Short communication Influence of an unexpected perturbation on adaptive gait behavior Christopher K. Rhea a, *, Shirley Rietdyk b a Department of Kinesiology, University of North Carolina at Greensboro, Greensboro, NC, USA b Department of Health and Kinesiology, Purdue University, West Lafayette, IN, USA 1. Introduction Functional mobility requires that gait is adapted to accommo- date or avoid obstacles in the environment. When stepping over an obstacle, maladaptation to the obstacle’s height could cause a trip and potentially a fall. Falls can have a tremendous financial cost and lead to decreased activity levels due to a lack of confidence or the inability to perform locomotor tasks [1,2]. To gain insight into the manner in which a trip is avoided, researchers have reported lower limb kinematics when approaching and clearing an obstacle [3,4]. Occasionally, obstacle contact spontaneously occurs [e.g., 5]. If enough contacts are observed, further insight into adaptive locomotor control can be gained. It is conceivable that obstacle contact may proactively change the behavior used to clear the obstacle in subsequent trials. For example, a subject may modify the position of the foot placement in front of the obstacle (termed ‘horizontal distance’) and/or raise their toe higher (termed ‘toe clearance’ and ‘peak toe elevation’) when stepping over the obstacle in subsequent trials to proactively reduce the likelihood of future obstacle contacts. Horizontal distance and toe clearance are correlated in non-perturbed adaptive gait [6], but it is unknown if they are both modulated following an unexpected perturbation. Furthermore, if gait is proactively modulated following obstacle contact, the magnitude and persistence of the modulation is unknown. Additionally, if the control of each limb is independent, as has been suggested [7–9], the changes should be limited to the perturbed limb. This study examined changes in gait behavior after a spontaneous obstacle contact. Two hypotheses were tested. First, a larger horizontal distance, toe clearance and peak toe elevation would be observed in the trials immediately following a trip, but these measures would decrease in subsequent trials. Second, the effect of the trip would only be observed in perturbed limb (i.e., limb independence would be observed). 2. Methods Data were examined from two previously reported studies in which at least one obstacle contact was observed [10,11]. Study protocol and written informed consent was approved by the local IRB. Participants stepped over an obstacle for either 40 [10] or 100 [11] consecutive trials with either unobstructed or partially obstructed vision. The obstacle was 10 cm by 78 cm by 0.5 cm (height by width by depth) and was held upright by L-brackets attached to the bottom. Vision was partially obstructed by goggles that blocked the lower visual field information up to approximately two steps ahead of the participants. Of the 15 participants from the previous studies, seven contacted the obstacle once and one contacted the obstacle twice. A total of nine contacts were observed in 1140 trials (0.8%) and all contacts were with the second limb that crossed the obstacle (termed ‘trail limb’). Six contacts occurred during partially obstructed vision, three with unobstructed vision. Six of the nine contacts had at least four trials before and eight trials after the trip to allow for comparison between pre- and post-contact. Five of those contacts were with partially obstructed vision, one was with unobstructed vision. This study focused on the five contacts with the trail limb that occurred with partially obstructed vision. Although five contacts is a relatively low number of trials to examine adaptive gait behavior, consistent behavior following the contact would Gait & Posture 34 (2011) 439–441 A R T I C L E I N F O Article history: Received 9 December 2010 Received in revised form 16 June 2011 Accepted 17 June 2011 Keywords: Gait Adaptive gait Biomechanics Perturbation A B S T R A C T During locomotion over uneven terrain, gait must be adapted to avoid a trip. In the event of a foot- obstacle contact, the body reactively responds to the perturbation. However, it is unknown if any proactive adjustments are made in subsequent strides to reduce the likelihood of another contact, and how long any proactive adaptations persist. This study examined gait behavior while stepping over a 10 cm obstacle placed in the middle of an 8 m walkway. The four obstacle crossings that preceded a spontaneous obstacle contact were compared to the eight obstacle crossings subsequent to the contact. Foot position before the obstacle was not modified following the obstacle contact. However, toe clearance and peak toe elevation increased in the limb that was tripped; the unperturbed limb showed no differences. These findings demonstrate that the sensory information of the perturbed limb proactively influenced the ipsilateral but not the contralateral limb, supporting the idea that the lead and trail limb are controlled independently during obstacle crossing. The proactive adaptation lasted for at least eight trials, suggesting that an unexpected perturbation influences the control of adafptive gait well after obstacle contact. ß 2011 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 4013684006. E-mail address: ckrhea@uncg.edu (C.K. Rhea). Contents lists available at ScienceDirect Gait & Posture jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/g aitp os t 0966-6362/$ – see front matter ß 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2011.06.011