Older adults experience obstacle-related collisions more frequently than younger adults when stepping over obstacles of a certain height (1), increasing their risk of falls (2–4). In particular, older adults typically demonstrate lower foot clearance step over obstacles, especially with the trailing limb (1, 5), and this clearance tends to be more variable (6–10). Given that these age-related changes increase the risk of tripping (9, 10), interventions aimed at reducing the incidence of obstacle collisions in older adults are necessary.

Following a tripping event, preventing falls requires adequate control over the forward shift of the body's center of gravity to maintain balance. Common strategies to recover balance include applying force to the ground with the supporting limb and/or executing a recovery step with the swing limb (11, 12). However, numerous studies have shown that aging may impair the ability to employ such strategies. For example, the peak ankle joint moment in the supporting limb is reduced in older adults (12), and the timing of muscle activation for the hip flexors and knee extensors is delayed when executing a rapid stepping strategy with the swing limb (13). Additionally, step reaction times also tend to be delayed in older adults (14). Given these age-associated limitations, the most effective approach for preventing a fall is to lift the foot sufficiently and avoid tripping altogether.

Recently, we demonstrated that placing the leading limb close to the obstacle after stepping over it effectively increased the toe height of the trailing limb, thereby reducing collisions (15). The intervention targeted trailing limb collision avoidance, as collisions involving the trailing limb occurred more commonly than collisions involving the leading limb (16–18). We conducted this experiment in a virtual reality (VR) environment so that physical collisions, which could dramatically change behavior (16) and hinder the effect of experimental manipulation to improve behavior, did not occur. In the study by Hakamata et al. (15), placing the leading limb close to the obstacle after crossing may cause the stance limb to be placed slightly further away from the obstacle, creating adequate space to avoid collisions of the trailing limb. Further, placing the leading limb close to the obstacle could also lead to careful control over limb movement due to a speed-accuracy trade-off. Notably, Chou and Draganich have already examined the effect of altering foot placement on contact rates (19). The difference between their study and Hakamata et al. is that, while Chou and Draganich altered the trail foot placement before the obstacle, Hakamata et al. altered the lead limb placement after the obstacle. This change was based on the suggestion from a previous study indicating that the trailing limb may have a lower priority for cognitive information processing when planning movement (20), possibly because the leading limb has a higher risk of causing a fall when it collides with an obstacle compared to the trailing limb (16). We therefore considered that altering the foot placement of the leading limb could avoid the risk of reduced control priority for the leading limb, as well as achieving similar benefits to modifying the foot placement of the trailing limb.

In that study, we compared three specified conditions for foot placement of the leading limb after stepping over the obstacle: a position relatively close to the obstacle (closer condition), a naturally selected position (middle condition), and a position relatively far from the obstacle (farther condition). A fourth condition allowed participants to choose foot placement freely without specification (control condition). The results showed that the trailing limb collision rate was lower for the closer condition than for the other three conditions. The results also showed that in the closer placement, the trailing limb passing speed decreased, suggesting greater careful control over limb movement. These findings suggest that, at least in young individuals, placing the leading limb closer to the obstacle after crossing may facilitate safe obstacle avoidance by the trailing limb.

The present study examined the effectiveness of this intervention in healthy older adults. In contrast to Hakamata et al. (15), this study was conducted in a real environment to avoid the potential risks of imbalance associated with walking while wearing a relatively heavy head-mounted display and to avoid VR sickness. Participants were divided into two groups: a closer group and a control group. This grouping was informed by previous findings, where only the closer condition demonstrated a reduction in trailing limb collisions (15), whereas the middle and farther conditions did not. The control group was included for direct comparison with the closer group. We hypothesized that an intervention involving closer foot placement of the leading limb relative to the obstacle would effectively reduce trailing limb collisions. We also hypothesized that by careful control of limb movement, indicated by slower passing speeds of the trailing limb, increased toe height, and reduced variability in toe height, would be observed. As explained, Chou and Draganich (19) have already examined the effect of altering foot placement on contact rates. The strengths of our current study, in comparison to their study, are the inclusion of a control group and the shorter duration required for the intervention.

Participant characteristics are summarized in Table 1. No significant differences were found between the two groups across any measurements.

The mean foot placement of the leading limb after stepping over the obstacle in each experimental group is shown in Figure 3. The ANOVA showed no main effect of group [F (1, 19) = 0.63, p = 0.43, η_p² = 0.03, ns]. However, a significant main effect of the test was observed [F (1, 19) = 21.81, p < 0.001, η_p² = 0.53], indicating that both groups showed a significant change in the mean foot placement of the leading limb from pre-test to post-test. The interaction between the two factors was not significant [F (1, 19) = 0.38, p = 0.54, η_p² = 0.01, ns].

The mean collision rate of the trailing limb for each experimental group is shown in Figure 4. The ANOVA conducted on the logit -transformed collision rate data showed that neither the main effect of group [F (1, 19) = 0.34, p = 0.56, η_p² = 0.01, ns] nor the main effect of the test [F (1, 19) = 0.75, p = 0.39, η_p² = 0.03, ns] was significant. However, a significant interaction effect was observed [F (1, 19) = 7.08, p = 0.01, η_p² = 0.27], with the collision rate in the closer group significantly lower in the post-test compared to the pre-test (p = 0.02).

The mean toe height of the leading limb in each experimental group is shown in Figure 5A. The ANOVA showed no significant main effect of group [F (1, 19) = 2.83, p = 0.10, η_p² = 0.12, ns]. However, a significant main effect of the test was observed [F (1, 19) = 9.81, p = 0.005, η_p² = 0.34], indicating that both groups showed a significant change in the mean toe height of the leading limb from pre-test to post-test. Additionally, the interaction between the two factors was not significant [F (1, 19) = 0.48, p = 0.49, η_p² = 0.02, ns].

The mean toe height of the trailing limb in each experimental group is shown in Figure 5B. The ANOVA showed no significant main effect of group [F (1, 19) = 0.01, p = 0.89, η_p² = 0.00, ns] and no significant main effect of the test [F (1, 19) = 1.37, p = 0.25, η_p² = 0.06, ns]. Additionally, the interaction between the two factors was not significant [F (1, 19) = 0.28, p = 0.59, η_p² = 0.01, ns].

The mean toe height variability of the leading limb in each experimental group is shown in Figure 6A. The ANOVA showed no significant main effect of group [F (1, 19) = 0.05, p = 0.81, η_p² = 0.00, ns]. However, a significant main effect of the test was observed [F (1, 19) = 24.47, p < 0.001, η_p² = 0.56], indicating that both groups showed a significant change in the mean toe height variability of the leading limb from pre-test to post-test. Additionally, the interaction between the two factors was not significant [F (1, 19) = 0.01, p = 0.89, η_p² = 0.00, ns].

The mean toe height variability of the trailing limb in each experimental group is shown in Figure 6B. The ANOVA revealed no significant main effect of group [F (1, 19) = 0.18, p = 0.67, η_p² = 0.00, ns] or test [F (1, 19) = 2.45, p = 0.13, η_p² = 0.11, ns]. However, a significant interaction effect was observed [F (1, 19) = 6.41, p = 0.02, η_p² = 0.25], with toe height variability in the closer group significantly lower in the post-test compared to the pre-test (p < 0.01).

The mean heel clearance of the leading limb in each experimental group is shown in Figure 7A. The ANOVA revealed a significant main effect of group was observed [F (1, 19) = 7.66, p = 0.01, η_p² = 0.28], indicating that the heel clearance of the leading limb differed between the groups. No significant main effect of test [F (1, 19) = 3.33, p = 0.08, η_p² = 0.14, ns] or interaction was observed [F (1, 19) = 0.20, p = 0.65, η_p² = 0.01, ns].

The mean walking speed at the obstacle crossing of the trailing limb in each experimental group is shown in Figure 7B. The ANOVA indicated no significant main effect of group [F (1, 19) = 2.88, p = 0.10, η_p² = 0.13, ns] or test [F (1, 19) = 1.37, p = 0.25, η_p² = 0.06, ns]. However, a significant interaction effect was observed [F (1, 19) = 4.56, p = 0.04, η_p² = 0.19]. In the closer group, walking speed was significantly slower in the post-test compared to the pre-test (p = 0.02), and post-test walking speed was significantly slower in the closer group than in the control group (p = 0.04).

In this study, we examined whether an intervention to place the foot placement of the leading limb closer after crossing an obstacle would reduce collisions in the trailing limb of healthy older adults. As hypothesized, the results showed that the collision rate of the trailing limb in the closer group was significantly lower in the post-test than in the pre-test (Figure 4). Additionally, reductions in both toe height variability and walking speed for the trailing limb further supported this hypothesis (Figures 6B, 7B). Although toe height tended to be greater in the closer group (Figure 5B), this increase was not statistically significant, providing partial support for the second hypothesis related to careful controlled limb movement. These findings suggest that an intervention involving placing the leading limb closer to the obstacle effectively enhances collision avoidance for the trailing limb, even among healthy older adults.

When the collisions of the trailing limb were reduced, careful control was observed. Previous research has shown that when attentional demands increase during stepping tasks, individuals reduce toe height and decrease variability in foot lift for both the leading and trailing limbs to minimize the risk of tripping and associated injury (33). Similarly, in tasks requiring greater attentional load during walking, older adults have been shown to reduce foot placement variability, maintain a consistent foot height (34), and reduce walking speed (35). These modifications in gait are understood as strategies to enhance stability and reduce the likelihood of tripping or falling (34, 35). In other words, during tasks that require heightened attention, older adults may exhibit careful control by reducing foot placement variability and decreasing walking speed. Additionally, it has been noted that older adults tend to place their leading limb closer to obstacles after stepping over them compared to younger adults (29). Considering these previous studies, the present findings suggest that the intervention to place the leading limb closer to the obstacle encouraged participants to adopt strategies that reduce toe height variability and walking speed, thereby supporting tripping and falling prevention. Moreover, both groups showed an increase in the mean foot height and a decrease in the variability of toe height for the leading limb in the post-test, with the placement of the leading limb's foot becoming closer to the obstacle (Figures 3, 5A, 6A). These changes were similar in both groups, indicating that the repeated obstacle-crossing task might have influenced older adults.

Careful control facilitated safe obstacle avoidance, consistent with the speed-accuracy trade-off principle a foundational concept in human motor control (36, 37). Previous research has shown that trade-offs apply to obstacle avoidance (15, 38), walking (39), and stepping movements of the lower limbs (40). Patla et al. (41) noted that the vertical speed of the trailing limb exceeds that of the leading limb during obstacle crossing. The observed reduction in trailing limb collisions among younger participants in a VR environment has been attributed to a slower anterior-posterior passing speed (15). Moreover, increased variability in obstacle clearance can elevate the risk of contact, potentially increasing the risk of tripping and falling (42). Based on these previous studies, the findings of this study suggest that strategies aimed at reducing toe height variability and slowing passing speed were employed when trailing limb collisions were reduced.

Placing the leading limb closer to an obstacle is generally considered to increase collision risk (43). However, careful control collision avoidance is more feasible for the leading limb than the trailing limb due to the availability of visual, particularly peripheral, input for controlling the leading limb's movements (18, 44). In this study, no collisions were observed for the leading limb. By placing the leading limb closer to the obstacle, conditions were established that reduced the likelihood for trailing limb collisions. Previous studies have proposed the possibility of an interaction between the leading and trailing limbs (45) and indicated that certain visual-cognitive processes involved in obstacle crossing are shared between the two limbs (46). Additionally, it has been suggested that the trailing limb may use proprioceptive information from the leading limb during crossing movements (47). Based on these previous studies, this study leveraged visual information from the leading limb to enable foot placement closer to the obstacle after crossing without incurring collisions. The trailing limb may reduce collision risk by utilizing shared information from the leading limb during the crossing movement.

One may assume that the intervention in the current study may increase the risk of the contact with the heel of the leading limb. Previous studies have indicated that the heel of the leading limb is more likely to be in contact with the obstacle than the toe (30, 31). Given that that recovering from a lead limb trip is more challenging than recovering from a trail limb trip and that the leading limb is more prone to collisions during the late swing phase (48), placing the heel of the leading limb too close to an obstacle after stepping over it is not advisable. To avoid the risk, it was necessary to appropriately place the marked position to be stepped on, ensuring enough space to avoid a collision, even if it meant landing closer to the obstacle than usual. The results of the current study showed that (a) the mean foot placement of the leading limb after stepping over the obstacle was approximately 20 cm in the post-test for the closer group (Figure 3) and (b) no collision with the leading limb occurred. Additionally, we confirmed that the heel clearance of the leading limb was sufficient to avoid colliding with the obstacle (Figure 7A). Although a difference in heel clearance was observed between the two groups, in the closer group, the value of heel clearance did not decrease significantly after the intervention, suggesting that the intervention did not reduce heel clearance. In fact, no collisions occurred in any of the trials. Taken collectively, these findings suggest that our setting did not increase the risk of contact with the heel of the leading limb. In other words, this study's intervention demonstrated a reduction in trailing limb collisions without causing leading limb collisions.

In this research, the collision rate during the pre-test was approximately 25%, which is higher than the 0.6% reported in a previous study (16). We consider that this relatively higher collision rate is primarily attributed to the experimental setup, where collision feedback was absent, making it difficult for participants to notice collisions. In support of this interpretation, Heijnen et al. reported that collision rates increase to approximately 47% when collision feedback is absent and obstacles are invisible (17). Additionally, age-related changes in the use of sensory input to monitor the movement of the trailing limb may have been involved. Previous research has indicated that older adults tend to rely more on their central vision than younger individuals while walking (49). Because the obstacle is out of sight when stepping over it with the trailing limb, older adults may have found it difficult to accurately monitor the movement of the trailing limb in situations where no collision feedback was provided.

This study has several limitations. Firstly, although the required number of participants was met according to the a priori power analysis, it cannot be ruled out that the unequal sample sizes between the closer group and the control group may have influenced the results (i.e., an increased risk of Type I errors). Second, the effectiveness of the intervention may be specific to a group of healthy older adults. Future research should examine whether the collision reduction effects extend to individuals with a history of falls or under conditions such as fractures. Third, the generalizability of results from an experimental setting in which physical collisions do not occur to everyday situations remains unclear. For example, a previous study reported a collision rate of 1.6% in older adults (50), whereas the average collision rate in both groups in the pre-test in this study was 25%. Therefore, further studies are required to confirm whether the experimental tasks employed here translate to real-life scenarios. Fourthly, this study only assessed the immediate effects of the intervention on leading limb placement. Similar to previous research (51), future studies should investigate the retention effects of this intervention over time.

In conclusion, this study demonstrated that an intervention to place the foot placement of the leading limb closer to the obstacle reduces collision avoidance by the trailing limb in older adults. This indicates that careful control was observed, as evidenced by the reduction in toe height variability and decreased passing speed of the trailing limb. Future research should investigate whether such interventions can further improve collision avoidance, particularly in populations more vulnerable to collisions, such as older adults with post-fall conditions.