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Plantaris muscle

The plantaris muscle is a small, slender muscle of the posterior compartment of the leg, lying between the gastrocnemius and soleus. Despite its small size, it has a very long tendon, sometimes referred to as the “freshman’s nerve” because it can be mistaken for a nerve during dissection. The muscle is often absent (in ~7–10% of people) and is considered vestigial, but it may play a minor role in proprioception and plantar flexion of the ankle.

Clinically, the plantaris is important because its tendon can mimic or accompany Achilles tendon injuries, and it may be harvested for tendon grafting. Plantaris rupture (“tennis leg”) can cause acute calf pain, often confused with deep vein thrombosis or gastrocnemius tear.

Synonyms

  • Freshman’s nerve (nickname for the tendon)

  • Small plantar muscle of leg

Origin and Insertion

  • Origin:

    • Inferior part of the lateral supracondylar line of femur

    • Oblique popliteal ligament of the knee joint

  • Course:

    • Small fusiform belly descends between the lateral head of gastrocnemius and popliteus

    • Forms a long, thin tendon that runs medially and inferiorly along the calf

    • Passes between gastrocnemius and soleus, continuing along the medial border of Achilles tendon

  • Insertion:

    • Into the posterior surface of the calcaneus, medial to the Achilles tendon

    • May blend with Achilles tendon in some individuals

Nerve Supply

  • Tibial nerve (S1–S2)

Arterial Supply

  • Popliteal artery branches

  • Posterior tibial artery muscular branches

Venous Drainage

  • Popliteal vein → posterior tibial vein

Function

  • Weak plantar flexion of the ankle

  • Weak flexion of the knee

  • Acts primarily as a proprioceptive organ, providing feedback for larger calf muscles

  • Considered functionally insignificant for strength, but clinically important in injuries

MRI Appearance

T1-weighted images:

  • Muscle belly shows low-to-intermediate signal intensity

  • Long tendon appears as a thin, dark linear structure

T2-weighted images:

  • Muscle belly shows low-to-intermediate signal

  • Acute rupture, edema, or hematoma appears as bright hyperintense signal along muscle belly or tendon path

STIR (Short Tau Inversion Recovery):

  • Normal tendon and muscle belly appear low signal

  • Injury, strain, or inflammation shows bright hyperintensity

Proton Density Fat-Saturated (PD FS):

  • Muscle belly and tendon show low-to-intermediate signal

  • Strain, partial tear, or peritendinous edema shows bright signal

T1 Fat-Sat Post-Contrast:

  • Normal tendon and muscle belly show mild uniform enhancement

  • Pathological changes (myositis, partial rupture, abscess) show heterogeneous or rim enhancement

CT Appearance

Non-Contrast CT:

  • Muscle belly appears as soft tissue density, small and slender

  • Long tendon may be difficult to visualize directly but can be inferred along the medial Achilles

  • Hematomas or calcification appear as abnormal densities along tendon course

Post-Contrast CT:

  • Muscle enhances mildly and homogeneously

  • Rupture or inflammation may show localized enhancement with surrounding fat stranding

  • Abscess appears as rim-enhancing low-attenuation lesion along muscle belly or tendon

MRI image

plantaris muscle anatomy image

MRI image

plantaris muscle anatomy MRI 3T image

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Plantaris muscle axial image

MRI image

Plantaris muscle mri image

CT image

Plantaris muscle ct axial image