Stability of the braced ankle. A biomechanical investigation

We measured the bare ankle and the braced angle-torque relationships in 12 uninjured volunteers under static and dynamic conditions within the full range of inversion motion. These relationships were measured with a specially designed mechanical device that allowed inversion movements with angular velocities up to 850 deg/sec. In testing the bare ankle under static conditions, the torque showed a 10-fold increase within the full range of motion (average, from 0.9 N-m at 7 degrees to about 8 N-m at 48 degrees of inversion). The slope of the angle-torque relationship increased under dynamic conditions giving higher torque values (up to 18 N-m on average). Both orthoses induced similar additional torques that increased linearly, up to about 6 N-m at 45 degrees, with higher angles of inversion. These additional torques are small compared with the amount of stress applied to the foot during a typical ankle sprain situation, such as recovering from a jump. Therefore, we propose that orthotic devices increase the ankle torque, counteracting the inversion movement, and also prevent the start of the inversion movement by preloading and maintaining the ankle in a proper anatomic position with optimal contact between the articular surfaces.