Pennate muscle
Encyclopedia
A pennate or pinnate muscle (also called a penniform muscle) is a muscle
with fascicles that attach obliquely (in a slanting position) to its tendon. These types of muscles generally allow higher force production but smaller range of motion
When a muscle contracts and shortens, the pennation angle increases
and these bundles of fasciculi are formed into bundles of fascicles
which compose the entire muscle itself. In a pennate muscle, aponeuroses run along each side of the muscle and attach to the tendon. The fascicles attach to the aponeurose and form an angle (the pennation angle) to the load axis of the muscle. If all the fascicles are on the same side of the tendon, the pennate muscle is called unipennate (Fig. 1A). Examples of this include certain muscles in the hand
. If there are fascicles on both sides of the central tendon, the pennate muscle is called bipennate (Fig. 1B). The rectus femoris, a large muscle in the quadriceps, is typical. If the central tendon branches within a pennate muscle, the muscle is called multipennate (Fig. 1C), as seen in the deltoid muscle
in the shoulder
.
The muscle cross sectional area (blue line in figure 1, also known as anatomical cross section area, or ACSA) does not accurately represent the number of muscle fibers in the muscle. A better estimate is provided by the total area of the crossections perpendicular to the muscle fibers (green lines in figure 1). This measure is known as the physiological cross sectional area (PCSA), and is commonly calculated and defined by the following formula (an alternative definition is provided in the main article
):
PCSA increases with pennation angle, and with muscle length. In a pennate muscle, PCSA is always larger than ACSA. In a non-pennate muscle, it coincides with ACSA.
However, only a component of that force can be used to pull the tendon in the desired direction. This component, which is the true muscle force (also called tendon force), is exerted along the direction of action of the muscle:
The other component, orthogonal to the direction of action of the muscle (Orthogonal force = Total force × sinΦ) is not exerted on the tendon, but simply squeezes the muscle, by pulling its aponeuroses toward each other.
Notice that, although it is practically convenient to compute PCSA based on volume or mass and fiber length, PCSA (and therefore the total fiber force, which is proportional to PCSA) is not proportional to muscle mass or fiber length alone. Namely, the maximum (tetanic
) force of a muscle fiber simply depends on its thickness (cross-section area) and type. By no means it depends on its mass or length alone. For instance, when muscle mass increases due to physical development
during childhood, this may be only due to an increase in length of the muscle fibers, with no change in fiber thickness (PCSA) or fiber type. In this case, an increase in mass does not produce an increase in force.
The speed at which a muscle fiber can shorten is partly determined by the length of the muscle fiber (i.e., by N). Thus, a muscle with a large pennation angle will contract more slowly than a similar muscle with a smaller pennation angle.
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
with fascicles that attach obliquely (in a slanting position) to its tendon. These types of muscles generally allow higher force production but smaller range of motion
When a muscle contracts and shortens, the pennation angle increases
Types of pennate muscle
Individual muscle fibers are formed into bundles of fasciculiMuscle fascicle
In anatomy, a fascicle is a bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue.Specialized muscle fibers in the heart that transmit electrical impulses from the Atrioventricular Node to the Purkinje Fibers are fascicles, also referred to as bundle branches...
and these bundles of fasciculi are formed into bundles of fascicles
Muscle fascicle
In anatomy, a fascicle is a bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue.Specialized muscle fibers in the heart that transmit electrical impulses from the Atrioventricular Node to the Purkinje Fibers are fascicles, also referred to as bundle branches...
which compose the entire muscle itself. In a pennate muscle, aponeuroses run along each side of the muscle and attach to the tendon. The fascicles attach to the aponeurose and form an angle (the pennation angle) to the load axis of the muscle. If all the fascicles are on the same side of the tendon, the pennate muscle is called unipennate (Fig. 1A). Examples of this include certain muscles in the hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
. If there are fascicles on both sides of the central tendon, the pennate muscle is called bipennate (Fig. 1B). The rectus femoris, a large muscle in the quadriceps, is typical. If the central tendon branches within a pennate muscle, the muscle is called multipennate (Fig. 1C), as seen in the deltoid muscle
Deltoid muscle
In human anatomy, the deltoid muscle is the muscle forming the rounded contour of the shoulder. Anatomically, it appears to be made up of three distinct sets of fibers though electromyography suggests that it consists of at least seven groups that can be independently coordinated by the central...
in the shoulder
Shoulder
The human shoulder is made up of three bones: the clavicle , the scapula , and the humerus as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder make up the shoulder joints. The major joint of the shoulder is the glenohumeral joint, which...
.
Physiological cross sectional area (PCSA)
One advantage of pennate muscles is that more muscle fibers can be packed in parallel, thus allowing the muscle to produce more force, although the fiber angle to the direction of action means that the maximum force in that direction is somewhat less than the maximum force in the fiber direction.The muscle cross sectional area (blue line in figure 1, also known as anatomical cross section area, or ACSA) does not accurately represent the number of muscle fibers in the muscle. A better estimate is provided by the total area of the crossections perpendicular to the muscle fibers (green lines in figure 1). This measure is known as the physiological cross sectional area (PCSA), and is commonly calculated and defined by the following formula (an alternative definition is provided in the main article
Physiological cross sectional area
In pennate muscles, the physiological cross-sectional area , as opposed to the anatomical one , is the total area of the cross-sections perpendicular to the muscle fibers.- Definition :...
):
PCSA increases with pennation angle, and with muscle length. In a pennate muscle, PCSA is always larger than ACSA. In a non-pennate muscle, it coincides with ACSA.
Relationship between PCSA and muscle force
The total force exerted by the fibers along their oblique direction is proportional to PCSA. If the specific tension of the muscle fibers is known (force exerted by the fibers per unit of PCSA), it can be computed as follows:However, only a component of that force can be used to pull the tendon in the desired direction. This component, which is the true muscle force (also called tendon force), is exerted along the direction of action of the muscle:
The other component, orthogonal to the direction of action of the muscle (Orthogonal force = Total force × sinΦ) is not exerted on the tendon, but simply squeezes the muscle, by pulling its aponeuroses toward each other.
Notice that, although it is practically convenient to compute PCSA based on volume or mass and fiber length, PCSA (and therefore the total fiber force, which is proportional to PCSA) is not proportional to muscle mass or fiber length alone. Namely, the maximum (tetanic
Tetanic contraction
A tetanic contraction occurs when a motor unit has been maximally stimulated by its motor neuron. This occurs when a muscle's motor unit is stimulated at a sufficiently high frequency of multiple impulses. Each stimulus causes a twitch. If stimuli are delivered slow enough, the tension in the...
) force of a muscle fiber simply depends on its thickness (cross-section area) and type. By no means it depends on its mass or length alone. For instance, when muscle mass increases due to physical development
Human development (biology)
Human development is the process of growing to maturity. In biological terms, this entails growth from a one-celled zygote to an adult human being.- Biological development:...
during childhood, this may be only due to an increase in length of the muscle fibers, with no change in fiber thickness (PCSA) or fiber type. In this case, an increase in mass does not produce an increase in force.
Lower velocity of shortening
In a pennate muscle, as a consequence of their arrangement, fibers are shorter than they would be if they ran from one end of the muscle to the other. This implies that each fiber is composed of a smaller number N of sarcomeres in series. Moreover, the larger is the pennation angle, the shorter are the fibers.The speed at which a muscle fiber can shorten is partly determined by the length of the muscle fiber (i.e., by N). Thus, a muscle with a large pennation angle will contract more slowly than a similar muscle with a smaller pennation angle.