Figure 18-21. (A) With a paralyzed quadriceps unable to pull the knee into extension, the body weight line falls behind the knee causing flexion. However, in a combined reversal of muscle action of the gluteus maximus and gas trocnemius muscles, knee extension during stance is possible. (B) In the closed-chain position, they pull the knee into extension The soleus assists by plantar flexing the dorsi- flexed ankle into a neutral ankle position. This puts the body weight line in front of the knee and ankle axes and allows the knee to remain extended.