Logan Steele
There are over 600 muscles in the human body, and they can be categorised in several ways. One way is by the shape of their muscle fibres. There are two basic structural patterns: fusiform and pennate (or pinnate). Fusiform muscles are spindle-shaped, with a wider centre that tapers down to narrow ends. The muscle fibres are arranged in parallel and run the entire length of the muscle. This arrangement makes up most human muscle. An example of a fusiform muscle is the biceps brachii, which is responsible for flexing the forearm.
| Characteristics | Values |
|---|---|
| Definition | A muscle that has a spindle shape, wider in the middle and narrowing towards both ends |
| Muscle fibres | Arranged in a parallel fashion and run the entire length of the muscle |
| Muscle architecture | The line of action runs in a straight line between the attachment points, which are often tendons |
| Force concentration | The force produced is concentrated into a small area |
| Examples | Biceps brachii, gastrocnemius |
| Types | Parallel, pennate (uni-, bi- and multi-), circular, convergent, and muscular hydrostats |
| Muscle fibre types | Type I (slow oxidative), Type IIa (fast oxidative), Type IIb (fast glycolytic) |
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What You'll Learn
Muscle architecture
Parallel muscle architecture is found in muscles where the fibres are parallel to the force-generating axis. An example of a parallel muscle is the sartorius muscle, which runs down the thigh to the knee. Fusiform muscles, such as the biceps brachii, are a type of parallel muscle that is spindle-shaped, with a wide centre that tapers down to narrow ends. The muscle fibres in a fusiform muscle run the entire length of the muscle.
Pennate muscles, on the other hand, have fibres that are oriented at an angle to the force-generating axis. The pennation angle can vary, and as the fibres shorten, the pennation angle increases, affecting the amount of force generated. Examples of pennate muscles include the lateral gastrocnemius (unipennate) and the rectus femoris of the quadriceps (bipennate).
Multipennate muscles, such as the deltoid in the human shoulder, have fibres that are oriented at multiple angles along the force-generating axis. The architectural gear ratio (AGR) is a measure that relates the contractile velocity of an entire muscle to the contractile velocity of a single muscle fibre, and it is influenced by the mechanical demands of a muscle during movement.
Muscular hydrostats function independently of a hardened skeletal system. The force produced by a muscle is influenced by several factors, including muscle volume, fibre length, fibre type, and pennation angle. The force is proportional to the cross-sectional area or the number of parallel sarcomeres present.
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Types of muscle fibres
A fusiform muscle is a type of muscle that is spindle-shaped, with a wide centre that tapers down to narrow ends. The muscle fibres in a fusiform muscle are arranged in a parallel fashion and run the entire length of the muscle. The fusiform arrangement makes up most human muscle, with the fibres largely arranged in parallel arrays along the muscle's longitudinal axis.
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles in the human body contain all three types, although in varying proportions. Muscle fibres can be classified based on two criteria: how fast the fibres contract relative to others, and how the fibres regenerate adenosine triphosphate (ATP), a molecule that releases energy when it is broken down.
Slow oxidative fibres, also called slow-twitch or Type I, contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They have a relatively small diameter and thus do not produce a large amount of tension. However, they are slow to fatigue because they can produce large amounts of ATP.
Fast oxidative fibres, also known as fast-twitch or Type IIa, have relatively fast contractions and primarily use aerobic respiration to generate ATP. They produce higher-tension contractions than slow oxidative fibres.
Fast glycolytic fibres, also referred to as fast-twitch or Type IIx or Type IIb, have relatively fast contractions and primarily use anaerobic glycolysis as their ATP source. They have a large diameter and high amounts of glycogen, which is used to generate ATP quickly and produce high levels of tension. However, they fatigue quickly and can only be used for short periods.
In addition to these three main types, there are also smooth and cardiac muscle fibres. Smooth muscles are involuntary and are present in the walls of hollow organs and vessels such as the stomach, intestines, bladder, arteries, and veins. Cardiac muscles are striated and are only found in the heart, where they contract in a coordinated way to allow the heart to beat.
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Examples of fusiform muscles
A fusiform muscle is a type of muscle that is spindle-shaped, with a wide centre that tapers down to narrow ends. The muscle fibres in a fusiform muscle are arranged in a parallel fashion and run the entire length of the muscle.
Biceps Brachii
The biceps brachii is a fusiform muscle that is located in the upper arm. It is responsible for flexing the elbow and rotating the forearm. The muscle is composed of two heads, the long head and the short head, that originate on the scapula and converge to insert on the radius bone in the forearm.
Gastrocnemius
The gastrocnemius is a fusiform muscle located in the back of the lower leg. It is one of the two muscles that make up the calf, with the other being the soleus muscle. The gastrocnemius is responsible for plantar flexion of the foot, which is pointing the toes downward, and it also plays a role in ankle stabilization.
Tenuissimus
Tenuissimus is a rare muscle that has been described in the right thigh of a cadaver by Green (1931). It presented as a fusiform muscle that arose from the deep surface of the gluteus maximus muscle and inserted onto the biceps femoris muscle in the thigh.
In addition to these examples, there are likely many other fusiform muscles in the human body, as this is one of the basic structural patterns of muscle fibres.
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How fusiform muscles work
Fusiform muscles are spindle-shaped, with a wide centre that tapers down to narrow ends. The muscle fibres in a fusiform muscle are arranged in a parallel fashion and run the entire length of the muscle. This arrangement makes up most human muscle. The line of action in this muscle type runs in a straight line between the attachment points, which are often tendons.
The force produced by fusiform muscles is concentrated into a small area. Fusiform muscles are arranged in a strap-like fashion to provide the greatest degree of shortening, thus enabling the muscle to produce a quick and wide range of motion. However, compared to penniform muscles, they are not a very powerful type of muscle.
An example of this architecture type is the biceps brachii in humans. Other examples of fusiform muscles include the sartorius, the brachioradialis, and the biceps femoris.
The muscle fibres in fusiform muscles can be arranged in several ways. They can run largely parallel to one another, converge, or form a circular arrangement. The ultrasound appearance of muscle is affected by many factors, including the state of contraction of the muscle, which alters its size and echogenicity.
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Other muscle types
There are several types of skeletal muscles, which are the voluntary muscles that support our skeleton and move our bones. These include parallel, circular, convergent, and pennate (uni-, bi-, and multipennate) muscles.
Parallel muscles are the most common type, with fascicles arranged in parallel to one another. They can be further divided into fusiform and non-fusiform types based on their shape. Parallel muscles can be long and thin, such as the sartorius muscle that runs down the thigh to the knee, or they can be flat bands or have large protrusions in their middle known as the belly of the muscle.
Circular muscles are striated muscles that form a circular shape. Examples include the orbicularis oris, the muscle surrounding the mouth, and the orbicularis oculi, the muscles surrounding the eyes.
Convergent muscles have a common point of attachment, but the individual fascicles do not necessarily run parallel to each other. This gives them a broader shape and allows them to cover a wider surface area. An example of a convergent muscle is the pectoralis major in humans.
Pennate muscles, also known as pinnate, are feather-shaped with fascicles that attach obliquely to a central tendon. The tendon runs through the length of the muscle. The deltoid muscle, which lies over the shoulder joint, is an example of a multipennate muscle.
In addition to these muscle types, there are also smooth muscles, which are involuntary. These muscles are found in the walls of hollow organs and vessels such as the stomach, intestines, bladder, arteries, and veins. They facilitate essential daily functions such as digestion and urination.
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Frequently asked questions
A fusiform muscle is a type of muscle that is spindle-shaped, wider in the middle, and tapers down to narrow ends. The muscle fibres are arranged in a parallel fashion and run the entire length of the muscle.
Examples of fusiform muscles include the biceps brachii and the gastrocnemius.
Due to their shape, the force produced by fusiform muscles is concentrated into a small area. Fusiform muscles are arranged in a strap-like fashion to provide the greatest degree of shortening, enabling the muscle to produce a quick and wide range of motion.
