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Muscle shape and strength: Why do some muscles produce more force
Muscle architecture refers to the physical arrangement of muscle fibers at a macroscopic level, which directly affects the mechanical function of the muscle. The architectural definition of muscles is usually divided into three types: parallel muscles, pennate muscles, and muscular hydrops. At the same time, different muscle parameters such as muscle length, fiber length, pennation angle and physiological cross-sectional area (PCSA) also affect the generation and transmission of force.
Architecture Types
Parallel muscles and pennate muscles are the two main types, while muscular hydrops can be considered a third subtype. Muscle architecture type is determined by the orientation of muscle fibers toward the axis of force generation. Depending on the muscle architecture, the force generated by a muscle is proportional to its cross-sectional area, meaning that the larger the cross-sectional area, the greater the force generated.
Parallel muscles
Parallel muscles are muscles whose fibers are aligned along the axis of force generation, are typically used for fast or wide range of motion, and can be measured by anatomical cross-sectional area (ACSA). Parallel muscles can be further divided into three major categories: strap muscles, spindle muscles, and fan muscles.
Strap muscles
Strap muscles are shaped like ribbons, with the fibers arranged longitudinally along the direction of contraction. These muscles have wider attachments than other muscle types and can be shortened to approximately 40% to 60% of their resting length. Strap muscles, such as the laryngeal muscles, play an important role in speech and singing.
Spindle muscle
Spindle muscles are cylindrical in the center and taper at the ends. This type of linear action is straight between the attachment points. Because of their shape, the force generated by spindle muscles is concentrated over a small area. The human biceps is an example of this type.
Convergence muscle
Convergent muscles are triangular muscles whose fibers converge on one side (usually a tendon) and fan out on the other side. The human pectoralis major is an example of a converging muscle, which has a weaker pulling force than other parallel muscle fibers but has the characteristic of changing the pulling direction.
Pendulous muscle
Unlike parallel muscles, pennate muscle fibers are aligned at an angle to the axis of force generation (the pennate angle) and usually insert into a central tendon. This structure allows the pennate muscles to have a relatively large number of fibers, thereby generating greater force. Pennate muscles can be divided into single-pinnate, double-pinnate and multiple-pinnate.
Single Feather
The fibers of a simplex muscle are arranged at a single angle on one side of the tendon, such as the lateral gastrocnemius.
Double Feathers
The fibers of biparietal muscles are located on both sides of the tendon, such as the ossicularis and rectus femoris in humans.
Multiple Feathers
In multi-feathered muscles, such as the human deltoid, the fibers are arranged at multiple angles along the force-generating axis.
Water-like muscles
Hydrogenous muscles operate independently of the rigid skeletal system and are usually supported by connective tissue, presenting a stable volume. Muscle fibers contract along three general lines of action: parallel, perpendicular, and spiral. These contractions allow the hydrocephalus to perform a variety of complex movements.
Power Generation
Muscle architecture directly affects force production, with force directly correlated to cross-sectional area through the variables of muscle volume, fiber length, fiber type, and pennation angle. Physiological cross-sectional area (PCSA) describes muscle force production more accurately than anatomical cross-sectional area (CSA).
Different muscle fiber types also affect the generation of power. Type I, IIa and IIb fibers each have their own unique characteristics and ways of generating force.
Pendulum angle and force relationship
The pennation angle is related to the contraction speed of the entire muscle and the contraction speed of a single fiber. By changing the pennation angle, we can obtain variable force generation capabilities under different exercise conditions, which allows different muscle designs to adapt to different exercise needs.
Architecture Gear Ratio
The architectural gear ratio (AGR) compares the contraction speed of the entire muscle to the contraction speed of individual fibers. Adjusting the pennation angle can lead to changes in the speed and force generation efficiency of the pennation muscle, thereby affecting the overall athletic performance of the muscle.
In our daily lives, how can we use the characteristics of these muscles to enhance athletic performance and become the key to fitness training?
