The Multi-Dimensional Process of Muscle Growth

by Josh Hodnik

Its no secret that resistance training leads to an increase in muscle mass, referred to as muscle hypertrophy. While scientists may agree on this, there is still much debate on how this very complex process takes place. To many, muscle growth occurs from simply lifting weights, eating ample nutrients, and resisting the trained muscle. The sequence of events that must occur in order for muscle tissue to grow appears to be much more complex than this.

Muscle hypertrophy is an increase in muscle mass due to an increase in size of individual muscle fibers. Skeletal muscle adapts to regular, increasing workloads that exceed what was once the capacity of the muscle fiber, and the muscle becomes more efficient at transmitting forces. By applying progressive levels of stress to skeletal muscle, it must also adapt by increasing the size and amount of contractile proteins within each muscle fiber, leading to an increase in the size of the muscle itself. This process may seem rather simple, but a closer look will prove the process to be much more complicated.

Resistance exercise causes trauma to skeletal muscle. The immune system then responds with a sequence of reactions that lead to inflammation in the area where the damage occurred. The purpose of the inflammation is to contain the damage, repair the damage, and clear the area of waste products. When muscles are damaged, cytokines are produced at the site of the damaged cells. Cytokines are proteins, which serve as directors of the immune system. They are responsible for the arrival of lymphocytes, neutrophils, monocytes, and other healer cells to the damaged area. The cytokines that produce most of the inflammatory response are Interleukin-1 (IL-1), Interleukin-6 (IL_6), and tumor necrosis factor (TNF.) These cytokines are responsible for protein breakdown, removal of damaged cells, and an increased production of hormone-like substances that help control the inflammation called prostaglandins. The immune system plays an important role in muscle repair and growth.
Overstraining syndrome often occurs when the immune system is not able to handle the amount of muscle tissue that is damaged, and this leaves a person vulnerable to injury and illness over time.

Satellite cells function to facilitate growth, maintenance, and repair damaged muscle tissue. Satellite cells are located on the outer surface of the muscle fiber. These cells are dormant, but become activated when the muscle receives any sort of damage, such as what occurs during resistance training. The activated satellite cells multiply and are drawn to the site where damage occurred. For example, if shoulders were trained, that is where these satellite cells would end up. After they reach the site, they fuse to the muscle fiber, donating their nuclei to help regenerate the muscle fiber. This process increases the size of the muscle fibers and the number of contractile proteins, (myosin and actin,) within the fiber. This satellite cell activation period lasts up to 48 hours after the muscle is damaged from training. This period solidifies the importance of rest for muscle growth to take place.

Hormones are chemicals which organs secrete in order to regulate the activity of an organ or group of cells in the body. Hormone function is affected by nutritional intake, lifestyle, and general health. There are several hormones that play an important role in muscle hypertrophy.

Growth hormone is a peptide hormone that stimulates insulin growth factor (IGF) in skeletal muscle, which promotes satellite cell activation. This results in an increase in contractile proteins present in the muscle fibers, and an increase in the size of the muscle fibers.

Cortisol, a steroid hormone, (non-anabolic,) is produced in the adrenal corters of the kidneys. This stress hormone stimulates gluconeogenesis, which is the formation of glucose from amino acids and fatty acids. It also inhibits the use of glucose by most cells, and this can initiate muscle catabolism. Unlike testosterone and growth hormone, cortisol has a negative impact on muscle growth and repair by increasing the rate of protein breakdown.

Of all hormones present, testosterone has the greatest impact on muscle hypertrophy. This hormone is an androgen, and the primary role of androgens is to promote growth and development of male organs and characteristics. With skeletal muscle, testosterone has an anabolic effect. It increases protein synthesis, which induces muscle growth.

Growth factors are highly specific proteins that are very involved in muscle hypertrophy. Without their involvement, muscle growth is not likely to occur.

Insulin, like growth factor (IGF) is secreted by muscle tissue. It regulates insulin metabolism and stimulates protein synthesis, which are both vital to hypertrophy. In response to resistance exercise, endogenous levels of IGF are substantially elevated. The importance of IGF for muscle growth has led to exogenous IGF to become very popular among bodybuilders.

Hepatocyte growth factor, (HGF,) is a cytokine that activates satellite cells and is responsible for causing satellite cells to travel to the injured muscle tissue for repair.

Fibroblast growth factor, (FGF,) is stored in skeletal muscle in nine different forms. Five of these forms initiate satellite cells to multiply, so they can aid in skeletal muscle repair and growth. The amount of FGF released is proportional to the degree of muscle damage.

Muscle growth is a multidimensional process, with many factors involved. It involves a complex interaction between satellite cells, the immune system, hormones, and growth factors to facilitate muscle growth. A deficiency in any one of these areas can slow muscle growth and ultimately lead to catabolism (muscle breakdown.) While an in depth knowledge of the processes of muscle hypertrophy is not required to experience muscle growth, a basic understanding on how the body repairs itself after resistance training is important for anyone wanting to maximize their muscle growth potential.