One important issue that surgeons need to deal during delicate procedures are the physiological motions induced by the breathing and the heartbeat. To avoid the use of external heart-lung machines, which implies in a great risk to the patient and a long recovery time, many research works have been developed in order to design a method able to cancel these motions and give to the surgeon a perception of a stable environment. This paper proposes a new force control scheme using an Active Observer based on the Kelvin Boltzmann interaction model. A feedforward term and a disturbance estimation are included in the control scheme to improve the disturbance estimation. This estimation is performed using the viscoelastic Kelvin Boltzmann tissue model and a quasi-periodic Fourier series model. The results show the ability of the proposed force control scheme to reject beating heart motions. Performance under parameters mismatch is also analyzed.