KIN 322 Kinesiology · Unit 3: Pelvis, hip, knee and ankle · on Exam 3
3.2 The knee
Study notes for The knee 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 tibiofemoral joint is a modified hinge: about 135° of flexion, plus rotation when the knee is bent. The menisci deepen the surface and absorb load: the medial one is C-shaped and attached to the MCL (less mobile, more often torn); the lateral one is more circular and mobile. At the patellofemoral joint, the patella (a sesamoid bone) lengthens the quadriceps’ moment arm.
- Ligaments: ACL (stops the tibia sliding forward on the femur; injured in noncontact cutting and landing), PCL (stops posterior sliding; “dashboard” injuries), MCL (resists valgus) and LCL (resists varus). A blow to the outside of the knee can cause the “unhappy triad”: ACL, MCL and medial meniscus.
- The screw-home mechanism: in the last 20–30° of extension the tibia rotates laterally on the femur (or, standing, the femur rotates medially on the tibia), locking the knee in its close-packed position. The popliteus unlocks it.
- Convex–concave at the knee: in open-chain extension the concave tibia rolls and glides anteriorly (same direction). Rising from a squat, the convex femur rolls anteriorly and glides posteriorly.
- Muscles: quadriceps (rectus femoris, which also flexes the hip, and the three vasti; femoral nerve), hamstrings (knee flexion and hip extension; sciatic nerve), gastrocnemius, popliteus, and the pes anserinus (sartorius, gracilis, semitendinosus). The Q-angle (ASIS to mid-patella to tibial tuberosity) is about 10–15°, and larger angles pull the patella laterally.
Worked example
A soccer player plants, cuts and hears a pop. In the Lachman test the tibia slides forward on the femur. Which ligament is injured, and what else is often hurt?
Show the solution
The ACL, which resists anterior tibial translation. If a valgus force was involved, the MCL and medial meniscus may also be torn (the unhappy triad).
Common mistakes
- Mixing the ACL (stops the tibia sliding forward) with the PCL (stops it sliding back).
- Attaching the lateral meniscus to the MCL. The medial one is attached.
- Forgetting that rectus femoris, the hamstrings and gastrocnemius each cross two joints.
Exam tipName each knee ligament by the force or translation it stops. That is how the special tests are described.
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