Alec Titan
The human body is a complex system of muscles, bones, and tendons that work together to enable movement. One of the key factors influencing a muscle's function is its shape, and muscles can be categorised into four groups based on their anatomical arrangement: circular, parallel, convergent, and pennate. Parallel muscles are characterised by fascicles that run parallel to one another, and they are the most common type of muscle architecture in the body. Some parallel muscles are flat sheets that expand at the ends to make broad attachments, such as the sartorius, while others have a spindle shape with a wider middle and tapered ends, known as fusiform muscles, including the biceps brachii. Flat, wide muscles with wide, flat tendons called aponeuroses are typical of abdominal wall muscles. The shape and arrangement of these muscles allow for a wide range of motion, although they are not very strong.
| Characteristics | Values |
|---|---|
| Muscle architecture | Parallel |
| Muscle fiber arrangement | Fibers are arranged parallel to the longitudinal axis of the muscle |
| Types | Strap, Fusiform, Fan-shaped, Quadrilateral |
| Function | Used for fast or extensive movements |
| Force | Less force produced by parallel muscles than pennate muscles |
| Range of motion | Greater range of motion than muscles with a different fiber arrangement but the same cross-sectional area |
| Joint velocity | Greater joint velocity than muscles with a different fiber arrangement but the same cross-sectional area |
| Examples | Sartorius, Sternocleidomastoid, Brachioradialis, Biceps Brachii, Sternohyoid, Psoas Major |
| Attachments | Broad attachments, can shorten to about 40-60% of their resting length |
| Tendons | Flat, broad tendons (aponeuroses) |
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What You'll Learn
- Quadrilateral muscles are short and flat, e.g. thyrohyoid muscle
- Strap muscles are narrow and belt-like, e.g. sartorius muscle
- Fusiform muscles are spindle-shaped, e.g. biceps brachii muscle
- Flat muscles with wide, flat tendons, e.g. abdominal wall muscles
- Flat muscles have sheets of connective tissue, called aponeuroses
Quadrilateral muscles are short and flat, e.g. thyrohyoid muscle
Skeletal muscles can be categorised into four groups based on their anatomical arrangement: parallel, convergent, pennate, or circular. Parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle. These muscles may narrow to a tendon at each end, forming a fusiform muscle, or they may form wide, flat muscles with wide, flat tendons (aponeuroses) typical of abdominal wall muscles.
One example of a flat, quadrilateral muscle is the thyrohyoid muscle, which is found in the anterior triangle of the neck. It is one of four members of the group of muscles known as the infrahyoid muscles, which also includes the sternohyoid, sternothyroid, and omohyoid muscles. The thyrohyoid muscle is the only infrahyoid muscle that is not innervated by the ansa cervicalis. Instead, nerve fibres from the anterior rami of the first cervical spinal nerve (C1) reach the muscle via the hypoglossal nerve (CN XII). The muscle is provided with arterial blood by branches of the superior thyroid artery and of the lingual artery.
The thyrohyoid muscle has two major functions. Firstly, it acts in conjunction with the other infrahyoid muscles to depress the hyoid bone, which is helpful after swallowing. Secondly, when the hyoid bone is stabilized by the suprahyoid muscles, the thyrohyoid elevates the larynx. This feature is important for vocalists who try to hit high notes. By controlling the position and shape of the larynx, the thyrohyoid muscle aids in making sound.
Other examples of flat, parallel muscles include the sartorius and biceps brachii muscles.
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Strap muscles are narrow and belt-like, e.g. sartorius muscle
Strap muscles, also known as fusiform muscles, are characterised by a parallel-fibre arrangement. This means that the muscle fibres are arranged parallel to the longitudinal axis of the muscle. They are narrow and belt-like, and can either narrow to a tendon at each end, forming a spindle shape, or form wide, flat muscles with wide, flat tendons. An example of a strap muscle is the sartorius muscle.
The sartorius muscle is the longest muscle in the human body, spanning the hip and knee joints. It is a long, thin, superficial muscle that runs down the length of the thigh in the anterior compartment. It is innervated by the femoral nerve, which receives its nerve supply from L2, L3, and L4 nerve roots. The femoral nerve innervates both the hip flexor and quadriceps muscle groups. The sartorius muscle originates from the anterior superior iliac spine, and part of the notch between the anterior superior iliac spine and anterior inferior iliac spine. It runs obliquely across the upper and anterior part of the thigh in an inferomedial direction. It passes behind the medial condyle of the femur to end in a tendon. This tendon curves anteriorly to join the tendons of the gracilis and semitendinosus muscles in the pes anserinus, where it inserts into the superomedial surface of the tibia. Its upper portion forms the lateral border of the femoral triangle, and the point where it crosses adductor longus marks the apex of the triangle.
The sartorius muscle can move the hip joint and the knee joint, although its actions are weak, making it a synergist muscle. At the hip, it can flex, weakly abduct, and laterally rotate the femur. At the knee, it can flex the leg, and when the knee is flexed, the sartorius medially rotates the leg. Sitting cross-legged demonstrates all four actions of the sartorius. The muscle may be absent in some people.
The name sartorius comes from the Latin word 'sartor', meaning patcher or tailor. This is likely in reference to the cross-legged position in which tailors once sat. Antique sewing machines required continuous crossbody pedalling, which resulted in tailors developing particularly strong sartorius muscles.
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Fusiform muscles are spindle-shaped, e.g. biceps brachii muscle
The shape of a muscle is an indicator of its specific action. For example, long, strap-like muscles typically provide large ranges of motion, whereas thick, short muscles typically provide large forces. Most muscles are one of four basic shapes: fusiform, triangular, rhomboidal, and pennate. However, muscles can also be rectangular, circular, radiate, and rhomboid in shape.
Fusiform muscles, also known as strap muscles, are spindle-shaped, with a wide center that tapers down to narrow ends. The muscle fibers in a fusiform muscle are arranged in a parallel fashion and run the entire length of the muscle. Fusiform muscles are generally associated with a greater range of motion and joint velocity compared to muscles with a different fiber arrangement but the same cross-sectional area. The tendons that attach fusiform muscles to bones are restricted to the ends of the muscle. The thickest part of the muscle is usually near its middle.
Examples of fusiform muscles include the sartorius, biceps brachii, and sternohyoid muscles. The biceps brachii is a muscle in the upper arm, and it is responsible for flexing the elbow and rotating the forearm. It has two origins and one insertion. The two heads of the biceps brachii originate on the scapula and join to form a single muscle belly that inserts on the radius bone in the forearm. The biceps brachii is a fusiform muscle because its fibers are arranged in a parallel fashion and run the entire length of the muscle, with tendons at the proximal and distal ends.
Another example of a fusiform muscle is the gastrocnemius, which is a muscle in the back of the lower leg that is responsible for plantar flexion of the ankle and flexion of the knee. The gastrocnemius is also a fusiform muscle because its fibers are arranged in a parallel fashion and run the entire length of the muscle, with tendons at the proximal and distal ends.
In summary, fusiform muscles are spindle-shaped muscles with a parallel-fiber arrangement that enables a greater range of motion and joint velocity compared to muscles with different fiber arrangements. Examples of fusiform muscles include the biceps brachii and the gastrocnemius.
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Flat muscles with wide, flat tendons, e.g. abdominal wall muscles
The abdominal wall is made up of five paired muscles and their respective aponeuroses, or flat, broad tendons. These muscles can be divided into two groups: flat muscles and vertical muscles. There are three flat muscles, situated laterally on either side of the abdomen, and two vertical muscles, situated near the midline of the body.
The three flat muscles are the external oblique, the internal oblique, and the transversus abdominis. The external oblique is the largest and most superficial flat muscle in the abdominal wall, with fibres that run inferomedially. The internal oblique is smaller and thinner, with fibres that run superomedially (perpendicular to the fibres of the external oblique). The transversus abdominis is the deepest of the flat muscles, with transversely running fibres.
The flat muscles of the abdominal wall help to stabilise the trunk and maintain internal abdominal pressure. They also assist in compressing the abdominal viscera and support processes such as defecation, micturition, and exhalation.
The abdominal muscles are susceptible to various conditions, such as strains, hernias, and prune belly syndrome. Strains can occur due to overstretching, overuse, or violent twisting of the trunk. Hernias can develop when an internal organ pushes through a weakened abdominal muscle, such as in the case of an inguinal hernia. Prune belly syndrome is a rare condition characterised by absent or weak abdominal wall muscles at birth, resulting in a wrinkled abdomen.
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Flat muscles have sheets of connective tissue, called aponeuroses
The human body is a complex system of muscles, bones, tendons, and connective tissues, all working in harmony to enable movement and stability. One such type of connective tissue is called aponeurosis, which plays a crucial role in muscle function and posture.
Aponeuroses are flat sheets or ribbons of tendon-like material that connect muscles with the parts they move. They are composed of dense fibrous connective tissue, containing fibroblasts and bundles of collagenous fibres. This tissue is similar to that found in tendons, which connect muscles to bones, but aponeuroses have a distinct structure and function. While tendons stretch and contract during muscle movements, aponeuroses remain stable and provide strength and rigidity to the body.
These flat sheets of connective tissue are found throughout the body and are integral to movement and posture. For example, the aponeurosis of the external oblique muscle helps the oblique muscles twist the trunk from side to side and facilitates the movement of the spine. Additionally, the plantar fascia, or aponeurosis, provides support and stability to the foot.
Aponeuroses are particularly important in flat, wide muscles, such as abdominal wall muscles. These muscles have a unique structure, with fascicles arranged in parallel and forming wide, flat tendons called aponeuroses. This design allows for a greater range of motion and joint velocity compared to muscles with different fibre arrangements. Parallel muscles, including flat muscles, are the most abundant and typical muscle type in the human body.
In summary, flat muscles have sheets of connective tissue called aponeuroses, which are essential for binding muscles to bones or to the fascia of other muscles. Aponeuroses provide strength, stability, and support to the body, enabling fluid movement and maintaining correct posture.
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Frequently asked questions
Flat parallel muscles are those with fascicles that run parallel to each other and are sometimes called strap muscles. They are normally long muscles that cause large movements and have good endurance.
Examples of flat parallel muscles include the sartorius, sternocleidomastoid, and thyrohyoid muscles.
Flat parallel muscles are characterised by their fascicles running parallel to one another and having a common point of attachment. They are typically long and cause large movements but are not very strong.
Flat parallel muscles differ from convergent or triangular muscles, which have a broad origin with fascicles converging towards a single tendon of insertion. They also differ from pennate muscles, which have fascicles attaching at an angle to a central tendon.
Flat parallel muscles are often used for fast or extensive movements. They have a wide range of mobility and are typically involved in producing large ranges of motion.
