Arresting forward falls with the upper extremities: Biomechanical factors affecting impact forces in young and old humans.

Abstract

Fall-related injuries costs the United States more than $50 billion per annum. This thesis addresses the biomechanics of using the upper extremities to arrest a forward fall to the ground. This common type of fall is associated with wrist fractures, elbow and shoulder soft-tissue injuries, and head injuries. A study of 20 healthy young and 20 older (>65 years) subjects showed that age, gender and perceived threat significantly affected how quickly subjects moved their upper extremities into a protective posture. Even the slowest subjects moved their hands into the protective posture in less than half the time available in a forward fall. A ballistic pendulum was used in similar groups of adults to demonstrate that peak hand reaction force is mainly determined by the inertial properties of the arm: wrist impact force decreased significantly as impact velocity decreased (50 percent/m/s), and as initial elbow flexion angle decreased (0.9 percent/degree). The impact force propagated to the elbow in 7 msec and shoulder in 30 msec, too rapidly for muscular reflexes to protect those structures after impact. When five healthy young males were released from a forward lean so that they fell 40 cm onto a padded surface. Peak impact forces on each hand varied from 0.7- to 1.5-times body weight. Within only four trials subjects were able to reduce the peak hand impact force by an average of 55%, compared to a stiff-arm landing. This reduction was achieved by slightly flexing their elbows and reducing their hand velocity prior to impact. A six link, rigid body, mathematical model was first validated using the above data. It was then used to simulate different fall arrest strategies, as well as the effects of aging on such strategies. Age-related losses in arm muscle strength in those over 65 years were predicted to substantially reduce the ability to arrest a forward fall without subsequent torso or head impact. Older females with below-average bone quality are at risk for wrist fracture when their center of mass falls through 23 cm or more.