Bent knee version is targeting more soleus along with other muscles in lower leg.
This move is often neglected in rehabilitation protocols.
The other option how to target primarily soleus muscle is seated calf raise.
In this video I will show you how to progress from very simple calf heel raises to 1 leg version on the step.
Generally, if you are able to do 20 repetitions without any problems feel free to progress into next step.
20 doesn’t mean grinding 20 with poor form.
Take your time and let your body adapt. I try to cover everything you need to know but if there is any questions just please ask in comments below. I ll try to answer ASAP