Impact severity in self-initiated sits and falls associates with center-of-gravity excursion during descent

Although the energy available during a fall from standing greatly exceeds that required to produce hip fracture, this occurs in only about 2% of falls in the elderly. This is thought to be due in part to one's ability to reduce the vertical impact velocity (nu(nu)) and kinetic energy (KE(nu)) of the body through energy absorption in the lower extremity muscles during descent. The present study tested the hypothesis that the magnitude and percent attenuation in nu(nu) and KE(nu) associate with the horizontal and vertical excursion of the body's center-of-gravity during descent. Measures were acquired of whole-body kinematics and lower extremity kinetics as young subjects underwent backward descents involving vertical drops of either thigh length (SIT) or lower extremity length (FALL), and horizontal pelvis excursions of either 33 or 66% of lower extremity length. In all trials, subjects attempted to "land as softly as possible." While attenuation in nu(nu) and KE(nu) (which averaged 62 and 92% respectively), did not associate with trial type, raw magnitudes of these parameters did, with nu(nu) averaging 2-fold greater, and KE(nu) averaging 6-fold greater, in 66% FALL than in 33% SIT or 66% SIT trials. This was due to a rapid increase in downward velocity accompanying the final stage of descent in 66% SIT and 66% FALL trials, which coincided with the knee moving posterior to the ankle. Accordingly, severe impacts likely accompany not only large fall heights, but also falls where the feet are thrown rapidly forward, as during a backward slip.