The influence of transitioning between grass and concrete surfaces on resultant tibial accelerations while running.

Running over 'softer' surfaces is thought to lower the magnitude of impact forces. Current research is limited and inconclusive regarding the influence that transitioning between surfaces has on impact forces. Adult runners (n = 18) ran outdoors over a 50 m pathway that was half concrete and half grass at a self-selected speed. We recorded tibial acceleration from inertial measurements units and calculated resultant tibial acceleration for each step. We performed a three-way repeated measures ANOVA with Cohen's d effect sizes to compare resultant tibial acceleration among surface type (concrete, grass), transition step (preparation, response), and period (transition, steady state). We observed no significant interactions among surface type, transition step, and period for resultant tibial acceleration. We observed no significant main effect for transition step but observed that resultant tibial acceleration was significantly greater on concrete than grass and when transitioning compared to steady state, with small effect sizes. The increase in tibial acceleration during the transition period likely occurs across multiple steps when preparing or responding to a new surface. Runners that are concerned about experiencing higher tibial accelerations may want to consider running on softer surfaces and limiting the number of surface transitions during a training session.