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 <WSD>principles</WSD> to combine them with for fast , fast muscle growth . 
Meaning of the word marked with <WSD> tags:  a rule or law concerning a natural phenomenon or the function of a complex system