Figure 1.

Schematic of the lumped-parameter model used to calculate spine forces during a backward fall onto the buttocks. The upper body, L4, L5, sacrum, pelvis, and skin overlying the ischial tuberosities were modeled as masses separated by linear springs and dampers, which represented the L3/L4, L4/L5, L5/S1, sacroiliac joints, and the soft tissue overlying the ischial tuberosities (buttocks). The floor was represented as a linear spring.