On minimizing injury risk in forward and lateral falls: Effects of muscle strength, movement strategy, and age.

Abstract

Fall-related injuries are a leading cause of morbidity and mortality, particularly in the elderly. We hypothesized that the severity of the impact force on each body part striking the ground in a fall depends several intrinsic factors including available muscle strength, gait speed, fall direction, reaction time, and pre- and post-ground contact body segment movement strategies. Healthy young males learned how to significantly reduce wrist impact by 15% in a sagittally-symmetric forward fall from waist height, and then retained that skill for three months (Chapter 2). Biomechanical computer simulations of those falls (Chapter 3 & 4) showed that a 25% decrease in upper extremity muscle strength required elbow flexion angles of less than 15° at impact in order to prevent buckling, but wrist impact increased by 56%. Similarly, the reason why healthy young women do not allow significant post-contact elbow flexion is their 31% smaller arm extensor muscle strength than men. Computer simulations of sagittally-symmetric falls from near-standing height (Chapters 5--7) showed that proper pre-contact upper and lower segment configurations reduced wrist impact forces by up to 58% and 66%, respectively, because of negative work done by lower extremity muscles and reduced potential energy loss during the fall. Increasing initial forward velocity of the body, from 0 to 2 m/s, increased impact severity 2- to 4-fold, depending on the strategy used. The usual age-related decline in muscle strength in older women necessitated the use of pre-contact knee flexion in order to avoid head impact. Young males who fell from near-standing height chose such a knee flexion strategy to land hands and knees simultaneously. Knee impact prior to hand impact increased knee impact force from 72% to 202% body-weight but decreased wrist impact from 97% to 70% body-weight. Computer simulations of lateral falls (Chapter 8) showed that use of the ipsilateral arm can reduce hip and shoulder impact by 26 and 56%, respectively. Without arm use, rolling the lateral aspect of the leg, and posterolateral aspect of the pelvis and torso onto the ground reduced hip impact force by up to 73%. This strategy was still effective in the presence of 30% muscle strength decline, but needed rapid decision making and proper sequential segmental coordination.