Background Ankle sprain is among the most frequent sports injuries and a significant contributor to time lost from sports participation. The injury mechanism is traditionally described as an inversion trauma, but a detailed description including joint kinetics is lacking.

Objective To provide an accurate description of an ankle sprain.

Design Case study.

Setting Sidestep cutting in 240 Hz motion analysis lab.

Patient Elite female team handball player (1.73 m, 63.7 kg, 22 years). Accidental grade 1 ankle sprain confirmed by orthopaedic surgeon.

Assessment and outcome Ankle kinetics and kinematics and ground reaction forces were compared between two normal trials and the injury trial.

Results In the injury trial we observed a sudden increase in ankle inversion (15° vs 4.9° and 4.8°) and ankle internal rotation (6.9° vs −1.2° and −3.6°) the first 50 ms. The injury trial centre of pressure had an increased lateral excursion after 50 ms (8.4 cm vs 3.3 and 3.0 cm). From 80 ms there was an increasing ankle inversion moment, reaching a peak of 72 Nm at 138 ms. At this time the ankle inversion angle had reached 33.1°, the ankle internal rotation angle was 53.7° and the ankle dorsiflexion was 26.5°. In the same period there appeared to be an attempted unloading of the foot, with a dorsiflexion of the ankle and reduced ground reaction force. The control trials displayed mainly eversion moment of the ankle throughout the stance phase, and joint angular deflections less than 6°.

Conclusion This study provides the most accurate and detailed description of ankle sprain dynamics to date. The injury involved inversion and internal rotation combined with an unloading of the foot that was initiated after approx 80 ms. The inversion moment likely caused the injury in the time period between 120 and 150 ms.