Sentence:  And they could n't have entrusted Henri to better hands because `` le professeur '' knows his muscles 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 , <WSD>fast</WSD> muscle growth . 
Meaning of the word marked with <WSD> tags:  acting or moving or capable of acting or moving quickly