Periods of extreme ankle displacement during one-legged standing.

The goal of this study was to describe the movement patterns of the ankle joint whilst standing on one leg. Ten healthy adult females (age 24 +/- 5.3 years) performed a one-legged standing task with eyes closed. Force platform recordings and video analyses were used to describe the kinematic and kinetic characteristics of the ankle joint during this task. A rocking movement of the foot (heel inversion-eversion) was documented by examining instances of extreme ankle displacement. Extreme ankle displacement was defined as any instant when the ankle position was more than +/- 2 SD away from the mean ankle joint position. Extreme values of lateral and medial ankle joint displacement were 14.8 and 9.2 mm, correspondingly. These instances of extreme foot inversion-eversion were characterized by large medial-lateral displacement of the gravity line (GLP) and center of pressure (COP) and large horizontal forces. Comparing instances of extreme ankle joint displacement to periods of non-extreme ankle displacement, the ankle joint moment remained fairly constant, averaging 8.4 +/- 4 and 6.9 +/- 3.5 Nm, respectively. The moment about the 'body-minus-foot' center of mass generated by the ankle joint reaction force, however, was on average over four times larger during instances of extreme ankle displacement (3.4 +/- 2.8 Nm), than during periods of non-extreme ankle displacement (0.8 +/- 0.4 Nm). In utmost situations, the moment due to the joint reaction force was up to 73% of the ankle joint moment. These results suggest that at least two different techniques are used to maintain balance during one-legged standing. The first technique, termed the ankle torque technique, involves a large restorative moment at a stationary ankle joint for balance maintenance. The other technique, the shear force technique, involves a large horizontal force at a moving ankle joint for balance maintenance. During non-extreme periods, balance was maintained primarily through the ankle torque technique. During extreme instances, a combination of both techniques was observed.