Context: Foot structure has been shown to affect aspects of neuromuscular control, including postural stability and proprioception. However, despite an association between pronated and supinated foot structures and the incidence of lateral ankle sprains, no one to our knowledge has measured muscle reaction time to a simulated ankle-sprain mechanism in participants with different foot structures.Objective: To determine whether pronated or supinated foot structures contribute to neuromuscular deficits as measured by muscle reaction time to a simulated ankle-sprain mechanism.Design: Cross-sectional study. Setting: University biomechanics laboratory.Patients or Other Participants: Thirty volunteers were categorized into 3 groups according to navicular-drop-height measures. Ten participants (4 men, 6 women) had neutral feet (navicular-drop height ¼ 5-9 mm), 10 participants (4 men, 6 women) had pronated feet (navicular-drop height ! 10 mm), and 10 participants (4 men, 6 women) had supinated feet (naviculardrop height 4 mm).Intervention(s): Three perturbations on a standing tilt platform simulating the mechanics of an inversion and plantarflexion ankle sprain.Main Outcome Measure(s): Muscle reaction time in milliseconds of the peroneus longus, tibialis anterior, and gluteus medius to the tilt-platform perturbation.Results: Participants with pronated or supinated foot structures had slower peroneus longus reaction times than participants with neutral feet (P ¼ .01 and P ¼ .04, respectively). We found no differences for the tibialis anterior or gluteus medius.Conclusions: Foot structure influenced peroneus longus reaction time. Further research is required to establish the consequences of slower peroneal reaction times in pronated and supinated foot structures. Researchers investigating lower limb muscle reaction time should control for foot structure because it may influence results.Key Words: tilt platform, arch height, injuries, neuromuscular control Key PointsReaction time of the peroneus longus muscle was slower in participants with pronated and supinated feet than in participants with neutral feet. Reaction times of the tibialis anterior and gluteus medius muscles were not different among groups. Further research is required to investigate if the risk of sustaining a lateral ankle sprain is greater in people with pronated or supinated feet than in people with neutral feet.