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Plantaris tendon

The plantaris tendon is the long, thin distal extension of the plantaris muscle, often referred to as the “freshman’s nerve” because of its slender cord-like appearance. Despite its small size, it has disproportionate clinical relevance, as injuries are often mistaken for Achilles tendon tears or contribute to posterior calf pain.

The tendon originates from the plantaris muscle belly, which arises from the lateral supracondylar ridge of the femur. The tendon courses distally along the posterior compartment of the leg, typically lying between the gastrocnemius and soleus, before inserting on the calcaneus, either independently or merging with the Achilles tendon.

Although its contribution to plantarflexion is minimal, the tendon plays a role in proprioception due to its high density of muscle spindles. It is frequently harvested as a graft for tendon reconstruction because it is dispensable.

Synonyms

  • Freshman’s nerve

  • Plantaris long tendon

Origin, Course, and Insertion

  • Origin: Plantaris muscle belly arises from the lateral supracondylar line of the femur and oblique popliteal ligament

  • Course: Tendon descends obliquely between gastrocnemius and soleus muscles, then travels distally along the medial border of the Achilles tendon

  • Insertion: Inserts into the posterior surface of the calcaneus, either independently or fused with the Achilles tendon

Tendon Attachments

  • Thin cord-like structure in the posterior leg

  • Often runs medially to the Achilles tendon near the ankle

  • Insertion varies: independent calcaneal insertion (20%), fusion with Achilles tendon (80%)

Relations

  • Superiorly: Lateral head of gastrocnemius

  • Inferiorly: Calcaneus and Achilles tendon insertion

  • Medially: Soleus muscle

  • Laterally: Gastrocnemius and fibula

  • Anteriorly: Popliteal vessels and posterior capsule of the knee

Function

  • Plantarflexion: Assists gastrocnemius and soleus in weak plantarflexion of the ankle

  • Knee flexion: Provides minimal assistance to knee flexion

  • Proprioception: Rich in sensory fibers, contributing to coordination of lower limb movement

  • Tendon graft use: Commonly harvested for tendon reconstruction (hand, foot, or ligament repairs)

Clinical Significance

  • Rupture: Commonly associated with “tennis leg,” often with gastrocnemius tear

  • Differential diagnosis: Plantaris rupture can mimic Achilles tendon injury clinically

  • Chronic pain: May cause posteromedial calf pain or be implicated in Achilles tendinopathy

  • Surgical relevance: Tendon used as autograft; absence noted in 7–20% of the population

  • Imaging importance: MRI and CT help distinguish plantaris injuries from Achilles tendon pathology

MRI Appearance

  • T1-weighted images:

    • Normal tendon: low signal (dark cord-like structure) medial to Achilles

    • Muscle belly: intermediate signal

    • Rupture: tendon discontinuity, proximal retraction, hematoma with intermediate-to-bright signal

  • T2-weighted images:

    • Normal tendon: low signal

    • Rupture or tendinopathy: bright hyperintense signal within or around tendon

    • Muscle belly edema: intermediate-to-bright signal, tendon remains darker than T1

  • STIR:

    • Normal tendon: dark

    • Pathology: bright hyperintense signal indicating edema, hematoma, or fluid around rupture site

  • Proton Density Fat-Saturated (PD FS):

    • Normal: uniformly dark tendon

    • Pathology: focal or diffuse bright signals indicating tendinitis, strain, or tear

    • Excellent for detecting subtle injuries in tendon or myotendinous junction

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: minimal enhancement

    • Pathology: enhancement in peritendinous soft tissues (inflammation, postoperative changes)

CT Appearance

Non-Contrast CT:

  • Tendon: fine low-density band, often poorly visualized

  • Indirect findings: peritendinous soft-tissue swelling, hematoma, or calcification in chronic injury

Post-Contrast CT (standard):

  • Muscle belly enhances normally

  • Injured tendon may show surrounding soft-tissue enhancement

  • Useful in differentiating plantaris vs. Achilles pathology

MRI image

Plantaris tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Plantaris tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Plantaris tendon axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002