Fast online corrections of tripping responses.

Tripping over obstacles is one of the main causes of falls. One potential hazard to actually fall when tripped is inadequate foot landing. Adequate landing is required to control the body's angular momentum, while avoiding dangerous surfaces (slippery patch, uneven ground). To avoid such dangers, foot trajectory needs to be controlled by inhibiting and adjusting the initiated recovery foot path during a tripping reaction. We investigated whether such adjustments can be made without jeopardizing balance recovery. Sixteen healthy young adults (25.1 ± 3.2 years) walked at their comfortable speed over a walkway equipped with 14 hidden obstacles. Participants were tripped 10 times in between a random number of normal walking trials; five trips included a projection of a forbidden zone (FZ, 30 × 50 cm) at the subject's preferred landing position. Participants were instructed to land their recovery foot outside the FZ, if the FZ was presented. Responses were evaluated in terms of foot position and body angular momentum at and following recovery foot landing. Participants successfully landed their recovery foot outside the FZ in 80% of trials, using strategies of either shortening their recovery steps (84%) or side stepping (16%). Their performance improved over trials, and some participants switched strategies. Angular momenta of the adjusted steps remained small at and following recovery foot landing. Young adults can quickly change foot trajectory after tripping by using different strategies, and without detrimental consequences on balance recovery, in terms of the angular momentum. These results open possibilities for training of tripping reactions.