Body mass is reported to influence myocardial performance. Recent studies have emphasised the importance of negative inotropic adipocyte-derived factors and their impact on cardiac contractile function. However, the underlying mechanisms remain unclear. We aimed to determine whether body mass impacts cardiac force development on the level of the contractile apparatus. We examined the influence of body mass index (BMI) (3 groups: group I >25, group II 25-30, group III >30) on the myocardial performance of skinned muscle fibres. Right atrial tissue preparations of 70 patients undergoing aortocoronary bypass operation (CABG, 48 patients, group a) and aortic valve replacement (AVR, 22 patients, group b) were obtained. The fibres were exposed to a gradual increase in the calcium concentration, and the force values were recorded. The statistical analysis was performed using Pearson's correlation (P30 (group III) was associated with less force (mean force 1.58±0.1 mN, P=0.02, max force 2.24±0.17 mN, P=0.02 vs. group II (mean force 1.8±0.3 mN, P=0.04, max force 2.59±0.2 mN, P=0.03) and group I (mean force 1.8±0.1 mN, P=0,03, max force 2.62±0.3 mN, P=0.03). Dividing the groups in the post-surgical procedure, the impact of BMI on force development in group III was more intense in the CABG group compared to the AVR group: 2.0±0.2 mN vs. 2.4±0.1 mN, P=0.04. In accordance with the literature, a BMI >30 is associated with reduced force capacities. Additionally, the underlying cardiac disease may aggravate the impact of weight on cardiac force. Further studies are needed to evaluate the clinical relevance of this experimental observation and the potential consequences for the treatment of cardiac function.