Differences in preferred reference frames for postural orientation shown by after-effects of stance on an inclined surface

This study reports a postural after-effect of leaning that follows a period of stance on an inclined surface with eyes closed. This leaning after-effect maintained the body-to-surface relationship as if subjects still stood on the incline. We examined the incidence and robustness of the leaning after-effect in 51 healthy subjects. The location of the center of pressure (CoP) under the feet and the alignment of the trunk and legs were measured before, during and after blindfolded subjects stood on a 5° toes-up inclined surface for 2.5 min. When the surface was inclined, all subjects stood with their trunk and legs aligned near to gravity-vertical, similar to the alignment adopted in the pre-incline period. When the surface returned to horizontal in the post-incline period, there was a continuum of postural alignment strategies across subjects. At one extreme, subjects leaned forward, with an average trunk lean near 5°. The leaned posture decayed exponentially toward baseline postural alignment across a period of up to 5 min. At the other extreme, subjects did not lean in the post-incline period, but instead, stayed aligned near upright with respect to gravity. Subjects were highly consistent in their post-incline postural behaviors upon repeated testing over days to months and across different directions of surface inclination. Our results suggest that individuals have well-established, preferred, sensory strategies for controlling postural orientation when vision is not available. Subjects who leaned in the post-incline period appear to depend more on the geometry of the support surface as a reference frame and to rely more on proprioceptive information to extract kinematic relationships, whereas subjects who did not lean appear to depend more on gravity as a reference frame and to rely more on sensory information related to forces and load.     

Postural after-effects of stepping on an inclined surface

In previous studies, blindfolded, healthy subjects exhibited an after-effect of leaning while standing on a horizontal surface after a period of standing on an inclined surface. We investigated whether this kinesthetic after-effect would transfer from one task to another by asking blindfolded subjects to stand on a horizontal surface after stepping-in-place on an incline. Results showed that all subjects demonstrated a forward trunk leaning after-effect lasting from half a minute to over 6 min after stepping on a 10 degrees -toes-up incline for 2.5 min. For 5/7 subjects, the amplitude of the leaning after-effect was very similar following stepping or standing on the inclined surface. The similarity of the post-incline lean between the standing and stepping conditions suggests a common underlying mechanism for the after-effect following standing and walking on a gradient and suggests that prolonged maintenance of a constant ankle or leg posture is not a prerequisite condition for the after-effect. The transfer of a postural effect built-up during a locomotor task to a postural after-effect during a standing task is consistent with a central adaptive mechanism that adjusts the surface-referenced set point for whole body postural orientation for both gait and posture.     

Local and global effects of neck muscle vibration during stabilization of upright standing

Neck muscle vibration (NMV) during upright standing is known to induce forward leaning, which has been explained as a global response to the (illusory) perception of a lengthening of the dorsal neck muscles. However, the effects of NMV both at the level of individual joints and on whole-body postural coordination, and its potential modulation by vision, have not yet been analyzed in detail. Eight healthy young adult participants completed a total of ten trials each, with a 10-s period of unperturbed standing followed by a 10-s period of NMV. Participants were instructed to stand “as still as possible”. This postural task was executed under two visual conditions: eyes open (EO) and eyes closed (EC). Postural responses were measured in terms of center of pressure (CoP) and center of mass (CoM) profiles, and whole-body kinematics. Responses to NMV at the level of individual body segments and joints were assessed by decomposing the time series into linear trends and residual fluctuations. Inter-segmental coordination was analyzed using a decorrelation technique, assessing motor-equivalent stabilization of four task-related variables: CoM position, trunk orientation, as well as head position and orientation. NMV induced a general forward leaning response under both visual conditions, visible in CoP, CoM, segment positions and orientations. Locally, NMV induced a pronounced extension of the atlanto-occipital joint. Residual fluctuations were higher with EC and unaffected by NMV. Coordination analysis showed that stabilization of different body parts was differentially affected by NMV. Head orientation was consistently stabilized across all conditions, with weaker coordination in the EC condition. In contrast, motor-equivalent stabilization of CoM and head position, and trunk orientation was only observed during no-vibration periods. Taken together, our results demonstrate specific effects of vision and proprioception on different aspects of local and global postural control. While perturbed neck proprioception seemed to affect the postural “set point” (inducing forward leaning), vision appeared to mainly serve in noise reduction (residual fluctuations) and control of head orientation.     

Influence of vision on adaptive postural responses following standing on an incline

Previous studies demonstrated a leaning after-effect (LAE) following standing or walking on an inclined surface consistent with a long-lasting, somatosensory memory for body orientation relative to the surface. Here, we asked whether providing a brief visual reference during LAE resets postural orientation to the new visual reference. The results showed that subjects immediately return to upright when eyes were opened briefly during the post-incline period. However, the subjects also immediately resumed leaning after closing their eyes again following 20 s of eyes open. The duration of LAE was not influenced by 1 or 2 brief periods of vision. Also, the amplitude of the lean following the brief vision period was often larger than when subjects had their eyes closed for the entire post-incline period. These results suggest a powerful somatosensory memory contribution to postural orientation in space that is not eliminated or recalibrated with brief exposure to a visual reference.     

Post-effect of forward and backward locomotion on body orientation in space during quiet stance

Neural circuits responsible for stance control serve other motor tasks as well. We investigated the effect of prior locomotor tasks on stance, hypothesizing that postural post-effects of walking are dependent on walking direction. Subjects walked forward (WF) and backward (WB) on a treadmill. Prior to and after walking they maintained quiet stance. Ground reaction forces and centre of foot pressure (CoP), ankle and hip angles, and trunk inclination were measured during locomotion and stance. In WF compared to WB, joint angle changes were reversed, trunk was more flexed, and movement of CoP along the foot sole during the support phase of walking was opposite. During subsequent standing tasks, WB induced ankle extension, hip flexion, trunk backward leaning; WF induced ankle flexion and hip extension. The body CoP was displaced backward post-WB and forward post-WF. The post-effects are walking-direction dependent, and possibly related to foot-sole stimulation pattern and trunk inclination during walking.     

Emergence of postural patterns as a function of vision and translation frequency.

Emergence of postural patterns as a function of vision and translation frequency. We examined the frequency characteristics of human postural coordination and the role of visual information in this coordination. Eight healthy adults maintained balance in stance during sinusoidal support surface translations (12 cm peak to peak) in the anterior-posterior direction at six different frequencies. Changes in kinematic and dynamic measures revealed that both sensory and biomechanical constraints limit postural coordination patterns as a function of translation frequency. At slow frequencies (0.1 and 0.25 Hz), subjects ride the platform (with the eyes open or closed). For fast frequencies (1.0 and 1.25 Hz) with the eyes open, subjects fix their head and upper trunk in space. With the eyes closed, large-amplitude, slow-sway motion of the head and trunk occurred for fast frequencies above 0.5 Hz. Visual information stabilized posture by reducing the variability of the head's position in space and the position of the center of mass (CoM) within the support surface defined by the feet for all but the slowest translation frequencies. When subjects rode the platform, there was little oscillatory joint motion, with muscle activity limited mostly to the ankles. To support the head fixed in space and slow-sway postural patterns, subjects produced stable interjoint hip and ankle joint coordination patterns. This increase in joint motion of the lower body dissipated the energy input by fast translation frequencies and facilitated the control of upper body motion. CoM amplitude decreased with increasing translation frequency, whereas the center of pressure amplitude increased with increasing translation frequency. Our results suggest that visual information was important to maintaining a fixed position of the head and trunk in space, whereas proprioceptive information was sufficient to produce stable coordinative patterns between the support surface and legs. The CNS organizes postural patterns in this balance task as a function of available sensory information, biomechanical constraints, and translation frequency.     

Differential integration of visual and kinaesthetic signals to upright stance

The present experiment was designed to assess the effect of active (deliberate) maintenance of a small forward (FL) or backward body lean (BL) (about 2° ankle flexion) with respect to the spontaneous direction of balance (or neutral posture, N) on postural balance. We questioned whether BL and FL stances, which impose a volitional proprioceptive control of the body-on-support angle, could efficiently reduce mediolateral displacements of the centre of pressure (CoP) induced by the visual motion of a room and darkness. Subjects (n = 15) were asked to stand upright quietly feet together while confronted to a large visual scene rolling to 10° on either side in peripheral vision (and surrounding vertical visual references in central vision) at 0.05 Hz. CoP displacements were recorded using a force platform. Analysis of medio-lateral CoP root-mean square showed that the effect of the moving room depends on the subject’s postural stability performance in the eyes open N stance condition. Two significant postural behaviours emerged. (1) The most stable subjects (G1) were not affected by the conditions of altered vision, but swayed more in BL stance than in the N stance. (2) The unstable subjects (G2) exhibited (i) larger CoP displacements in altered visual conditions and a greater coupling of the CoP with the motion of the visual scene, (ii) enhanced visual dependency with postural leaning, and (iii) decreased CoP displacements when leaning forward in the eyes open motionless scene. Interestingly, the visual quotient positively correlated with the proprioceptive quotient, indicating that the more the subjects relied heavily on the visual frame of reference (FOR) the more they were influenced by body leaning. This result suggested hence a lesser ability to use efficiently body-ground proprioceptive cues. On the whole, the present findings indicate that body leaning could provide a useful mean to assess the subject’s ability to use body-ground proprioceptive cues not only to improve postural stability during eyes opening (especially during forward leaning), but also as a mean to disclose subjects’ visual dependency and their associated difficulties to shift from visual to proprioceptive-based FOR.     

Somatosensory loss increases vestibulospinal sensitivity

To determine whether subjects with somatosensory loss show a compensatory increase in sensitivity to vestibular stimulation, we compared the amplitude of postural lean in response to four different intensities of bipolar galvanic stimulation in subjects with diabetic peripheral neuropathy (PNP) and age-matched control subjects. To determine whether healthy and neuropathic subjects show similar increases in sensitivity to galvanic vestibular stimulation when standing on unstable surfaces, both groups were exposed to galvanic stimulation while standing on a compliant foam surface. In these experiments, a 3-s pulse of galvanic current was administered to subjects standing with eyes closed and their heads turned toward one shoulder (anodal current on the forward mastoid). Anterior body tilt, as measured by center of foot pressure (CoP), increased proportionately with increasing galvanic vestibular stimulation intensity for all subjects. Subjects with peripheral neuropathy showed larger forward CoP displacement in response to galvanic stimulation than control subjects. The largest differences between neuropathy and control subjects were at the highest galvanic intensities, indicating an increased sensitivity to vestibular stimulation. Neuropathy subjects showed a larger increase in sensitivity to vestibular stimulation when standing on compliant foam than control subjects. The effect of galvanic stimulation was larger on the movement of the trunk segment in space than on the body's center of mass (CoM) angle, suggesting that the vestibular system acts to control trunk orientation rather than to control whole body posture. This study provides evidence for an increase in the sensitivity of the postural control system to vestibular stimulation when somatosensory information from the surface is disrupted either by peripheral neuropathy or by standing on an unstable surface. Simulations from a simple model of postural orientation incorporating feedback from the vestibular and somatosensory systems suggest that the increase in body lean in response to galvanic current in subjects with neuropathy could be reproduced only if central vestibular gain was increased when peripheral somatosensory gain was decreased. The larger effects of galvanic vestibular stimulation on the trunk than on the body's CoM suggest that the vestibular system may act to control postural orientation via control of the trunk in space.     

Two kinematic synergies in voluntary whole-body movements during standing

We used a particular computational approach, the uncontrolled manifold hypothesis, to investigate joint angle covariation patterns during whole-body actions performed by standing persons. We hypothesized that two kinematic synergies accounted for the leg/trunk joint covariation across cycles during a rhythmic whole-body motion to stabilize two performance variables, the trunk orientation in the external space and the horizontal position of the center of mass (COM). Subjects stood on a force plate and performed whole-body rhythmic movements for 45 s under visual feedback on one of the four variables, the position of the center of pressure or the angle in one of the three joints (ankle, knee, or hip). The Fitts-like paradigm was used with two target amplitudes and six indices of difficulty (ID) for each of the four variables. This was done to explore the robustness of kinematic postural synergies. A speed-accuracy trade-off was observed in all feedback conditions such that the movement time scaled with ID and the scaling differed between the two movement amplitudes. Principal-component (PC) analysis showed the existence of a single PC in the joint space that accounted for over 95% of the joint angle variance. Analysis within the uncontrolled manifold hypothesis has shown that data distributions in the joint angle space were compatible with stabilization of both trunk orientation and COM location. We conclude that trunk orientation and the COM location are stabilized by co-varied changes of the major joint angles during whole-body movements. Despite the strong effects of movement amplitude and ID on performance, the structure of the joint variance showed only minor dependence on these task parameters. The two kinematic synergies (co-varied changes in the joint angles that stabilized the COM location and trunk orientation) have proven to be robust over a variety of tasks.     

From balance regulation to body orientation: two goals for muscle proprioceptive information processing?

This study was based on the assumption that the central processing of proprioceptive inputs that arise from numerous muscles contributes to both awareness and control of body posture. The muscle-spindle inputs form a “proprioceptive chain” which functionally links the eye muscles to the foot muscles. Here, we focused on the specific contribution of two links in the control of human erect posture by investigating how proprioceptive messages arising from ankle and neck muscles may be integrated by the central nervous system. Single or combined mechanical vibrations were applied to different muscle tendons at either one (ankle or neck) or both (ankle plus neck) body levels. The amplitude and the specific direction of the resulting oriented body tilts were analyzed by recording the center of foot pressure (CoP) through a force platform with four strain gauges. The results can be summarized as follows: (1) the vibration-induced whole-body tilts were oriented according to the muscles stimulated; furthermore, the tilts were in opposite directions when neck or ankle muscles on the same side of the body were stimulated; (2) except for the ankle antagonist muscles, co-vibrating adjacent or antagonist muscles at the same body level (ankle or neck) resulted in body sways, whose orientation was a combination of those obtained by stimulating these muscles separately; and (3) likewise, co-vibrating ankle and neck muscles induced whole-body postural responses, whose direction and amplitude were a combination of those obtained by separate vibration. We conclude that the multiple proprioceptive inputs originating from either one or both body levels may be co-processed in terms of vector-addition laws. Moreover, we propose that proprioceptive information from ankle and neck muscles may be used for two tasks: balance control and body orientation, with central integration of both tasks. Received: 16 September 1997 / Accepted: 16 July 1998     

Motor equivalent control of the center of mass in response to support surface perturbations

To claim that the center of mass (CM) of the body is a controlled variable of the postural system is difficult to verify experimentally. In this report, a new variant of the method of the uncontrolled manifold (UCM) hypothesis was used to evaluate CM control in response to an abrupt surface perturbation during stance. Subjects stood upright on a support surface that was displaced in the posterior direction. Support surface translations between 0.03 and 0.12 m, each lasting for 275 ms, were presented randomly. The UCM corresponding to all possible combinations of joints that are equivalent with respect to producing the average pre-perturbation anterior–posterior position of the center of mass (CM_AP) were linearly estimated for each trial. At each point in time thereafter, the difference between the current joint configuration and the average pre-perturbation joint configuration was computed. This joint difference vector was then projected onto the pre-perturbation UCM as a measure of motor equivalence, and onto its complementary subspace, which represents joint combinations that lead to a different CM_AP position. A similar analysis was performed related to control of the trunk’s spatial orientation. The extent to which the joint velocity vector acted to stabilize the CM_AP position was also examined. Excursions of the hip and ankle joints both increased linearly with perturbation magnitude. The configuration of joints at each instance during the perturbation differed from the mean configuration prior to the perturbation, as evidenced by the joint difference vector. Most of this joint difference vector was consistent, however, with the average pre-perturbation CM_AP position rather than leading to a different CM_AP position. This was not the case, however, when performing this analysis with respect to the UCM corresponding to the control of the pre-perturbation trunk orientation. The projection of the instantaneous joint velocity vector also was found to lie primarily in the UCM corresponding to the pre-perturbation CM_AP position, indicating that joint motion was damped in directions leading to a change away from the pre-perturbation CM_AP position. These results provide quantitative support for the argument that the CM position is a planned variable of the postural system and that its control is achieved through selective, motor equivalent changes in the joint configuration in response to support surface perturbations. The results suggest that the nervous system accomplishes postural control by a control strategy that considers all DOFs. This strategy presumably resists combinations of DOFs that affect the stability of important task-relevant variables (CM_AP position) while, to a large extent, freeing from control combinations of those DOFs that have no effect on the task-relevant variables (Schöner in Ecol Psychol 8:291–314, 1995).     

Subjective, physiological and biomechanical responses to prolonged manual work performed standing on hard and soft surfaces

The aim of this laboratory study was to examine the subjective, physiological and biomechanical responses to prolonged light repetitive manual work during standing on soft (polyurethane standard mat) and hard (aluminum casting) surfaces. The subjects stood on the hard (10 subjects) and on the soft surfaces (11 subjects) for 2 h. Intensity of unpleasantness, shank circumference, electromyograph (EMG) activities from the right soleus and tibialis anterior muscles, mean amplitude and total angular displacement around the left and right ankle in the saggital plane, centre of pressure (CoP) displacement in the frontal and saggital planes, calf surface temperature, and pain intensity in experimentally induced muscle pain were recorded. Maximal voluntary contraction and fatigue tests were performed before and after the 2 h experiment. Standing on a soft surface caused a lower intensity of unpleasantness. During standing on a hard surface compared to a soft one the results showed an enhanced swelling of the shank, an increased EMG activity (right soleus muscle) of the lower leg, a greater amplitude and total angular displacement, and a larger CoP displacement in the frontal plane. Indications of more pronounced muscle fatigue while standing on the hard surface were also noticed. After 105 min, experimental muscle pain was elicited by injecting hypertonic saline. The intensity of the induced pain was lower when standing on the soft surface. Amplitude, angular distance and CoP displacement showed a tendency to be greater after injection of the hypertonic saline. It was found that the experimentally induced pain influenced postural activity, underlining central interactions between proprioceptors and nociceptors. The results highlighted a higher feeling of comfort when standing on the soft surface. In addition, postural activity was lower when standing on the soft surface, but the activity was sufficient to prevent swelling of the lower legs. Accepted: 27 June 1997     

Effects of neck muscles vibration on the perception of the head and trunk midline position

The present study focused on the influence of neck vibration on the perception of the head and trunk midline position (orientation and localization). The orientation of the head and trunk was investigated by the rolling adjustment of a rod on their midline while their localization was investigated by the adjustment of the position of a visual dot as being straight-ahead the eyes or the sternum. The first experiment investigated whether a head–trunk dissociation was induced by the unilateral vibration of neck muscles in upright and restrained subjects. Results showed that the subjective orientation and localization of whole-body midline were shifted toward the vibrated side. The second experiment determined the effect of the neck muscles vibration when the subjects were lying on their side. The effect of vibration disappeared when the side of vibration was opposed to the side of postural inclination and it was stronger than in the upright position when the side of vibration and the side of postural inclination were congruent. Whereas, results suggested that the input from neck muscle proprioceptors participates directly to the elaboration of the egocentric space, the question may be raised as to how the sensory cues interacted in their contribution to the neural generation of an egocentric, body centred coordinate system.     

Postural coordination modes and transition: dynamical explanations

While research to date has been successful in quantifying postural behaviour, this paper examines the causes of transition between postural coordination mode using dynamical variables and, by inference, efficient control strategies underlying postural behaviour. To this end, six subjects in bipedal stance were instructed to maintain a constant distance between their head and a visual target that oscillated along the line of sight. Within sessions, participants were exposed to gradual changes in increasing target motion frequency. Kinematic results showed a sudden transition between in-phase and anti-phase postural coordination modes in visual target tracking. The dynamical analysis pointed out that (1) the center of pressure (CoP) position parameter is a crucial parameter in the determination of the adopted coordination mode, (2) the change occurred in response to limits bordered by the system: the interaction between equilibrium constraints (A/P displacements of CoP), physiological limits (net joint moments) support the emergence of different postural behaviours and, (3) finally, the anti-phase mode presents a better distribution of muscular moment between hip and ankle joints and is more effective to achieve high frequency oscillations with limited CoP displacements.     

The effects of stance configuration and target distance on reaching

The aim of the present study was to investigate the relationship between the focal and postural components of a functional movement during the preparatory phase of a task. The contribution of the arms, trunk, and legs were varied by having subjects reach for two targets within and two beyond arm's length. In addition, the degree of postural stability was manipulated by varying the size of the base of support (BoS). Nine subjects reached and grasped a dowel placed at four locations while standing on a force plate with their feet in a parallel or step stance (right foot forward) under simple reaction-time (RT) conditions. Anticipatory postural adjustments (APAs) occurring prior to arm movement and RTs were analyzed. APAs varied depending on the demands of the task. For movements within arm's length, subjects selected different strategies to initiate the movement. However, for movements beyond arm's length, all subjects used the same strategy: the center of pressure (CoP) was shifted posteriorly, which resulted in the center of mass (CoM) moving towards the target. Target distance and BoS had no effect on the onset of APAs. In contrast, amplitude and duration of APAs increased linearly with target distance, and amplitude was always greater during the more posturally stable BoS configuration. Although wrist RT increased linearly with movement amplitude for both stance configurations, the rate of change was less under the more stable BoS. These results suggest that, during the performance of a functional task, dynamic changes that occur in the trunk and lower extremities prior to initiation of arm movement serve not only to stabilize the body, but are also used to initiate and assist whole-body reaching.     

Postural coordination patterns as a function of rhythmical dynamics of the surface of support

This study investigated the organization of postural coordination patterns as a function of the rhythmical dynamics of the surface of support. We examined how the number and nature of the dynamical degrees of freedom in the movement coordination patterns changed as a function of the amplitude and frequency of support surface motion. Young adult subjects stood on a moving platform that was translated sinusoidally in anterior-posterior (AP) direction with the task goal to maintain upright bipedal postural balance. A force platform measured the kinetics at the surface of support and a 3D motion analysis system recorded torso and joint kinematics. Principal components analysis (PCA) identified four components overall, but increasing the average velocity of the support surface reduced the modal number of components of the postural coordination pattern from three to two. The analysis of joint motion loadings on the components revealed that organizational properties of the postural pattern also changed as a function of platform dynamics. PC1 (61.6–73.2 %) was accounted for by ankle, knee, and hip motion at the lowest velocity conditions, but as the velocity increased, ankle and hip variance dominated. In PC2 (24.2–20.2 %), the contribution of knee motion significantly increased while that of ankle motion decreased. In PC3 (9.7–5.1 %) neck motion contributed significantly at the highest velocity condition. Collectively, the findings show that the amplitude and frequency of the motion of the surface of support maps redundantly though preferentially to a small set of postural coordination patterns. The higher platform average velocities led to a reduction in the number of dynamical degrees of freedom of the coordination mode and different weightings of joint motion contributions to each component.     

Limited control strategies with the loss of vestibular function

When subjects stand on an unstable or compliant support surface, rather than a stable one, vestibular information becomes more important for the control of posture. We investigated how subjects with bilateral vestibular loss (BVL) controlled their upright posture, with and without light-touch contact at the fingertip, while standing on a support surface, sinusoidally rotating at different frequencies. Subjects stood with eyes closed on a platform that rotated +/-1.2 degrees around an axis directly beneath the midline of the ankle for frequencies ranging from 0.01 to 0.4 Hz for two sensory conditions: (1) with light, nonsupportive touch (less than 1 N vertical force) on a stationary surface; or (2) with the fingertip held in a position directly above the contact surface (no contact). Gain, phase, and variability of the center of mass (CoM) and the finger were analyzed to compare BVL subjects with healthy controls in the no-touch and light-touch conditions. Three important results were observed: First, CoM gain and variability of BVL subjects was distinctly higher than control subjects with no-touch contact, particularly at the higher platform frequencies. Second, with light-touch contact, BVL and control subjects showed equivalent gain, variability, and phase. Third, multiple relationships between the finger and the CoM were observed in control subjects, whereas BVL subjects implemented a single finger/CoM control scheme. The results are explained in terms of three interacting factors: the transfer function of the vestibular system, a sensory reweighting mechanism, and the inertial properties of the body. Moreover, multiple control strategies observed in control subjects suggest a more flexible control system than that of individuals with severely diminished vestibular function.     

Coordination between equilibrium and hand trajectories during whole body pointing movements

We examined the coordination between equilibrium and voluntary pointing movements executed from the standing position, using the whole body. It has previously been shown that trunk movement has little effect upon kinematic characteristics of hand pointing when movements are executed in the sitting position. The present study asked if elements of hand trajectory are modified by requirements of large trunk displacements and fine equilibrium control when pointing movements are executed from the standing position. To achieve this, center of pressure (CoP) and center of mass (CoM) displacements were analyzed along with the kinematics of the pointing hand. Results showed that the CoM was not stabilized (it displaced between 23% and 61+/-21% of the foot's length), confirming that instead of a compensation of mechanical perturbations due to arm and trunk movements, the present equilibrium strategy consisted of controlling CoM acceleration towards the target. Hand paths were curved and were not distance or speed invariant. Rather than simple inefficiencies in programming or execution, path curvature suggested that different hand movement strategies were chosen as a function of equilibrium constraints. In light of these results, we hypothesize that postural stability may play a role in the generation of hand trajectory for complex, whole-body pointing movements, in addition to constraints placed upon end-effector kinematics or the dynamic optimization of upper-limb movements. A dependent regulation of equilibrium and spatial components of the movement is proposed.     

Is the erect posture in microgravity based on the control of trunk orientation or center of mass position?

In the present experiments carried out in microgravity two questions were addressed. First, when the subject was instructed to adopt a vertical erect posture in microgravity with his feet fixed to the floor of the space cabin, would he control anteroposterior position with respect to the ankle joint axis of the ”vertical projection” of his center of mass (CM) or trunk axis orientation with respect to the ”vertical” (perpendicular to the floor of the space cabin)? Secondly, is CM anteroposterior position regulated during upper trunk movements in microgravity, in the absence of equilibrium constraint? Two subjects were tested in a long-term space flight. Video camera recordings were performed and analyzed off line. The results show that during erect vertical posture in microgravity, the trunk axis with respect to the ”vertical” is inclined some 7° forward. The anteroposterior position of the CM ”vertical” projection is not shifted forward, as might be expected in view of the trunk inclination, but remains close to the ankle joint axis. At the end of the upper trunk forward or backward bending movement, the final position of the vertical CM projection remains close to the ankle joint axis in microgravity. These results are interpreted as indicating that CM anteroposterior position continues to be accurately controlled in microgravity; the forward inclination of the trunk axis observed in microgravity is interpreted as being due to a misevaluation of the ”vertical” axis on the basis of biased information from proprioceptive inputs.     

Effect of the hip motion on the body kinematics in the sagittal plane during human quiet standing

Human quiet stance is often modeled as a single-link inverted pendulum pivoting only around the ankle joints in the sagittal plane. However, several recent studies have shown that movement around the hip joint cannot be negligible, and the body behaves like a double-link inverted pendulum. The purpose of this study was to examine how the hip motion affects the body kinematics in the sagittal plane during quiet standing. Ten healthy subjects were requested to keep a quiet stance for 30 s on a force platform. The angular displacements of the ankle and hip joints were measured using two highly sensitive CCD laser sensors. By taking the second derivative of the angular displacements, the angular accelerations of both joints were obtained. As for the angular displacements, there was no clear correlation between the ankle and hip joints. On the other hand, the angular accelerations of both joints were found to be modulated in a consistent anti-phase pattern. Then we estimated the anterior–posterior (A–P) acceleration of the center of mass (CoM) as a linear summation of the angular acceleration data. Simultaneously, we derived the actual CoM acceleration by dividing A–P share force by body mass. When we estimated CoM acceleration using only the angular acceleration of the ankle joint under the assumption that movement of the CoM is merely a scaled reflection of the motion of the ankle, it was largely overestimated as compared to the actual CoM acceleration. Whereas, when we take the angular acceleration of the hip joint into the calculation, it showed good coincidence with the actual CoM acceleration. These results indicate that the movement around the hip joint has a substantial effect on the body kinematics in the sagittal plane even during quiet standing.