( Concussion ( ( is ( ( the ( ( most common ) form ) ) ( of ( ( head injury ) ( for athletes ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Concussion)) (VP (VBZ is) (NP (NP (DT the) (ADJP (RBS most) (JJ common)) (NN form)) (PP (IN of) (NP (NP (NN head) (NN injury)) (PP (IN for) (NP (NNS athletes))))))) (. .)))
Concussion is the most common form  of head injury for athletes.
(ROOT (S (NP (DT A) (PRN (-LRB- -LRB-) (NP (NN n)) (-RRB- -RRB-)) (NN concussion)) (VP (VBZ is) (NP (NP (DT the) (ADJP (RBS most) (JJ common)) (NN type)) (PP (IN of) (NP (NN brain) (NN injury))))) (. .)))