The Collaborative International Dictionary of English v.0.48:

Overstrain \O`ver*strain"\, v. i. [imp. & p. p. Overstrained;
   p. pr. & vb. n. Overstraining.]
   To strain one's self to excess. --Dryden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Overstrain \O`ver*strain"\, v. t.
   To stretch or strain too much; as, to overstrain one's
   nerves. --Ayliffe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overstrain
    n 1: too much strain
    v 1: strain excessively; "He overextended himself when he
         accepted the additional assignment" [syn: overstrain,
         overextend]