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An eversion ankle sprain case study analysed using the model-based image-matching method.Ankle sprain injuries are common in National Football League (NFL) athletes, and can lead to notable time out of competition. While there is some evidence investigating inversion ankle sprain injury mechanisms across a variety of sports, very little is known about eversion sprains. Eversion ankle sprains tend to be more severe, but less common, and are a frequent type of ankle injury in NFL athletes. The aim of this study was to quantify the kinematics of an eversion ankle sprain injury occurring in an NFL athlete.Uncalibrated video footage showing three separate camera views of a single case study injury were obtained. The video footage was processed and rendered into a single video and imported to three-dimensional animation software. The model-based image-matching method was applied to quantify ankle joint kinematics for the eversion sprain. Foot ground contact was determined visually, and the foot orientations are reported relative to the shank segment.At touchdown, the ankle joint was dorsiflexed, externally rotated but neutral in the frontal plane. Peak eversion was 50°, occurring 0.2 s after foot-ground contact, with a maximum eversion velocity of 426°/s. In the sagittal plane, the ankle joint plantarflexed briefly after touchdown before rapidly dorsiflexing. Peak dorsiflexion was at 64°, with a dorsiflexion velocity of 573°/s. Rotation in the transverse plane remained constant throughout ground contact.A combination of dorsiflexion and eversion is associated with an eversion sprain, with dorsiflexion at touchdown prompting the ankle to evert. In American Football, athletes should aim to land with the foot in a neutral position to prevent the sprain injury mechanism. Further research into eversion sprains is required before this pattern of motion can be described as the eversion sprain injury mechanism.