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A unipennate muscle is a type of skeletal muscle with fascicles that attach obliquely (at an angle) to a tendon on one side. The term pennate comes from the Latin pinnatus, meaning feathered or winged, as the fascicles attach like the plumes of a feather. Unipennate muscles include certain muscles in the hand, such as the extensor digitorum of the forearm, and the Peronei. In a unipennate muscle, the tendon may extend along one side of the belly, with the fibres converging to one side of the tendon. This is in contrast to bipennate muscles, where the tendon runs through the centre of the belly, and multipennate muscles, where there are multiple tendinous intersections.
| Characteristics | Values |
|---|---|
| Definition | A unipennate muscle is a type of skeletal muscle with fascicles that attach obliquely to a tendon on one side. |
| Examples | Extensor digitorum of the forearm, certain muscles in the hand, and the Peronei. |
| Muscle Architecture | Unipennate muscles are a type of pennate or pinnate muscle architecture, where the muscle fibres are at an angle to the force-generating axis. |
| Force | Unipennate muscles can produce high force due to the ability to pack in more muscle fibres, but the maximum force in the direction of action is less than the maximum force in the fibre direction. |
| Range of Motion | The range of motion is smaller in pennate muscles compared to other muscle architectures. |
| Speed | The speed of contraction is determined by the length of the muscle fibre, with larger pennation angles resulting in slower contraction speeds. |
| Muscle Fibre Length | In unipennate muscles, the fibres are shorter than in other muscle types. |
| Number of Muscle Fibres | Unipennate muscles have a higher number of muscle fibres compared to other muscle types. |
| Cross-Sectional Area | The cross-sectional area of unipennate muscle fibres is larger than in non-pennate muscles, increasing with the pennation angle. |
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What You'll Learn
Unipennate vs bipennate muscles
A unipennate muscle is a type of skeletal muscle with fascicles that attach obliquely (in a slanting position) to its tendon. In a unipennate muscle, the obliquely set fascicles fan out on only one side of a central muscle tendon. Examples of this include certain muscles in the hand, the flexor pollicis longus, and the vastus lateralis.
A bipennate muscle, on the other hand, is a muscle with fascicles on both sides of the central tendon. The rectus femoris, a large muscle in the quadriceps, is a typical example of a bipennate muscle. The gastrocnemius is another example of a bipennate muscle, with fibres obliquely set on both sides of a central tendon.
Pennate muscles are flattened, and either or both of the tendons extend for some distance along the length of the belly. The tendon in a unipennate muscle extends along one side of the belly, whereas in a bipennate muscle, it courses through the centre of the belly. As pennation increases (unipennate to bipennate and beyond), the muscle fibres become shorter, the number of fibres increases, and the cross-sectional area of the fibres increases. This results in a decreased shortening of the muscle as a whole upon contraction and an increase in the force produced.
Pennate muscles generally allow higher force production but a smaller range of motion. When a muscle contracts and shortens, the pennation angle increases. The larger the pennation angle, the shorter the muscle fibres, and the slower the contraction. The total force exerted by the fibres along their oblique direction is proportional to the physiological cross-sectional area (PCSA), which increases with the pennation angle and muscle length.
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Muscle architecture
The first type, parallel muscles, have fibres that are arranged in the same direction as the long axis of the muscle. Most 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, while others 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 and sartorius.
The second type, pennate muscles, have fibres that attach obliquely (in a slanting position) to the tendon. This type of muscle generally allows for higher force production but a smaller range of motion. The term pennate comes from the Latin "pinnatus", meaning feathered or winged. Pennate muscles may be further classified as unipennate, bipennate, or multipennate, depending on the number of similarly angled sets of fibres attaching to the central tendon. Unipennate muscles, such as the extensor digitorum of the forearm, have fibres that converge to one side of a tendon, like the plumes of a quill pen. Bipennate muscles, such as the rectus femoris, have fascicles on both sides of the tendon, similar to the arrangement of a single feather. Multipennate muscles, such as the deltoid muscle in the shoulder, have fibres that insert on multiple tendons, converging towards a common tendon, like multiple feathers coming together at a central point.
The third type, muscular hydrostats, function independently of a hardened skeletal system and are typically supported by a membrane of connective tissue that holds the volume constant.
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Skeletal muscles
The physical arrangement of muscle fibres at the macroscopic level is known as muscle architecture, and this determines a muscle's mechanical function. There are several different muscle architecture types, including parallel, pennate, and hydrostats. Force production and gearing vary depending on different muscle parameters such as muscle length, fibre length, pennation angle, and the physiological cross-sectional area (PCSA).
The two main types of muscle architecture are parallel and pennate (or pinnate). A third subcategory is muscular hydrostats. Architecture type is determined by the direction in which the muscle fibres are oriented relative to the force-generating axis. The force produced by a given muscle is proportional to the cross-sectional area, or the number of parallel sarcomeres present.
Parallel muscle architecture is found in muscles where the fibres are parallel to the force-generating axis. 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 at 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 skeletal muscle tissue, 10-100 endomysium-sheathed muscle fibres are organised into perimysium-wrapped bundles known as fascicles. Each muscle is composed of a number of fascicles grouped together by a sleeve of connective tissue, known as an epimysium. In a pennate muscle, aponeuroses run along each side of the muscle and attach to the tendon. The fascicles attach to the aponeuroses and form an angle (the pennation angle) to the load axis of the muscle.
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Fascicles
A unipennate muscle is a type of skeletal muscle with fascicles that attach obliquely (in a slanting position) to its tendon. In a unipennate muscle, the fascicles are located on one side of the tendon. The extensor digitorum of the forearm is an example of a unipennate muscle.
There are several types of fascicle arrangements, including:
- Parallel: Fascicles are arranged in the same direction as the long axis of the muscle.
- Circular: Found in sphincter muscles that surround openings such as the mouth or anus.
- Convergent: Fascicles come to a single, common attachment point, which could be a tendon, an aponeurosis, or a raphe.
- Pennate: Fascicles attach to the aponeuroses and form a pennation angle to the load axis of the muscle. The larger the pennation angle, the shorter the muscle fibers, resulting in slower contraction.
The differing ways in which fascicles attach to tendons create a variety of skeletal muscle sizes and shapes. The fascicular arrangement contributes to the functional capabilities of skeletal muscles.
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Muscle fibres
A unipennate muscle is a type of skeletal muscle with a specific structure and function. Let's delve into the details of muscle fibres in the context of unipennate muscles.
- Muscle fibres, also known as muscle cells, are the fundamental units of a muscle. In skeletal muscle, each fibre is covered by a layer of connective tissue called endomysium, providing structural support and allowing for the transmission of electrical impulses during muscle contraction.
- In a unipennate muscle, the muscle fibres are arranged obliquely, forming an angle with the load axis of the muscle. This arrangement is similar to the plumes of a feather, with the fibres converging towards one side of the tendon.
- The tendon in a unipennate muscle extends along the length of the muscle, and the fibres attach to it obliquely. This architectural arrangement allows for greater force production, as more muscle fibres can be packed in parallel.
- The speed of muscle fibre contraction is influenced by its length. In a unipennate muscle, the fibres are shorter, resulting in a slower contraction speed compared to muscles with longer fibres.
- The cross-sectional area of the muscle fibres, known as the physiological cross-sectional area (PCSA), is larger in unipennate muscles. This increased PCSA contributes to the higher force production capacity of these muscles.
- Muscle fibres in unipennate muscles experience varying degrees of fibre strain due to their architectural arrangement. The pennation angle, or the angle formed between the muscle fibres and the load axis, influences the force generated by the muscle.
- Unipennate muscles, such as the extensor digitorum in the forearm, showcase a unique arrangement of muscle fibres that enhances their force-generating capacity while potentially sacrificing contraction speed.
In summary, the muscle fibres in unipennate muscles exhibit a distinct architectural pattern, resulting in specific functional characteristics. This arrangement of fibres allows for greater force production while impacting contraction speed and overall muscle function.
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Frequently asked questions
A unipennate muscle is a type of skeletal muscle with fascicles attached to only one side of the tendon.
The term "pennate" comes from the Latin "pinnatus", meaning "feathered" or "winged". Unipennate muscles get their name from the fact that their fascicles attach to one side of the tendon like the plumes of a feather.
Examples of unipennate muscles include certain muscles in the hand and the extensor digitorum of the forearm.
A bipennate muscle has fascicles attached to both sides of the tendon, like the rectus femurs. A unipennate muscle, on the other hand, has fascicles attached to just one side.
Unipennate muscles can pack more muscle fibres in parallel, allowing them to produce more force. However, this increased force comes at the cost of a smaller range of motion.
