The paper deals with research into the effectiveness of operation of a seat suspension system as a means to protect industrial and construction machinery operators from vibrations. The basic categories of the sources of mechanical vibrations that disturb the functioning of heavy machinery have been discussed. The root mean square acceleration values of the vibrations to which the operators of the most popular heavy machinery are exposed during their work have also been given. The input signals have been determined that induce seat vibrations and whose spectral characteristics are representative for the excitations encountered in cabins of industrial and construction machinery of different kinds. A biomechanical model of the human body has been developed as a discrete mechanical system with many degrees of freedom and with viscoelastic links. The vibro-isolating properties of a semi-active and active seat suspension system with a scissor linkage mechanism used to guide the isolated object were investigated within the work carried out. The general forms of mutually opposing vibro-isolation criteria have been given, which were used to evaluate the effectiveness of operation of the seat suspension systems. Moreover, the vibro-isolating properties of a semi-active and an active seat suspension system were determined from simulation tests where the biomechanical model of the human body was applied and from experimental tests carried out with participation of seated humans and these data have been presented in the final part of this paper.