Logan Steele
Pennate muscles, also known as penniform or pinnate muscles, are a type of skeletal muscle with a unique structure that resembles the shape of a feather. This structure, with its diagonal orientation of muscle fibres, maximizes the muscle's force potential, allowing for the generation of large forces. The architectural gear ratio (AGR) of pennate muscles is an important concept, referring to the ratio between muscle shortening and fibre shortening. The pennation angle, or angle of the muscle fibres to the tendon, plays a crucial role in the overall function and force production of these muscles. Pennate muscles have shorter fibres than fusiform muscles and can be classified as uni-, bi-, or multipennate depending on the location of the fascicles in relation to the tendon. They are commonly found in muscles such as the rectus femoris and the gastrocnemius, which are responsible for producing large forces to support or propel the body's weight.
| Characteristics | Values |
|---|---|
| Muscle type | Skeletal |
| Muscle shape | Resembles a feather |
| Muscle fibres | Shorter than fusiform muscles |
| Muscle structure | More muscle fibres can be packed in parallel |
| Muscle function | Higher force production but smaller range of motion |
| Muscle contraction | Unable to reduce length to the same degree as a parallel muscle |
| Muscle gearing | Velocity advantages come at the cost of reduced force |
| Muscle architecture | Internal architecture impacts function |
| Muscle excursion | Less overall excursion due to pennation |
| Muscle force | Maximises force potential |
| Muscle angle | Pennation angle increases with contraction |
| Architectural gear ratio | Ratio of muscle shortening to fibre shortening |
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What You'll Learn
Pennate muscle structure
A pennate muscle is a type of skeletal muscle that resembles the shape of a feather. It has shorter fibres than a fusiform muscle and inserts its tendons in various ways, forming unipennate, bipennate, and multipennate muscles. The classification of a pennate muscle depends on the location of the fascicles in relation to the tendon. If all the fascicles are on the same side of the tendon, it is called a unipennate muscle. If there are fascicles on both sides of the central tendon, it is called a bipennate muscle. If the central tendon branches within the muscle, it is called a multipennate muscle.
The structure of a pennate muscle maximizes its force potential. The diagonal orientation of the fibres allows for a greater number of muscle fibres, resulting in increased muscular tension. This design, however, restricts the range of motion of the muscle. When a pennate muscle contracts, the fibres can only pull at an angle, resulting in a smaller range of movement compared to non-pennate muscles.
The pennation angle also affects the speed of contraction in a pennate muscle. As the pennation angle increases, the fibres shorten. Therefore, a larger pennation angle results in slower contraction.
The architectural gear ratio (AGR) is a feature of pennate muscles that describes the ratio between the longitudinal strain of the muscle and muscle fibre strain. It also represents the ratio between muscle-shortening velocity and fibre-shortening velocity.
Pennate muscles are found in various parts of the body, including the rectus femoris and gastrocnemius in the thigh, the deltoid muscle in the shoulder, and certain muscles in the hand.
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Advantages of pennate muscles
The advantages of pennate muscles are largely due to their unique structure and function. This type of muscle is characterised by muscle fibres that attach obliquely to a central tendon, resembling the shape of a feather. This design allows for a greater number of muscle fibres within a given volume, increasing the cross-sectional area and, consequently, the force-generating capacity of the muscle.
Increased Force Generation
The pennate muscle architecture, with its diagonal fibre orientation, increases the muscle's force potential. This design enables a greater number of muscle fibres to fit within the muscle compared to similarly sized muscles with parallel fibres. The increased cross-sectional area, known as the physiological cross-sectional area (PCSA), is a strong predictor of muscle strength. The PCSA increases with the pennation angle and muscle length, further enhancing the force-generating capacity of pennate muscles.
Variable Gearing
Pennate muscles exhibit variable gearing, allowing them to automatically adjust their output based on the load. During low-load contractions, pennate muscles favour velocity output, functioning at a higher gear ratio. Conversely, during contractions against high loads, they shift to a lower gear, prioritising force output. This variable gearing provides a mechanism for modulating muscle performance during diverse mechanical functions.
Training Adaptability
Pennate muscles also demonstrate adaptability in response to strength training. Resistance training can increase the pennation angle, leading to a more significant increase in single muscle fibre contractile strength than anatomical muscle cross-sectional area and volume. This suggests that the morphology, architecture, and contractile capacity of pennate muscles are interrelated and can be positively influenced by specific training adaptations.
Efficient Design
The pennate design efficiently maximises strength while optimising muscle volume. Although the oblique arrangement of fibres may initially seem inefficient, it is an evolutionary advantage. By packing more fibres at an angle, pennate muscles achieve greater force generation without requiring larger muscle volumes. This design is particularly beneficial in muscles that need to produce large forces to support or propel the body, such as the rectus femoris and gastrocnemius.
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Types of pennate muscles
Pennate muscles are a type of skeletal muscle that resembles the shape of a feather. They have shorter fibres than fusiform muscles and insert into their tendons in various ways, forming unipennate, bipennate, and multipennate muscles. The classification of a pennate muscle depends on the location of the fascicles in relation to the tendon.
Unipennate muscles
Unipennate muscles have fascicles that attach obliquely to a tendon on only one side. An example of this is the extensor digitorum of the forearm.
Bipennate muscles
Bipennate muscles have fascicles that attach to both sides of a tendon. The rectus femoris, a large muscle in the quadriceps, is an example of a bipennate muscle.
Multipennate muscles
Multipennate muscles have a complex, branching network of fascicles and tendons. The deltoid muscle of the shoulder is an example of a multipennate muscle.
The diagonal orientation of the fibres in pennate muscles allows for a higher density of muscle fibres, maximising the muscle's force potential. However, the angular arrangement of the fibres means that the maximum force in the direction of action is less than the maximum force in the fibre direction.
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Architectural gear ratio
The architectural gear ratio (AGR) of a muscle is the ratio of the velocity of muscle fiber shortening to the velocity of the muscle-tendon unit shortening. This ratio is determined by the muscle's architecture, particularly the arrangement of muscle fibers and the angle at which they connect to the associated tendon, known as the pennation angle. Muscles are made of fibers arranged in specific patterns: parallel or pennate. In parallel muscles, fibers run parallel to one another and insert directly into their tendon, as seen in the biceps brachii. On the other hand, pennate muscles attach to their tendon at an angle, or obliquely. The quadriceps, gastrocnemius, and deltoids are examples of pennate muscles.
The pennation angle is pivotal in determining a muscle's AGR. A larger pennation angle allows more fibers to be packed into a muscle, resulting in a higher AGR and velocity amplification. In pennate muscles, segments with higher pennation angles produce less force per shortening muscle fiber. High AGR muscles with high pennation angles are more forceful but slower, while low AGR muscles are faster but produce less force. The AGR plays a crucial role in understanding muscle injuries and their ability to generate force and contraction speed, which are vital for efficient movement and stability.
Empirical studies have shown that a muscle's AGR varies from contraction to contraction. The AGR is highest during lengthening contractions, indicating that lengthening can occur with relatively little stretch of the muscle. This suggests that a high gear ratio may protect pennate muscles against the damaging effects of active lengthening. During low-force contractions, muscles operate at a high gear, while during high-force contractions, muscles operate at a low gear. This variable gearing has a significant impact on muscle performance, favoring muscle speed during fast contractions and muscle force during slow, high-force contractions.
The architectural gear ratio in segmented musculature, such as pennate muscles, increases when muscle fibers increase in angle with respect to the medial axis, along with the direction and amount of muscle bulging. This phenomenon is known as the spatially varying gear ratio, providing new insights into muscle biology, specifically "inhomogenous muscle mechanics." The model proposed by Emanuel Azizi, known as the segmented muscle model, demonstrates that the architectural gear ratio increases with muscle bulging. The model also shows that as the muscle bulges more in dorsoventral height, the muscle fibers shorten further, resulting in a higher architectural gear ratio.
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How pennate muscles contract
A pennate muscle is a type of skeletal muscle that resembles the shape of a feather. It has shorter fibres than a fusiform muscle and inserts on its tendons in various ways, forming unipennate, bipennate, and multipennate muscles. The classification of a pennate muscle depends on the location of the fascicles in relation to the tendon. In a pennate muscle, the fascicles attach obliquely (in a slanting position) to its tendon, forming an angle known as the pennation angle. This angle increases when the muscle contracts and shortens.
The architectural gear ratio (AGR) is a feature unique to pennate muscles. It is defined by the ratio between the longitudinal strain of the muscle and muscle fibre strain or the ratio between muscle-shortening velocity and fibre-shortening velocity. During contraction, the fibres in a pennate muscle rotate to greater angles of pennation, resulting in a velocity along its line of action that can exceed the velocity of the contracting fibres. This velocity amplification is another characteristic of the AGR.
The physiological cross-sectional area (PCSA) of a pennate muscle is larger than its anatomical cross-sectional area (ACSA). The total force exerted by the fibres along their oblique direction is proportional to the PCSA. However, only a component of that force can be used to pull the tendon in the desired direction, known as the true muscle force or tendon force. The remaining force is exerted orthogonally to the direction of action, squeezing the muscle by pulling its aponeuroses together.
The pennate structure allows for a greater number of muscle fibres, resulting in higher force production but a smaller range of motion compared to other muscle types. This makes pennate muscles well-suited for generating large forces to support or propel the weight of the body. Resistance training can increase the pennation angle, leading to an increase in single muscle fibre CSA and maximal contractile strength.
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Frequently asked questions
A pennate muscle is a type of skeletal muscle that resembles the shape of a feather. They are also called penniform or pinnate muscles.
The diagonal orientation of the fibres in pennate muscles maximises the muscle's force potential. This allows the muscle to produce more force. However, the angular arrangement of the fibres means that the range of motion is smaller.
Examples of pennate muscles include the rectus femoris and the gastrocnemius. The quads (quadriceps femoris) are also pennate muscles.
