Regularity of Center of Pressure Trajectories in Expert Gymnasts during Bipedal Closed-Eyes Quiet Standing.

We compared postural control of expert gymnasts (G) to that of non-gymnasts (NG) during bipedal closed-eyes quiet standing using conventional and nonlinear dynamical measures of center of foot pressure (COP) trajectories. Earlier findings based on COP classical variables showed that gymnasts exhibited a better control of postural balance but only in demanding stances. We examined whether the effect of expertise in Gymnastic can be uncovered in less demanding stances, from the analysis of the dynamic patterns of COP trajectories. Three dependent variables were computed to describe the subject's postural behavior: the variability of COP displacements (ACoP), the variability of the COP velocities (VCoP) and the sample entropy of COP (SEnCoP) to quantify COP regularity (i.e., predictability). Conventional analysis of COP trajectories showed that NG and G exhibited similar amount and control of postural sway, as indicated by similar ACoP and VCoP values observed in NG and G, respectively. These results suggest that the specialized balance training received by G may not transfer to less challenging balance conditions such as the bipedal eyes-closed stance condition used in the present experiment. Interestingly, nonlinear dynamical analysis of COP trajectories regarding COP regularity showed that G exhibited more irregular COP fluctuations relative to NG, as indicated by the higher SEnCoP values observed for the G than for the NG. The present results showed that a finer-grained analysis of the dynamic patterns of the COP displacements is required to uncover an effect of gymnastic expertise on postural control in nondemanding postural stance. The present findings shed light on the surplus value in the nonlinear dynamical analysis of COP trajectories to gain further insight into the mechanisms involved in the control of bipedal posture.

Sensory Re-Weighting in Human Bipedal Postural Control: The Effects of Experimentally-Induced Plantar Pain.

The present study was designed to assess the effects of experimentally-induced plantar pain on the displacement of centre of foot pressure during unperturbed upright stance in different sensory conditions of availability and/or reliability of visual input and somatosensory input from the vestibular system and neck. To achieve this goal, fourteen young healthy adults were asked to stand as still as possible in three sensory conditions: (1) No-vision, (2) Vision, and (3) No-vision - Head tilted backward, during two experimental conditions: (1) a No-pain condition, and (2) a condition when a painful stimulation was applied to the plantar surfaces of both feet (Plantar-pain condition). Centre of foot pressure (CoP) displacements were recorded using a force platform. Results showed that (1) experimentally-induced plantar pain increased CoP displacements in the absence of vision (No-vision condition), (2) this deleterious effect was more accentuated when somatosensory information from the vestibular and neck was altered (No-vision - Head tilted backward condition) and (3) this deleterious effect was suppressed when visual information was available (Vision condition). From a fundamental point of view, these results lend support to the sensory re-weighting hypothesis whereby the central nervous system dynamically and selectively adjusts the relative contributions of sensory inputs (i.e. the sensory weightings) in order to maintain balance when one or more sensory channels are altered by the task (novel or challenging), environmental or individual conditions. From a clinical point of view, the present findings further suggest that prevention and treatment of plantar pain may be relevant for the preservation or improvement of balance control, particularly in situations (or individuals) in which information provided by the visual, neck proprioceptive and vestibular systems is unavailable or disrupted.

Do somatosensory conditions from the foot and ankle affect postural responses to plantar-flexor muscles fatigue during bipedal quiet stance?

The present study investigated the effects of somatosensory conditions at the foot and ankle on postural responses to plantar-flexor muscle fatigue during bipedal quiet stance. Twenty-two young healthy adults were asked to stand upright as still as possible with their eyes closed in three somatosensory conditions (normal, altered and improved) both prior to and after exercises inducing plantar-flexor muscle fatigue. In the normal condition, the postural task was executed on a firm support surface constituted by the force platform. In the altered condition, a 2-cm thick foam support surface was placed under the subjects' feet. In the improved condition, increased cutaneous feedback at the foot and ankle was provided by strips of athletic tape applied across both their ankle joints. Muscle fatigue was induced in the plantar-flexor muscles of both legs through the execution of a repeated standing heel raise exercise. Centre of foot pressure displacements were recorded using a force platform. Results showed that plantar-flexor muscle fatigue yielded increased centre of foot pressure displacements under normal foot and ankle sensory conditions. Furthermore, this effect was exacerbated under altered foot and ankle sensory conditions and mitigated under improved foot and ankle sensory conditions. Altogether, the present findings suggested an increased reliance on somatosensory information from the foot and ankle for controlling upright posture in the presence of plantar-flexor muscle fatigue.

Control of bipedal posture following localised muscle fatigue of the plantar-flexors and finger-flexors.

The present experiment investigated the control of bipedal posture following localised muscle fatigue of the plantar-flexors and finger-flexors. Twelve young healthy adults voluntarily participated in this study. They were asked to stand upright as still as possible with their eyes closed in two randomly ordered experimental sessions. Each session consisted of pre- and post-fatigue bipedal static postural control measurements immediately before and after a designated fatiguing protocol for plantar-flexor and finger-flexor muscles. Centre of foot pressure (CoP) displacements were recorded using a force platform. The results showed that the postural effects of localised muscle fatigue differed between the muscles targeted by the fatiguing procedures. Indeed, localised muscle fatigue of the plantar-flexors yielded increased CoP displacements, whereas localised muscle fatigue of the finger-flexors had no significant effect on the CoP displacements. In other words, fatigue localised to muscles which are involved in the performance of the postural task (plantar-flexors) degraded postural control, whereas fatigue localised to muscles which are not involved in the performance of the postural task did not. Taken together, the present findings support the recent conclusions that the effects of localised muscle fatigue on upright postural control is joint- and/or muscle-specific, and suggest that localised muscles fatigue of the plantar-flexors could mainly affect bipedal postural control via sensorimotor rather than cognitive processes.

Effects of plantar-flexor muscle fatigue on the magnitude and regularity of center-of-pressure fluctuations.

Control of bipedal posture is highly automatized but requires attentional investment, the amount of which varies between participants and with postural constraints, such as plantar-flexor muscle fatigue. Elevated attentional demands for standing with fatigued plantar flexors have been demonstrated using a stimulus-response reaction-time paradigm. Recently, a direct relation between the regularity of center-of-pressure (COP) fluctuations and the amount of attention invested in posture was proposed, according to which more regular COP fluctuations are expected with muscle fatigue than without. To study this prediction, we registered anterior-posterior COP fluctuations for bipedal stance with eyes closed prior to and after a plantar-flexor muscle fatiguing exercise protocol in 16 healthy young adults. We quantified the magnitude of COP fluctuations with conventional posturography and its regularity with sample entropy. The magnitude of COP fluctuations increased significantly with fatigued plantar flexors. In addition, more regular COP fluctuations were observed with fatigued plantar flexors, as evidenced by significantly lower sample entropy values. These findings corroborated our hypotheses. Moreover, COP regularity assisted in qualifying the change in sway magnitude with fatigue. Whereas increased sway is customary taken to reflect impaired postural control, we interpret it as a functional, but attention-demanding adaptation to the alteration of important posture-specific information.

Contribution of each leg to the control of unperturbed bipedal stance in lower limb amputees: new insights using entropy.

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.

Center-of-pressure regularity as a marker for attentional investment in postural control: a comparison between sitting and standing postures.

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.

Effects of mirror feedback on upright stance control in elderly transfemoral amputees.

OBJECTIVE: To investigate the effects of mirror feedback on upright stance control in elderly transfemoral amputees. DESIGN: Before and after intervention trials. SETTING: University medical bioengineering laboratory. PARTICIPANTS: Elderly transfemoral amputees (N=12). INTERVENTIONS: Participants were asked to stand upright, as immobile as possible, in 2 Eyes-open and Mirror-feedback experimental conditions. The latter experimental condition consisted of supplying the participants with their frontal reflection by positioning a mirror in front of them. MAIN OUTCOME MEASURES: Weight-bearing symmetry and the displacements of the center of foot pressure under the nonaffected limb and the prosthetic limb. RESULTS: Mirror feedback did not modify weight-bearing symmetry and had different effects on the center of foot pressure displacements under the nonaffected limb and prosthetic limb: a decreased center of foot pressure surface area was observed in the Mirror-feedback condition relative to the Eyes-open condition under the nonaffected limb, whereas no significant difference between the Eyes-open condition and the Mirror-feedback condition was observed under the prosthetic limb. CONCLUSIONS: Results suggest that elderly transfemoral amputees were able to integrate augmented visual biofeedback through the use of mirror-reflected body image to improve their upright stance control. The present findings could have implications for clinical practice and rehabilitation.