Which of the following exercises works best in isolating the soleus muscle?

Standing heel raises keep the knee extended, allowing the gastrocnemius to remain fully active at optimal length, so the exercise targets both the gastrocnemius and soleus rather than isolating the soleus.

Performing ankle extensions with the knees straight still allows the gastrocnemius to function efficiently since it is not slackened, preventing true soleus isolation.

The gastrocnemius is a biarticular muscle crossing both the knee and ankle, while the soleus is monoarticular, crossing only the ankle. When the knee is flexed, the gastrocnemius is placed in a shortened, mechanically disadvantaged position and cannot contribute significantly to plantar flexion, thereby isolating the soleus during the heel raise movement.

Half squats with external hip rotation primarily target the quadriceps and hip abductors and do not create the knee flexion condition needed to isolate the soleus.