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The architectural design of muscle: What factors determine its mechanical function?
In our daily lives, muscles play a vital role. Whether walking, running, or performing various sports, they all rely on muscle contraction and relaxation. The concept of "architectural design of muscles" is the key to our understanding of how muscles work.
Muscle building types
The structural layout of a muscle defines the mechanical function of the muscle at a macroscopic level and depends on how the fibers are arranged. At present, the main types of muscle building known are parallel type, feather type and water slide type.
Force generation and mechanical ratios will vary based on changes in different muscle parameters such as muscle length, fiber length, pennation angle, and physiological cross-sectional area (PCSA).
Parallel muscles
Parallel muscles refer to muscles whose fibers are parallel to the force-producing axis. They are usually suitable for rapid or large-range movements, and their anatomical cross-sectional area (ACSA) is often used for measurement.
Girdle muscles
Strand muscles are shaped like a belt, with fibers arranged longitudinally along the direction of contraction. For example: The sartorius, the longest muscle in humans, not only has a unique shape, but also plays an important role in speech and singing.
Spindle muscles
These muscles are wider in the middle and gradually narrower on the sides. They are shaped like a spindle, like the biceps, which is this type of muscle.
Convergence muscles
A convergent muscle (or deltoid) whose fibers are fused at one end (usually on a tendon) and fan out at the other end. The human pectoralis major muscle belongs to this type.
Pennate muscles
In pennate muscles, the fibers are distributed at an angle to the force-generating axis, which often results in changes in the force transmission pattern relative to physiological cross-sectional area (PCSA).
Unipennate muscles
The fibers of a unipennate muscle are connected to a tendon on one side, such as the lateral gastrocnemius muscle.
Bipennate muscles
The bipennate muscles have fibers on both sides of the tendon, such as the human rectus femoris.
Pennate muscles
Pennate muscles such as the deltoid muscle have fibers connected to the force-producing axis from different angles.
Water sliding muscles
Water-sliding muscles operate independently of a hard skeleton. They are internally supported by connective tissue membranes to maintain a constant volume, thereby supporting the stability of the entire muscle structure.
Generation of force
The architectural design of muscles directly affects force production, which is related to muscle volume, fiber length, fiber type and pennation angle.
In muscles, physiological cross-sectional area (PCSA) is the most accurate measure of force generation and is primarily affected by pennation angle.
Fiber length is also a key variable in muscle anatomy. Fiber length is determined by the individual myofibrils in the fiber and their individual lengths. As fibers change length, individual myofibers shorten or lengthen, but the total number remains the same.
Feathered horns
The pennation angle is the angle between the longitudinal axis of the entire muscle and the fibers. In muscle fibers, as tension increases, pennation angle also increases.
Building gear ratio
Architectural Gear Ratio (AGR) involves the relationship between the contraction speed of an entire muscle and the contraction speed of individual muscle fibers. AGR is determined by the mechanical demands of the muscles during exercise.
Variations in pennation angle help achieve varying gear ratios in the pennation muscle, which also affects the muscle geometry.
High gear ratios will result in low force, high speed contractions of the entire muscle, while low gear ratios are associated with high force, low speed contractions.
