We built a device to manipulate force and length of isolated intact cardiomyocytes (guinea pig), using a pair of computer-controlled piezo-positioned carbon fibers (CF). CF-cell connections were firm enough to allow lifting cells off the glass surface and stretching to sarcomere lengths up to mmm. Cells were paced at 2 Hz and superfused with tyrode solution with 1 mM Ca²⁺ at 37°C. Both fiber distance and sarcomere-length were monitored in real time; passive and active forces were calculated from CF bending. The force profile from an auxotonic contraction (i.e. no active movement of the CF) was used to create a counter-movement profile to control afterload. The counter-movement of the piezo-positioner could be used to control cell shortening, from isotonic up to isometric contractions. Preload was varied by increasing diastolic CF distance. Altering the gain of the counter-movement to switch from isometric to isotonic and back allowed to create force-length workloops for intact myocytes. End-systolic force-length relation curves were always near linear within the physiological range, whether pre- or afterload was varied in a workloop, for isometric, isotonic or auxotonic contractions. These results are consistent with whole-heart findings where end-systolic pressure-volume relations are linear under physiological loading conditions and nearly load-independent. The ability to control both pre- and afterload adds a valuable tool for studies into single cardiomyocyte mechanics and their interrelation to whole-heart patho-physiology.