Gastrocnemius

Noun: the gastrocnemius muscle in the back part of the leg that forms the greater part of the calf, responsible for the plantar flexion of the foot; the largest muscle in the calf of the leg, the action of which extends the foot, raises the heel, and assists in bending the knee.

"Her legs are telling that she's a dancer: gastrocnemiuses look strong, perfect shape of shank, back is not slouching." 

 Origin: First recorded 1670–80;  < Neo-Latin  < Greek gastroknēm ( ía ) calf of the leg + Latin -ius  noun suffix

Related forms  - gastrocnemial, gastrocnemian - adjectives

Since it's a Latuin suffix I would think the plural should really be gastrocnemii.  But then does one refer to hippopotamuses or hippopotami??