Movement is a fundamental characteristic of living beings, ranging from the flow of protoplasm within a single cell to the complex activities of entire organisms. While all locomotion is movement, not all movement is locomotion. Locomotion refers specifically to the voluntary movement of an individual from one place to another. In the animal kingdom, the methods of locomotion are incredibly diverse, driven by the coordinated action of the muscular and skeletal systems.

The basis of movement in multicellular animals is the contractile property of muscular tissue. There are three primary types of muscle, distinguished by their structure, location, and function. Skeletal muscle, also known as striated or voluntary muscle, is attached to the skeletal framework and is responsible for body posture and all voluntary movements. Visceral muscle, or smooth muscle, is found in the inner walls of hollow internal organs like the alimentary canal and blood vessels; its contractions are involuntary. Cardiac muscle, found exclusively in the heart, is also striated but its contractions are involuntary, powering the rhythmic pumping of blood.

A closer look at the structure of a skeletal muscle reveals a highly organized arrangement. Each muscle is composed of numerous muscle fibers (cells), bundled together. Each muscle fiber contains a large number of parallelly arranged myofilaments within a cytoplasm called sarcoplasm. These myofilaments, or myofibrils, have characteristic light and dark bands, giving the muscle its striated appearance. These bands are formed by the specific arrangement of two main contractile proteins: a thin filament called actin and a thick filament called myosin. The functional unit of muscle contraction is the sarcomere, which is the region of a myofibril between two successive Z-lines.

The mechanism of muscle contraction is best explained by the Sliding Filament Theory. According to this theory, the contraction of a muscle fiber occurs when the thin actin filaments slide past the thick myosin filaments. This process is initiated by a signal from the central nervous system via a motor neuron. The release of a neurotransmitter at the neuromuscular junction triggers an action potential in the muscle fiber. This electrical impulse causes the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm. The calcium ions then bind to a subunit of troponin on the actin filaments, causing a conformational change that exposes the active sites for myosin to bind. Using energy derived from the hydrolysis of ATP, the myosin heads bind to these active sites on the actin, forming a cross-bridge. The myosin heads then pivot, pulling the actin filaments towards the center of the sarcomere, causing the muscle to shorten and contract. The cycle of cross-bridge formation and breaking continues as long as calcium and ATP are available.

For locomotion and movement to occur effectively, muscles need a framework to act upon. This is provided by the skeletal system, which serves as the primary supportive structure of the body, protects vital organs, and provides attachment points for muscles. The human skeleton is composed of 206 bones and is divided into two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton, forming the main axis of the body, consists of the skull, the vertebral column, the sternum, and the ribs. The appendicular skeleton is composed of the bones of the limbs (arms and legs) and their corresponding pectoral (shoulder) and pelvic (hip) girdles, which connect the limbs to the axial skeleton.

The interaction between bones is facilitated by joints, which are points of articulation. Joints are essential for all types of movement involving bony parts of the body. They are classified into three major structural types. Fibrous joints, such as the sutures of the skull, are immovable and do not allow any movement. Cartilaginous joints, found between adjacent vertebrae, allow for limited movement. Synovial joints are the most common type and are characterized by a fluid-filled synovial cavity between the articulating bones. These joints, such as the ball-and-socket joint of the shoulder or the hinge joint of the knee, allow for considerable movement and are crucial for locomotion.

The muscular and skeletal systems are susceptible to various disorders. Common ailments include myasthenia gravis, an autoimmune disorder affecting the neuromuscular junction; muscular dystrophy, a progressive genetic disorder causing muscle degeneration; arthritis, an inflammation of the joints; and osteoporosis, a condition characterized by decreased bone mass, leading to an increased risk of fractures.