Walking is necessary for many activities of daily living and is associated with higher perceived quality of life (Bechtold et al., 2021). Walking can pose risks for those with impaired balance, as falls often happen in dynamic situations (Lord et al., 1996). While falls can occur in any direction, most falls occur laterally (Sparto et al., 2018). This is consistent with well-established literature that balance in walking is passively stabilized in the anteroposterior direction (Kuo, 1999, McGeer, 1990), but is unstable in the mediolateral direction (Bauby and Kuo, 2000, MacKinnon and Winter, 1993). To maintain stable walking, a favorable relationship between the projection of the center of mass (COM) on the ground and base of support (BOS) must be maintained, which is done by coordinating various strategies (Bruijn and Van Dieën, 2018, Hof et al., 2005, Kuo, 1999, MacKinnon and Winter, 1993, Reimann et al., 2018a).

Foot placement is the primary strategy for maintaining stability (Bruijn and Van Dieën, 2018, Reimann et al., 2018a). During steady walking, up to 80 % of the variance in foot placement can be predicted by the COM state (Wang and Srinivasan, 2014). In perturbed walking, foot placements are adjusted to account for perturbations to the COM, suggesting foot placements are selected based on COM dynamics (Hof et al., 2010, Rankin et al., 2014). While foot placement is the primary defense against a fall in walking, alternate strategies are needed to maintain stability when foot placement is constrained or impaired.

Other strategies used to stabilize walking include regulation of lateral ankle roll, ankle push-off, and angular momentum through trunk postural movements (Brough et al., 2021, Bruijn and Van Dieën, 2018, Reimann et al., 2018a). The lateral ankle roll strategy can adjust the location of the center of pressure under the foot, particularly during single support (Hof et al., 2010, Van Leeuwen et al., 2022). This strategy has a small range to adjust the center of pressure, but can act quickly and complement foot placement (Best and Wu, 2020, Brough et al., 2021, Hof et al., 2010, Reimann et al., 2017, van Leeuwen et al., 2021). The ankle push-off strategy can modulate the ground reaction force on the hind leg during double support, thereby changing the moment about the COM (Brough et al., 2021, Bruijn and Van Dieën, 2018, Reimann et al., 2018a). Increasing ankle push-off increases acceleration towards the foreleg in the anterior and medial directions, thereby influencing mediolateral balance (Reimann et al., 2018a). As the trunk, head, and arms make up approximately two-thirds of the body’s mass, regulation of their posture is an additional requirement for stability (Hurt et al., 2010, Reimann et al., 2018a). Generating angular momentum by angularly accelerating the trunk or linearly accelerating segments with respect to the COM causes ground reaction forces that affect COM motion (Bruijn and Van Dieën, 2018, Kuo, 1999, Van Leeuwen et al., 2022). However, there are biomechanical limits to how much the trunk can rotate, so reliance on this strategy depends on availability of other strategies (van den Bogaart et al., 2022, Van Leeuwen et al., 2022).

These four strategies can be used to improve stability during walking (Kuo, 1999, Reimann et al., 2018a), yet their simultaneous occurrence and interactions during walking make it difficult to disentangle their individual contributions to stability (Reimann et al., 2017, van den Bogaart et al., 2022, van Leeuwen et al., 2021). One recent study investigated the responses of the four major strategies to perturbations during treadmill walking (Brough et al., 2021, Brough and Neptune, 2022). Here, we examine these strategies during overground walking perturbations, which may better reflect daily-life activities (Hollman et al., 2016, Lee and Hidler, 2008, Taylor et al., 2022, Watt et al., 2010), and calculate their contributions to restoring stability.

The first objective was to describe the responses of the four major strategies to foot perturbations of various sizes in overground walking in healthy individuals. The second objective was to determine the contribution of each strategy to restoring stability in response to perturbations. The long-term goal is to demonstrate the importance of each strategy in restoring stability to suggest which strategies would be good candidates for training in populations with poor balance (Van Leeuwen et al., 2022).