Conceptually, there are two mechanisms by which muscles grow: hypertrophy and hyperplasia. While both hypertrophy and hyperplasia contribute to muscle growth, there are key differences between the two.
While both muscle hypertrophy and hyperplasia indicate the enlargement of the muscle fibers, hypertrophy specifically means the increase in volume due to cell size while hyperplasia specifically means the increase in volume due to cell number or proliferation.
While hypertrophy was primarily attributed to skeletal muscular enlargement from exercise, evidence suggests hyperplasia plays a role as well.
Being one of the primary goals of gym-goers and athletes alike, muscle growth is an essential component of training and exercise. Responsible for producing movement and locomotion in the body, the muscular system is composed of three types of muscles: smooth muscles, cardiac muscles, and skeletal muscles.
Smooth muscles are involuntary non-striated muscles which means that these muscles are not consciously controlled and they do not exhibit striation patterns that other muscle types do. Smooth muscles are typically found around the visceral organs such as the ones in the gastrointestinal tract.
While smooth muscles can be found in abundance, the cardiac muscles can only be found in the walls of the heart. Compared to smooth muscles, cardiac muscles are striated which means they exhibit a striated pattern due to the framework of the muscle fibers. Like smooth muscles, cardiac muscles are also involuntary.
Skeletal muscles are attached to the skeletal system, they are voluntary and striated. In terms of training, exercise, and performance, it is often skeletal muscles being discussed.
Muscle growth occurs when the muscle fibers are repaired or replaced during rest. This is why resistance training such as lifting weights can increase muscle volume since the stress induced by the exercise can activate certain pathways that will promote muscle growth.
Physiologically, there are three main mechanisms by which muscles can increase in volume: muscle tension, muscle damage, and metabolic stress.
Even prior to sufficient wear and tear, muscle tension can help activate the necessary pathways for muscle growth. This is why people progressively lift heavier weights. Lifting heavier weights signals to the body that more musculature is needed, thus growth occurs.
Muscle damage is another mechanism of muscle growth that many people utilize. Muscle damage can especially be felt when an individual partakes in a rigorous exercise that can induce DOMS (delayed-onset muscle soreness). In the event of muscle damage, the body activates certain cells that repair the muscles. Typically, muscles are then restored to a greater degree as the body attempts to prevent muscle damage from occurring again.
Lastly, muscle growth can be induced by metabolic stress. However, unlike muscle tension and muscle damage, muscle growth from metabolic stress can be incredibly short-lived.
During metabolic stress, the body can temporarily provide an individual with increased muscle volume. This is immediately experienced by individuals who go to the gym who have experienced the “pump” or temporary swelling of the muscles from exercise. However, this phenomenon can be mainly attributed to the addition of muscle glycogen and not necessarily the alteration of the muscle fibers themselves.
Biochemically, there are certain hormones that can help trigger muscle growth such as testosterone, growth hormone, and insulin-like growth factor.
It is emphasized that muscle growth does not occur during exercise itself but during rest. During rest, the body is at a anabolic state which can promote healing and repair in the areas that were rigorously trained.
An integral component in muscle growth is myosatellite cells (also known as satellite cells or muscle stem cells). These cells are small multipotent cells which means that these are the precursor cells that eventually give rise to skeletal muscle cells.
Studies have shown that satellite cells are necessary for muscle growth. A 2011 study published in Development investigated muscle growth in satellite cell-depleted muscles. For this study, the researchers used a novel mouse strain that was genetically engineered to lack more than 90% of its satellite cells in mature skeletal muscles. After putting the mice in muscular overload, the researchers found that the mice did not develop significant muscle growth compared to the controls.
Studies have also found that training and exercise are capable of activating satellite cells. A 2004 study published in The Journal of Physiology investigated the effects of high intensity exercise on satellite cell proliferation. Eight volunteers were tasked to perform a single bout of high intensity exercise with one leg, while the other leg served as the control.
Observing the vastus lateralis muscle (the largest muscle of the quadriceps group) of the exercised leg after four and eight days, the researchers found that a single bout of high intensity exercise was enough to activate the satellite cells to re-enter the cell cycle. However, a single bout of high intensity exercise was not sufficient for the satellite cells to differentiate into muscle cells.
Muscle protein turnover is constantly occurring – the sum total of muscle protein synthesis and muscle protein breakdown. Essentially, an imbalance in these two processes will incur either a net gain (hypertrophy) or a net loss (atrophy). For the purpose of gaining muscle, athletes and trainers aim to maximize a net gain between muscle protein synthesis and muscle protein breakdown to achieve muscle growth.
Hypertrophy is defined by the increase and growth of the muscles due to the increase in the size of the component cells. An overload of stimulus can cause perturbations in the muscle matrix which can trigger certain signaling pathways (i.e., Akt/mammalian target of rapamycin pathway, mitogen-activated protein kinase pathway, calcium-dependent pathways) to promote an increase in muscle cell size. In the muscular system, there are two types of hypertrophy: myofibrillar and sarcoplasmic hypertrophy.
Myofibrillar hypertrophy refers to the enlargement of the muscles due to the increase in the size of the muscle contraction parts such as the myofibril. This type of hypertrophy increases strength and speed as it activates contractor muscles.
Sarcoplasmic hypertrophy refers to the enlargement of the muscles due to the increase in the volume of sarcoplasmic fluid in the muscle cell. Without conferring any additional strength or speed, sarcoplasmic hypertrophy increases energy storage and endurance as it activates glycogen storage in the muscles.
Compared to hypertrophy, hyperplasia is defined by the increase and growth of the muscles due to the proliferation of the muscle fibers or cells.
Historically, muscle hyperplasia was primarily observed to be induced by stretch. In 1973, an experiment was conducted on a bird. A weight (approximately 10% of the weight of the bird) was attached to one wing for a period of time. The weight imposed a stretch on the back muscles of the bird such as the anterior latissimus dorsi. The researchers found that the imposed weight resulted in an increase in muscle fiber numbers on the side where the weight was attached.
There are two main ways hyperplasia can occur in the muscles. Either new muscle fibers are developed from satellite cells, or larger muscle fibers split into two or more smaller muscle fibers. The main obstacle of measuring hyperplasia in humans is the mere difficulty in counting the individual muscle fibers, instead of the relatively easier methods to measure hypertrophy.
Numerous studies have been conducted that shows exercise primarily induce muscular hypertrophy. Hypertrophy can easily be measured – usually by measuring the cross-sectional area of a muscle.
However, a 1986 review published in Sports Medicine criticized that some experimental methods were erroneous and biased towards hypertrophy. Methodological bias was also pointed out in studies measuring cross-sectional areas of muscles due to fibers terminating intrafascicularly, overestimation of longitudinal fiber growth, and use of muscles where fibers do not run parallel to the longitudinal axis of the muscle.
Nowadays, several studies can be cited that show evidence supporting muscle growth attributed to both hypertrophy and hyperplasia. Methods for observing hyperplasia greatly differ from the methods observing hypertrophy. Instead of measuring muscle cross-sections, hyperplasia can be observed by direct fiber counts following nitric acid digestion of the muscle.
While exercise has been proven to be a major proponent of muscular growth, there are several factors in play such as exercise intensity, volume, type, repetitions, and rest intervals. One may argue that the resting periods are the most important part of building muscle.