Figure 2. A simplified spring-damper-mass model used in the computer simulation. The components of dynamics system presented are: lower body rigid mass (m1) and wobbling mass (m2), upper body rigid mass (m3) and wobbling mass (m4), compressive spring (k1) and damper (c1) that connect the upper and lower rigid bodies, spring (k3) and spring}damper unit (k2, c2) connecting the lower wobbling mass to the upper and lower rigid bodies, spring (k5) and spring}damper unit (k4, c4) connecting the upper wobbling mass to the upper rigid mass (Liu et al., 1998).