Asymmetrical ground impact of the hands after a trip-induced fall: experimental kinematics and kinetics

Abstract

Background: Distal radius fractures are among the most common fall-related fractures. The manner in which the upper extremities are used for protection during a fall may exert a considerable influence on the incidence of injury. Here, we sought to determine the degree to which the assumption of sagittal plane symmetry was valid in unexpected falls after a trip, and to quantify the effects of asymmetrical upper extremity motion on impact kinematics and kinetics.

Methods: The motion of eight healthy older women who fell after being unexpectedly tripped was quantified. Impact kinematics and kinetics of 36 adults who intentionally fell onto force plates with their hands positioned either symmetrically or asymmetrically were quantified.

Findings: Just prior to safety harness engagement the wrists of the older women were not positioned or moving symmetrically relative to the midpoint between the shoulders. Asymmetry did not affect the peak reaction force magnitude, but increased the degree to which force was directed along the axis of the radius (axial component of the unit vector k = 0.949 versus k = 0.932, P = 0.026). Asymmetry resulted in greater wrist dorsiflexion (47 degrees versus 43 degrees , P = 0.019) compared to symmetrical trials and increased temporal offset (33 ms versus 11 ms, P<0.001) between right and left ground impacts.

Interpretation: Kinetics and kinematics arising from asymmetric impact may meaningfully affect the fracture strength of the distal radius. Because trip-induced falls in older women may result in asymmetric upper extremity impact, these differences in landing kinematics and kinetics due to asymmetry merit consideration when developing clinical interventions to prevent fall-related fractures.

Figure 1

Experimental setup showing the locations of the four targets; when subjects lay prone their nose touched the floor directly between the target 2 locations. In the symmetrical falls (a) each hand targeted the same target number. In asymmetrical falls, the non-dominant hand was moved from either target 4 (b) or 2 (c) to another position and the dominant hand remained at its original location (target 4 or 2, respectively).

Figure 2

Two typical subjects whose arms are moving asymmetrically immediately prior to harness engagement. Subject video (left) compared with a three-dimensional view showing wrist, shoulder, sacral, knee, and ankle marker locations, and wrist and midshoulder velocities (vectors). Velocity vectors are at 1/3 scale relative to the axis labels and wrist velocities are taken relative to the shoulder midpoint.

Figure 3

A summary of the changes in kinematics and kinetics when a hand is moved from a symmetrical position to an asymmetrical one.

Figure 4

Frontal plane reflective marker positions and wrist velocity vectors, relative to the shoulder midpoint, immediately prior to harness engagement.