What is muscle growth, and how does it happen?

Increasing muscle size through strength training is key to improving body composition. This is why lifting weights is essential to bodybuilding, personal training, and strength coaching alike.

In recent years, a great deal of research has been published (and continues to be published) exploring the effects of strength training on muscle size. This is an exciting opportunity for all of us working in the fitness industry, because it enables us to improve the effects of our training programs continually, by adding to our knowledge over time.

To integrate each study as it is published, I think it helps to have a basic framework in which to set new information. This framework should help you identify the strengths and limitations of new research, as well as link concepts from different papers together, to create a clear understanding of how strength training works to produce muscle growth.

Here is my basic framework. It starts by defining what each measurement of muscle growth actually means, then sets out a clear picture of how muscle fibers increase in size, and finally provides a model of how a strength training workout stimulates the increase in muscle protein synthesis that causes fiber hypertrophy.


What are muscle growth, hyperplasia, and fiber hypertrophy?

Muscles often increase in volume (and therefore in mass) after long-term strength training. Since they are made up of many individual fibers, muscles can theoretically increase in volume because either (1) the number of fibers increases (called hyperplasia), or (2) the volume of each muscle fiber increases (called fiber hypertrophy).

Either of these processes involve an increase in the protein content of the whole muscle, which is known as hypertrophy.


#1. Hyperplasia

Research in rodents has found increases in fiber number after mechanical loading, with greater increases being observed after exposure to higher forces at longer muscle lengths. The new fibers are often smaller than the older ones, and some researchers have suggested that this happens because fibers split, so that they can multiply.

Split fibers are often observed in tandem with increases in fiber number in rodent studies. However, fibers also split when muscles experience contusions that do not stimulate muscle growth, so whether this splitting represents a useful adaptation or is a side effect of severe muscle damage is unclear.

In humans, researchers have observed signs of fiber splitting after very strenuous programs of voluntary strength training, but to date we have no really solid indications that long-term strength training causes increases in muscle fiber number.


#2. Fiber hypertrophy

Increases in the protein content (and therefore the volume) of individual muscle fibers can occur because either (1) they increase in diameter or cross-sectional area, or (2) they increase in length.

It can seem odd to think about muscle fibers increasing in length after training, because the locations of the origin and insertion of the whole muscle cannot change. Even so, the whole muscle can increase in length after training, by bulging out slightly in the middle, even while its starting and ending points are fixed.

Many studies in humans have shown that muscle fascicle length (fascicles are bundles of muscle fibers) increases after long-term strength training. This happens particularly often when the strength training program involves eccentric-only contractions, or when the peak contraction of the exercise occurs at long muscle lengths.

Similarly, researchers have found that the diameter of individual muscle fibers also increases after long-term strength training. Increases in diameter are sometimes greater in type II fibers, likely because type I fibers are more commonly (but not always!) linked with lowest threshold motor units, and generally only the higher threshold motor units increase in size after strength training.


How do we measure muscle growth?

Researchers can measure muscle growth in several ways, which can be subdivided into (1) those that assess the whole body, (2) those that assess muscles, and (3) those that assess muscle fibers.


#1. Whole body

One common approach to measuring changes in muscle mass after strength training is to use whole body scanning with X-rays (DEXA), which allows researchers to estimate lean (non-fat) body mass. By combining these data with other measurements, such as total body volume using air-displacement plethysmography, and total body water using bioelectrical impedance, a more accurate assessment can be achieved.

This type of measurement is valuable, because it gives us a good overview of changes in whole body composition after strength training. However, it does not tell us much about how each muscle itself has adapted.


#2. Muscle

Other scanning methods, such as magnetic resonance imaging (MRI), computed tomography (CT), and ultrasonography can give us an insight into how individual muscles change in size, in each of their dimensions. However, muscles do not increase in size in all directions equally after strength training, and this affects how we interpret the results of each scanning measurement.

When multiple axial scans are done along the length of the muscle, this produces a series of cross-sectional images. Combining these together allows researchers to calculate whole muscle volume. This type of measurement is useful, because it does not matter whether individual muscle fibers increase in length or diameter, nor whether the arrangement of the fibers inside the muscle alters after training, nor whether different regions of the muscle increase in size more than others.

Sometimes, multiple scans are not performed, and only a single, axial cross-sectional image is recorded (the image is described as “axial” when it is perpendicular to the body in the anatomical position). This measurement is called the anatomical cross-sectional area. Unlike measures of muscle volume, recording anatomical cross-sectional area can cause us to underestimate or overestimate a real change in muscle volume, if the arrangement of the fibers inside the muscle alters after training, or if different regions of the muscle increase in size more than others.

Indeed, the arrangement of fibers inside a muscle *does* change after strength training. Their angle relative to the line of pull (called the pennation angle) increases in conjunction with increases in fiber diameter. Similarly, many muscles do increase in size in some regions more than others, depending on the exercise used in training, because they have functional subdivisions, each of which is best-suited to producing force in a different direction or at a different joint angle.

If you visualize muscle fibers as running from one end of a muscle to the other, it can seem odd to think about them changing their angle within a muscle after strength training. However, fibers often run diagonally across muscles, between broad sheets of collagen tissue (called aponeuroses) on either side. After strength training, as muscle fibers increase in pennation angle, they become less parallel to the aponeuroses, and more perpendicular.

More recently, ultrasound has been adopted by some researchers to assess changes in muscle size, because the equipment is inexpensive. The most common measurement recorded by ultrasound is muscle thickness. This refers to the perpendicular, linear distance between the superficial and distal aponeuroses. It is fairly closely related to anatomical cross-sectional area, and is subject to the same limitations.


#3. Individual fiber

Some studies record changes in single fiber diameter after strength training. To do this requires taking a muscle biopsy both before and after the long-term strength training program, and then taking cross-sectional slices of the muscle tissue, before performing staining procedures and imaging to identify the borders (and therefore the diameter) of each muscle fiber.

Often, the staining procedures allow researchers to identify muscle fibers of different types, and this means that average changes in fiber type-specific cross-sectional area can be measured.

Since muscle growth in humans seems to arise mainly from increases in single fiber volume, studying changes in the diameter of individual muscle fibers is an attractive method. However, the main downside is that fibers can also increase in length, and this measurement does not get recorded by this approach.


What happens inside a muscle fiber during hypertrophy?

In humans, muscles increase in size predominantly through increases in the volume of single muscle fibers. These muscle fibers increase in volume mainly because of increases in their diameter, but also partly because of increases in their length.

When muscle fibers increase in diameter, this involves an increase in the number of sarcomeres in parallel. When muscle fibers increase in length, this involves an increase in the number of sarcomeres in series.

Sarcomeres are short sections of actin and myosin myofibrils and their associated cytoskeletal structures, which allow muscles to produce force. These sections are joined together in long strings, down the length of each fiber. Each fiber involves many myofibrillar strings arranged in parallel. Rodent and human studies have shown us that there can be 1,000 –1,500 myofibrils inside a single fiber.

Whether the volume of a muscle fiber is increased because of an increase in the number of sarcomeres in each myofibrillar string, or because of an increase in the number of myofibrillar strings inside the muscle fiber, this requires an increase in the protein content of the fiber, which happens through an increase in the rate of muscle protein synthesis.

This additional protein comprises the various molecules that are needed to create the new sarcomeres, and their surrounding sarcoplasmic support structures.


Can myofibrillar hypertrophy and sarcoplasmic hypertrophy happen independently?

Some researchers have speculated that the density (number per unit cross-sectional area) of myofibrillar strings in parallel inside each muscle fiber could vary. This would affect the strength of a muscle fiber (and therefore of a muscle) relative to its size, since it is the myofibrillar strings that are responsible for producing force.

This hypothesis arose in order to explain why strength increases by far more than muscle size after strength training, and why strength increases by more after strength training with heavy loads than after strength training with light loads, despite similar gains in muscle size.

Specifically, it was suggested that strength training with heavier loads might cause more myofibrillar hypertrophy, and less sarcoplasmic hypertrophy, compared to strength training with lighter loads.

However, this probably does not happen.

Research has shown that the number of myofilaments in a muscle fiber increases in proportion to its cross-sectional area after strength training, and this is probably why the force that a single muscle fiber can exert relative to its cross-sectional area tends to remain constant after strength training.

Also, we now know that strength training increases strength by more than size because there are many other adaptations that contribute to an increased ability to produce force, and these adaptations are preferentially stimulated when using heavy weights. So the hypothesis that myofibrillar and sarcoplasmic hypertrophy can occur independently is unnecessary.


What stimulates hypertrophy to occur?

Three mechanisms have been proposed to trigger the growth of muscle fibers, which are: (1) mechanical tension, (2) muscle damage, and (3) metabolic stress. Currently, however, there is only strong evidence for the role of mechanical tension.

Muscle fibers are able to detect mechanical tension using receptors located at the cell membrane. When these receptors inside the muscle fiber detect the presence of mechanical loading, this triggers a sequence of signaling events in a process known as mechanotransduction.

It is very important to note that it is the muscle fiber that detects the presence of mechanical loading, and not the whole muscle. We know that muscles can experience one type of mechanical loading, while individual muscle fibers inside them experience a completely different stimulus.

For example, when carrying out eccentric training, mechanical tension causes both muscle lengthening and individual muscle fiber lengthening. Yet, the extent to which the muscle and each individual muscle fibers lengthen differs, because of (1) tendon elasticity, and (2) regional subdivisions within the muscle. And since it is the stimulus on the individual muscle fiber, and not on the whole muscle, that triggers adaptations to the individual muscle fibers, it is the fiber lengthening that determines the extent to which sarcomeres are added in series to any of the fibers.

This happens because muscle fibers are grouped together inside the muscle, and are constantly pushing and pulling on their collagen structures and neighboring fibers. This means that the mechanical loading they experience is very different from the mechanical load placed upon the whole muscle.


What is the takeaway?

Muscle growth in humans happens predominantly through increases in the volume of single muscle fibers, although not all measurement methods are ideal for recording this.

Single muscle fibers increase in volume mainly because of increases in their diameter, but also partly because of increases in their length. This increase in volume involves an increase in protein content, which happens by an increase in the rate of muscle protein synthesis, and the increase in fiber size involves proportional increases in both myofibrillar and sarcoplasmic elements.

Single muscle fibers are triggered to grow when they detect the presence of mechanical loading, by receptors located at their cell membranes. This mechanical loading can be different from the mechanical loading experienced by the muscle as a whole, but it is the stimulus detected by the individual fiber that determines how it adapts to the strength training program.