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    Saddle Joints
    e.g., thumb joint
    Cranial Bones
    Bones of the skull that protect the brain.
    Acetabulum
    The acetabulum is the pelvic socket that connects with the femoral head to form the hip joint, vital for stability, movement, and weight-bearing.
    Gluteus Maximus
    Largest muscle in the buttocks responsible for hip extension.
    Ulna
    Forearm bone on the pinky side.
    Humerus
    Upper arm bone connecting the shoulder to the elbow.
    Lacrimal Bones
    Bones forming part of the eye socket and housing the tear ducts.
    Soleus
    Calf muscle responsible for plantarflexion of the foot.
    Thoracic Cage
    Ribs and sternum forming the protective cage for the heart and lungs.
    Ellipsoidal (Condyloid) Joints
    e.g., wrist
    Acromioclavicular Ligament
    Ligament that connects the acromion to the clavicle.
    Patella
    Knee cap, protecting the knee joint.
    Rotator Cuff Tendons
    Tendons of the rotator cuff muscles.
    Hamstrings
    Biceps Femoris, Semitendinosus, Semimembranosus.
    Lumbar Vertebrae (L1 - L5)
    Vertebrae in the lower back (L1-L5).
    Biceps Tendon
    Tendon that attaches the biceps muscle to the bone.
    Anterior Cruciate Ligament (ACL)
    Knee ligament that stabilizes the joint.
    Tibia
    Shin bone, the larger bone in the lower leg.
    Metacarpals (5 bones)
    5 bones forming the palm of the hand.
    Vomer Bone
    Bone forming the nasal septum.
    Coracoacromial Ligament
    Ligament that connects the acromion to the coracoid process.
    Anterior Longitudinal Ligament
    Spinal ligament running along the front of the vertebral column.
    Hinge Joints
    e.g., elbow, knee
    Occipital Bone
    Bone forming the back and base of the skull.
    Femur
    Thigh bone, the longest and strongest bone in the body.

    Achilles Tendon

    Reviewed by our medical team

    Tendon connecting the calf muscle to the heel bone.

    1. Overview

    The Achilles tendon, also known as the calcaneal tendon, is the largest and strongest tendon in the human body. It connects the calf muscles to the heel bone and plays a pivotal role in walking, running, and jumping. Despite its strength, it is a common site for injury, especially in athletes and active individuals. The tendon is named after the Greek mythological hero Achilles, whose only vulnerable spot was his heel.

    2. Location

    The Achilles tendon is located in the posterior compartment of the lower leg:

    • Proximally: It originates from the merging of the gastrocnemius and soleus muscles (collectively known as the triceps surae).

    • Distally: It inserts into the posterior surface of the calcaneus (heel bone).

    • It runs superficially down the back of the ankle and is easily palpable above the heel.

    3. Structure

    The Achilles tendon is composed primarily of dense regular connective tissue:

    • Collagen fibers (Type I):

      • Provide tensile strength; fibers are arranged longitudinally for maximum load-bearing capacity.

    • Paratenon:

      • A thin, flexible sheath surrounding the tendon, facilitating movement and supplying blood vessels.

    • Fascicles and tenocytes:

      • The tendon is organized into fascicles, maintained by tenocytes (specialized fibroblasts).

    The Achilles tendon lacks a true synovial sheath and instead relies on the paratenon for nutrition and lubrication.

    4. Function

    The Achilles tendon serves several essential biomechanical functions:

    • Force transmission: Transmits contraction force from the gastrocnemius and soleus muscles to the calcaneus, producing plantarflexion of the foot.

    • Shock absorption: Acts as a spring during locomotion, storing and releasing elastic energy to minimize muscular workload.

    • Postural support: Helps maintain balance and posture when standing and walking.

    5. Physiological role(s)

    Beyond basic movement, the Achilles tendon supports a range of physiological and functional roles:

    • Gait mechanics: Essential for push-off during walking and running cycles.

    • Energy efficiency: Stores mechanical energy during dorsiflexion and releases it during plantarflexion, aiding in efficient movement.

    • Neuromuscular coordination: Works in coordination with proprioceptive receptors and lower leg muscles to respond to dynamic changes in terrain.

    • Adaptation to stress: Adapts to physical activity through remodeling and hypertrophy with appropriate mechanical loading.

    6. Clinical Significance

    The Achilles tendon is commonly involved in orthopedic and sports-related conditions:

    • Achilles tendinitis:

      • Inflammation of the tendon, usually due to overuse or improper footwear. Presents with pain, swelling, and stiffness.

    • Tendinosis:

      • Degenerative changes in the tendon without significant inflammation, often due to chronic microtrauma.

    • Achilles tendon rupture:

      • Partial or complete tear, typically occurring during explosive activities like sprinting or jumping. Patients may report a sudden "pop" with loss of plantarflexion.

      • Requires surgical or conservative treatment followed by prolonged rehabilitation.

    • Insertional Achilles tendinopathy:

      • Pain at the point where the tendon inserts into the calcaneus, commonly seen in runners and older individuals.

    • Haglund’s deformity:

      • Excessive bony enlargement of the heel that irritates the Achilles tendon, often resulting in bursitis and discomfort.

    • Post-surgical considerations:

      • Rupture repair requires careful rehabilitation to avoid re-rupture and ensure return to full function.

    Did you know? Your spinal cord is protected by the vertebral column.