Lower Leg Anatomy: Bones, Muscles, Tendons, Nerves, and Vessels Below the Knee

What are the parts of the leg below the knee?

The leg below the knee, anatomically known as the crus, comprises a complex interplay of bones, muscles, tendons, ligaments, nerves, and blood vessels working synergistically to facilitate movement, support body weight, and maintain balance during locomotion.

Understanding the Lower Leg: An Anatomical Overview

The human lower leg is a marvel of biomechanical engineering, crucial for bipedal locomotion, shock absorption, and propulsion. Far more than just a "shin," this segment of the limb extends from the knee joint proximally to the ankle joint distally, housing structures vital for virtually every aspect of upright movement. For fitness professionals, athletes, and kinesiologists, a precise understanding of its components is fundamental to effective training, injury prevention, and rehabilitation.

Bones of the Lower Leg

The skeletal framework of the lower leg consists of two primary bones:

• Tibia (Shin Bone): This is the larger, medial, and more robust of the two bones. The tibia is the primary weight-bearing bone of the lower leg, forming the main articulation with the femur at the knee joint and contributing significantly to the ankle joint. Its prominent anterior border is palpable as the "shin."
• Fibula: Located laterally to the tibia, the fibula is a much thinner bone. While it bears minimal weight directly, it plays a critical role in providing attachment points for numerous muscles and, crucially, forms the lateral malleolus, a key stabilizing component of the ankle joint. The fibula articulates with the tibia both proximally and distally, and with the talus bone of the foot at the ankle.

These two bones are connected along their length by the interosseous membrane, a strong fibrous sheet that provides additional surface area for muscle attachment and helps to maintain the relative positions of the tibia and fibula.

Muscles of the Lower Leg

The muscles of the lower leg are typically organized into three distinct compartments by strong fascial septa. This compartmentalization is important for understanding their actions, innervation, and potential for conditions like compartment syndrome.

Anterior Compartment

Located on the front of the leg, these muscles are primarily responsible for dorsiflexion (lifting the foot upwards) and toe extension. They are innervated by the deep fibular (peroneal) nerve.

• Tibialis Anterior: The largest muscle in this compartment, responsible for strong dorsiflexion and inversion of the foot.
• Extensor Digitorum Longus: Extends the lateral four toes and assists in dorsiflexion.
• Extensor Hallucis Longus: Extends the great toe (hallux) and assists in dorsiflexion.
• Fibularis (Peroneus) Tertius: A small muscle often considered part of the EDL, assists in dorsiflexion and eversion.

Lateral Compartment

Positioned on the lateral side of the leg, these muscles are primarily responsible for eversion (turning the sole of the foot outwards) and assist in plantarflexion. They are innervated by the superficial fibular (peroneal) nerve.

• Fibularis (Peroneus) Longus: Plantarflexes and powerfully everts the foot. Its tendon passes under the foot to attach to the first metatarsal and medial cuneiform.
• Fibularis (Peroneus) Brevis: Plantarflexes and everts the foot. Its tendon attaches to the base of the fifth metatarsal.

Posterior Compartment

This is the largest compartment, located on the back of the leg, and is further subdivided into superficial and deep groups. These muscles are primarily responsible for plantarflexion (pointing the foot downwards) and toe flexion. They are innervated by the tibial nerve.

Superficial Posterior Group

• Gastrocnemius: The most superficial muscle, forming the bulk of the calf. It is a bi-articular muscle, crossing both the knee and ankle joints, thus responsible for powerful plantarflexion and knee flexion.
• Soleus: Lies deep to the gastrocnemius. It is a uni-articular muscle, crossing only the ankle joint, making it a primary plantarflexor, particularly important for endurance activities and posture.
• Plantaris: A small, slender muscle with a long tendon, often absent. It assists weakly in plantarflexion and knee flexion.

Deep Posterior Group

• Tibialis Posterior: The deepest muscle, crucial for plantarflexion and inversion of the foot, playing a significant role in supporting the medial longitudinal arch of the foot.
• Flexor Digitorum Longus: Flexes the lateral four toes and assists in plantarflexion.
• Flexor Hallucis Longus: Flexes the great toe and powerfully assists in plantarflexion, especially during push-off phase of gait.

Tendons and Ligaments

Tendons connect muscles to bones, while ligaments connect bones to bones, providing stability. Key structures in the lower leg include:

• Achilles (Calcaneal) Tendon: The largest and strongest tendon in the body, formed by the confluence of the gastrocnemius and soleus muscles, inserting into the calcaneus (heel bone). It is essential for plantarflexion.
• Retinacula: Strong bands of connective tissue (e.g., extensor retinaculum, flexor retinaculum, peroneal retinaculum) that hold tendons close to the bones, preventing bowstringing during movement.
• Ankle Ligaments: A complex network of ligaments stabilizing the ankle joint, including the lateral collateral ligaments (anterior talofibular, posterior talofibular, calcaneofibular) and the medial collateral (deltoid) ligament.
• Interosseous Membrane: As mentioned, a strong fibrous sheet connecting the tibia and fibula.

Nerves and Blood Vessels

The lower leg is richly supplied with nerves and blood vessels, ensuring proper function and sensation.

Nerves

• Tibial Nerve: A branch of the sciatic nerve, it supplies all muscles in the posterior compartment of the leg and provides sensation to the sole of the foot.
• Common Fibular (Peroneal) Nerve: Also a branch of the sciatic nerve, it divides into:
  • Deep Fibular (Peroneal) Nerve: Supplies muscles of the anterior compartment.
  • Superficial Fibular (Peroneal) Nerve: Supplies muscles of the lateral compartment and provides sensation to the dorsum of the foot.
• Sural Nerve: Provides sensory innervation to the lateral aspect of the lower leg and foot.

Blood Vessels

• Popliteal Artery: Divides into the main arteries of the lower leg:
  • Anterior Tibial Artery: Supplies the anterior compartment, continuing as the dorsalis pedis artery in the foot.
  • Posterior Tibial Artery: Supplies the posterior and lateral compartments, continuing as the medial and lateral plantar arteries in the foot.
  • Fibular (Peroneal) Artery: A branch of the posterior tibial artery, supplying the lateral compartment.
• Veins: Deep veins generally accompany the arteries (e.g., anterior tibial veins, posterior tibial veins, fibular veins). Superficial veins include the Great Saphenous Vein (medial aspect) and Small Saphenous Vein (posterior aspect), both crucial for venous return.

Functional Significance

The intricate arrangement of bones, muscles, and neurovascular structures in the lower leg is fundamental to:

• Locomotion: Enabling walking, running, jumping, and intricate foot movements.
• Balance and Proprioception: Providing continuous feedback on body position and movement.
• Shock Absorption: Mitigating impact forces during activities like running and landing.
• Power Generation: Providing the propulsive force for push-off during gait.

Conclusion

The leg below the knee is a highly specialized and robust anatomical region. Its comprehensive understanding—from the weight-bearing tibia to the intricate network of muscles, tendons, nerves, and blood vessels—is paramount for anyone involved in human movement, be it for performance enhancement, injury prevention, or rehabilitation. This foundational knowledge empowers practitioners to design more effective training programs and provide superior care.