What is Muscle Hypertrophy? Physiology of Muscle Hypertrophy Explained

Sep 22, 2022

Photo Credit: Nicole Head/Pixabay

By: Danielle Abel, MSN, FNMS, CSCS(c)

Have you ever wondered how a muscle actually gets bigger? In this article, we're going to break down this process all the way from muscle damage to the recruitment of satellite cells all the way to the repair and growth of muscle fibers to produce muscle hypertrophy.

We'll use the bicep for example, if your bicep is smaller than you want right now and you want a bigger bicep, here's how the process will work from a physiology perspective. 

Muscle Layers

When we think about the muscle belly, the belly is composed of many muscle fascicles; fascicles are just groups of muscle fibers; fascicles are also known as motor units. 

Within each fiber of the fascicle, there are smaller subunits called "myofibrils." Around the fiber (which consists of many different myofibrils) are undifferentiated cells called satellite cells. 

Muscle cells have multiple nuclei (the myofibrisl), which are unlike other cells in the body that only have 1 nucleus. When it comes to hypertrophy, having multiple nuclei is a huge benefit because each nuclei can expand in size. 

Muscle Damage

Training causes muscular microtrauma to the muscle fibers. Damage occurs from mechanical tension, metabolic stress, and/or muscle damage. (According to Schoenfeld).

  • High rep training for more metabolic stress, for example
  • Heavy load training for mechanical tension, for example 
  • Eccentric training for muscle damage, for example

When damage occurs, portions of the fiber are sheared away, creating small areas of damaged tissue within the myofibrils - areas on the actin and myosin are damaged. 

Muscle Protein Synthesis

After microtrauma has occurred, there is a hormonal cascade that initiates the process of muscle protein synthesis. For example, testosterone, IGF-1 (insulin-like growth factor 1), and growth hormone that are going to signal that there's been damage. This signaling will begin activating satellite cells. 

Satellite Cells 

Satellite cells are DNA-level, undifferentiated cells. So these are cells that could become Type I or Type II muscle fibers. DNA can code or transcribe amino acids, which can be grouped together to form myofibrils or, simply put, more muscle tissue. 

When myofibrils are added, sarcomeres are added, which makes the muscle bigger. Making the muscle bigger is the same thing as muscle hypertrophy.

Muscle Fiber Expansion

Just to be even more clear, it's important to differentiate muscle hyperplasia compared to muscle hypertrophy. When more myofibrils (nuclei) are added to the muscle fiber (cell), this process is known as hyperplasia. Still, research is unclear whether this happens in humans as it does in animal studies. 

What is more likely is that muscle hypertrophy is occurring, in which the existing myofibrils themselves are repaired with more actin and myosin (sarcomeres) inside of them, which makes the myofibril larger itself by expanding and size of the myofibril. 

From a sequential perspective, muscle hypertrophy from satellite cell activation occurs in the following fashion: 

  • Quiescent (dormant) satellite cell activated by a hormonal cascade
  • Satellite cell gets activated and proliferates (replicates)
  • Myogenic progenitor forms (myoblasts form)
  • Differentiation of myoblasts occurs
  • Myocytes are formed following differentiation
  • Myocytes fuse together to form myotubes
  • Maturation occurs, and the myotubes combined form myofibril tissue inside the myofiber itself 

So now you know how muscle hypertrophy actually works, all the way from tissue damage to repair to your muscles becoming larger. 

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