( ( Since ( muscles ( ( can only ) ( ( ( work ( by contracting ) ) , ) ( in ( order ( for ( the ( organism ( to ( accomplish ( effective movements ) ) ) ) ) ) ) ) ) ) ) ) ( , ( ( each muscle ) ( ( must ( be ( ( ( partnered ( with ( another muscle ) ) ) or ) ( set ( of ( muscles ( that ( have ( opposite actions ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN Since) (S (NP (NNS muscles)) (VP (MD can) (ADVP (RB only)) (VP (VB work) (PP (IN by) (NP (NN contracting))) (, ,) (PP (IN in) (NP (NP (NN order)) (PP (IN for) (NP (DT the) (NN organism) (S (VP (TO to) (VP (VB accomplish) (NP (JJ effective) (NNS movements))))))))))))) (, ,) (NP (DT each) (NN muscle)) (VP (MD must) (VP (VB be) (VP (VP (VBN partnered) (PP (IN with) (NP (DT another) (NN muscle)))) (CC or) (VP (VBN set) (PP (IN of) (NP (NP (NNS muscles)) (SBAR (WHNP (WDT that)) (S (VP (VBP have) (NP (JJ opposite) (NNS actions))))))))))) (. .)))
Since muscles can only work by contracting, in order for the organism to accomplish effective movements, each muscle must be partnered with another muscle or set of muscles that have opposite actions.
(ROOT (S (NP (JJ Skeletal) (NNS muscles)) (VP (MD must) (VP (VB work) (PP (IN in) (NP (NNS pairs))) (S (VP (TO to) (VP (VB move) (NP (NNS bones)) (ADVP (RB back) (CC and) (RB forth)) (PP (IN at) (NP (NP (DT a) (NN joint)) (SBAR (IN because) (S (NP (PRP they)) (VP (MD can) (ADVP (RB only)) (VP (VB contract)))))))))))) (. .)))