( ( Protein ( by itself ) ) ( ( ( is not ) ( ( a stimulus ) ( for ( muscle growth ) ) ) ) . ) )
(ROOT (S (NP (NP (NNP Protein)) (PP (IN by) (NP (PRP itself)))) (VP (VBZ is) (RB not) (NP (NP (DT a) (NN stimulus)) (PP (IN for) (NP (NN muscle) (NN growth))))) (. .)))
Protein by itself is not a stimulus for muscle growth.
(ROOT (S (NP (NNS Muscles)) (VP (VBP need) (NP (NN stimulus)) (PP (TO to) (NP (NN contract)))) (. .)))