KIN 322 Kinesiology · Unit 3: Pelvis, hip, knee and ankle · on Exam 3
3.3 Leg and ankle
Study notes for Leg and ankle in KIN 322 Kinesiology at Montana State University: the big ideas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- The talocrural (ankle) joint is a hinge between the talus and the mortise formed by the tibia and fibula: about 20° dorsiflexion and 50° plantarflexion. The talus is wider in front, so the joint is close-packed and most stable in dorsiflexion; sprains happen in plantarflexion with inversion.
- The subtalar joint (talus on calcaneus) provides most inversion (about 35°) and eversion (about 15°). Open-chain pronation = dorsiflexion + eversion + abduction; supination = plantarflexion + inversion + adduction.
- Lateral ligaments: anterior talofibular (the most often sprained), calcaneofibular and posterior talofibular. The medial deltoid ligament is so strong that eversion injuries often break bone instead. A “high ankle sprain” injures the distal tibiofibular syndesmosis.
- Compartments. Anterior (tibialis anterior, extensor hallucis longus, extensor digitorum longus): dorsiflexion, deep fibular nerve. Lateral (fibularis longus and brevis): eversion, superficial fibular nerve. Superficial posterior (gastrocnemius, soleus, plantaris, joining in the Achilles tendon): plantarflexion, tibial nerve. Deep posterior (tibialis posterior, FDL, FHL, “Tom, Dick and Harry” behind the medial malleolus): inversion and plantarflexion.
- The gastrocnemius crosses the knee, so it is strongest with the knee straight; the soleus crosses only the ankle and dominates with the knee bent. Tibialis anterior lowers the foot eccentrically after heel contact; if it is weak (common fibular nerve injury at the fibular head), the foot slaps or drops. Other problems: Achilles tendinopathy or rupture, medial tibial stress syndrome and compartment syndrome.
Worked example
A basketball player lands on another player’s foot and rolls the ankle so the sole turns inward. Which ligament is most likely sprained, and why that one?
Show the solution
The anterior talofibular ligament. Landing in plantarflexion and inversion puts the narrow back of the talus in the mortise, where the joint is least stable, and stretches the ATFL first; it is the weakest lateral ligament.
Common mistakes
- Stretching the soleus with the knee straight. Bend the knee to slacken the gastrocnemius.
- Placing inversion and eversion at the talocrural joint. They happen mainly at the subtalar joint.
- Mixing the nerves: deep fibular (dorsiflexors), superficial fibular (evertors), tibial (plantarflexors and invertors).
Exam tipTie each compartment to one action and one nerve: anterior, dorsiflex, deep fibular; lateral, evert, superficial fibular; posterior, plantarflex and invert, tibial.
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