Background: Several case studies observed that the lateral ankle sprain resulted from a sudden increase in ankle inversion accompanied by internal rotation. However, without sufficient ankle kinetics and muscle activity information in the literature, the detailed mechanism of ankle sprain is still unrevealed. The purpose of our case report is to present 2 accidental ankle giving way incidents for participants with chronic ankle instability (CAI) and compare to their normal trials with data of kinematics, kinetics, and electromyography (EMG).

Case description:Two young female participants accidentally experienced the ankle giving way when landing on a 25° lateral-tilted force plate. 3D kinematics, kinetics, and muscle activity were recorded for the lower extremity. Qualitative comparisons were made between the giving way trials and normal trials for joint angles, angular velocities, moments, centers of pressure and EMG linear envelopes.

Results: One participant's giving way trial displayed increased ankle inversion and internal rotation angles in the pre-landing phase and at initial contact compared to her normal trials. Another participant's giving way trial exhibited greater hip abduction angles and delayed activation of the peroneus longus muscle in the pre-landing phase versus her normal trials.

Conclusion: A vulnerable ankle position (i.e., more inverted and internally rotated), and a late activation of peroneus activity in the pre-landing phase could result in the ankle giving way or even sprains. A neutral ankle position and early activation of ankle evertors before landing may be helpful in preventing ankle sprains.