What causes a crossover step when walking on uneven ground? A study in healthy young women.

We hypothesized that, during gait, the perturbation caused by stepping on a 1.2-cm high protuberance with the medial forefoot would not only alter stance foot kinematics and kinetics, but also alter subsequent step location and timing, even to the point of causing a crossover step. Twelve healthy young women performed at least three flat surface and three perturbation trials while walking along a level walkway. To obstruct visual monitoring of their upcoming foot placement they used both hands to carry a large tray in front of their abdomen. Kinematic data from optoelectronic markers and force plate data were recorded at 50 and 100Hz, respectively. The results showed that the medial forefoot perturbation significantly decreased the net ankle eversion moment (p<0.001) and increased the maximum inversional foot acceleration (p=0.007). Step width (SW) and time (ST) of the post-perturbation recovery step were significantly decreased compared with normal steps (p=0.002 and 0.049, respectively). The maximum inversional foot acceleration correlated negatively with SW and ST of the recovery step (both p=0.001). Finally, four of 36 (12%) recovery steps were crossover steps. We conclude that a medial forefoot perturbation results in an increased inversional acceleration of the stance foot followed by a decrease in recovery SW and ST. In some cases, a medial forefoot perturbation can result in a crossover step, an extreme form of a narrow recovery step.