The soleus muscle is one of the two muscles that comprise the calf, the other is the gastrocnemius and together they become the Achilles tendon at the heel region. The difference is the gastrocnemius muscle, the outer calf, crosses the knee joint while the inner calf muscle, soleus, does not. See link below.
https://www.yogauonline.com/yoga-anatomy/gastrocnemiussoleus-complex-yoga
Normally the strength of soleus should be 30% stronger than gastrocnemius. Unfortunately, many times it is not (especially in runners), and this sets the stage for Achilles tendon injuries including ruptures. The link below shows several different ways to strengthen the soleus muscle all are performed with the knee bent which removes training the gastrocnemius at the same time. There is also some suggestion that a stronger soleus reduces the risk of ACL injuries.
Remember if anything hurts or doesn’t feel right check stop and consult your trainer, myself, or a health professional.
