We know that the soleus has a knee extensor function due to its posterior pull on the tibia during ground contact with knee flexion. In fact the soleus has a stronger neural relationship with the quads than it does with the medial gastroc in certain tasks https://www.ncbi.nlm.nih.gov/pubmed/29107958 

Afferent feedback from the soleus is linked with the excitation of the motor units in the quads which seems to work both ways. Similar neural relationship exist between medial gastroc and hamstrings, as well as soleus and peroneals https://www.ncbi.nlm.nih.gov/pubmed/8299754 

An interesting study using Alter-G treadmill compared running biomechanics & muscle EMG activity at 60 & 80% bodyweight to full bodyweight, found similar pattern of muscle recruitment between soleus & quads during breaking phase which differed from gastroc https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5167401/

So it might be worth including some soleus strength work when rehabbing above the knee injuries, as well as for calf/achilles/ankle injures!

That would be an ideal scenario but a big challenge in today’s world!