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

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° versus 43°, 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.