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TECHNICAL PAPERS: Joint/Whole Body

Determining Fall Direction and Impact Location for Various Disturbances and Gait Speeds Using the Articulated Total Body Model

[+] Author and Article Information
Cécile Smeesters1

Department of Mechanical Engineering, Université de Sherbrooke, 2500, boul. de l'Université, Sherbrooke (Quebec) J1K 2R1 CanadaCecile.Smeesters@USherbrooke.ca

Wilson C. Hayes

 Orthopedic Biomechanics Laboratory, Department of Orthopedic Surgery, Beth Israel Deaconess Medical Center and Harvard Medical School, 330 Brookline Avenue, RN115, Boston, MA 02215

Thomas A. McMahon2

Biomechanics Laboratory, Division of Engineering and Applied Sciences, Harvard University, Pierce Hall 110A, Cambridge, MA 02138; and Orthopedic Biomechanics Laboratory, Department of Orthopedic Surgery, Beth Israel Deaconess Medical Center and Harvard Medical School, 330 Brookline Avenue, RN115, Boston, MA 02215

1

Corresponding author.

2

Professor McMahon unexpectedly passed away on February 14, 1999, at the age of 55. He is greatly missed.

J Biomech Eng 129(3), 393-399 (Aug 06, 2006) (7 pages) doi:10.1115/1.2737432 History: Received March 21, 2004; Revised August 06, 2006

Because fall experiments with volunteers can be both challenging and risky, especially with older volunteers, we wished to develop computer simulations of falls to provide a theoretical framework for understanding and extending experimental results. To perform a preliminary validation of the articulated total body (ATB) model for passive falls, we compared the model predictions of fall direction, impact location, and impact velocity as a function of disturbance type (faint, slip, step down, trip) and gait speed (fast, normal, slow) to experimental results with young adult volunteers. The three-dimensional ATB model had 17 segments and 16 joints. Its physical characteristics, environment definitions, contact functions, and initial conditions were representative of our experiment. For each combination of disturbance and gait speed, the ATB model was left to fall passively under gravity once disturbed, i.e., no joint torques were applied, until impact with the floor occurred. Finally, we also determined the sensitivity of the model predictions to changes in the model’s parameters. Our model predictions of fall angles and impact angles were qualitatively in agreement with those observed experimentally for ten and seven of the 12 original simulations, respectively. Quantitatively, the model predictions of fall angles, impact angles, and impact velocities were within one experimental standard deviation for seven, three, and nine of the 12 original simulations, respectively, and within two experimental standard deviations for ten, nine, and 11 of the 12 original simulations, respectively. Finally, the fall angle and impact angle region did not change for 92% and 95% of the 74 input variation simulations, respectively, and the impact velocities were within the experimental standard deviations for 78% of the 74 input variation simulations. Based on our simulations and a sensitivity analysis, we conclude that our preliminary validation of the ATB model for passive falls was successful. In fact, these ATB model simulations represent a significant step forward in fall simulations. We believe that with additional work, the ATB model could be used to accurately simulate a variety of human falls and may be useful in further understanding the etiology and mechanisms of fall injuries such as hip fractures.

FIGURES IN THIS ARTICLE
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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

The standard human body configuration of the ATB model with 17 segments and 16 joints as used in the two sets of initial conditions: (a) 50% swing, used for faints (not shown) and trips, has the initial positions of all the joints aligned except for the right hip, right knee, and ankles which are flexed at 20, 80, and 90 deg, respectively; (b) double support, used for slips (not shown) and steps down, has the initial positions of all the joints aligned except for the right hip, left hip, ankles, right shoulder, and left shoulder which are flexed at 20, −20, 90, 20, and −20 deg, respectively. In both cases, the arms are also abducted at 5 deg. Initial velocities are as shown. Gait speed (v), angular velocity (ω), lower torso (LT), right and left upper arm (RUA, LUA), right and left upper leg (RUL, LUL), right lower leg (RLL), arm length (lA), leg length (lL), and lower leg length (lLL).

Grahic Jump Location
Figure 2

(a) Contact and penetration (or deflection) of one segment, plane, or obstacle into the other at any given time. (b) For any given segment-segment contact (e.g., between the lower torso and a foot), loading occurred along the baseline force-deflection curve illustrated. When the maximum deflection was reached, contact proceeded down the quadratic unloading force-deflection curve defined by an energy recovery factor (R) of 0.7, so that 30% of the energy was absorbed by the contact (shaded area). The friction coefficient (μ) was 0.25.

Grahic Jump Location
Figure 3

At impact: (a) A top view of the simulation is shown. The fall angle measures the fall direction in the floor plane (θf=90 deg, example shown). (b) The transverse cross-section of the lower torso segment that included the first impact point is shown. The impact angle measures the impact location on the pelvis ellipse with the floor plane (θi=45°, example shown). The virtual midpoint between the two ankle joints (MA), center of the lower torso (CLT), virtual midpoint between the two shoulder joints (MS), virtual leg segment (VL), virtual torso segment (VT), fall angle (θf), walking direction (WD), and impact angle (θi) are also shown.

Grahic Jump Location
Figure 4

Top view of the simulations illustrating the fall direction of the ATB model at impact for all disturbance types and gait speeds. For the slow gait speed step down, the ATB model configuration when the left knee impacted the floor plane is shown since the ATB model never impacted its lower torso with the floor plane. The locations of the step and the trip obstacle are dotted.

Grahic Jump Location
Figure 5

Front view of the simulations illustrating the impact location of the ATB model at impact for all disturbance types and gait speeds. For the slow gait speed step down, the ATB model configuration when the left knee impacted the floor plane is shown since the ATB model never impacted its lower torso with the floor plane. The locations of the trip obstacle are dotted.

Grahic Jump Location
Figure 6

On the fall circle, fall directions for the fast (●), normal (▴), and slow (∎) gait speeds for the experiment (mean±standard deviation, closed symbols) (6) and for the ATB model (open symbols). Since negative and positive fall angles are reported as equivalent, they are labeled only on the left half and plotted only on the right half. Special cases: ATB model landed on one of its feet (∗). ATB model never impacted the floor plane (†).

Grahic Jump Location
Figure 7

On the pelvis ellipse, impact locations for the fast (●), normal (▴), and slow (∎) gait speeds for the experiment (mean±standard deviation, closed symbols) (6) and for the ATB model (open symbols). Since negative and positive impact angles are reported as equivalent, they are labeled only on the left half and plotted only on the right half. Special cases: ATB model landed on one of its feet (∗). ATB model never impacted the floor plane (†).

Grahic Jump Location
Figure 8

Impact velocities for the fast, normal, and slow gait speeds for the experiment (mean±standard deviation, columns) (6) and for the ATB model (diamonds). The mean (solid line) – standard deviation (dashed line) impact velocity needed to fracture an elderly femur is also shown (6,10,13-14). Special cases: ATB model landed on one of its feet (∗). ATB model never impacted the floor plane (†).

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