“An analysis of perturbation threshold stimulus required to elicit reactive stepping in four cardinal directions – an exploratory study”

Balance is defined as the ability to maintain the body’s center of mass within the base of support to ensure both stability and orientation. When the center of mass moves beyond this base and cannot be restored through ankle or hip strategies, reactive or compensatory stepping serves as the final protective response to prevent a fall. Maki and McIlroy (1999) emphasized that reactive stepping is essential for balance recovery in daily life, and age-related impairments in this mechanism significantly increase fall risk. Research indicates that the direction of perturbation plays a key role in balance recovery. Mille et al. (2013) found that older adults are more susceptible to lateral and backward perturbations compared to forward ones, suggesting that perturbation direction affects reactive stepping efficiency and fall avoidance. Similarly, Batcir et al. (2020) and Pai et al. (1998) demonstrated that the threshold required to elicit reactive stepping varies according to perturbation direction. Different experimental models have been developed to induce reactive stepping. The waist-pull method (Pai et al., 1998) has been commonly used but may not replicate real-world balance disturbances. In contrast, surface translation techniques—as employed by Batcir et al. (2020) and Muthukumar et al. (2025)—utilize moving platforms or treadmills that simulate natural perturbations more effectively. These studies also introduced the concept of using a percentage of body weight as a threshold to standardize stimulus intensity. Understanding threshold variations across directions can provide valuable insights for balance assessment and direction-specific rehabilitation. Therefore, the present study aims to determine the threshold stimulus (as a percentage of body weight) required to induce reactive stepping in forward, backward, and lateral directions using a moving platform.