How do Muscles Grow?

Article by G Rieger

Posted October 2, 2014

Last updated October 25, 2017

How do Muscles Grow?  It would seem in the 21st Century there would be a clear answer to this question.  Although there is an enormous amount of research on the subject.   The answer is not so clear cut.  Stem cell research has shed a great deal of light on what can effect positive growth in muscle tissue. (15)  The whole complete process on how skeletal muscle tissue responds to increasing loads is not known. 

The amazing process of skeletal muscle cell hypotrophy is a multi-level consideration, which opens the potential to unlock the key to anti-aging and longevity. (15)  Skeletal muscle cell or fiber hypotrophy is an increase in muscle mass.  The opposite, which results in a decrease of muscle mass is known as atrophy.

What an incredible direction mankind is moving in.  In an age of Medical Spas and Medical Tourism, humanity is searching for long life and well being and understanding.  An understanding of how muscles grow may eventually facilitate an end to the search.

The first question that needs to be answered is "what is skeletal muscle cell hypertrophy"?  Hypertrophy is the increase in dimension of individual muscle cells as in diameter but not length. (13)   As any die hard body builder knows skeletal muscle cell hypertrophy or muscles grow with the development of cross-sectional muscle cell mass.  In other words the individual cell increases it's size resulting in a ripped body build.  The unique way that each person's DNA directs this process results in our various body types.  It goes without saying that not every body type will produce a championship physique. 

Exercise is an important component of how muscles grow and a healthy lifestyle.  Most people will agree with this statement.  A unique part of this concept is that skeletal muscle is not the only muscle that responds to regular increasing workloads.  The cardiac (heart) muscle also responds to workloads that exceed the existing capacity of the muscle cells.  Aerobic as well as anaerobic exercise is vital for the well being of the human form.  The delivery of O2 to all the cells of the body helps maintain our health and overall sense of wellbeing. 

With aerobic exercise the cardiac muscle improves its ability to push more blood volume out of the heart chambers.  Skeletal muscle on the other hand become more efficient at transmitting force through the tendons, ligaments and joints.  (13)  The two main functions of skeletal muscle fibers are to produce contraction that moves a joint resulting in locomotion of the human form and to provide for stability and posture within the body. 

Each skeletal muscle within the human body must generate a different force from a unique level of tension in order to accomplish this remarkable feat.  How muscles grow plays a dominant role within the balance of this system.  Each skeletal muscle is able to adapt to variable loads by increasing the size and the amount of contractile proteins within the muscle fiber. 

The Physiological Factors that Effect How Muscles Grow

  • Satellite Cells
  • Immunology
  • Growth Factors
  • Fibroblast Growth Factor
  • Insulin-Like Growth Factor
  • Hepatocyte Growth Factor
  • Growth Hormone
  • Cortisol
  • Testosterone

 In discussing skeletal muscle cell physiology it is necessary to introduce the role and interaction of satellite cells, as well as provide an overview of the immune response and growth factor proteins.  In addition, to develop a more complete picture it is necessary to discuss cortisol, growth hormone and testosterone.  Each of these factors will help to expand our understanding of how muscles grow. 

Satellite cells are not found in cardiac (heart) muscle tissue.  Satellite cells are only activated when skeletal muscle cells (fibers) are damaged. (6) Their specialized function is to facilitate repair, growth and maintenance of damaged muscle fibers.  They have been labeled satellite because they are found on the outside of the sarcolemma, which is the uppermost layer of the muscle fiber membrane. (6)

Unlike skeletal muscle cells, satellite cells have only one nucleus that comprises most of the volume of the cell.  They are only activated when fiber damage calls them into action.  This damage can be a result of injury or trauma, but is more likely the result of resistance training or increased loads on the muscle. (6)  The satellite cell divides and generates a daughter cell which migrates into the damaged region of the fiber, it then fuses with the fiber donating its nuclei to the damaged fiber to begin repair.   This process of repair and maintenance explains in part how muscles grow.

It is important to point out that satellite cells do not generate new fibers.  They repair cells by increasing the size and number of actin and myosin contractile proteins within the skeletal muscle fiber.   Satellite cells are primarily responsible for muscle hypotrophy increasing muscle cell mass or bulk. 

The ability to create muscle bulk from resistance over loading of muscle cells, is a result of micro-tears to individual fibers activating satellite cells to increase the size and number of contractile proteins actin and myosin which form the myofibril. (13)  This is a major consideration when an athlete desires to understand how muscles grow. 

The muscle cell contractile unit is known as the sarcomere and is formed by the myofibril containing actin and myosin proteins.  The muscle cell is made up of a chain of sarcomeres extending the length of the fiber.  The longest muscle in the body is the sartorius, this muscle, therefore contains the longest muscle fibers or cells in the body.

The satellite cells will continue to proliferate and increase muscle mass for approximately 48 hours after a resistance training over load trauma. (6) The actual number of the available satellite cells present depends entirely on the type of muscle fiber involved.  We will discuss fiber types in more detail later in this article. 

The inflammatory response plays a vital role in how muscles grow.  It is an immunological response to the micro-tearing of the muscle fibers responsible for containing the damage and cleaning up the waste by products. (14)  A sequential series of responses is triggered by the initial damage. 

The satellite cell fuses its nucleus to the damaged fiber, providing communication and control to the immunological cascade of repair activities.  The inflammatory response calls in the macrophage, white blood cells responsible for engulfing and destroying cellular debris. (10)  They also secrete cytokines, growth factors and other substances involved in cellular repair. 

The Importance of Cytokines in Skeletal Muscle Repair

Cytokines are proteins that are secreted by white blood cells, they are the first responders known as macrophages.  Cytokines serve the body as directors of immune response by facilitating cell to cell communication.  Cytokines stimulate the arrival of other cells involved in injury and repair response such as monocytes, neutrophils and lymphocytes, these cells are all involved in healing the injured tissue. 

There are three cytokines (proteins) relevant to exercise and over load skeletal muscle micro-tearing & tissue repair.  Interleuken-1   (IL-1), Interleukin-6 (IL-6), and tumor necrosis factor (TNF), these are the cytokines responsible for the inflammatory response sometimes called the pro-inflammatory cytokines. (10)

Let's recap our discussion so far.  The overloading of skeletal muscle fibers during resistance training produces micro-tearing of the fiber.  This kick starts the satellite cell into producing daughter cells that fuse to the torn fiber segments.  The satellite cell initiates the production of actin and myosin protein increasing the bulk or mass of the muscle belly, but no the length of the muscle nor does it increase the number of muscle fibers.  The inflammatory response activated by the micro-tearing calls in macrophages, which results in the production of cytokines (proteins) that trigger the cascade of other healing cells to repair the muscle cell. 

The three pro-inflammatory cytokines are responsible for the breakdown and removal of damaged muscle fibers, as well as the production of prostaglandins. (1)  Prostaglandins are hormone like secretions that help to control the inflammation.  Inflammation from micro-trauma initiates the entire process.

The Role of Growth Factors in How Muscles Grow

Growth factors are intricately involved in how muscles grow.  Skeletal muscle cell hypertrophy is controlled and facilitated by growth factors. (1)  Growth factors are proteins which can include prostaglandins, hormones and cytokines.

The role of growth factors is that they stimulate the division and diversification of cells.  In relation to skeletal muscle cells fibroblast growth factor, insulin-like growth factor and hepatocyte growth factor all work together in muscle fiber hypertrophy.  The complexity of how muscles grow is a fascinating study of how the body regenerates.

Insulin-like growth factor is a hormone secreted by skeletal muscle cells to regulate insulin metabolism and protein synthesis. (3)  It is also responsible for the proliferation of satellite cells.  Skeletal muscles that have undergone resistance training show elevated levels of insulin-like growth factor which results in skeletal muscle hypertrophy.

The next growth factor we need to discuss is fibroblast growth factor which has 5 identified forms and is stored within the skeletal muscle cell.  It is released when the cell experiences trauma or micro-trauma.  The amount of fibroblast growth factor that is released is proportionate to the degree of cell trauma. (17) The release of fibroblast growth factor results in skeletal muscle cell hypertrophy.

Hepatocyte growth factor plays various roles in tissue development.  Within the skeletal muscle fiber, hepatocyte growth factor has been shown to initiate satellite cell activity and migration to damaged fibers. (6)  It is a cytokine protein that plays a role in how muscles grow.

The Role of Hormones in How Muscles Grow

Hormones are chemicals secreted by tissues and endocrine glands to regulate the cells and organs in another part of the body.  Hormone function can be affected by nutritional intake.  Accounting for disease and developmental problems, both physical and mental in many parts of the world.   

There are three hormones that affect skeletal muscle hypertrophy.  They are growth hormone, cortisol and testosterone.  All three of these hormones can be affected by nutritional factors.

Growth hormone is a peptide that stimulates the production of insulin-like growth factor which promotes the activation of satellite cells.  (4)  Growth hormone clearly plays a role in muscle cell development under overload due to resistance training, but it is believed to play a larger role in fluid retention and connective tissue development than it does in myofibril development.  Fluid retention and connective tissue structures play a significant role in how muscles grow. (4)

Cortisol is a steroid hormone which allows it to pass through a cell wall without a receptor site.  Cortisol is produced in the adrenal cortex of the kidney.  Cortisol is a stress hormone that plays a role in the breakdown of proteins. (7)  The presence of cortisol inhibits skeletal muscle cell hypertrophy. (7)  Therefore stress remains a negative expression of our health and wellbeing and can adversely impact a competitor's development of muscle mass. 

Testosterone accounts for the differences between the male and female skeletal muscle tissue development.  It is an androgen or male sex hormone.  The purpose of androgens is to promote the growth and development of male characteristics and organs.  (17)  Testosterone has a physiological effect on the nervous system, skeletal muscle, reproductive system, bone marrow as well as skin and hair.   

Testosterone increases protein synthesis, which increases skeletal muscle hypertrophy and helps to explain how muscles grow. (17)  Since it naturally occurs in higher quantities in men than in women, it accounts for the different ratio of muscular development among male and female competitors.  Some female competitors supplement their testosterone levels to promote muscle cell hypertrophy. 

Skeletal Muscle Cell Types &
How Fiber Distribution Creates Your Unique Physique

  • Type I Fibers
  • Type II Fibers
  • Type II-a Fibers
  • Tpe II-b Fibers

The DNA directs the distribution of skeletal muscle fibers.  The infinite possible combinations of skeletal muscle cell type results in various body shapes.  The fiber type that support a long distance or marathon runner vs. the fiber type that support a sprinter.  Taking a closer look at the types of fibers found within skeletal muscle will increase your understanding of your own physical structure. 

Skeletal muscle fibers are classified into two categories, slow twitch type I and fast twitch type II.  The two types of skeletal muscle fibers can be differentiated by several factors.  These factors can include blood supply and capillary density. (12) Metabolism, contractile velocity and neuromuscular differences also play a role in separating the fast twitch from the slow twitch muscle fibers. 

Glycogen stores and the actual response to hypertrophy also are factors which differentiate the slow twitch from the fast twitch muscle cell. (12)  Slow twitch type I muscle fibers are considered postural muscle fibers.  Slow twitch muscle fibers are designed for endurance and a higher capacity for sustained work load.  This type of muscle fiber would have a greater distribution within the physique of a long distance or marathon runner as they are able to generate tension for long periods of time.

It is commonly known that resistance and overload training will increase the regional bulk of type I muscle fibers.  It is important to point out that aerobic activity is also known to increase hypertrophy of slow twitch type I muscle fibers to some degree.  Type I fibers utilize fats and carbohydrates better because they have an increase reliance on oxidative metabolism, which is the breakdown of nutrients into ATP utilizing O2.  Adenosine triphosphate is produced by the mitochondria of the muscle fiber. 

On the other hand fast twitch type II fibers produce bursts of energy.  They are found in greater distribution within the physique of sprinters providing their burst of speed for short distances only. (12)  They can be found in greater distribution in the quadriceps muscles such as vastus lateralis and in the gastrocnemius, both of which play a large role in generating high tension force and speed for sprinters. 

Type II or fast twitch skeletal muscle fiber can be broken down into two subcategories Type II-a and Type II-b.  Fast twitch II-a are also called fast twitch oxidative glycolytic.  They are differentiated because they have characteristics of both fast and slow twitch fibers.  They are placed in between because they possess both anaerobic and oxidative characteristics.  (12) Anaerobic metabolism produces energy without the use of oxygen.   

Type II-b fast twitch fibers can be referred to as fast twitch glycolic fibers.  These fibers have high amounts of glycolic enzymes because they rely solely on anaerobic metabolism (no O2). (5)  They produce the greatest tensile force.  They have an increased nerve supply which facilitates an increase in action potential.  They do not have as dense a network of capillaries, because type II-b fast twitch muscle fibers do not depend on oxidative metabolism.  

An interesting way to visualize them within the mind's eye is to consider chicken.  The breast meat is called white meat because it is made up of Type II-b fast twitch fibers, which have less blood vessels and more nerve endings. The dark meat is made up of a distribution of Type I, slow twitch fibers and Type II-a fibers, which have more blood vessels and less nerve endings. 

How does muscle fiber type effect how muscles grow? 

The number of satellite cells within muscle fibers has been shown to relate to the type of muscle cell.  Type I slow twitch fibers, have 5 to 6 times more satellite cells than type II fast twitch fibers.  Type I, slow twitch muscle fibers have a greater blood supply and utilize oxidative metabolism.  In human locomotion type I muscle fibers are utilized with the greatest frequency, which may account for the greater distribution of satellite cells.

In addition type II-a fibers convert to type II-b fibers as a result of resistance training. (8)  Research indicates that resistance training increases the oxidative capacity of the muscle fiber.   This adaptation is seen as a positive change, since type II-b fibers have a higher capacity for oxidative metabolism.

The conversion of a fibers to b fibers is a fundamental factor in how muscles grow.  It also explains the reverse effect of atrophy.  When an individual no longer exercises with resistance training the converted type II-b fibers will adapt again and become type II-a fibers.  This change accounts for the decrease in size of the overall muscle or muscle group.

In conclusion, it can be clearly seen that the answer to how muscles grow is a multidimensional process.  This article has briefly discussed some of the many factors involved in skeletal muscle fiber hypertrophy.  It is hoped that continued stem cell studies will open the doors to greater understanding of the body's ability to regenerate. (15)  When considering how muscles grow it is vital to participate in regular exercise and good nutritional habits.  (12) The goal of any professional trainer is to develop better and better protocols for effectively training the human body.  It can be just as important for trainers, clients, and all of us to understand how the body adapts to over loading stimuli as a result of resistance training. 

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