Postural control is a highly automatized basic activity that requires limited attentional investments. These investments have been shown to increase from balancing experts to controls, and from controls to persons with impaired postural control. Such between-subject comparisons led to a proposed direct relation between the regularity of center-of-pressure (COP) fluctuations and the amount of attention invested in posture. This study aims to expand this relation to a within-subject comparison of conditions that differ in balance demands. Specifically, more regular COP fluctuations were expected for standing than sitting, as stimulus-response reaction-time studies showed that the required attentional demands are lower for sitting than standing. COP registrations were made for fifteen healthy young adults in seated and standing postures. COP regularity was quantified with sample entropy. As expected, COP fluctuations were found to be more regular for standing than sitting, as evidenced by significantly lower sample entropy values. These findings expand the relation between COP regularity and the amount of attention invested in posture to postural tasks that vary in balance demands. An assessment of COP regularity may thus not only be instrumental in the examination of attentional investment in posture in between-subject designs, but also for different postures in within-subjects designs.

The present study was designed to assess the relative contribution of each leg to unperturbed bipedal posture in lower limb amputees. To achieve this goal, eight unilateral traumatic trans-femoral amputees (TFA) were asked to stand as still as possible on a plantar pressure data acquisition system with their eyes closed. Four dependent variables were computed to describe the subject's postural behavior: (1) body weight distribution, (2) amplitude, (3) velocity and (4) regularity of centre of foot pressure (CoP) trajectories under the amputated (A) leg and the non-amputated (NA) leg. Results showed a larger body weight distribution applied to the NA leg than to the A leg and a more regular CoP profiles (lower sample entropy values) with greater amplitude and velocity under the NA leg than under the A leg. Taken together, these findings suggest that the NA leg and the A leg do not equally contribute to the control of unperturbed bipedal posture in TFA. The observation that TFA do actively control unperturbed bipedal posture with their NA leg could be viewed as an adaptive process to the loss of the lower leg afferents and efferents because of the unilateral lower-limb amputation. From a methodological point of view, these results demonstrate the suitability of computing bilateral CoP trajectories regularity for the assessment of lateralized postural control under pathological conditions.