means that the more simple and stable your set up, the more you are able to accomplish your hypertrophy goals!
This also means that the more complex the exercise is or if you have more balance demands from an exercise, this will in turn not make it optimal for hypertrophy.
For example a single leg RDL on a Bosu Ball can be a fun exercise, but not a great hypertrophy exercise.
Good vs Bad Exercises for Maximizing HTMU Recruitment:
*These are just examples and not the only options!
Hack Squat & Leg Extensions vs. Single Leg Bosu Ball Squats
Barbell Deadlifts & Seated Leg Curls vs. Single Leg Standing DB RDL
DB Bench Press & Chest Press vs. Cross Body Cable Press
Chest-Supported DB Rows & Hammer Rows vs. Standing Single Arm Cable Row
Seated Barbell Military Press vs. Standing KB Shoulder Press
These exercises allow you the stability necessary to maximize HTMU recruitment, and thus hypertrophy. This is why different exercises can cause similar amounts of hypertrophy despite drastic differences in load. Since mechanical tension is the involuntary slowing of the muscle despite maximal effort, both sets have involuntary slowing despite maximal effort despite the different loading parameters.
Ex. Taking a set of 5-8 reps to failure will cause similar amounts of hypertrophy as a set of 20 reps taken to failure.
HOWEVER
The set of 5-8 reps taken to failure will cause significantly less fatigue while also recruiting more HTMUs due to lesser amounts of fatigue!
Mechanical Tension in Practice
To fully understand the importance of mechanical tension, let's break it down in a practical scenario:
Example: When performing a squat with a heavy barbell on your back, mechanical tension is applied to your quadriceps, hamstrings, glutes, and core. As you descend into the squat (eccentric phase), the muscles are lengthened under load, creating significant tension. When you push back up to standing (concentric phase), the muscles shorten under load, maintaining tension throughout the movement.
To maximize hypertrophy in the squat:
Lift heavy: Choose a weight that challenges you but still allows for proper form.
Progressive overload: Gradually increase the weight or reps at that weight as you get stronger.
Studies have shown that mechanical tension is the most important and well-supported mechanism for hypertrophy. Without sufficient tension on your muscles, there simply isn’t enough stimulus to trigger growth.
A key study by Schoenfeld (2010) highlighted the importance of mechanical tension in resistance training as the primary driver for muscle growth, especially when combined with progressive overload.
To maximize muscle growth, you must consistently challenge your muscles by applying mechanical tension through resistance training. Focus on stable movements and progressively increase the weight to stimulate the best possible hypertrophy response.
Next, we'll explore Metabolic Stress.
Metabolic Stress – The "Pump"
What is Metabolic Stress?
Metabolic stress refers to the build-up of metabolites in the muscles during exercise. This is often experienced as the “burn” or “pump” you feel during high-repetition sets, or when your muscles feel swollen during or after a workout. This is due to the accumulation of byproducts like lactate, hydrogen ions, and other substances during muscular contraction.
How Does Metabolic Stress Contribute to Hypertrophy?
Metabolic stress only contributes to hypertrophy when combined with mechanical tension. If mechanical tension is not present, growth does not occur. Thus, metabolic stress on its own is ineffective. There are three primary ways that people claim metabolic stress promotes muscle growth:
Cell Swelling (The Pump): The temporary swelling of muscle cells creates tension on the cellular structure. The pump itself isn't a direct cause of hypertrophy, but the cellular responses to this stress are usually paired up with mechanical tension because of the eventual involuntary slowing, thus why it is a common misconception.
Metabolite Accumulation: The build-up of metabolites is commonly present with other favorable anabolic signals. This was the logic used by bodybuilders for decades to emphasize short rest periods between sets. However, this is another misconception as these things can be present during mechanical tension, and research has shown longer rest periods will result in greater amounts of hypertrophy. Research has shown the hypertrophy response is very different in “acute (minutes, hours)” vs “chronic (days, weeks, months)” spikes/elevations of GH/testosterone/IGF-1 (Van Every et. al, 2024).
Fiber Recruitment: During intense sets where perception of effort is high, the body recruits more muscle fibers, especially fast-twitch fibers, which have greater growth potential. This backs the mechanical tension model more than the metabolic stress model.
In addition, sarcoplasmic hypertrophy does not occur independently of myofibrillar hypertrophy (Vann et al, 2020). As written, “sarcoplasmic hypertrophy is either: (i) a transient symptom of training-induced edema, (ii) a transient mechanism for muscle fiber growth, and/or (iii) an outcome of a myofibrillar protein accretion threshold being reached in well-trained individuals.” (Roberts et. al) This supports mechanical tension as BFR training results in involuntary slowing (mechanical tension), while the mere presence of metabolites alone does not result in hypertrophy.
Muscle Damage – The Soreness Myth
What is Muscle Damage?
Muscle damage refers to cellular damage/death from training. Muscle fibers do not get micro-tears, as this is the outdated “popping sarcomeres” theory.
How has this been debunked?
Muscle damage can be high during low forces
Muscle damage can occur during aerobic exercise
Muscle damage can be high during concentric contractions
Muscle damage can occur post-workout
In the past, it was believed that this damage directly led to muscle growth, with soreness often being seen as an indicator of a successful workout. However, modern research has shown that muscle damage is not required for hypertrophy, and excessive damage can actually hinder recovery and slow down progress.
How Muscle Damage Happens
Muscle damage is caused by calcium ions (triggering a cascade of calpains and phospholipases) and increased levels of calcium ions via stretch-activated channels.
Once the mostly biochemical damage occurs, the body’s natural repair processes kick in. This repair process involves increased protein synthesis. However, too much muscle damage can overwhelm the body’s ability to recover, leading to prolonged soreness and slower progress in the long term. Research has shown that this is part of the reason the Repeated Bout Effect exists, as your first time completing the workout results in a greater amount of resources being used to repair instead of build. You simply get more efficient at handling calcium ions and preventing less of them from entering.
The Myth of Soreness and Muscle Growth
One of the most common misconceptions in fitness is that muscle soreness equals muscle growth. Many people associate the soreness they feel after a tough workout (known as delayed onset muscle soreness, or DOMS) with how much muscle they’ve built. However, this isn’t necessarily the case.
Soreness is a byproduct of muscle damage and inflammation, but it doesn’t directly correlate with hypertrophy. Just because you’re sore doesn’t mean you’re growing more muscle. Similarly, a lack of soreness doesn’t mean you didn’t have an effective workout. Muscle growth is more closely tied to progressive overload and mechanical tension, not how sore you feel the next day.
The Role of Satellite Cells in Repair
The body has a built-in mechanism to repair damage—satellite cells. These cells are activated when muscle damage occurs and they play a critical role in repairing cellular damage. After a workout that causes muscle damage, satellite cells move to the site of the damage and fuse with them in order to donate their nucleus. However, hypertrophy occurs even without satellite cells (think of them as fixers, not builders), thus proving the mechanical tension model (addition of contractile proteins to skeletal muscle via protein synthesis).
Why You Shouldn’t Chase Muscle Damage
While some muscle damage is unavoidable when training hard, trying to maximize muscle damage through excessive volume or intensity is not an effective approach to hypertrophy. Instead, focus on smart training that balances mechanical tension, metabolic stress, and recovery.
Actionable Tips for Muscle Growth
Here are some practical tips for applying these principles to your training:
Progressive Overload: Focus on gradually increasing the weight in your workouts over time.
Recovery: Give your muscles time to repair and grow by getting enough rest, sleep, and proper nutrition.
Conclusion
The science of muscle growth is complex, but understanding the key drivers—mechanical tension, metabolic stress, and muscle damage—can help you optimize your workouts for the best results. Remember, mechanical tension is the primary driver of hypertrophy, while metabolic stress and muscle damage are not necessary. Focus on smart, progressive training and recovery for long-term gains.
References
Schoenfeld B. J. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. Journal of Strength and Conditioning Research.
Wackerhage H., Schoenfeld B. J., et al. (2019). Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise. Journal of Applied Physiology.
Loenneke, J. P., et al. (2014). Does blood flow restriction result in skeletal muscle damage? A critical review of available evidence. Scandinavian Journal of Medicine & Science in Sports.
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