Perceptibility of body position in anteroposterior direction while standing with eyes closed.

The perception of body position in the anteroposterior direction was investigated by evaluating the reproducibility of the position from a quiet standing posture to forward or backward leaning posture with eyes closed. The subjects were 10 healthy male undergraduates, aged 20 to 28 years. The standing position was represented by the pressure center of the foot, which was shown by the relative distance (%) from the heel to the length of the foot. The reference positions of the pressure center of the foot were set at 10% increments from 20 to 80% of the length of the foot. The subjects attempted to reproduce each reference position 10 times, and the absolute and constant errors of the reproduced position were analyzed. The absolute errors at reference positions of 30 to 60% were distinctly larger than those at the other reference positions. This indicated that the perception of standing positions from 30 to 60% was less accurate. The constant errors at the reference positions of 40 to 60% were significantly positive, which meant that the reproduced position was located farther forward than the reference position.     

Perceived standing position after reduction of foot-pressure sensation by cooling the sole

We investigated the influence of the reduction of foot-pressure sensation by cooling the sole of the foot, at 1 degree C for 30 or 40 minutes, on the perception of standing position varied in the anteroposterior direction. The subjects were 16 healthy undergraduates. Firstly, for 4 of the subjects, cooling the sole of the foot decreased sensory information from the mechanoreceptors in the sole, by testing for an increase in the threshold for two-point discrepancy discrimination on the sole of the foot and for the disappearance of postural change with vibration to the sole. Next, the perception of standing position was measured by reproduction of a given standing reference position involving forward or backward leaning under both normal and cooled conditions of the feet. Standing position was varied in relation to the location of the center of foot pressure, defined as distance from the heel in percentage of the length of the foot. The reference positions, representing various locations of the center of foot pressure, were set at 10% increments from 20% to 80% of the length of the foot. With eyes closed, the subject first experienced the reference position and then attempted to reproduce it. The mean location of the center of foot pressure in the quiet standing posture was 45.7%. At the 40%, 50%, and 60% reference positions, those closest to quiet standing, absolute errors of reproduction were significantly larger than at other reference positions in both the normal and the cooled conditions. They were significantly larger in the cooled than in the normal condition. The 50% and 60% reference positions were reproduced significantly further forward in the cooled than in the normal condition. These results may be explained as due to an absence of marked changes in sensory information from both muscular activity and foot pressure when moving to reference positions close to the quiet standing posture.     

Perceptibility of large and sequential changes in somatosensory information during leaning forward and backward when standing

11 healthy young men served as subjects in two experiments on perceptibility of (1) large changes in foot pressure and muscle activity induced by body leaning and (2) sequential changes in pressure at the first toe and the head of the first metatarsalis when leaning forward. The effects of reduced sensitivity on that perceptibility were also studied by repeating the experiments while cooling localized plantar areas of the sole (the head of the first metatarsalis, the first toe, and the heel). Under the normal (noncooled) condition, all subjects accurately perceived maximum pressure at the head of the first metatarsalis, but most subjects misperceived the second large increase in pressure at the first toe and in muscle activity as the first large increase. Under the cooling condition, localized cooling did not affect the perceptibility of maximum pressure at the head of the first metatarsalis or the activity in the tibialis anterior, but the perceptibility of pressure at the first toe and activity of the abductor hallucis were reduced. There were individual differences in perceptibility of activity of the rectus femoris when the heel was cooled. Perceptibility of sequential changes in the pressure was affected differently by the localized cooling of each region. Given these findings, we discussed the role and interrelatedness of pressure sensation in perceiving large and sequential changes in somatosensory information while standing and leaning forward and backward.     

State-dependent corrective reactions for backward balance losses during human walking

We investigated corrective reactions for backward balance losses during walking. Several biomechanical studies have suggested that backward falling can be predicted from the horizontal position and velocity of the body center of mass (COM) related to the stance foot. Our hypothesis was that corrective reactions for backward balance losses depend on whether the body moves forward or backward after a perturbation. Using a split-belt treadmill, backward balance losses during walking were induced by rapid decreases of belt speed from 3.5 km/h to 2.5, 2.0, 1.5 and 1.0 km/h. We measured kinematic data and surface electromyography (EMG) during corrective reactions while walking on the treadmill. Phase portrait analysis of COM trajectories revealed that backward balance stability was decreased by the perturbations. When the perturbed belt speed was 1.0 km/h, the COM states at toe-off were significantly lower than the stability limit; a rapid touch-down of the swing foot posterior to the stance foot then occurred, and the gait rhythm was modulated so that the phase advanced. EMG recordings during perturbed steps revealed a bilateral response, including modulation of the swing leg during the recovery. For weaker perturbations, the swing foot placements were anterior to the stance foot and there was a phase delay. In contrast to the bilateral responses for stronger perturbations, unilateral EMG responses were observed for weaker perturbations. The differences in joint kinematics and EMG patterns in the unperturbed swing leg depended on the COM states at toe-off, suggesting the existence of different responses consisting of ongoing swing movements and rapid touch-down. Thus, we conclude that corrective reactions for backward balance losses are not only phase-dependent but also state-dependent. In addition, the control system for backward balance losses predicts the feasibility of forward progression and modulates swing movement and walking rhythm according to backward balance stability.     

Postural movement pattern and muscle action sequence associated with self-paced bilateral arm flexion during standing

Investigated were postural movement pattern and action sequence of postural muscles while subjects rapidly flexed both arms during standing. The arm movement was started at the subject's own pace. Subjects were healthy individuals; 48 men and 53 women. Postural movement pattern was classified based on the movement angles of foot-leg (ankle joint) and leg-trunk (hip joint). Electromyograms were recorded from the anterior deltoid, biceps femoris, and erector spinae. The time difference between action onsets of the latter two muscles and the anterior deltoid was analyzed. Movement angles of the ankle and hip for both sexes were distributed on a similar linear regression line (y = -2.092x - 2.552 (r = -.71). The postural movement pattern was categorized based on the distribution into three types: hip flexion (in the 2nd quadrant), backward leaning (the 3rd), and hip extension (the 4th). The proportion of subjects was 26% in the hip flexion type, 55% in backward leaning type, and 19% in hip extension type. The action of biceps femoris and erector spinae significantly preceded that of anterior deltoid in the backward leaning and hip extension types but did not in the hip flexion type.     

The 'extrapolated center of mass' concept suggests a simple control of balance in walking

Next to position x and velocity v of the whole body center of mass (CoM) the 'extrapolated center of mass' (XcoM) can be introduced: xi = chi + nu/omega 0, where omega 0 is a constant related to stature. Based on the inverted pendulum model of balance, the XcoM enables to formulate the requirements for stable walking in a relatively simple form. In a very simple walking model, with the effects of foot roll-over neglected, the trajectory of the XcoM is a succession of straight lines, directed in the line from center of pressure (CoP) to the XcoM at the time of foot contact. The CoM follows the XcoM in a more sinusoidal trajectory. A simple rule is sufficient for stable walking: at foot placement the CoP should be placed at a certain distance behind and outward of the XcoM at the time of foot contact. In practice this means that a disturbance which results in a CoM velocity change Deltav can be compensated by a change in foot position (CoP) equal to Deltav/omega 0 in the same direction. Similar simple rules could be formulated for starting and stopping and for making a turn.     

Anticipatory Postural Adjustments for Altering Direction During Walking

The authors examined how individuals adapt their gait and regulate their body configuration before altering direction during walking. Eight young adults were asked to change direction during walking with different turning angles (0deg;, 45deg;, 90deg;), pivot foot (left, right), and walking speeds (normal and fast). The authors used video and force platform systems to determine participants' whole-body center of mass and the center of pressure during the step before they changed direction. The results showed that anticipatory postural adjustments occurred during the prior step and occurred earlier for the fast walking speed. Anticipatory postural adjustments were affected by all 3 variables (turn angle, pivot foot, and speed). Participants leaned backward and sideward on the prior step in anticipation of the turn. Those findings indicate that the motor system uses central control mechanisms to predict the required anticipatory adjustments and organizes the body configuration on the basis of the movement goal.     

Anticipatory postural adjustments for altering direction during walking

The authors examined how individuals adapt their gait and regulate their body configuration before altering direction during walking. Eight young adults were asked to change direction during walking with different turning angles (0 degree, 45 degree, 90 degree), pivot foot (left, right), and walking speeds (normal and fast). The authors used video and force platform systems to determine participants' whole-body center of mass and the center of pressure during the step before they changed direction. The results showed that anticipatory postural adjustments occurred during the prior step and occurred earlier for the fast walking speed. Anticipatory postural adjustments were affected by all 3 variables (turn angle, pivot foot, and speed). Participants leaned backward and sideward on the prior step in anticipation of the turn. Those findings indicate that the motor system uses central control mechanisms to predict the required anticipatory adjustments and organizes the body configuration on the basis of the movement goal.     

Effects of amount of attention allocated to the location of visual stimulus pairs on perception of simultaneity

This study examined whether a change in the amount of attention equally allocated to two locations affects judgments of the simultaneity or successiveness of stimuli presented at those locations. Observers were cued to expect two brief flashes either to the left and right of fixation or above and below fixation. Stimulus onset asynchrony was randomly varied. On a small proportion of trials, the stimuli appeared at the unexpected locations. Observers were more likely to report the stimuli as simultaneous when they appeared in the unexpected locations. A model proposed to account for the data assumes that a brief stimulus event is represented by a probability distribution reflecting the uncertainty in determining the time of the event’s occurrence, and two events are judged to be simultaneous if they are perceived to fall within some critical temporal interval, c, which is a function of the amount of attention allocated to the task.     

Effect of Heel Heights on Female Postural Control During Standing on a Dynamic Support Surface With Sinusoidal Oscillations

The authors aimed to investigate female balance or stability control with comparative analysis of the center of pressure trajectory and plantar pressure distribution with different high-heeled shoes while standing on a dynamic surface with multidirectional perturbations. College females with at least 2 years' history of wearing high-heel shoes voluntarily participated in the test with a Novel Pedar insole (Novel, GmBH, Munich, Germany. With heels height increasing, the pressure time integral obviously transfer to the medial forefoot region, with center of pressure trajectory medially deviated significantly, either under anteroposterior or mediolateral perturbations. The passive plantarflexion position of ankle incurred by high heel increased the range of motion in the frontal plane but decreased ankle stability, thus increasing the challenge of body balance maintenance.     

Differential effects of main error correction versus secondary error correction on motor pattern of running

The aim of this study was to gain a better understanding of how the run pattern varies as a consequence to main error correction versus secondary error correction. Twenty-two university students were randomly assigned to one of two training-conditions: ‘main error’ (ME) and ‘secondary error’ (SE) correction. The rear-foot strike at touchdown was hypothesized as the ‘main error’, whereas an incorrect shoulder position (i.e., behind the base of support) as the ‘secondary error’. In order to evaluate any changes in run pattern at the foot touchdown instant, the ankle, knee and hip joint angles, the height of toe and heel (with respect to the ground), and the horizontal distance from the heel to the projected center of mass on the ground were measured. After the training-intervention, the ME group showed a significant improvement in the run pattern at the foot touchdown instant in all kinematic parameters, whereas no significant changes were found in the SE group. The results support the hypothesis that the main error can have a greater influence on the movement patterns than a secondary error. Furthermore, the findings highlight that a correct diagnosis and the correction of the ‘main error’ are fundamental for greater run pattern improvement.     

Challenging horizontal movement of the body during sit-to-stand: impact on stability in the young and elderly

There are 3 significant challenges to sit-to-stand: (a) bringing the center of mass forward, (b) vertically raising the center of mass from the sitting to standing position, and (c) transition from a relatively large and stable base of support in sitting to a considerably smaller base of support when standing. The authors explored the challenges to stability control following sit-to-stand when the requirement for horizontal movement of the center of mass was influenced by foot position and their potential effect on the preceding phases of sit-to-stand. Eleven healthy young and 11 healthy elderly individuals performed the sit-to-stand with their feet further away and closer to the chair. Kinetic and kinematic data were recorded. Regardless of foot position, challenges in stability were greater in elderly participants than young participants despite their similar movement time and shear forces. The greater instability in elderly participants, despite their comparable movement characteristics, emphasizes the importance of stability control following sit-to-stand performance. For both young and elderly participants, the sit-to-stand duration and the shear forces were greater in the far condition. However, foot position did not affect the stability measures (i.e., duration of the stabilization phase and the total center of pressure path during the 1st second of the stabilization phase).     

Postural orientation: age-related changes in variability and time-to-boundary

The relation between age-specific postural instability and the detection of stability boundaries was examined. Balance control was investigated under different visual conditions (eyes open/closed) and postural orientations (forward/backward lean) while standing on a force platform. Dependent variables included center of pressure variability and the time-to-contact of the center of pressure with the stability boundaries around the feet (i.e., time-to-boundary). While leaning maximally, older individuals (ages 55-69) showed increased center of pressure variability compared to no lean, while younger subjects (ages 24-38) showed a decrease. These significant differences were found only in anterior-posterior direction. No significant age-specific differences were found between eyes open and eyes closed conditions. Time-to-boundary analysis revealed reduced spatio-temporal stability margins in older individuals in both anterior-posterior and medio-lateral directions. Time-to-boundary variability, however, was not significantly different between the groups in both medio-lateral and anterior-posterior direction. These results show the importance of boundary relevant center of pressure measures in the study of postural control, especially concerning the lateral instability often observed in older adults.     

Examining the effects of postural constraints on estimating reach

The tendency to overestimate has consistently been reported in studies of reachability estimation. According to one of the more prominent explanations, the postural stability hypothesis, the perceived reaching limit depends on the individual's perceived postural constraints. To test that proposition, the authors compared estimates of reachability of 38 adults (a) in the seated posture (P1) and (b) in the more demanding posture of standing on one foot and leaning forward (P2). Although there was no difference between conditions for total error, results for the distribution and direction of error indicated that participants overestimated in the P1 condition and underestimated in the P2 condition. It therefore appears that perceived postural constraints could be a factor in judgments of reachability. When participants in the present study perceived greater postural demands, they may have elected to program a more conservative strategy that resulted in underestimation.     

Postural leaning direction challenges the manifestation of tendon vibration responses at the ankle joint

In this study, we examined the interaction between central and peripheral proprioceptive afferent pathways by applying ankle tendon vibration during postural leaning in different directions. Twenty young participants stood for 60s over the midline of two adjacent force platforms in (a) neutral stance distributing Body Weight (BW) equally between the platforms, (b) forward leaning transferring 80% of BW to the front platform and (c) backward leaning transferring 80% of BW to the rear platform. Participants controlled the degree of leaning by receiving on-line visual feedback of BW distribution matched to a target line. Vibration (80 Hz, 1.5–1.8 mm) was applied over the Achilles or tibialis anterior tendon during the middle 20s of standing. This induced a postural shift towards the vibration side and an increase in the variability of the BW distribution that was greater in backward compared to forward leaning. EMG responses to tendon vibration were independent of the leaning direction. Antagonistic activity also increased in response to vibration, the amplitude of this increase however was direction dependent. These results favor the hypothesis about the central co-modulation of the vibration evoked proprioceptive inflow based on postural and visual feedback rather than muscle tension constraints.     

Visual motion shifts saccade targets

Saccades are made thousands of times a day and are the principal means of localizing objects in our environment. However, the saccade system faces the challenge of accurately localizing objects as they are constantly moving relative to the eye and head. Any delays in processing could cause errors in saccadic localization. To compensate for these delays, the saccade system might use one or more sources of information to predict future target locations, including changes in position of the object over time, or its motion. Another possibility is that motion influences the represented position of the object for saccadic targeting, without requiring an actual change in target position. We tested whether the saccade system can use motion-induced position shifts to update the represented spatial location of a saccade target, by using static drifting Gabor patches with either a soft or a hard aperture as saccade targets. In both conditions, the aperture always remained at a fixed retinal location. The soft aperture Gabor patch resulted in an illusory position shift, whereas the hard aperture stimulus maintained the motion signals but resulted in a smaller illusory position shift. Thus, motion energy and target location were equated, but a position shift was generated in only one condition. We measured saccadic localization of these targets and found that saccades were indeed shifted, but only with a soft-aperture Gabor patch. Our results suggest that motion shifts the programmed locations of saccade targets, and this remapped location guides saccadic localization.     

Effect of localized muscle fatigue on vertical ground reaction forces and ankle joint motion during running

The purpose of this study was to examine the changes in the vertical ground reaction force (VGRF) and ankle joint motion during the first 50% of the stance phase of running following fatiguing exercise of either the dorsiflexors or the invertors of the foot. VGRFs, sagittal and rearfoot kinematic data were collected from 11 female recreational runners running at 2.9 m/second on a treadmill prior to and following localized muscle fatigue of either the invertors or dorsiflexors of the right foot. Loading rate of the impact peak force significantly increased following fatiguing exercise of the dorsiflexors, while the peak magnitudes of the impact and push-off forces remained unchanged. There were significant decreases in dorsiflexion at heel contact, but no significant difference in any rearfoot motion parameters tested following dorsiflexor fatigue. Following fatiguing exercise of the invertors, impact peak magnitude, push-off peak magnitude and the rate of decline of the impact peak force significantly decreased; there was no change in the loading rate of the impact peak force. Invertor fatigue also resulted in a less inverted foot position at heel contact, but there were no significant differences in any other kinematic parameters tested. The results demonstrate that localized muscle fatigue of either the invertors or dorsiflexors can have a significant effect on the loading rates, peak magnitudes and ankle joint motion seen during running. These changes, due to localized muscle fatigue, may play a role in many common lower extremity running injuries.     

Examining the Effects of Postural Constraints on Estimating Reach

The tendency to overestimate has consistently been reported in studies of reachability estimation. According to one of the more prominent explanations, the postural stability hypothesis, the perceived reaching limit depends on the individual's perceived postural constraints. To test that proposition, the authors compared estimates of reachability of 38 adults (a) in the seated posture (P1) and (b) in the more demanding posture of standing on one foot and leaning forward (P2). Although there was no difference between conditions for total error, results for the distribution and direction of error indicated that participants overestimated in the P1 condition and underestimated in the P2 condition. It therefore appears that perceived postural constraints could be a factor in judgments of reachability. When participants in the present study perceived greater postural demands, they may have elected to program a more conservative strategy that resulted in underestimation.     

Sensori-motor integration during stance: Time adaptation of control mechanisms on adding or removing vision

Sudden addition or removal of visual information can be particularly critical to balance control. The promptness of adaptation of stance control mechanisms is quantified by the latency at which body oscillation and postural muscle activity vary after a shift in visual condition. In the present study, volunteers stood on a force platform with feet parallel or in tandem. Shifts in visual condition were produced by electronic spectacles. Ground reaction force (center of foot pressure, CoP) and EMG of leg postural muscles were acquired, and latency of CoP and EMG changes estimated by t-tests on the averaged traces. Time-to-reach steady-state was estimated by means of an exponential model. On allowing or occluding vision, decrements and increments in CoP position and oscillation occurred within about 2 s. These were preceded by changes in muscle activity, regardless of visual-shift direction, foot position or front or rear leg in tandem. These time intervals were longer than simple reaction-time responses. The time course of recovery to steady-state was about 3 s, shorter for oscillation than position. The capacity of modifying balance control at very short intervals both during quiet standing and under more critical balance conditions speaks in favor of a necessary coupling between vision, postural reference, and postural muscle activity, and of the swiftness of this sensory reweighing process.     

Exercise training can improve spatial characteristics of time-critical obstacle avoidance in elderly people

Fall prevention programs have rarely been evaluated by quantitative movement analysis methods. Quantitative movement analyses could provide insight into the mechanisms underlying the effects of training. A treadmill obstacle avoidance task under time pressure has recently been used to evaluate a fall prevention exercise program for community-dwelling elderly people and it showed that participants improved their obstacle avoidance success rates. The mechanism, by which the increased success rates were achieved, however, remained to be determined. Participants were elderly who had fallen at least once in the year prior to participation. They were assigned to either the exercise or the control group. The control group did not receive any specific treatment. The exercise group was administered a five week exercise program, which consisted of exercises on a functionally oriented obstacle course, walking exercises, and practice of fall techniques. Pre- and post-intervention laboratory obstacle avoidance tests were conducted. Three possible determinants of success were investigated, namely avoidance reaction times, the distribution of avoidance strategies, and three spatial parameters (toe distance, foot clearance and heel distance). Analysis yielded significant TimexGroup interactions in heel distances. The exercise group increased heel distance, while the control group did not. Increased heel distance may result in reduced risk of heel contact with the obstacle and, consequently, larger success rates. The remaining parameters showed no effect of training. In conclusion, the training program was effective in improving time-critical obstacle avoidance skills. In every day life, these effects of training may contribute to less obstacle-related fall incidents in elderly. In addition, these findings could indicate that the execution of other time-critical events, like an actual fall, could also be improved by training.