The value of muscle: classification, function and muscle physiology

**Show off your muscles: The classification and value of muscles in the body**

Muscles are an essential component of the body, the foundation of bodily activity. The cooperation of the body's 600 muscles helps us defy gravity, enabling us to walk, jump, and even climb steep rocks. Muscles are like a magical engine, propelling us to perform bodily actions through pulling and stretching. Therefore, muscle fibers control every movement of our body, from the smallest eye movements that allow us to see clearly, to blinking, making eye contact, and smiling-all accomplished through the pulling action of muscles. Thousands upon thousands of tiny fibers aggregate to form muscle bundles, thus forming a complete muscular system. Most muscles are skeletal muscles, connected to bones by tendons. These tightly bound tendon fibers function like rubber bands; the muscles in our hands and fingertips allow us to grasp extremely small objects. Muscles are the body's second heart; they are not only a symbol of health and strength but also a crucial factor in maintaining abundant energy and a good physical condition. Muscles are small in volume but high in mass, meaning they are dense and have firm, beautiful, elastic lines. Therefore, people with more muscle tend to weigh more than those with less muscle.

Muscles are primarily composed of muscle tissue. Muscle cells are long and slender, resembling fibers; therefore, they are commonly referred to as muscle fibers. There are several principles for naming muscles: First, by shape, such as trapezius and deltoid muscles; second, by location, such as supraspinatus, infraspinatus, anterior tibialis, and intercostal muscles; third, by origin and insertion, such as sternocleidomastoid and sternohyoid muscles; fourth, by location and size, such as pectoralis major, pectoralis minor, and psoas major; fifth, by function, such as supinator, adductor magnus, flexor, and extensor muscles; sixth, by structure, such as semitendinosus and semimembranosus muscles; seventh, by structure and location, such as biceps brachii and quadriceps femoris; and eighth, by location and fiber direction, such as external oblique and transverse abdominis muscles.

The main function of muscles is to contract and pull on bones, thereby driving joint movement. Based on the lever principle, there are three basic forms of movement: first, balancing lever movement, where the fulcrum is between the load and the force point, such as the head tilting and lowering movements performed by the atlanto-occipital joint; second, effort-saving lever movement, where the load is located between the fulcrum and the force point, such as the ankle joint movement when lifting the heel; and third, speed lever movement, where the force point is located between the load and the fulcrum, such as the elbow joint movement when lifting a heavy object.

Human muscles can be classified into three types based on their structure and function: smooth muscle, cardiac muscle, and skeletal muscle. Based on their shape, they can be further divided into long muscles, short muscles, broad muscles, and orbicularis oculi muscles. Head muscles can be divided into facial muscles (also called expression muscles) and masticatory muscles. Trunk muscles can be divided into back muscles, chest muscles, abdominal muscles, and diaphragm. Lower limb muscles are classified by location into hip muscles, thigh muscles, calf muscles, and foot muscles. Lower limb muscles are generally thicker than upper limb muscles, which is related to supporting body weight, maintaining upright posture, and walking. In addition, there are neck muscles, shoulder muscles, arm muscles, forearm muscles, and hand muscles, among others.

Smooth muscle mainly constitutes the internal organs and blood vessels, and is characterized by slow, sustained contraction and low fatigue. Cardiac muscle forms the heart wall; both contract involuntarily, hence they are also called involuntary muscles. Smooth muscle is found in the digestive system, blood vessels, bladder, respiratory tract, and the uterus in women. The ability of smooth muscle to maintain tension for extended periods, and the fact that it contracts involuntarily, means that the nervous system automatically controls it without conscious thought. For example, the muscles in the stomach and intestines perform their tasks daily, but people are generally unaware of them.

Skeletal muscles are distributed throughout the head, neck, trunk, and limbs, typically attaching to bones. They contract rapidly and powerfully, fatigue easily, and can contract and relax at will, hence the name voluntary muscles. The human body has over 600 skeletal muscles, widely distributed, accounting for approximately 40% of body weight. Each skeletal muscle, regardless of size, has a specific shape, structure, location, and accessory apparatus, and is richly innervated by blood vessels and lymphatic vessels. Under a microscope, skeletal muscles appear striated, hence the name striated muscles. Skeletal muscles are the power source of the musculoskeletal system; under the control of the nervous system, their contraction pulls on bones to produce movement. Each skeletal muscle can be considered an organ, a type of muscle that can be seen and felt. Skeletal muscles attach to bones and appear in pairs; one muscle moves the bone in one direction, and the other in the opposite direction. These muscles typically contract at will, meaning the nervous system issues a command to contract them. Skeletal muscles can perform brief, single contractions, such as the twitching we often experience, or prolonged, sustained contractions, such as in the case of tetanus. When people exercise, they are working on their skeletal muscles.

Cardiac muscle-exists only in the heart. Its greatest characteristics are endurance and strength. It can stretch to a limited extent like smooth muscle, or it can contract with the force of skeletal muscle. It is a twitching muscle that does not contract involuntarily.