During everyday life, healthy adults occasionally trip over an obstacle that they knew was there. These 'spontaneous' trips can provide insight into the circumstances leading to trips and falls. The goal of this study was to describe the errors in foot placement and/or foot elevation that resulted in a spontaneous contact with a fixed, visible obstacle in young, healthy adults. Fifteen subjects stepped over an obstacle (height set to 25 % leg length) placed in the middle of an 8 m walkway, up to 300 times. Three subjects never contacted the obstacle and 12 subjects contacted the obstacle 1-4 times, totaling 24 contacts in 3,843 trials (0.6 %). Most of the contacts (92 %) were with the trail limb. Minimum foot clearance of the trail limb (trail MFC) decreased linearly (average slope of -1 mm/trial) with repeated trials. The majority of subjects (70 %) continued the linear decrease of trail MFC until they contacted the obstacle. The remaining contacts resulted from an apparent misjudgment of foot placement and/or foot elevation. Following contact, trail MFC increased 75 % in the subsequent trials and remained elevated at least up to 30 trials post-contact, but the trajectory of the unperturbed lead limb did not change, further supporting the idea of independent control for the lead and trail limbs during obstacle crossing. Possible causes of the progressive decrease in trail MFC until obstacle contact are considered.

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