Figure 1

Kinematics variables during the SEBT task, experimental setting and description of subtasks and events. Illustration of the six conditions at the SEBT (3 reaching directions per limb; AM: anteromedial, M: medial, PM: posteromedial) and profiles (mean ± 1 standard deviation; n = 3 trials) of the vertical position of the reaching foot (A), global body CoM (CoMgl) (B and C) and joint amplitude of motion of the stance limb (D) in a typical healthy subject in the PM reaching direction. The central grey rectangle, called the central phase around foot contact, represents the critical period used for the analyses. The lowest vertical position of the tip of the reaching foot was used to subdivide the task, which corresponds to the “foot contact” event (vertical line at 50% of the task). The first subtask is characterized by body lowering (B and D) and alignment of the reaching limb for foot contact (A). In the second subtask which occurred after foot contact, a rapid straightening up of the entire body (B and C) is performed while returning from perturbation in single-limb stance. The transition between subtasks represents a highly challenging period for stability and motor control as a change in movement direction takes place at the perceived limits of stability. This transition period, called the central phase, was defined as -1 s to +1 s after foot contact. Further analysis about the quality of motor control was performed during this phase.