An Analysis of the Effect of Lower Extremity Strength on Impact Severity During a Backward Fall

[+] Author and Article Information
Reuben Sandler, Stephen Robinovitch

Biomechanics Laboratory, Department of Orthopaedic Surgery, San Francisco General Hospital and University of California, San Francisco, San Francisco, CA 94110

J Biomech Eng 123(6), 590-598 (May 16, 2001) (9 pages) doi:10.1115/1.1408940 History: Received July 02, 1999; Revised May 16, 2001
Copyright © 2001 by ASME
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Typical temporal variations in the vertical (downward) velocity of the pelvis, vertical kinetic energy of the body, link inclinations (with respect to the vertical), joint rotations, and joint torques during a simulated fall with the three link model. Ankle torque is dorsiflexor and eccentric throughout descent. Knee torque is extensor and eccentric until the final stage of descent. Hip torque is initially flexor and concentric, but switches to extensor (and eccentric) when the trunk moves forward to the vertical. This is accompanied by abrupt declines in the slopes of the downward velocity and kinetic energy traces.
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Association between total joint work (Wtot) and (a) vertical kinetic energy (KEv) at the instant of impact and (b) pelvis velocity (vv) at the instant of impact. For each of the three models, Wtot associated with KEv and vv (see text for correlation statistics). However, variations in KEh and knee flexion at impact caused considerable scatter in these relations for the three-link model.
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Effect of available strength factors on total joint work during descent (Wtot), change in potential energy during descent (ΔPE), vertical kinetic energy at impact (KEv), horizontal kinetic energy at impact (KEh), and rotational kinetic energy at impact (KErot). Conservation of energy dictates that ΔPE=Wtot+KEv+KEh+KErot. Declines in available strength factors cause an increase in KEv, by reducing Wtot and (for the three-link model) decreasing KEh. When compared to the one link model, ΔPE and KEv values are lower in simulations with the two-link and three-link models. This is due to hip flexion and impact of the trunk in a nearly vertical orientation.
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One-link inverted pendulum model. See Appendix A for definition of variables.
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Two-link inverted pendulum model. See Appendix A for definition of variables.
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Typical descent kinematics associated with: (a) one-link model, (b) two-link model, and (c) three-link model (Ta=ankle torque; Tk=knee torque; Th=hip torque; θa=ankle rotation; θk=knee rotation, θh=hip rotation).




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