Postural stability in trail and cross-country runners: Time-to-boundary metrics and training insights

Trail and cross-country runners frequently train on uneven and unpredictable terrain, elevating their risk of ankle injury. Time to Boundary (TtB) is a measure of postural stability that has emerged as a valuable indicator of lower limb injury risk, with lower values reflecting compromised stability. This study examined postural control across three groups: amateur trail runners (TR), NCAA Division I cross-country runners (CCR), and non-runner controls (NRC). Participants completed single-leg quiet stance trials on their dominant leg under two conditions: eyes open and eyes closed. Center of pressure data from a force platform were used to calculate TtB mean, standard deviation, and minimum values in both the anteroposterior (AP) and mediolateral (ML) directions. A 2 (condition) × 3 (group) repeated-measures ANOVA (with Sidak correction) revealed no significant interaction effects, but several main effects for group. TR demonstrated lower ML variability than both CCR and NRC (p' < .001), and lower AP variability than CCR (p < .05). CCR exhibited a greater AP TtB mean than NRC (p < .05), while TR showed the highest AP TtB minimum values (p < .05). These results suggest that trail runners have greater postural control, possibly reflecting adaptations to the proprioceptive and neuromuscular demands of off-road training. Future research should identify specific environmental or training variables that drive these adaptations, to better inform evidence-based training strategies.