Sentence:  And they could n't have entrusted Henri to better hands because `` le professeur '' knows his <WSD>muscles</WSD> from the sterno-cleido mastoideus of the neck right down to the tibialis anticus of the leg - and better still , he knows just what exercises work best for them and what Weider principles to combine them with for fast , fast muscle growth . 
Meaning of the word marked with <WSD> tags:  one of the contractile organs of the body