The repeated impact loading of the musculoskeletal system that occurs during running has the potential to cause injury. This research investigates initial ankle posture effects upon impact loading and energy return. Four initial ankle postures were used (12.5\sp∘ of dorsiflexion; a flatfooted impact; 12.5\sp∘ of plantarflexion; 25\sp∘ of plantarflexion). A human pendulum modality (Lafortune and Lake, 1995c), which provided control of the initial impact conditions, was used to simulate running impacts. Impact force and tibial and head accelerations were recorded to quantify impact severity and energy return. A joint model was also developed to estimate ankle joint compressive loads at the time of the impact peak. The flatfooted contact produced the most severe impacts but the smallest joint force. The dorsiflexed ankle posture produced less severe impacts but larger joint forces. The two plantarflexed conditions resulted in the least severe impacts and the largest joint forces. The plantarflexed conditions were also more effective at storing and returning energy to the system.