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]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "overstrain":
   beat, burn out, carry too far, debilitate, do in, do up, enervate,
   exaggerate, exhaust, fag, fag out, fatigue, flag, frazzle,
   go overboard, go to extremes, go too far, harass, haul, heave,
   inflate, jade, knock out, knock up, know no bounds, overact,
   overcarry, overdevelop, overdistend, overdo, overdraw, overdrive,
   overelaborate, overemphasize, overexercise, overexert, overexpand,
   overexpend, overextend, overfatigue, overindulge, overlabor,
   overplay, overreact, overspend, overstress, overstretch, overstudy,
   overtask, overtax, overtire, overtrain, overuse, overweary,
   overwork, pass all bounds, poop, poop out, press, prostrate,
   protest too much, pull, rack, strain, strain every nerve, stress,
   stretch, sweat blood, swell, tax, tense, tire, tire out,
   tire to death, tucker, tug, use up, weaken, wear on, wear out,
   weary, wilt, wind