( ( Its antagonist ) ( ( is ( ( the triceps ) ( on ( ( the back ) ( of ( the arm ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP$ Its) (NN antagonist)) (VP (VBZ is) (NP (NP (DT the) (NNS triceps)) (PP (IN on) (NP (NP (DT the) (NN back)) (PP (IN of) (NP (DT the) (NN arm))))))) (. .)))
Its antagonist  is the triceps on the back of the arm.
(ROOT (S (NP (DT The) (NNS triceps)) (VP (VBP help) (S (NP (DT the) (NN body)) (VP (VB extend) (NP (DT the) (NN arm))))) (. .)))