from looking at pics of the gastrocnemius muscle (at the back of the lower leg, main portion from the knee, ending about midway down the lower leg, attaches to

from looking at pics of the gastrocnemius muscle (at the back of the lower leg, main portion from the knee, ending about midway down the lower leg, attaches to the achilles tendon) and the soleus muscle (underneath the gastrocnemius, extending from the knee down to the ankle), the gastrocnemius shouldn't activate during the motion. It seems like you could put your hand on the back of your calf, close to the knee, and ensure that the gastrocnemius doesn't flex/stays loose during the motion.