Total force of muscle

Dear all,

we are wondering what would be the best way to calculate the total force of e.g. the soleus muscle. In our model the soleus consist of 3 medial and 3 lateral parts.

For muscle activation I understand that it would be best to calculate the envelope. However, for muscle force should we also calculate the envelope over lets say the 3 lateral parts to get the muscle force of the lateral soleus or should we sum those up? If we sum all up, values appear high...
Greetings,
Jo