Maximus Gage
The human body is made up of more than 600 muscles, which help us do everything from moving our bodies to breathing and staying alive. Skeletal muscles, which are attached to the bones of a skeleton, are one of the three types of vertebrate muscle tissue, the others being cardiac and smooth muscle. Skeletal muscles can be categorised into four basic structural patterns: circular, parallel, convergent, and pennate. The shape of a muscle is often reflected in its name, for example, the deltoids have a delta or triangular shape, and the rhomboid major is a diamond shape.
| Characteristics | Values |
|---|---|
| Shape | Triangular/Delta/Deltoid |
| Saw-like/Serrated/Serratus | |
| Rhomboid/Diamond | |
| Round/Teres | |
| Trapezoid/Trapezius | |
| Spindle-shaped | |
| Cylindrical | |
| Pear-shaped/Piriformis | |
| Flat/Platys | |
| Slender/Gracilis | |
| Comblike/Pectinate | |
| Fan-shaped | |
| Circular/Orbicular |
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What You'll Learn
Triangular or fan-shaped muscles
The human body has more than 600 muscles, and they come in various shapes and sizes. One of the four basic muscle shapes is triangular or fan-shaped. These muscles have a broad origin, with fascicles converging toward a single tendon of insertion.
The pectoralis major muscle of the anterior thorax is an example of a triangular or fan-shaped muscle. The deltoids are another example of a muscle with a triangular shape. The gluteus medius is also triangular.
The shape of a muscle is an important indicator of its specific action. For instance, long, strap-like muscles typically provide large ranges of motion, whereas thick, short muscles typically provide large forces.
Triangular muscles have expansive proximal attachments that converge to a small distal attachment. In a pennate pattern, the fascicles are short and attach obliquely to a central tendon that runs the length of the muscle.
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Rhomboid-shaped muscles
The human body has more than 600 muscles that help us do everything from moving our body to breathing and staying alive. Skeletal muscles are named based on many different factors, including their location, origin and insertion, number of origins, shape, size, direction, and function.
The rhomboids are important in upper limb movement and stability of the shoulder girdle and scapula. They work with other muscles like the serratus anterior, trapezius, and levator scapulae to anchor the scapula and prevent winging. The main action of the rhomboid muscles is scapular retraction around the scapulothoracic joint. This involves the simultaneous sliding of the scapula superiorly and medially along the trunk, rotating the glenoid cavity inferiorly and dropping the shoulder girdle.
The rhomboids are innervated by the dorsal scapular nerve, a branch of the brachial plexus. They receive arterial blood from three sources: the dorsal scapular artery, the deep branch of the transverse cervical artery, and the dorsal branch of the posterior intercostal arteries.
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Circular muscles
The gastrointestinal system breaks down ingested food into molecular forms through digestion, which are then transferred to the internal environment through absorption. Circular muscles play a role in this process by generating radial closure pressure to create a local peristaltic closure wave. This pressure is highest where circular muscle fibres are most concentrated.
In contrast, longitudinal muscles have two functions, one physiological with mechanical implications, and one purely mechanical. The mechanical function of longitudinal muscle shortening (LLS) is to reduce the level of pressure required to maintain closure. The physiological function is deduced by combining the laws of mechanics with concurrent measurements of intraluminal pressure and changes in cross-sectional muscle area.
Sphincters are a type of circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. There are over 60 types of sphincters in the human body, some of which are microscopic in size. For example, the lower oesophageal sphincter, or cardiac sphincter, at the upper portion of the stomach, is closed most of the time, preventing the contents of the stomach from moving upwards into the oesophagus.
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Parallel muscles
The human body has over 600 muscles, which help us do everything from moving our bodies to breathing and staying alive. Skeletal muscles, which are responsible for voluntary movements, can be categorised into four groups based on their anatomical arrangement: circular, parallel, convergent, and pennate.
This answer will focus on parallel muscles. Parallel muscles are the most abundant and typical type of muscle, with fascicles arranged parallel to one another. They can be further divided into fusiform and non-fusiform types based on their shape. Fusiform muscles are more spindle-shaped (their diameter at the centre is greater than at either end), whereas non-fusiform muscles are more rectangular with a constant diameter. Parallel muscles can also be categorised into three main categories: strap, fusiform, or fan-shaped.
Strap muscles are shaped like a strap or belt and have fibres that run longitudinally to the contraction direction. These muscles have broad attachments compared to other muscle types and can shorten to about 40-60% of their resting length. An example of a strap muscle is the sartorius muscle, which is the longest muscle in the human body.
Fusiform muscles are wider and cylindrically shaped in the centre and taper off at the ends. This shape is often referred to as a spindle. An example of a fusiform muscle is the biceps brachii, which is responsible for flexing the forearm.
Fan-shaped muscles, also known as convergent or triangular muscles, have a broad origin with fascicles converging toward a single tendon of insertion. An example of a convergent muscle is the pectoralis major in humans. These muscles have a weaker pull on the attachment site compared to other parallel fibres due to their broad nature.
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Pennate muscles
The human body has more than 600 muscles that help us do everything from moving our bodies to breathing and staying alive. Skeletal muscles can be arranged into four basic structural patterns: circular, parallel, convergent, and pennate.
There are three forms of pennate muscles: unipennate, bipennate, and multipennate. In unipennate muscles, the fascicles insert into only one side of the tendon, whereas in bipennate muscles, the fascicles insert into the tendon from opposite sides. If the central tendon branches within a pennate muscle, it is called multipennate. An example of a bipennate muscle is the rectus femoris in the thigh, which is responsible for its flexion.
The larger the pennation angle, the shorter the muscle fibres. The speed at which a muscle fibre can shorten is determined by its length. Therefore, a muscle with a large pennation angle will contract more slowly than a similar muscle with a smaller pennation angle. The architectural gear ratio (AGR) is a feature of pennate muscles, defined by the ratio between the muscle-shortening velocity and the fibre-shortening velocity.
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Frequently asked questions
There are three types of muscle in the human body: skeletal muscle, cardiac muscle and smooth muscle.
Skeletal muscles are attached to bones and are the only type of muscle that can be controlled consciously. They make up around 40% of body weight in healthy young adults.
Cardiac muscles are found in the heart and are responsible for pumping blood around the body.
Smooth muscles are generally found in the internal organs and are responsible for keeping the body working properly.
The different shapes of skeletal muscle are circular, convergent, parallel, pennate, and fusiform.
