Logan Steele
Skeletal muscles can be categorised into four groups based on their anatomical arrangement: parallel, convergent, pennate, and circular (or sphincter). Parallel muscles are the most common type, with fascicles arranged parallel to one another. They can be further divided into three categories: strap, fusiform, and fan-shaped. Fusiform muscles, such as the biceps brachii, are wider in the centre and taper off at the ends, resembling a spindle. They are associated with a greater range of motion and joint velocity compared to muscles with a different fibre arrangement.
| Characteristics | Values |
|---|---|
| Muscle architecture | Parallel |
| Muscle fascicles | Run parallel to one another |
| Muscle contraction | Acts as an extension of the contraction of a single muscle fiber |
| Muscle shape | Flat bands, spindle-shaped, or with large protrusions in the middle (belly of the muscle) |
| Muscle types | Fusiform and non-fusiform |
| Fusiform shape | Spindle-shaped, wider in the center, and tapered at the ends |
| Non-fusiform shape | Rectangular with a constant diameter |
| Example | Biceps brachii |
| Function | Flexing the forearm |
| Advantages | Greater range of motion, greater joint velocity, faster contractile speeds, and larger excursions |
| Disadvantages | Produce lower force compared to pennate muscles |
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What You'll Learn
Parallel muscles are the most common type
Skeletal muscles can be categorised into four groups based on their anatomical arrangement: parallel, convergent, circular (or sphincter), and pennate. Parallel muscles are the most common type.
Parallel muscles are characterised by fascicles that run parallel to one another, and the contraction of these muscle groups acts as an extension of the contraction of a single muscle fibre. They are also called parallel-fibered muscles, and they can be further divided into fusiform and non-fusiform types based on their shape. Fusiform muscles, also known as strap muscles, are wider and cylindrically shaped in the centre and taper off at the ends. This shape is often referred to as a spindle. Non-fusiform muscles, on the other hand, are more rectangular with a constant diameter. Examples of fusiform parallel muscles include the biceps brachii, sartorius, and sternohyoid muscles. The biceps brachii is responsible for flexing the forearm.
Parallel muscles can also be categorised into three main shapes: strap, fusiform, or fan-shaped. Strap muscles are shaped like a strap or belt and have fibres that run longitudinally to the contraction direction. They have broad attachments compared to other muscle types and can shorten to about 40–60% of their resting length. The sartorius muscle, which is the longest muscle in the human body, is an example of a strap muscle. Studies have suggested that it may control the fundamental frequency used in speech production and singing.
In terms of function, parallel muscles are associated with speed and extensive movement. They can generate larger excursions and achieve faster contractile speeds compared to other muscle types. This is because they have more sarcomeres in series, resulting in shorter fibre lengths. This allows for more fibres to be present in a given muscle, enabling faster contractions. Additionally, the force produced by a given muscle is proportional to the cross-sectional area or the number of parallel sarcomeres present.
Compared to pennate muscles, which have fibres arranged at an angle, parallel muscles have a greater range of motion and joint velocity for the same cross-sectional area. However, pennate muscles can generate higher maximum forces due to their greater number of sarcomeres in parallel, resulting in a larger total fibre cross-sectional area.
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They have a common point of attachment
The human body has over 600 muscles, and skeletal muscles make up the majority of them. These muscles are attached to the bones of the skeleton, and their connections determine the force, speed, and range of movement. The end of the muscle that attaches to the bone being pulled is called the muscle's insertion, and the end of the muscle attached to a fixed or stabilized bone is called the origin.
Skeletal muscles can be categorised into four groups based on their anatomical arrangement: parallel, circular, convergent, and pennate. Parallel muscles are the most abundant and typical, with fascicles arranged parallel to one another and to the long axis of the muscle. They can be further divided into fusiform and non-fusiform types based on their shape. Fusiform muscles, also known as spindle-shaped muscles, have a large belly that tapers as it extends to its origin and insertion. Examples of fusiform parallel muscles include the biceps brachii and the sartorius muscle. On the other hand, non-fusiform muscles have a more rectangular shape with a constant diameter.
Convergent muscles, on the other hand, have a common point of attachment, but their individual fascicles do not necessarily run parallel to each other. Instead, they converge towards a single tendon of insertion, resulting in a broader muscle shape. The pectoralis major, the large muscle on the chest, is an example of a convergent muscle. It converges on the greater tubercle of the humerus via a tendon. Another example of a convergent muscle is the temporalis muscle of the cranium.
Pennate muscles, named for the Latin word "feather," have fascicles that attach obliquely to a central tendon that runs through the muscle's central region. This tendon resembles the quill of a feather, with the muscle fascicles arranged like feathers. Due to this design, the muscle fibres in a pennate muscle can only pull at an angle, resulting in shorter tendon movement during contraction. The deltoid muscle of the shoulder is an example of a multipennate muscle, with multiple feathers converging on a central point.
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They can be fusiform or non-fusiform
Muscles that are arranged in parallel exhibit a unique structural organization that maximizes their contractile efficiency and mechanical advantage. In terms of their shape, these muscles can be categorized into two distinct types: fusiform and non-fusiform.
Fusiform muscles, also known as spindle-shaped muscles, are characterized by their tapered ends, resembling a spindle or an elongated oval. The middle portion, or belly, of the muscle is thicker, while the ends, or tendons, are thinner and more pointed. This shape allows for optimal force generation and is commonly observed in muscles that require powerful contractions, such as the biceps brachii in the arm.
Non-fusiform muscles, on the other hand, deviate from the typical spindle shape. They exhibit a variety of forms, including triangular, strap-like, or circular configurations. These muscles often have a broader area of attachment and may have a more complex arrangement of fibers. An example of a non-fusiform muscle is the deltoid muscle in the shoulder, which has a triangular shape and functions to abduct and stabilize the arm.
The shape of a muscle, whether fusiform or non-fusiform, is intimately linked to its function and anatomical location. Fusiform muscles are typically involved in producing powerful, concentrated movements, such as flexion or extension of joints. Their tapered ends provide a mechanical advantage, allowing them to generate substantial force during contractions. Non-fusiform muscles, with their diverse shapes, often have roles in stabilization, fine motor control, or complex movements that require a broader area of attachment.
The arrangement of muscle fibers within these shapes can also vary. In some cases, the fibers may be parallel to each other throughout the entire muscle, creating a simple and efficient contractile mechanism. In other instances, the fibers may be arranged in complex, overlapping patterns, allowing for a greater range of motion and adaptability in the muscle's function. This diversity in muscle architecture reflects the intricate ways in which the human body has evolved to accommodate a wide range of movements and functional demands.
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They favour speed over force
Skeletal muscles can be categorised into four groups based on their anatomical arrangement: parallel, convergent, pennate, and circular (or sphincter). Parallel muscles are the most common and typical, with fascicles arranged parallel to one another. They can be further divided into fusiform and non-fusiform types based on their shape.
Parallel muscles favour speed over force. This is due to their structure, which allows them to generate larger excursions and achieve faster contractile speeds. This is in contrast to pennate muscles, which generate higher maximum forces for a given muscle volume. The trade-off between force and speed is determined by the architecture of the muscle.
Parallel muscles have fascicles that extend nearly the entire length of the muscle and are parallel to the muscle's axis of force transmission. This means that they can contract over a larger range of motion and achieve greater joint velocity compared to other muscle types with the same cross-sectional area. Strap muscles, such as the laryngeal muscles, are an example of a parallel muscle type. They are thought to control the fundamental frequency used in speech production and singing. The sartorius, the longest muscle in the human body, is another example of a strap muscle.
Fusiform muscles, which are a type of parallel muscle, are wider and cylindrically shaped in the centre, tapering off at the ends. This shape is often referred to as a spindle. Examples of fusiform muscles include the biceps brachii, sartorius, and sternohyoid muscles. Fusiform muscles are associated with a greater range of motion and joint velocity compared to muscles with a different fibre arrangement but the same cross-sectional area.
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They can be strap, fusiform or fan-shaped
Muscles that are arranged in parallel exhibit a unique structural organization that maximizes their functionality and adaptability. Within this arrangement, muscle fibers are oriented in a parallel fashion, enabling efficient force generation and movement. These muscles can vary in shape, including strap-like, fusiform, or fan-shaped configurations, each serving distinct purposes in the body.
Strap-shaped muscles, also known as strap muscles, possess a long and narrow structure resembling a strap or a band. They are typically responsible for stabilizing and supporting adjacent structures. An excellent example of a strap muscle is the serratus anterior, which originates from the upper eight or nine ribs and
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Frequently asked questions
Parallel muscles are characterised by fascicles that run parallel to one another. They are the most common type of muscle and can be further divided into fusiform and non-fusiform types.
Examples of fusiform parallel muscles include the biceps brachii, sartorius, and sternohyoid muscles.
In pennate muscles, the tendon runs through the length of the muscle, and the fascicles attach at an angle. This means that they do not move as far during contraction compared to parallel muscles. However, pennate muscles have more muscle fibres and can therefore carry more tension.
Parallel muscles can generate larger excursions and achieve faster contractile speeds. They also generally produce a greater range of motion and greater joint velocity compared to muscles with the same cross-sectional area but with a different fibre arrangement.
