Diabetic heart disease typically features a restrictive/ hypertrophic cardiac phenotype characterised by diastolic dysfunction with preserved systolic performance. Little is known about the early adaptational mechanisms that underpin maintenance of systolic performance in diabetic diastolic dysfunction and how this may contribute to disease progression. This study aimed to evaluate cardiomyocyte contractility in a high-fat diet (HFD) mouse model of diastolic dysfunction. Echocardiography (GE Vivid 9) was performed in 33 wk old male C57Bl/6 mice fed a high-fat diet (HFD, 43% kcal fat, 24 wks duration). Isolated cardiomyocytes (paced 2Hz, 2.0mM Ca 2+ , 37C) were subjected to progressive axial stretch. Sarcomere length/ shortening, tension and intracellular calcium transients (Fura-2AM, 5μM) were simultaneously measured (Myostretcher, Ionoptix). HFD hearts displayed in vivo (22.2±1.4 vs 16.7±1.0 E/E’; p<0.05) and in vitro diastolic dysfunction (0.244±0.039 vs 0.075±0.017 nN/pl/ %cell stretch; p<0.05). HFD cardiomyocytes exhibited length-dependent hypercontractility with augmented developed tension amplitude at maximal stretch (4.891±0.942 vs 2.484±0.677 nN/pl; p<0.05), but not basal stretch. Hypercontractility could not be attributed to a difference in Ca 2+ transient amplitude but was correlated with length dependence of Ca 2+ sensitivity (n=17 cells, R 2 =0.4039 p<0.05). During progressive cardiomyocyte stretch, systolic tension slopes were strongly correlated with diastolic tension slopes (cardiomyocyte stiffness) (n=30 cells; R 2 =0.8442, p<0.05) while there was no difference in the ratio of systolic/diastolic tension slopes indicating maintained length-dependent activation. These findings provide the first evidence that cardiomyocyte hypercontractility and stiffness are strongly linked. High-fat diet cardiomyocytes may operate near maximal contractile capacity to maintain basal cardiac output, potentially limiting capacity for cardiac output increase in response to physiological stressors. An aberrant myofilament post-translational modification profile represents a potential mechanism through which cardiac dysfunction arises during the development of diabetes.