Anatomy of Key Human Muscles and Joints
The diaphragm is a dome-shaped muscle that forms the floor of the chest cavity and the roof of the abdominal cavity. It is concave below and convex above. Its central part is a fascial tendon called the central tendon, surrounded by muscular fibers. These fibers originate from anatomical elements forming the lower costal opening. The thickest part is the crus of the diaphragm, with a larger right crus and a smaller left crus. The right and left crura ascend and join to form the aortic hiatus, through which the aorta passes. Superior to this, the esophageal hiatus is formed, allowing passage for the esophagus. All fibers converge at the central tendon, which has a cloverleaf shape. A rightward opening in the central tendon accommodates the inferior vena cava. The central tendon is typically at the level of the 5th intercostal space, but its position can vary from the 3rd to the 5th intercostal space. Normally, the diaphragm moves up and down about 2-3 cm during respiration.
During contraction, the diaphragm flattens and moves downward, increasing the craniocaudal diameter of the chest cavity. This action facilitates inspiration.
Abdominal Muscles: External Oblique Muscle
The external oblique muscle, also known as the superficial oblique muscle, is located on the lateral side of the abdomen. It is the largest of the abdominal muscles. Its origin is from the lateral aspects of the 5th to 12th ribs, with jagged slips that interlace with the serratus anterior and latissimus dorsi muscles. The fibers run downward and forward. The line of attachment extends from the iliac crest to the outer edge of the rectus abdominis aponeurosis. Some fibers reach the anterior superior iliac spine, arching superiorly near the pubis to form the superficial inguinal ring, through which the spermatic cord (in males) passes.
Muscle: Quadriceps – Vastus Medialis
The vastus medialis is located between the vastus lateralis and vastus intermedius, occupying the anterior and medial aspects of the femur, except for the distal 2 centimeters. It attaches to the superior edge of the patella and the quadriceps tendon, contributing to knee extension.
Soleus Muscle
The soleus muscle is a broad, thick muscle in the posterior compartment of the lower leg, situated below the gastrocnemius (twins). It originates from the head and posterior border of the fibula, the oblique line of the tibia, and the soleal line. Inferiorly, it tapers into a narrow aponeurosis that, along with the gastrocnemius, forms the Achilles tendon, inserting into the posterior calcaneus. The tibial nerve innervates the soleus muscle.
Radio-humeral Joint
The radio-humeral joint is a ball-and-socket joint, though it functions more like a condyloid joint. It involves the dome-shaped radial head articulating with the lateral condyle of the humerus.
Humeral-ulnar Joint
The humeral-ulnar joint is a trochlear joint with a slightly oblique axis. The proximal ulna, designed to bear the weight of the hand and forearm during flexion, has a well-developed trochlear notch (greater sigmoid cavity) that articulates with the trochlea of the humerus, located in the medial half of its distal articular surface.
Radio-ulnar Joint
The radio-ulnar joint is a pivot (trochoid) joint. The enlarged radial head articulates with the radial notch of the ulna. The proximal radio-ulnar joint is formed by the lesser sigmoid cavity of the ulna and the annular ligament, which surrounds the radial head.