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The strength of a muscle is determined by its architecture, which is the physical arrangement of muscle fibres at the macroscopic level. There are several different muscle architectures, including parallel, pennate, and hydrostats. The force produced by a given muscle is proportional to the cross-sectional area, or the number of parallel sarcomeres present. In a pennate muscle, the complex arrangement of connective tissue, tendons, and relatively
| Characteristics | Values |
|---|---|
| Muscle shape | Pennate muscles resemble the shape of a feather |
| Muscle fibres | More muscle fibres can be packed in parallel |
| Muscle force | Pennate muscles can generate higher maximum forces for a given muscle volume |
| Muscle volume | Muscle volume is determined by the cross-sectional area |
| Muscle length | PCSA increases with muscle length |
| Muscle fibre length | The larger the pennation angle, the shorter the muscle fibres |
| Muscle action | The number of sarcomeres contributing to muscle action is higher in pennate muscles |
| Muscle contraction | The pennation angle increases when a pennate muscle contracts and shortens |
| Muscle architecture | The pennation angle is the angle of the muscle fibres to the force-generating axis |
| Muscle fibre angle | The fibre angle to the direction of action means that the maximum force in that direction is less than the maximum force in the fibre direction |
| Muscle fibre type | Muscle fibre type does not change with training |
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What You'll Learn
- The diagonal orientation of muscle fibres maximises force potential
- The pennation angle increases with muscle contraction
- The muscle's cross-sectional area does not accurately represent the number of muscle fibres
- The muscle fibre pennation angle increases with resistance training
- The muscle fibre arrangement in pennate muscles allows for more sarcomeres in parallel
The diagonal orientation of muscle fibres maximises force potential
The structure of a muscle is a good indicator of its specific action. Pennate muscles, also known as penniform muscles, are a type of skeletal muscle with fibres that attach obliquely to a tendon. The name 'pennate' comes from the Latin 'pinnatus', meaning feathered or winged. The muscle fibres are oriented at an angle to the muscle's axis of force transmission, or line of action. This angle is known as the pennation angle.
The pennation angle also affects the speed of muscle contraction. When a muscle contracts and shortens, the pennation angle increases. This allows for more sarcomeres to be in parallel, which increases the total fibre cross-sectional area or physiological cross-sectional area. A larger cross-sectional area means that more acto-myosin cross-bridges can be activated in parallel during contraction, resulting in a higher maximum force. However, the diagonal orientation of muscle fibres also means that the maximum force in the direction of the muscle action is somewhat less than the maximum force in the fibre direction. This is because only the component of fibre force that is in line with the muscle axis of force transmission will contribute to muscle force.
The force produced by pennate muscles is greater than that produced by parallel muscles. This is because the pennation angle results in a lateral, as well as longitudinal component to the shortening process. This means that the overall excursion of the muscle is less, but the contraction generates more whole muscle force. This is due to the larger physiological cross-sectional area of pennate muscles, which is measured perpendicular to the axis of the muscle fibres, not to the axis of the whole muscle.
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The pennation angle increases with muscle contraction
The pennation angle, or the angle formed between the muscle fascicles and the load axis of the muscle, increases with muscle contraction. This is because, during contraction, the muscle fibres bulge radially and shorten, which increases the pennation angle. This radial bulging also results in forces that are exerted off-axis from the muscle's line of action, loading the connective tissues that surround the muscle fibres, fascicles, and the entire muscle.
The larger the pennation angle, the shorter the muscle fibres. This is because the fibres do not run the full length of the muscle in a pennate muscle. Instead, they are relatively short and are packed in parallel, allowing for a greater number of fibres within the muscle. This results in a larger cross-sectional area, or physiological cross-sectional area (PCSA), which is a better estimate of the number of muscle fibres than the anatomical cross-sectional area (ACSA). The PCSA increases with the pennation angle and muscle length, and the total force exerted by the fibres is proportional to the PCSA.
The pennate muscle structure is found in muscles that are required to produce large forces to support or propel the body weight, such as the rectus femoris and the gastrocnemius. These muscles have a large force-generating capacity due to their larger PCSA, which is proportional to the maximal force-generating capacity of a given muscle. Therefore, the larger pennation angle results in a greater force-generating capacity in pennate muscles.
The pennation angle also affects the speed of muscle contraction. A larger pennation angle results in slower contractile velocity since the muscle fibres are shorter and have fewer sarcomeres in series. However, the pennate arrangement of fibres allows for more sarcomeres in parallel, which results in a higher maximum force for a given muscle volume. Therefore, the pennate arrangement of fibres favours force over speed.
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The muscle's cross-sectional area does not accurately represent the number of muscle fibres
The muscle cross-sectional area, or anatomical cross-sectional area (ACSA), does not accurately represent the number of muscle fibres in a pennate muscle. This is because the fibres in a pennate muscle are oriented at an angle to the force-generating axis, or load axis, of the muscle, forming a pennation angle. This angle allows for more muscle fibres to be packed in parallel, resulting in a larger cross-sectional area than fusiform or parallel-fibered muscles. However, the ACSA does not account for this oblique orientation of the fibres.
The ACSA assumes that the fibres are parallel to the muscle's long axis, which is true for fusiform muscles, but not for pennate muscles. In a pennate muscle, the fibres are shorter and are arranged at an angle, resulting in a complex arrangement of connective tissue, tendons, and relatively short fibres. This unique structure creates a larger cross-sectional area than fusiform fibres because more sarcomeres, or contractile units, can pack into a given volume of muscle.
To accurately represent the number of muscle fibres in a pennate muscle, the physiological cross-sectional area (PCSA) is used. The PCSA takes into account the pennation angle and is calculated as the muscle volume divided by the fibre length. By considering the muscle volume and fibre length, the PCSA provides a better estimate of the true cross-sectional area perpendicular to the muscle fibres. This measurement is crucial in understanding the force-generating capacity of a pennate muscle, as the force produced by a muscle is proportional to the cross-sectional area and the number of parallel sarcomeres present.
The PCSA increases with the pennation angle and muscle length. As the pennation angle increases, the fibres become shorter, allowing for a greater number of fibres to be packed in parallel. This results in a higher force production by the muscle. However, it is important to note that the maximum force in the direction of the muscle's action is slightly less than the maximum force in the fibre direction due to the fibre angle.
In summary, while the muscle cross-sectional area or ACSA may provide information about the muscle's size, it does not accurately reflect the number of muscle fibres in a pennate muscle due to its unique structure and fibre orientation. The PCSA, which considers the pennation angle and muscle volume, provides a more accurate representation of the cross-sectional area and the force-generating capacity of pennate muscles.
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The muscle fibre pennation angle increases with resistance training
The pennation angle of a muscle refers to the angle formed by the fascicles (bundles of muscle fibres) and the load axis of the muscle. In other words, it is the angle at which the muscle fibres are attached to the tendon. This angle is important because it affects the muscle's force-generating capacity.
When a muscle contracts and shortens, the pennation angle increases. The larger the pennation angle, the shorter the muscle fibres, and the greater the muscle's force-generating capacity. This is because the total force exerted by the fibres is proportional to the physiological cross-sectional area (PCSA), which increases with the pennation angle.
Studies have shown that resistance training can increase the pennation angle of human pennate muscles, leading to an increase in single muscle fibre cross-sectional area (CSAfibre) and maximal contractile strength. This is because the morphology, architecture, and contractile capacity of human pennate muscles are interrelated. Resistance training causes specific adaptation responses that result in an increase in the pennation angle, leading to a greater PCSA and, consequently, a higher force-generating capacity.
However, it is important to note that the relationship between muscle morphology and function is complex, and not all studies have found a direct correlation between CSAfibre and anatomical muscle CSA or volume. Additionally, the increase in pennation angle and its effects on muscle strength may vary depending on the specific muscle and training regimen.
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The muscle fibre arrangement in pennate muscles allows for more sarcomeres in parallel
The arrangement of muscle fibres in pennate muscles allows for more sarcomeres in parallel. This is because the fibres are oriented at an angle to the muscle's axis of force transmission, or load axis, rather than being parallel to it. This is known as the pennation angle.
The pennation angle means that the fibres in a pennate muscle are shorter than they would be if they ran from one end of the muscle to the other. This allows for a greater number of fibres to fit within the muscle, and therefore a greater number of sarcomeres in parallel.
The pennation angle also increases the muscle's cross-sectional area, or physiological cross-sectional area (PCSA). This is the area of the cross-sections perpendicular to the muscle fibres. The PCSA is larger than the anatomical cross-sectional area (ACSA) in a pennate muscle, and it is this measure that represents the number of muscle fibres in the muscle.
The larger PCSA means that more sarcomeres can be activated in parallel during contraction, and so the muscle can generate a higher maximum force for a given muscle volume. This is why pennate muscles are stronger than parallel-fibre muscles, despite having a slower contractile velocity.
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Frequently asked questions
The force produced by a given muscle is proportional to the cross-sectional area, or the number of parallel sarcomeres present. The pennate arrangement of muscle fibres allows for more sarcomeres in parallel, thus allowing the muscle to produce more force.
A pennate 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 "pinnatus" meaning "feathered" or "winged".
Examples of pennate muscles include the rectus femoris, gastrocnemius, and the deltoid muscle.
The pennation angle is the angle formed by the fascicles to the load axis of the muscle. As the pennation angle increases, the muscle fibre length decreases, allowing for more muscle fibres to be present in a given muscle. This results in increased muscle strength.
The force generated by a pennate muscle is influenced by muscle volume, with greater muscle volume resulting in increased muscle strength. This relationship is due to the ability of pennate muscles to pack more sarcomeres into a given volume.
