( ( ( ( Straightening ( the arms ) ) and ) ( tensing ( the triceps ) ) ) ( hurts . ) )
(ROOT (S (S (VP (VP (VBG Straightening) (NP (DT the) (NNS arms))) (CC and) (VP (VBG tensing) (NP (DT the) (NNS triceps))))) (VP (VBZ hurts)) (. .)))
Straightening the arms and tensing the triceps hurts.
(ROOT (S (NP (DT The) (NNS triceps)) (VP (VBP help) (S (NP (DT the) (NN body)) (VP (VB extend) (NP (DT the) (NN arm))))) (. .)))