Dynamic Models for Sideways Falls From Standing Height

Abstract

Despite our growing understanding of the importance of fall mechanics in the etiology of hip fracture, previous studies have largely ignored the kinematics and dynamics of falls from standing height. Beginning from basic principles, we estimated peak impact force on the greater trochanter in a sideways fall from standing height. Using a one degree-of-freedom impact model, this force is determined by the impact velocity of the hip, the effective mass of that part of the body that is moving prior to impact, and the overall stiffness of the soft tissue overlying the hip. To determine impact velocity and effective mass, three different paradigms of increasing complexity were used: 1) a falling point mass or a rigid bar pivoting at its base; 2) two-link models consisting of a leg segment and a torso; and 3) three-link models including a knee. The total mechanical energy of each model before falling was equated to the total mechanical energy just prior to impact in order to estimate the hip impact velocity. In addition, the configuration of the model just before impact was used to estimate the effective mass. Our model predictions were compared with the results of an earlier experimental study with young subjects falling on a 10-inch thick mattress. Values from literature were used to estimate the soft tissue stiffness. For the models, predicted values for hip impact velocity and effective mass ranged from 2.47 to 4.34 m/s and from 15.9 to 70.0 kg, respectively. Predicted values for the peak force applied to the greater trochanter ranged from 2.90k to 9.99k N. Based on comparisons to the experimental falls, impact velocity and impact force were best predicted by a simple two-link model with the trunk at 45 degrees to the vertical at impact. A three-link model with a quadratic spring incorporated in the knee of the model was the best predictor of effective mass. Using our most accurate model, the peak impact force was 2.90k N for a 5th percentile female and 4.26k N for a 95th percentile female, thereby confirming the widely held perception that "the bigger they are, the harder they fall".

Effect of compliant flooring on impact force during falls on the hip

“…First, evidence suggests that sideways falls from standing (onto a rigid floor) generate impact forces that are just above the mean value required to fracture the elderly cadaveric femur. 13,18,19 Therefore, even a small reduction in impact force may prevent fractures. Second, while hip protectors have been shown to reduce fracture risk in clinical trials by as much as 50%, 20 those same devices reduce peak impact force by only 15-20% in laboratory testing.…”

Experimental Validation of Subject-Specific Dynamics Model for Predicting Impact Force in Sideways Fall

“…Where α is the angle between the vertical and the projection of the shank on y-z plane; β is the angle between the shank and its projection on x-z plane; γ is the angle between the vertical and the projection of the trunk on y-z plane. The angle between the trunk and its projection on x-z plane, κ, is considered as a constant in a fall [9]. However, it may have different value in different fall depending on the initial conditions.…”

“…The mass (M) is the so-called effective mass, representing only the part of the body that has contribution to the impact force. The method for calculating the effective mass is described in [9] and the main formulas are summarized in Appendix C. The effective mass is related to the kinematic configuration of the body before the impact. Therefore, it is not a constant even for the same subject.…”

“…It is necessary to study fall dynamics and hip fracture risk to effectively prevent hip fractures. Van den Kroonenberg proposed a three-link dynamic model for studying sideways fall from standing height [9]. Laing and his colleagues attempted to identify hip stiffness by studying experimentally obtained force-deflection relation of the pelvis affected by a lateral impact to the hip [10].…”

“…Therefore, they have very limited accuracy in predicting fall-induced impact force for a specific individual. In this study, a previously developed dynamics model [9] is improved to make it subject-specific. The improved model is then validated by controlled and protected fall tests using young volunteers.…”