To investigate contractile profiles of Guinea pig single isolated cardiac myocytes in isometric and isotonic contractions, we used a pair of compliant, computer-controlled and piezo-translator (PZT) positioned carbon fibers (CF), attached glue-free to opposite cell-ends, and an optical force-length (FL) feedback system. Guinea pigs hearts were quickly excised post mortem, and cells were enzymatically isolated, using established Langendorff perfusion-based protocols. Once CF attachment was established, cells were lifted off the coverslip and paced at 2 Hz in normal Tyrode solution (37°C). CF-tip distance was monitored in real time, and passive/active forces were calculated from CF bending. Isometric and isotonic contractions were imposed by applying appropriate PZT commands, both at moderate (initial sarcomere length, SL, ~1.94 μm) and high (initial SL ~2.05 μm) preloads. Contractile properties, both during isometric and isotonic contractions, were quantified via analysis of instantaneous elastance curves, which describe the time-course of changes in the slope of a line connecting the instantaneous position in force-length space with the intersection point of end-diastolic and end-systolic force-length relations (minimum point). Altering preload did not affect time to peak elastance, neither in isometric nor isotonic conditions, but led to a significant slowing of relaxation at high afterload (i.e. during isometric contractions). Increasing afterload, i.e. switiching from isotonic to isometric contraction at a given preload, increased time to peak elastance (see Table 1 and Fig. 1). These results are consistent with previous findings on the shortening-induced decrease, and stretch-induced increase, in Ca²⁺ affinity of myofilaments [1]. Thus, combining the CF technique with dynamic FL control enables one to trace elastance curves, instead of either force or shortening, which aids comparison of the contractile profiles via a single parameter (elastance) for various loading conditions, including isometric and isotonic (where the traditional approach is limited).