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

At least 280,000 hip fractures occur annually in the U.S., at an estimated cost of $9 billion. While over 90 percent of these are caused by falls, only about 2 percent of all falls result in hip fracture. Evidence suggests that the most important determinants of hip fracture risk during a fall are the body’s impact velocity and configuration. Accordingly, protective responses for reducing impact velocity, and the likelihood for direct impact to the hip, strongly influence fracture risk. One method for reducing the body’s impact velocity and kinetic energy during a fall is to absorb energy in the lower extremity muscles during descent, as occurs during sitting and squatting. In the present study, we employed a series of inverted pendulum models to determine: (a) the theoretical effect of this mechanism on impact severity during a backward fall, and (b) the effect on impact severity of age-related declines (or exercise-induced enhancements) in lower extremity strength. Compared to the case of a fall with zero energy absorption in the lower extremity joints, best-case falls (which involved 81 percent activation of ankle and hip muscles, but only 23 percent activation of knees muscles) involved 79 percent attenuation (from 352 J to 74 J) in the body’s vertical kinetic energy at impact $KEv,$ and 48 percent attenuation (from 3.22 to 1.68 m/s) in the downward velocity of the pelvis at impact $vv.$ Among the mechanisms responsible for this were: (1) eccentric contraction of lower extremity muscles during descent, which resulted in up to 150 J of energy absorption; (2) impact with the trunk in an upright configuration, which reduced the change in potential energy associated with the fall by 100 J; and (3) knee extension during the final stage of descent, which “transferred” up to 90 J of impact energy into horizontal (as opposed to vertical) kinetic energy. Declines in joint strength reduced the effectiveness of mechanisms (1) and (3), and thereby increased impact severity. However, even with reductions of 80 percent in available torques, $KEv$ was attenuated by 50 percent. This indicates the importance of both technique and strength in reducing impact severity. These results provide motivation for attempts to reduce elderly individuals’ risk for fall-related injury through the combination of instruction in safe falling techniques and exercises that enhance lower extremity strength.