A tearing injury to muscle. Usually causes some degree of bleeding within the muscle tissue (haematoma). (biology) a group of organisms within a species that differ in trivial ways from similar groups; a new strain of microorganisms.(psychology) nervousness resulting from mental stress; his responsibilities were a constant strain; the mental Strain of staying alert hour after hour was too much for him.A variation of a particular species that possesses minor differences in its characteristics though still remain distinguishable. A different group of species(microbiology).