Logan Steele
Skeletal muscles are a type of muscle that humans can control voluntarily. They are responsible for moving our bones and supporting our skeletons. Skeletal muscles can be further classified into three types of muscle architecture: parallel, pennate, and hydrostats. Pennate muscles, also known as pinnate or penniform muscles, are characterised by their fascicles that attach obliquely to their tendon. Depending on the number of similarly angled sets of fibres attached to the central tendon, pennate muscles can be classified as unipennate, bipennate, or multipennate. Unipennate muscles are those where the muscle fibres are oriented at one fibre angle to the force-generating axis and are all on the same side of a tendon. Examples of unipennate muscles include the extensor digitorum of the forearm and certain muscles in the hand.
| Characteristics | Values |
|---|---|
| Definition | A unipennate muscle is a type of skeletal muscle with fascicles that attach obliquely (in a slanting position) to its tendon. |
| Muscle Fibers | The fibers are oriented at one fiber angle to the force-generating axis and are all on the same side of a tendon. |
| Pennation Angle | The pennation angle in unipennate muscles typically varies from 0° to 30°. |
| Examples | Extensor digitorum of the forearm, flexor pollicis longus, certain muscles in the hand, lateral gastrocnemius |
| Force | Unipennate muscles produce greater force than parallel muscles. |
| Speed | Unipennate muscles contract more slowly than similar muscles with a smaller pennation angle. |
| Muscle Volume | Determined by the cross-sectional area. |
| Architectural Gear Ratio | Defined by the ratio between the longitudinal strain of the muscle and muscle fiber strain. |
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What You'll Learn
Unipennate vs bipennate muscles
A pennate or pinnate muscle is a type of skeletal muscle with fascicles that attach obliquely (in a slanting position) to its tendon. The term "pennate" comes from the Latin pinnātus ("feathered, winged"), from pinna ("feather, wing"). Pennate muscles are characterised by their flattened shape, with one or both tendons extending along the length of the belly.
Unipennate muscles are a type of pennate muscle where the fascicles are attached to only one side of the tendon. Examples of unipennate muscles include certain muscles in the hand, the flexor pollicis longus, and the vastus lateralis.
Bipennate muscles, on the other hand, are pennate muscles with fascicles attached to both sides of a central tendon. The rectus femoris, a large muscle in the quadriceps, is an example of a bipennate muscle. Other examples include the gastrocnemius and dorsal interossei.
The main difference between unipennate and bipennate muscles lies in the arrangement of their fascicles or muscle fibres relative to the tendon. Unipennate muscles have fascicles on only one side of the tendon, while bipennate muscles have fascicles on both sides of a central tendon. This gives rise to distinct structural and functional characteristics.
As pennation increases from unipennate to bipennate muscles, the muscle fibres become shorter, and the number of fibres increases, leading to an increase in the cross-sectional area of the fibres. This results in a decreased shortening of the muscle as a whole upon contraction and an increase in the force produced. Bipennate muscles, therefore, have a higher force production capacity compared to unipennate muscles. However, this increased force production comes at the cost of a smaller range of motion, as the fibre angle to the direction of action affects the maximum force in that direction.
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Muscle architecture
The parallel muscle architecture is the most common type, with fascicles arranged in the same direction as the long axis of the muscle. These muscles have a greater range of motion compared to pennate muscles. An example of this type of muscle architecture is the sartorius, the longest muscle in the human body.
The pennate muscle architecture is characterised by fibres that are at an angle to the force-generating axis, forming a "feathered" or "winged" structure. This angle is known as the pennation angle. Due to this arrangement, pennate muscles produce greater force but have a smaller range of motion than parallel muscles. The pennation angle varies among different types of pennate muscles, with unipennate muscles exhibiting angles from 0° to 30° at resting length.
Unipennate muscles, the focus of this discussion, are a subtype of pennate muscles where all the fascicles are on the same side of the tendon. This arrangement allows for a higher number of muscle fibres, resulting in increased force production. Examples of unipennate muscles include certain muscles in the hand, the extensor digitorum of the forearm, and the lateral gastrocnemius.
Bipennate muscles, on the other hand, have fascicles on both sides of the tendon, resembling the arrangement of a single feather. The rectus femoris, a large muscle in the quadriceps, is an example of a bipennate muscle.
The third subtype of pennate muscles is the multipennate architecture, where fibres are oriented at multiple angles along the force-generating axis. The deltoid muscle in the shoulder is an example of a multipennate muscle.
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Skeletal muscles
Parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle. The majority of skeletal muscles in the body have this type of organisation. Some parallel muscles are flat sheets that expand at the ends to make broad attachments, such as the sartorius. Other parallel muscles have a larger central region called a muscle belly that tapers to tendons on each end. This arrangement is called fusiform, and examples include the biceps brachii.
Pennate muscles are a type of skeletal muscle with fascicles that attach obliquely (in a slanting position) to its tendon. This type of muscle generally allows higher force production but a smaller range of motion. When a muscle contracts and shortens, the pennation angle increases. The term pennate comes from the Latin "pinnatus" ("feathered, winged"), from "pinna" ("feather, wing"). In a pennate muscle, as a consequence of their arrangement, fibres are shorter than they would be if they ran from one end of the muscle to the other. This implies that each fibre is composed of a smaller number of sarcomeres in series. Moreover, the larger the pennation angle, the shorter the fibres. The speed at which a muscle fibre can shorten is partly determined by the length of the muscle fibre. Thus, a muscle with a large pennation angle will contract more slowly than a similar muscle with a smaller pennation angle.
Pennate muscles can be further classified as unipennate, bipennate, or multipennate, depending on the number of similarly angled sets of fibres that attach to the central tendon. Unipennate muscles are those where the muscle fibres are oriented at one fibre angle to the force-generating axis and are all on the same side of a tendon. Examples of this include certain muscles in the hand and the extensor digitorum of the forearm. Bipennate muscles, such as the rectus femoris, have fascicles on both sides of the tendon, like the arrangement of a single feather. Multipennate muscles, such as the deltoid muscle in the shoulder, have fibres that are oriented at multiple angles along the force-generating axis.
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Muscle volume
PCSA is influenced by the pennation angle and muscle length. In pennate muscles, the fibres are at an angle to the force-generating axis, which results in a shorter fibre length compared to parallel muscles. This architectural difference allows for a greater number of fibres within a given muscle, increasing the PCSA.
The larger the pennation angle, the shorter the muscle fibres, and the higher the PCSA. This relationship also means that as pennation increases from unipennate to multipennate muscles, the cross-sectional area of the fibres increases, leading to a greater force production.
The PCSA is a critical factor in force production. The force generated by a muscle is proportional to the cross-sectional area and the number of parallel sarcomeres present. The maximum force of a muscle fibre depends on its thickness (cross-sectional area) and type, rather than its mass or length alone. Therefore, an increase in muscle mass does not necessarily lead to an increase in force unless accompanied by an increase in PCSA or a change in fibre type.
In summary, muscle volume is a key determinant of muscle function, with higher volumes leading to increased force production. This relationship is particularly evident in pennate muscles, where the fibre angle plays a crucial role in determining the PCSA and, consequently, the force-generating capacity of the muscle.
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Muscle fibres
Parallel muscles have fibres that are arranged in the same direction as the long axis of the muscle. Most skeletal muscles in the body are parallel. Some are flat sheets that expand at the ends to make broad attachments, such as the sartorius. Others have a larger central region called a muscle belly that tapers to tendons at each end. This arrangement is called fusiform, and an example is the biceps brachii. Circular muscles are also called sphincters and are concentrically arranged bundles of muscle fibres that increase or decrease the size of an opening, such as the orbicularis oris muscle around the mouth.
Pennate muscles are a type of skeletal muscle with fibres that attach obliquely (in a slanting position) to its tendon. This type of muscle generally allows higher force production but a smaller range of motion. When a muscle contracts and shortens, the pennation angle increases. The term "pennate" comes from the Latin "pinnatus" or "feathered, winged". In a pennate muscle, the fibres are shorter than they would be if they ran from one end of the muscle to the other, and the larger the pennation angle, the shorter the fibres. As the pennation increases, the muscle fibres become shorter, the number of fibres increases, and the cross-sectional area of the fibres increases, resulting in a decreased shortening of the muscle as a whole upon contraction and an increase in force produced.
Pennate muscles can be further classified into unipennate, bipennate, or multipennate, depending on the number of similarly angled sets of fibres that attach to the central tendon. Unipennate muscles have fibres oriented at one fibre angle to the force-generating axis and are all on the same side of a tendon. Examples include the flexor pollicis longus and the extensor digitorum of the forearm. Bipennate muscles have fibres on both sides of a tendon, such as the rectus femoris of the quadriceps. Multipennate muscles have fibres that are oriented at multiple angles along the force-generating axis, such as the deltoid muscle in the shoulder.
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Frequently asked questions
A unipennate muscle is a type of skeletal muscle where all the fascicles are on the same side of the tendon.
The term "pennate" comes from the Latin "pinnatus", meaning "feathered" or "winged". This refers to the arrangement of the muscle fibres, which attach obliquely to the tendon, resembling the plumes of a feather.
Examples of unipennate muscles include the extensor digitorum of the forearm, the flexor pollicis longus, and certain muscles in the hand.
Unipennate muscles can produce more force than parallel muscles. This is because they can pack in more muscle fibres, which are arranged at an angle to the force-generating axis, resulting in a shorter fibre length.
Apart from unipennate muscles, there are two other types of pennate muscles: bipennate and multipennate. Bipennate muscles have fascicles on both sides of the tendon, while multipennate muscles have fibres oriented at multiple angles along the force-generating axis.
