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Leg and knee anatomy
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How many times have a layman’s language and anatomy ever matched? Almost never. For example, take a look at the leg. When your non-health science friends talk about the ‘leg’, they mean the entire lower extremity. However, in the world of anatomy, the ‘leg’ strictly means the portion between the knee and the ankle joints. It is composed of several structures:
- Tibia
- Fibula
- Leg muscles
In this page, we will take a look at all of the above as well as the anatomy of the knee.
| Bones of the leg |
Tibia: the largest and most medial leg bone, forming both the knee and ankle joints. It facilitates weight-bearing and movement Fibula: the thinnest and most lateral leg bone, forming only the ankle joint. It mainly facilitates movement. Mnemonic: FibuLA is LAteral |
| Knee joint |
Bones: tibia, fibula, patella Articulations: tibiofemoral, patellofemoral Menisci: lateral, medial Ligaments: frontal, medial/lateral, dorsal, cruciate, anterolateral Bursae: suprapatellar, prepatellar, infrapatellar |
| Muscles of the leg |
Anterior group: tibialis anterior, extensor hallucis longus, extensor digitorum longus, fibularis tertius Lateral group: fibularis longus, fibularis brevis Posterior group: - Superficial layer: gastrocnemius, soleus, plantaris - Deep layer: popliteus, tibialis posterior, flexor digitorum longus, flexor hallucis longus |
Tibia and fibula
The tibia and fibula are two long bones that run parallel to each other, forming the scaffold of the leg and providing attachment points for many muscles. The tibia, commonly known as the ‘shin bone’, is the largest and most medial of the two. You can palpate its anterior border when you run your finger down the anterior aspect of your leg. You can also certainly feel it when you hit it against a surface by accident. The tibia is the weight-bearing bone of the leg and is directly involved in the formation of both the knee and ankle joints. Therefore, it facilitates movement.
The fibula is smaller, thinner, and laterally positioned compared to the tibia. As a result, it doesn’t play any crucial role in weight bearing. However, the fibula function is more nuanced - it plays a secondary role in the articulation of the ankle joint, facilitating its movement, and provides the proximal attachment point for a number of muscles. The fibula also articulates with the tibia in two locations: slightly distal to the knee (superior tibiofibular joint) and just superior to the ankle (inferior tibiofibular joint). These joints are surrounded and reinforced by strong ligaments, which also prevent excessive and potentially damaging movements. Here’s a diagram with the tibia bone labelled, as well as the fibula, showcasing all their surface landmarks.
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They look quite complicated and daunting at a first glance, right? Actually, if you learn them systematically, they are easier than you think. Complete the study unit below to master the anatomy of the tibia and fibula.
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Knee joint
Bones and soft tissues
The main parts of the knee joint are the femur, tibia, patella, and supporting ligaments. The condyles of the femur and of the tibia come in close proximity to form the main structure of the joint. The patella, commonly known as the ‘kneecap’, is a sesamoid bone that sits within the tendon of the quadriceps femoris. It serves a protective function for the knee and facilitates movement by acting as a pulley for the quadriceps femoris muscle.
The knee joint is quite a complex structure. In fact, it actually consists of two separate joints housed within one single joint capsule:
- Tibiofemoral joint (medial and lateral articulations); between the corresponding femoral and tibial condyles
- Patellofemoral joint; situated between the patella and the femur
Since the knee joint must strike the perfect balance between strength for weight bearing and flexibility for movement purposes, it is reinforced by various soft tissue structures. The joint is surrounded by several fat pads and bursae, which are synovial fluid-filled sacs that reduce the friction caused by the tendons and muscles as they move against bony areas. The joint contains two shock-absorbers (menisci), which also function to improve congruency at the knee.
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In addition, the knee joint is strengthened by various ligaments, such as the patellar ligament, tibial and fibular collateral ligaments, oblique popliteal ligament and anterolateral ligement. Inside the joint, there are additional reinforcing ligaments, such as the transverse ligament, together with the anterior (ACL) and posterior (PCL) cruciate ligaments. These intracapsular knee ligaments prevent dislocation of the joint, but can be commonly injured amongst professional athletes.
| Articulations |
Tibiofemoral (lateral and medial condyles of the femur + tibial plateaus of the tibia) Patellofemoral (patellar surface of the femur + posterior surface of the patella) |
| Menisci | Medial, lateral |
| Ligaments |
Frontal ligaments - they stabilize the patella: patellar ligament (extension of the quadriceps femoris muscle), retinaculum (of the vastus medialis and lateralis) Medial/lateral ligaments - prevent excessive sideways movement: medial (tibial) collateral ligament, lateral (fibular) collateral ligament Dorsal - they prevent overextension of the knee: oblique popliteal ligament, arcuate popliteal ligament Cruciate - they prevent the slippage of the femur on the tibia: anterior cruciate ligament (ACL), posterior cruciate ligament (PCL) |
| Joint capsule | Fibrous capsule with the inner synovial membrane; popliteus muscle penetrates the lateral side of the capsule |
| Bursae | Suprapatellar, prepatellar, infrapatellar |
Here’s a labelled knee diagram to see how everything fits together:
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If you want to learn more about the anatomy of the knee joint, including its complex range of motion (flexion, extension, rotation, and the knee’s terminal locking mechanism), simply take a look at the following resources!
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Muscles
The knee joint itself is moved by several muscles, the majority of which are part of the anterior compartment of the thigh. These include the sartorius and the four muscles of the quadriceps femoris (rectus femoris, vastus medialis, intermedius, and lateralis), all of which extend the leg at the knee joint. They receive innervation via the femoral nerve (the neurovasculature of the lower extremity can be a daunting task, but we're here to help you out!)
| Sartorius |
Origins: Anterior superior iliac spine (ASIS) Insertions: Medial surface of proximal tibia (via Pes anserinus) Innervation: Femoral nerve (L2-L3) Functions: Thigh flexion, abduction, and external rotation at the hip joint; leg flexion and internal rotation at the knee joint |
| Quadriceps femoris |
Origins: Anterior inferior iliac spine and superior margin of acetabulum (rectus femoris), intertrochanteric line of femur (vastus medialis), linea aspera of femur and greater trochanter of femur (vastus lateralis), anterior surface of femoral shaft (vastus intermedius) Insertions: Tibial tuberosity (via patellar ligament), medial and lateral femoral condyles (vastus medialis and lateralis) Innervation: Femoral nerve (L2-L4) Functions: Thigh flexion at the hip joint and leg extension at the knee joint (rectus femoris), leg extension at the knee joint (vastus medialis, lateralis, and intermedius) |
The antagonistic muscles of the knee joints are called the hamstrings. They are also part of the thigh, but they are located in the posterior compartment. These muscles include the biceps femoris, semitendinosus, and semimembranosus muscles, all of which flex the leg at the knee joint. They receive their innervation via the sciatic nerve.
| Semimembranosus |
Origins: (superolateral impression of) Ischial tuberosity Insertions: Medial condyle of tibia Innervation: Tibial division of sciatic nerve (L5-S2) Functions: Thigh extension and internal rotation at hip joint; leg flexion and internal rotation at knee joint; pelvis stabilization |
| Semitendinosus |
Origins: (posteromedial impression of) Ischial tuberosity Insertions: Proximal end of tibia below medial condyle of tibia (via pes anserinus) Innervation: Tibial division of sciatic nerve (L5-S2) Functions: Thigh extension and internal rotation at hip joint; leg flexion and internal rotation at knee joint; pelvis stabilization |
| Biceps femoris |
Origins: Ischial tuberosity (long head), linea aspera of femur (lateral lip), lateral supracondylar line of femur (short head) Insertions: Head of fibula Innervation: Tibial division of sciatic nerve (long head) and common fibular division of sciatic nerve (short head) Functions: Thigh extension and external rotation at hip joint; Leg flexion and external rotation at knee joint |
One other specific muscle, called the gracilis, helps with flexion of the knee joint but also with its internal rotation. This muscle is part of the medial compartment of the thigh and receives innervation via the obturator nerve.
Muscles of the leg
Now that we have covered the proximal articulation of the leg, let’s move further and take a look at the muscles that move the more distal articulation (the ankle, or talocrural, joint). The leg is divided into three major anatomical compartments, each one containing a specific group of muscles:
- anterior group
- lateral group
- posterior group
Here’s a leg muscles diagram to give you an overview:
As the name suggests, the anterior leg muscles are located along the anterior aspect of the leg. There are four muscles in this compartment: tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis tertius. They receive their innervation via the deep fibular nerve. Their principal functions are dorsiflexion, inversion, and eversion of the foot at the ankle joint. Some are also responsible for toe (hallux) extension.
| Tibialis anterior |
Origins: Lateral surface of tibia, Interosseous membrane Insertions: Medial cuneiform bone, Base of metatarsal bone 1 Innervation: Deep fibular nerve (L4, L5) Functions: Foot dorsiflexion at the talocrural joint; foot inversion at the subtalar joint; Supports medial longitudinal arch of foot |
|
Extensor hallucis longus |
Origins: (Middle third of ) Medial surface of fibula, Interosseous membrane Insertions: Base of distal phalanx of great toe Innervation: Deep fibular nerve (L5, S1) Functions: Toe extension at the metatarsophalangeal and interphalangeal joints; foot dorsiflexion at the talocrural joint |
|
Extensor digitorum longus |
Origins: (Proximal half of) Medial surface of fibula, Lateral tibial condyle, Insertions: Distal and middle phalanges of digits 2-5 Innervation: Deep fibular nerve (L5, S1) Functions: Toe extension at the metatarsophalangeal and interphalangeal joints 2-5; foot dorsiflexion at the talocrural joint; foot inversion at the subtalar joint |
| Fibularis tertius |
Origins: (Distal third of) Medial surface of fibula Insertions: Dorsal surface of base of metatarsal bone 5 Innervation: Deep fibular nerve (L5, S1) Functions: Foot dorsiflexion at the talocrural joint; foot eversion at the subtalar joint |
| Mnemonic |
Tiny Elegant Fire Extinguishers (stands for: Tibialis anterior, Extensor hallucis longus, Fibularis (peroneus) tertius, Extensor digitorum longus) |
The lateral compartment of the leg is the smallest of them all, containing only two muscles: fibularis (peroneal) longus and fibularis brevis. Due to their slightly complicated trajectory as they wrap-around the foot, their actions might not be that obvious. These muscles plantar flex and evert the foot by moving the ankle joint. They receive their innervation via the superficial fibular nerve.
| Fibularis longus |
Origins: Head of fibula, superior two-thirds of lateral surface of fibula, intermuscular septa Insertions: Medial cuneiform bone, metatarsal bone 1 Innervation: Superficial fibular nerve (L5, S1) Functions: Foot plantar flexion at the talocrural joint; foot eversion at the subtalar joint; supports longitudinal and transverse arches of foot |
| Fibularis brevis |
Origins: Distal 2/3 of lateral surface of fibula Insertions: Tuberosity of metatarsal bone 5 Innervation: Superficial fibular nerve (L5, S1) Functions: Foot plantar flexion at the talocrural joint; foot eversion at the subtalar joint |
| Mnemonic |
Fly Long Superficial Fly Boy (stands for: Fibularis Longus, Superficial fibular nerve (innervation for both), Fibularis Brevis |
Last but not least, let’s move to the posterior compartment of the leg, which is the largest one. There are seven muscles in total, separated into two layers. The superficial muscles are the gastrocnemius, soleus (together forming the triceps surae) and plantaris.
| Gastrocnemius |
Origins: Lateral head: Posterolateral surface of lateral femoral condyle; Medial head: Posterior surface of medial femoral condyle, Popliteal surface of femoral shaft Insertion: Posterior surface of calcaneus (via calcaneal tendon) Innervation: Tibial nerve (S1, S2) Functions: Foot plantar flexion at the talocrural joint and knee flexion at the knee joint |
| Soleus |
Origins: Soleal line, Medial border of tibia, Head of fibula, Posterior border of fibula Insertion: Posterior surface of calcaneus (via calcaneal tendon) Innervation: Tibial nerve (S1, S2) Functions: Foot plantar flexion at the talocrural joint |
| Plantaris |
Origins: Lateral supracondylar line of femur, Oblique popliteal ligament of knee Insertion: Posterior surface of calcaneus (via calcaneal tendon) Innervation: Tibial nerve (S1, S2) Functions: Foot plantar flexion at the talocrural joint and knee flexion at the knee joint (weak) |
The deep layer consists of many more, namely the popliteus , tibialis posterior, flexor digitorum longus, and flexor hallucis longus. As the name suggests, all these muscles run along the posterior aspect of the leg, producing plantar flexion of the ankle joint. Some of them also help with foot inversion and toe flexion around the metatarsophalangeal joints. They receive innervation via the tibial nerve.
| Popliteus |
Origins: Lateral femoral condyle, posterior horn of lateral meniscus of knee joint Insertions: Posterior surface of proximal tibia Innervation: Tibial nerve (L5-S2) Functions: Unlocks knee joint; knee joint stabilization |
| Tibialis posterior |
Origins: Posterior surface of tibia, posterior surface of fibula, Interosseous membrane Insertions: Tuberosity of navicular bone, all cuneiform bones, bases of metatarsal bones 2-4 (cuboid bone) Innervation: Tibial nerve (L4, L5) Functions: Foot plantar flexion at the talocrural joint; foot inversion at the subtalar joint; Supports medial longitudinal arch of foot |
| Flexor digitorum longus |
Origins: Posterior surface of tibia (inferior to soleal line) Insertions: Bases of distal phalanges of digits 2-5 Innervation: Tibial nerve (S1, S2) Functions: Toe flexion at the metatarsophalangeal and interphalangeal joints 2-5; foot plantar flexion at the talocrural joint; foot inversion at the subtalar joint |
| Flexor hallucis longus |
Origins: (Distal 2/3 of) Posterior surface of fibula, Interosseous membrane Insertions: Base of distal phalanx of great toe Innervation: Tibial nerve (S2, S3) Functions: Toe flexion at the metatarsophalangeal and interphalangeal joint 1; foot plantar flexion at the talocrural joint; foot inversion at the subtalar joint |
The gastrocnemius and plantaris muscles deserve a special mention. Due to their origins from the femur, they are also important players in flexing the leg around the knee joint. In order to retain these muscles, it’s important for you to consistently revise them.
Test your knowledge on the main muscles of the lower limb with this integrated quiz.
In order to remember all muscles that belong to the posterior leg compartment you can use the following mnemonic;
Great Plans Sometimes Tie Friends and Partners Forever
Superficial group:
- Gastrocnemius
- Plantaris
- Soleus
Deep group:
- Tibialis posterior
- Flexor hallucis longus
- Popliteus
- Flexor digitorum longus
You can start by taking a look at the following study unit:
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Learn more about the leg and knee anatomy by taking our special quiz, customized to focus on bones, muscles, nerves and vessels of this region!
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