Appendicular muscles are the skeletal muscles that move and stabilize the limbs. These muscles attach to the appendicular skeleton, which includes the pectoral girdle (shoulder), upper limbs, pelvic girdle (hip), and lower limbs. They are distinct from the axial muscles associated with the trunk, head, and neck. While axial muscles maintain posture and facilitate breathing, appendicular muscles enable movements such as walking, running, and manipulating objects. Axial muscles often stabilize the trunk, providing a firm base for the appendicular muscles to act upon.
Muscles of the Pectoral Girdle and Upper Limbs
The muscles of the pectoral girdle and upper limbs control the shoulder, arm, forearm, and hand. Muscles that position the pectoral girdle are located on the thorax. Anteriorly, the pectoralis minor and serratus anterior move the scapula (shoulder blade), with the serratus anterior pulling it forward around the rib cage. Posteriorly, the rhomboid major and minor move the scapula medially, pulling the shoulder backward.
Movement of the humerus, or the upper arm bone, is controlled by large muscles. The pectoralis major, a large fan-shaped muscle on the anterior thorax, and the latissimus dorsi on the back are two axial muscles that act on the humerus. The deltoid, a thick muscle giving the shoulder its rounded shape, is the primary muscle for abducting (lifting away) the arm. A group of four muscles known as the rotator cuff (supraspinatus, infraspinatus, teres minor, and subscapularis) originates from the scapula and provides shoulder rotation and stability.
The upper arm muscles are divided into anterior and posterior compartments. The anterior compartment contains the primary forearm flexors, including the biceps brachii and the brachialis. The biceps brachii also helps supinate the forearm (turning the palm upward). The posterior compartment contains the triceps brachii, the primary forearm extensor, which straightens the elbow.
Muscles of the Pelvic Girdle and Lower Limbs
Muscles of the pelvic girdle and lower limbs are large and powerful, necessary to support and propel the body. The pelvic girdle is firmly attached to the sacrum, providing a stable foundation for the lower limbs. Muscles moving the femur (thigh bone) originate from the pelvic girdle. The iliopsoas group (psoas major and iliacus) is a major flexor of the thigh, while the gluteal muscles (gluteus maximus, medius, and minimus) extend and abduct the femur.
The thigh is separated into distinct compartments. The anterior compartment houses the quadriceps femoris, a group of four muscles that are the primary extensors of the lower leg at the knee. This group includes the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The posterior compartment contains the hamstrings (biceps femoris, semitendinosus, and semimembranosus), which flex the leg at the knee and extend the thigh.
Lower leg muscles are organized to move the ankle, foot, and toes. The gastrocnemius, the prominent calf muscle, is a powerful plantar flexor (pointing the toes downward). The tibialis anterior on the front of the shin is responsible for dorsiflexion (pulling the foot toward the shin). These opposing actions enable walking and running.
Functional Roles in Movement
Appendicular muscles create a wide variety of movements through coordinated actions at the joints. These movements are described in opposing pairs. Flexion decreases the angle between two bones (bending the elbow), while extension increases that angle (straightening the knee). Abduction moves a limb away from the body’s midline, while adduction moves it toward the midline. Rotation involves turning a bone around its long axis, either medially (inward) or laterally (outward).
Muscles work in groups to produce smooth, controlled motions. The primary muscle in a movement is the agonist or prime mover. Synergists are muscles that contract to assist the agonist. For example, the brachialis muscle is a synergist to the biceps brachii during forearm flexion.
Muscles that perform the opposite action are antagonists. They maintain position and control rapid movements. A classic example is the relationship between the biceps and triceps brachii; when the biceps (agonist) contracts to flex the elbow, the triceps (antagonist) relaxes. When the triceps contracts to extend the elbow, the biceps becomes the antagonist. This opposition allows for precise limb movement.
Relationship to the Nervous System
Muscle contraction is initiated by signals from the nervous system in a process called innervation. Motor neurons connect to every appendicular muscle, transmitting electrical impulses that command the muscle fibers to shorten. Without this neural input, muscles cannot function. These nerves are bundled into complex networks called plexuses.
The muscles of the pectoral girdle and upper limbs receive their nerve supply from the brachial plexus. This network originates from spinal nerves in the neck (C5-T1) and forms all major nerves of the upper limb, including the radial, median, and ulnar nerves. Damage to the brachial plexus can cause significant weakness or paralysis of the upper limb.
The lower limbs are innervated by nerves from the lumbar and sacral plexuses. The lumbar plexus (T12-L4) forms nerves like the femoral nerve, which supplies the anterior thigh muscles. The sacral plexus (L4-S4) forms the sciatic nerve, the largest nerve in the body, which innervates the posterior thigh and most of the leg and foot. This organized system ensures that every muscle receives the precise signals needed for coordinated movement.