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An ultrasensitive, optical-fiber-based force transducer was developed to measure the microscopic force of contraction of single heart cells. The original design permits approximately 1% shortening of cell length to occur during twitch contractions. It is shown that the shortening can be reduced significantly by adding a piezoelectric bimorph actuator and closed-loop control. As a result, the effective stiffness of the transducer can be increased by a factor of about 100, and cell shortening reduced to approximately 0.01%. For the force probes typically used, this is equivalent to a movement of less than 20 nm for a typical value of 100 nN peak cell force in single frog ventricular cells. The gain in stiffness is obtained without sacrificing sensitivity, although at the expense of frequency response. The design permits control of cell length and is applicable to studies of the mechanical stiffness of cardiac cells.