WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "overextend":
   haul, heave, inflate, overdevelop, overdistend, overdraw,
   overexert, overexpand, overstrain, overstretch, overtax, press,
   pull, rack, strain, strain every nerve, stress, stretch,
   sweat blood, swell, tax, tense, tug