The development of mechanical models of interaction of the human-bed systems aims to determine the distribution of forces of mutual effect of the technical item and the human body. By using mesh phantoms, which are a reflection of the human body of the 5th, 50th and 95th centile (Fig. 8.55), the distribution of forces working in its support points can be determined. The calculation scheme of the mechanical human-bed system can be reduced to the form of a multi-joint beam with a length that corresponds to the height of the user’s body. In the initial phase of building the model, the place of occurrence of reaction forces of the base has been established (like in the human-seat system) on the basis of the distribution of stresses of the body on the mattress, measured using a sensory mat of Force Sensitive Applications® system (Fig. 8.56) (Smardzewski et al. 2008).
Mass loads have been determined based on the coordinates of the position of the centres of gravity of individual parts of the body (Gedliczka 2001). The static scheme of the joint beam with points of application of active forces, corresponding to gravity forces of individual body parts, and reaction forces of the base have been illustrated in Fig. 8.57.
The calculations conducted show that the share percentage of pressure in a selected point of support in relation to the sum value of support forces of the tested body model is constant and does not depend on the anthropometric measure. Share of pressures of the smallest values were reported in the support point of the head (approx. 6 %) and feet (approx. 8 %). However, it should be noted that the contact of the body in these places takes place on a very small area, which in turn leads to the formation of very high stresses inside the tissues. The largest value of the base reaction on the user’s body is observed at the contact site of the torso with the base. This is reflected in about a 44 % share of the total sum of all reaction forces.
Fig. 8.55 Models of the user s body corresponding to the measures of the 5th, 50th and 95th centile
Fig. 8.56 Determining the distribution of reaction forces of the base for the user in a lying down position
Fig. 8.57 Static scheme of the human-bed system, where: Rg—base reaction at the contact site of the head, Rt—base reaction at the contact site of the torso, Ru—base reaction at the contact site of the thigh, Rp—base reaction at the contact site of the shank, Rs—base reaction at the contact site of the foot, Rsv—base reaction at the contact site of the additional support of the foot, Qg—centre of gravity of the head, Qt—centre of gravity of the torso, Qu—centre of gravity of the thigh, Qp— centre of gravity of the shank, Qs—centre of gravity of the foot
By increasing the number of supports (places of contact of the body with the base), not only the values of reaction forces can be reduced, but also the adverse high values of bending moments. In Fig. 8.58, the courses of bending moments have been compared, in the user’s body, before and after introducing additional supports of the torso and thigh. Based on this, a clear reduction in the value of the bending moments can be observed, and as a result, an increase in the comfort of using the furniture piece.
The analysis of the mechanical human-bed system provides information about the locations of concentrated forces occurring, being the reaction of the base’s impact on the body. The analytical model also enables to specify the distribution of internal forces, especially bending moments, acting on a body that is resting on an upholstered base.