Further evidence on the dynamics of self-injurious behaviors: impact forces and limb motions.

The dynamics of self-injurious behaviors (SIBs) were examined in 8 adults with mental retardation. The trajectories of the arm movements and the impact forces of the head blows were determined from a dynamic analysis of videotapes of discrete bouts of self-injury. The results revealed a high degree of cycle-to-cycle consistency in the qualitative dynamics of the limb motions, indicating that the motions involved in SIB are often stereotyped in nature. The resultant individual peak impact forces ranged from 50 to 1560 N. The impact forces of SIB as a percentage of body mass are either near or at the low end of forces generated in boxing blows and karate hits.