Motor proteins are individual molecules that hydrolyze ATP and use the released energy to move forward along a polymer. These microscopic engines operate in an overdamped regime where Brownian motion is a nonegligible contribution to the physics. We provide a new definition for the efficiency of an engine in the overdamped Brownian realm and discuss how a high efficiency can be reached.

PACS numbers: 05.40.--a, 82.40.--g, 87.15.--v