A Positive Quadriceps Active Test, without the Quadriceps Being Active

Figure 4

Illustration of gastrocnemius contraction reducing the posterior subluxation. Diagrammatic depiction of the knee joint showing posterior subluxation of the tibia with the gastrocnemius relaxed (a) and anterior translation to a reduced position with the gastrocnemius contracted (b). This translation is driven by the gastrocnemius acting like the string of a bow, aided by the normal posterior slope of the tibia and origin of the gastrocnemius being anterior to the posterior border of the subluxed tibia.

(a)

(b)