Muscle Growth and Repair

Muscle injury and repair are a common cycle in human development. As we move through the world, trauma occurs, and the body must respond accordingly. While initial muscle growth and development occurs in-utero and continues through childhood, the body reaches a point where it is not creating new muscle, but adapting the tissue that already exists. Understanding this process can help us know how to stay healthy and strong.

Human skeletal muscle is about 40 % of the body mass and contains two main cells that play a key role in the formation of new muscle tissue. First, myoblasts exist in an undifferentiated state until the process of regeneration is activated. At this point, the cells can adapt to the type needed for repair. Second, satellite cells are skeletal stem cells that lie dormant between the plasma membrane and basal lamina of a muscle fiber. Once the repair process is activated, these cells are able to multiply at a highly rapid rate, allowing repair to occur more quickly. These satellite cells are deposited throughout the body during initial development while in-utero, meaning they exist in a finite number. When an injury occurs, these cells work together to create specified myotubes that can fuse with damaged myofibers to strengthen points of injury.

Skeletal muscle injuries can stem from a variety of events: direct trauma such as muscle lacerations and contusions; indirect insults such as strains; and degenerative diseases such as muscular dystrophies. Skeletal muscle can regenerate completely and spontaneously in response to minor injuries, such as strain. In contrast, after severe injuries, muscle healing is incomplete, often resulting in the formation of fibrotic tissue that impairs muscle function.

After injury, skeletal muscle repair goes through four main phases. First is degeneration, which immediately follows the injury occurrence. It can be characterized by necrosis of the myofibers. This immediately begins the second phase – inflammation – when the body sends additional fluids to the injury area to protect the compromised fibers. These fluids are also rich in oxygen and nutrients that will support the healing process as the third phase – regeneration – begins. During regeneration the body engages in a process known as phagocytosis whereby the cells engulf and metabolize damaged cells. As the body destroys unneeded cells and prepares to eliminate them as waste, the satellite cells are activated and begin to build new myotubes which grow into myofibers. Our final stage, known as remodeling, has two possible results. First, the myofibers begin to fuse with the healthy cells of a muscle to restore the functional capacity of a muscle group. Second, in cases where there are few healthy cells to fuse with, the cell matrix forms dense patterns known as fibrosis, or scar tissue.

Skeletal muscle has a remarkable capacity to undergo hypertrophy, which is an increase in size. It has a positive response to physical activities, particularly those based on resistance exercise. When a muscle is exposed to resistance related exercise (being asked to lift a load heavier than it can typically handle over a long period of time or being asked to lift a moderate load multiple times in rapid succession), this causes trauma, or micro tears, to the tissue fibers. The body begins the regeneration process, creating new tissue growth over the injured tissue. This effect is compounding, which means it can be repeated several times over; however, that does not mean it is always healthy to continue to repeat this process. At a certain point, even if the tissue continues to increase in size, it will lose its pliability, compromising the muscle group’s functionality.

There are many reasons this “bulking” response of the muscle tissue can fail, or function at a slower rate. First, age is an important factor to consider. Body functions naturally slow down over time, meaning we do not react as quickly, or as efficiently. Satellie cells, the stem cells planted in our tissue at birth, are created in a finite amount, which means at a certain point there are simply no more resources to draw on to help the body repair and heal after major injury. Next, we must consider how hydration and nutrition influence muscle repair. If the body does not have appropriate resources to draw on, it cannot complete the healing phases properly. Another possible hinderance to muscle repair is appropriate rest. This can mean a proper sleep schedule, but it can also mean creating and adhering to an appropriate work out cycle. If you are constantly and consistently overloading your muscle tissue, you are not allowing the body the time necessary to begin the healing process. This can cause long term issues with muscle movement or development, and it can create circumstances for more severe, possibly irreparable injury.