MINIMUM TOE CLEARANCE ADAPTATIONS TO FLOOR SURFACE IRREGULARITY AND GAIT SPEED IN OLDER ADULTS 1 Brian Schulz, 1 Stephanie Hart-Hughes, and 1 Tatjana Bulat 1 VA HSR&D/RR&D Center of Excellence in Maximizing Rehabilitation Outcomes, James A. Haley VA Hospital, Tampa, Florida, USA email: Brian.Schulz@va.gov INTRODUCTION Toe speed during gait generally nears its maximum while its height reaches a local minima halfway through swing phase. Trips are thought to frequently occur at these local minima (minimum toe clearance or MTC events) and trip risk has been quantified using the minimum distance between the toe and ground here (MTC) [1]. Previous work has shown age to have little to no effect on MTC [2], but overground MTC on surfaces with obstacles has never before been evaluated in older adults. METHODS Unimpaired younger (7 male, 7 female, age=26±5) and older (7 male, age=73±7) subjects each traversed a 4.88m walkway 4 times at slow, preferred, and fast speeds across surfaces with no obstacles, visible obstacles, and hidden obstacles. Both surfaces with obstacles had the same random obstacle configuration. Shoe and body segment motions were tracked using passive markers and MTC and kinematics calculated using previously- described methods [3]. Also as per these methods [3], MTC events were defined to occur at a point in the swing phase where the following criteria were met: 1) The minimum toe clearance was at a local minima (value is less than that for preceding or following two frames), 2) Toe segment centroid speed (mean speed of the four toe markers) was within the upper quartile for that step, and 3) The minimum toe clearance was lower than the minimum heel clearance. If these criteria were met by more than one point per gait cycle (common on surfaces with obstacles), then the point with the smaller MTC value was used. SAS was used for all statistical analyses and main effects were considered significant at p<0.014 after Bonferroni corrections for multiple statistical tests. RESULTS AND DISCUSSION Older subjects had similar MTC values as younger subjects (Older=22.7±12.5mm & Young=20.0±9.4), but they adapted the shape of their minimum toe clearance time histories more markedly to the presence of obstacles (see Fig. 1 for exemplar data). This resulted in a smaller percentage of valid MTC events (MTC event occurrence) for the same number of opportunities in older subjects (Fig. 2, p=0.0006). The only other significant age effect was a 12º greater downward head tilt in the older subjects (p=0.001) to look at obstacles on the floor. All other effects were similar to those for younger subjects [3]. All MTC and kinematic variables tested significantly increased with faster instructed gait speed except the likelihood of MTC event occurrence (local minima in minimum toe clearance trajectory when foot is in upper quartile of speed). MTC events were less frequent on surfaces with obstacles (80% vs. 98% for no obstacles). MTC values, when present, were doubled by the presence of visible obstacles (23.9±8.1mm vs. 10.4±5.3mm) and further increased to 28.3±8.2mm when these obstacles were hidden (all comparisons p<0.0001). Ankle dorsiflexion and knee and hip flexion angles at MTC all significantly increased (Table 1, p<0.0001) with flooring surfaces challenge, but foot segment angles were not affected by flooring surface. CONCLUSIONS Younger and older subjects adapt similarly to changes flooring surface and instructed gait speed. The greater head tilt and reduced frequency of minimum toe clearance event occurrence in the older subjects may indicate less confidence in gait across these surfaces and greater caution, despite the similarities in MTC values and joint kinematics.