I am sorry I can post out the link about the book, coz I got the book long time ago and i just can't find out the link for you via the internet.

About soleus problem,
during standing, the plantarflexor muscle will reverse its functions, due to reverse origin-insertion relationship, thus normal soleus during wieghtbearing its functions is to shifting the tibia from anterior to posterior directions.
Thus, a weak soleus will allow the tibia to shift forward and result in a proning for the case to have a more knee flexed position.

for the rectus femoris and hamstrings, you should check about the lower-cross syndrome to find out its relationship with pelvic.

Also, its muscle balance is crucial in maintain a upright posture.