Sloped Walking Surface Characteristics Modulate Unconstrained Slip Mechanics and Subsequent Fall Risk

Slips are a common cause of community-based falls, however existing literature has not examined slips during sloped surface walking, which is biomechanically distinct from level walking in ways that likely increase the risk of a slip. We examined the influence of various sloped walking characteristics on slip mechanics, compensatory stepping reactions, and fall rates. Twenty-one young adults experienced 27 unconstrained slip perturbations while walking on a treadmill inclined to 0°, 5°, or 10° in an uphill, downhill, or cross-slope direction while full-body motion capture and harness load cell data were recorded. Fall rates increased with slope angle, decreased with later slip onset, and were not dependent on slope direction. Slips shortened, slowed, and changed from anteriorly to posteriorly directed at later onset phases, eliciting longer, more anteriorly placed compensatory steps as a result. Steeper slope angles caused longer, faster slips that required shorter compensatory steps. Slope directions influenced slip directions and compensatory steps: uphill slopes were associated with more posterior slips and longer anterior steps, while downhill slopes caused the opposite. On cross-slopes, up-slope foot slips were shorter, slower, contralateral, and led to more anteriorly placed compensatory steps compared to down-slope foot slips, but fall rates did not differ. Our results are surprising given the slope direction-specific kinetics of sloped walking, suggesting that other factors play outsized roles in determining slip-and-fall risk. Perturbation-based balance training should incorporate sloped walking contexts in comprehensive protocols to assess if balance recovery skills can be enhanced beyond what is attainable from level training alone.