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PubMed2006

Anal sphincter lacerations and upright delivery postures--a risk analysis from a randomized controlled trial.

OBJECTIVE: To evaluate obstetric sphincter lacerations after a kneeling or sitting position at second stage of labor in a multivariate risk analysis model. MATERIALS AND METHODS: Two hundred and seventy-one primiparous women with normal pregnancies and spontaneous labor were randomized, 138 to a kneeling position and 133 to a sitting position. Medical data were retrieved from delivery charts and partograms. Risk factors were tested in a multivariate logistic regression model in a stepwise manner. RESULTS: The trial was completed by 106 subjects in the kneeling group and 112 subjects in the sitting group. There were no significant differences with regard to duration of second stage of labor or pre-trial maternal characteristics between the two groups. Obstetrical sphincter tears did not differ significantly between the two groups but an intact perineum was more common in the kneeling group (p<0.03) and episiotomy (mediolateral) was more common in the sitting group (p<0.05). Three grade IV sphincter lacerations occurred in the sitting group compared to none in the kneeling group (NS). Multivariate risk analysis indicated that prolonged duration of second stage of labor and episiotomy were associated with an increased risk of third- or fourth-degree sphincter tears (p<0.01 and p<0.05, respectively). Delivery posture, maternal age, fetal weight, use of oxytocin, and use of epidural analgesia did not increase the risk of obstetrical anal sphincter lacerations in the two upright postures. CONCLUSION: Obstetrical anal sphincter lacerations did not differ significantly between a kneeling or sitting upright delivery posture. Episiotomy was more common after a sitting delivery posture, which may be associated with an increased risk of anal sphincter lacerations. Upright delivery postures may be encouraged in healthy women with normal, full-term pregnancy.

Adult↗
PubMed2003

Impact of postural deformities and spinal mobility on quality of life in postmenopausal osteoporosis.

The objective of the study was to evaluate the impact of postural deformities and spinal mobility on quality of life (QOL) in patients with spinal osteoporosis. A total of 157 postmenopausal women aged over 60 years with osteoporosis were divided into five groups according to their postural deformities: round back (RB, n=41), hollow round back (HRB, n=33), whole kyphosis (WK, n=40), lower acute kyphosis (LAK, n=18), and normal posture (NP, n=25). QOL was evaluated using the Japanese Osteoporosis QOL Questionnaire (JOQOL) proposed by the Japanese Society for Bone and Mineral Research. This questionnaire contains six domains, with higher scores indicating higher levels of QOL. The number of vertebral fractures, thoracic kyphosis and lumbar lordosis angles, and spinal range of motion (ROM) during maximum flexion and extension were also measured with radiographs. Total QOL scores in RB, HRB, WK, and LAK groups were significantly lower than those in the NP group, and those in WK group were even lower compared with the other groups ( P<0.05). All the groups with postural deformities, but not the NP group, showed significant positive correlations between total QOL score and spinal ROM (0.521</= r</=0.747, P<0.05). Total QOL score showed a significant correlation with age, number of vertebral fractures, lumbar lordosis angle, and spinal ROM in a total of 157 patients. However, multiple regression analysis revealed that spinal ROM best correlated with total QOL score. We concluded that QOL in patients with osteoporosis was impaired by postural deformities, especially by whole kyphosis, and that spinal mobility has a strong effect on QOL in these patients.

Activities of Daily Living↗
PubMed2001

Building anticipatory postural adjustment during childhood: a kinematic and electromyographic analysis of unloading in children from 4 to 8 years of age.

The present study addressed the question of how do anticipatory postural adjustments (APA) develop during childhood, in the range from 4 to 8 years, during a bimanual load-lifting task. This task required maintaining the stabilisation of the forearm position despite imposed or voluntary unloading of the forearm. Elbow angle and multiple surface EMG were recorded on the child postural forearm supporting a load. During voluntary unloading, the elbow flexion of the postural forearm was calculated as a percentage of the elbow flexion measured during the imposed situation. Improvement of the forearm stabilisation was observed mainly between 4--6 and 7--8 years of age, but the oldest children did not reach the adult level of stabilisation of the postural forearm. Moreover, a clear developmental sequence in the acquisition of APA was reported: first the selection of an efficient EMG pattern underlying the forearm stabilisation, and second the mastering of timing adjustments. In fact, regression of the co-contraction pattern was observed with age, together with selection of the adult-like reciprocal pattern. Mastering of the timing adjustments of the reciprocal pattern, characterised in adults by a well-synchronised co-ordination between onset of the flexor muscle contraction of the manual arm and onset of the flexor muscle inhibition of the postural forearm, progressively improves during development. Moreover, these results suggest that the internal representation of the consequences of unloading on the forearm stabilisation, underlying anticipatory function during a bimanual co-ordination task, slowly build up during childhood.

Adult↗
PubMed2003

Differential approach to strategies of segmental stabilisation in postural control.

The present paper attempts to clarify the between-subjects variability exhibited in both segmental stabilisation strategies and their subordinated or associated sensory contribution. Previous data have emphasised close relationships between the interindividual variability in both the visual control of posture and the spatial visual perception. In this study, we focused on the possible relationships that might link perceptual visual field dependence-independence and the visual contribution to segmental stabilisation strategies. Visual field dependent (FD) and field independent (FI) subjects were selected on the basis of their extreme score in a static rod and frame test where an estimation of the subjective vertical was required. In the postural test, the subjects stood in the sharpened Romberg position in darkness or under normal or stroboscopic illumination, in front of either a vertical or a tilted frame. Strategies of segmental stabilisation of the head, shoulders and hip in the roll plane were analysed by means of their anchoring index (AI). Our hypothesis was that FD subjects might use mainly visual cues for calibrating not only their spatial perception but also their strategies of segmental stabilisation. In the case of visual cue disturbances, a greater visual dependency to the strategies of segmental stabilisation in FD subjects should be validated by observing more systematic "en bloc" functioning (i.e. negative AI) between two adjacent segments. The main results are the following: 1. Strategies of segmental stabilisation differed between both groups and differences were amplified with the deprivation of either total vision and/or static visual cues. 2. In the absence of total vision and/or static visual cues, FD subjects have shown an increased efficiency of the hip stabilisation in space strategy and an "en bloc" operation of the shoulder-hip unit (whole trunk). The last "en bloc" operation was extended to the whole head-trunk unit in darkness, associated with a hip stabilisation in space. 3. The FI subjects have adopted neither a strategy of segmental stabilisation in space nor on the underlying segment, whatever the body segment considered and the visual condition. Thus, in this group, head, shoulder and hip moved independently from each other during stance control, roughly without taking into account the visual condition. The results, emphasising a differential weighting of sensory input involved in both perceptual and postural control, are discussed in terms of the differential choice and/or ability to select the adequate frame of reference common to both cognitive and motor spatial activities. We assumed that a motor-somesthetics "neglect" or a lack of mastering of these inputs/outputs rather than a mere visual dependence in FD subjects would generate these interindividual differences in both spatial perception and postural balance. This proprioceptive "neglect" is assumed to lead FD subjects to sensory reweighting, whereas proprioceptive dominance would lead FI subjects to a greater ability in selecting the adequate frame of reference in the case of intersensory disturbances. Finally, this study also provides evidence for a new interpretation of the visual field dependence-independence dimension in both spatial perception and postural control.

Adult↗
PubMed2003

Development of postural adjustments during reaching in sitting children.

We evaluated the development of postural adjustments accompanying reaching movements in sitting children. Twenty-nine typically developing children aged, 2-11 years, and ten adults were studied with multiple surface electromyograms (EMGs) and kinematics during reaching in four conditions: sitting with the seat-surface oriented horizontally with and without an additional task load, and sitting with the seat-surface tilted 15 degrees forward and 15 degrees backward. The development of postural adjustments during reaching in a sitting position turned out to have a non-linear and protracted course, which is not finished by the age of 11 years. The development of these adjustments is characterised by variation, yet specific developmental sequences could be distinguished. Firstly, the development of postural adjustments during reaching from the age of 2 years onwards lacked a preference for an en bloc strategy, which consists of an in concert activation of the direction-specific neck and trunk muscles. Secondly, anticipatory postural muscle activity, which was consistently present in adults, was virtually absent between 2 and 11 years of age. Thirdly, the data demonstrated that with increasing age the head gradually becomes the dominant frame of reference. In addition, the study suggested that, in terms of postural control, the forward-tilted position is the most efficient one.

Adult↗
PubMed2003

Postural feedback responses scale with biomechanical constraints in human standing.

We tested whether human postural responses can be described in terms of feedback control gains, and whether these gains are scaled by the central nervous system to accommodate biomechanical constraints. A feedback control model can describe postural responses for a wide range of perturbations, but biomechanical constraints-such as on the torque that can be exerted on the ground-make a single set of feedback gains inappropriate for all perturbations. To observe how postural responses change with perturbation magnitude, we applied fast, backward perturbations of magnitudes 3-15 cm to 13 healthy young volunteers (4 men, 9 women, aged 20-32 years). We used a 3-segment, sagittal-plane biomechanical model and a linear state feedback controller to reproduce the observed postural responses. Optimization was used to identify the best-fit feedback control gains for each trial. Results showed that trajectories of joint angles and joint torques were scaled with perturbation magnitude. This scaling occurred gradually, rather than abruptly changing at magnitudes where biomechanical constraints became active. Feedback gains were found to fit reasonably well with data ( R(2)=0.92) and to be multivariate and heterogenic in character, meaning that the torque produced at any joint is generally a function of motions not only at the same joint, but other joints as well. Hip gains increased and ankle gains decreased nearly linearly with perturbation magnitude, in accordance with biomechanical limitations on ground reaction torque. These results indicate that postural adjustments can be described as a single feedback control scheme, with scalable heterogenic gains that are adjusted according to biomechanical constraints.

Adult↗
PubMed2005

Supervised learning of postural tasks in patients with poststroke hemiparesis, Parkinson's disease or cerebellar ataxia.

Supervised learning of different postural tasks in patients with lesions of the motor cortex or pyramidal system (poststroke hemiparesis: 20 patients), nigro-striatal system (Parkinson's disease: 33 patients) and cerebellum (spinocerebellar ataxia: 37 patients) was studied. A control group consisted of 13 healthy subjects. The subjects stood on a force platform and were trained to change the position of the center of pressure (CP) presented as a cursor on a monitor screen in front of the patient. Subjects were instructed to align the CP with the target and then move the target by shifting the CP in the indicated direction. Two different tasks were used. In "Balls", the target (a ball) position varied randomly, so the subject learned a general strategy of voluntary CP control. In "Bricks", the subject had to always move the target in a single direction (downward) from the top to the bottom of the screen, so that a precise postural coordination had to be learned. The training consisted of 10 sessions for each task. The number of correctly performed trials for a session (2 min for each task) was scored. The voluntary control of the CP position was initially impaired in all groups of patients in both tasks. In "Balls", there were no differences between the groups of the patients on the first day. The learning course was somewhat better in hemiparetic patients than in the other groups. In "Bricks", the initial deficit was greater in the groups of parkinsonian and cerebellar patients than in hemiparetic patients. However, learning was more efficient in parkinsonian than in hemiparetic and cerebellar patients. After 10 days of training, the hemiparetic and cerebellar patients completed the acquisition at a certain level whereas the parkinsonian patients showed the ability for further improvement. The results suggest that motor cortex, cerebellum, and basal ganglia are involved in voluntary control of posture and learning different postural tasks. However, these structures play different roles in postural control and learning: basal ganglia are mainly involved in learning a general strategy of CP control while the function of the motor cortex chiefly concerns learning a specific CP trajectory. The cerebellum is involved in both kinds of learning.

Adult↗
PubMed2005

Spatiotemporal interactions between audition and touch depend on hand posture.

We report two experiments designed to assess the consequences of posture change on audiotactile spatiotemporal interactions. In Experiment 1, participants had to discriminate the direction of an auditory stream (consisting of the sequential presentation of two tones from different spatial positions) while attempting to ignore a task-irrelevant tactile stream (consisting of the sequential presentation of two vibrations, one to each of the participant's hands). The tactile stream presented to the participants' hands was either spatiotemporally congruent or incongruent with respect to the sounds. A significant decrease in performance in incongruent trials compared with congruent trials was demonstrated when the participants adopted an uncrossed-hands posture but not when their hands were crossed over the midline. In Experiment 2, we investigated the ability of participants to discriminate the direction of two sequentially presented tactile stimuli (one presented to each hand) as a function of the presence of congruent vs incongruent auditory distractors. Here, the crossmodal effect was stronger in the crossed-hands posture than in the uncrossed-hands posture. These results demonstrate the reciprocal nature of audiotactile interactions in spatiotemporal processing, and highlight the important role played by body posture in modulating such crossmodal interactions.

Acoustic Stimulation↗
PubMed2006

Adaptation of postural orientation to changes in surface inclination.

We previously showed that standing on an inclined surface resulted in an after-effect of leaning in many healthy, blindfolded subjects when they returned to standing on a horizontal surface (Kluzik et al. in Exp Brain Res 162:474-489, 2005). The direction of leaning depended on the direction of prior surface inclination, always in a direction that preserved the relative alignment between the body and the support surface. For example, subjects leaned forward after they stood on a toes-up-inclined surface. In the present study, we investigated how the amplitude of surface inclination affected postural muscle activity, joint position, body segment orientation, and body center of mass (CoM) and foot center of pressure (CoP) locations before, during, and after subjects stood on an inclined surface. We asked whether the mechanism that underlies the lean after-effect involves regulation of local postural variables, such as the position of the ankle joint or the level of muscle activity, or whether instead, the mechanism involves regulation of global, whole-body postural variables that can only be determined by multisensory processing, such as orientation of the trunk or the body's CoM. In one experiment, we found that varying the amplitude of a toes-up surface inclination between 2.5 degrees and 10 degrees had a systematic, linear, effect on the post-incline orientation of the trunk and head, but did not systematically affect the post-incline orientation of the legs, position of the ankle joint, the level of EMG activity, or the location of the CoP. In a second experiment, we found that preventing the legs from leaning in the post-incline period did not abolish leaning of the upper body. These findings suggest that (1) the body-to-support-surface relationship is an important reference for the CNS internal representation of postural orientation which is subject to adaptive modification and (2) the adaptive mechanism underlying the post-incline after-effect of leaning acts at the level of global, whole-body postural variables.

Adaptation, Physiological↗
PubMed2000

Postural invariance in three-dimensional reaching and grasping movements.

The question of whether the final arm posture to be reached is determined in advance during prehension movements remains widely debated. To address this issue, we designed a psychophysical experiment in which human subjects were instructed to reach and grasp, with their right arm, a small sphere presented at various locations. In some trials the sphere remained stationary, while in others (the perturbed trials) it suddenly jumped, at movement onset, to a new unpredictable position. Our data indicate that the final configuration of the upper limb is highly predictable for a given location of the sphere. For movements directed at stationary objects, the variability of the final arm posture was very small in relation to the variability allowed by joint redundancy. For movements directed at "jumping" objects, the initial motor response was quickly amended, allowing an accurate grasp. The final arm posture reached at the end of the perturbed trials was neither different from nor more variable than the final arm posture reached at the end of the corresponding stationary trials (i.e. the trials sharing the same final object location). This latter result is not trivial, considering both joint redundancy and the motor reorganization imposed by the change in sphere location. In contrast to earlier observations, our data cannot be accounted for by biomechanical or functional factors. Indeed, the spherical object used in the present study did not constrain the final arm configuration or the hand trajectory. When considered together, our data support the idea that the final posture to be reached is planned in advance and used as a control variable by the central nervous system.

Adult↗
PubMed2001

Abnormal vestibular control of gaze and posture in a strain of a waltzing rat.

The waltzing behavior is usually attributed to vestibular dysfunction. However, the vestibular control of gaze and posture has not yet been measured quantitatively in any waltzing mutant. Therefore, this study was aimed at investigating the relationship between inner-ear morphology, the circling behavior, and the vestibular control of gaze and posture in a new strain of waltzing rats. Light- and electron-microscopy studies of these mutants did not reveal any structural abnormalities of the vestibular neural epithelia. In addition, the expression of Calretinin and 200-kD phosphorylated and non-phosphorylated neurofilaments was also found to be normal in the vestibular neural epithelia and ganglion cells. In contrast, the mutants showed severe dysfunctions of the vestibular control of gaze and posture. The skeletal geometry of the alert unrestrained animals was studied using cineradiography. At rest, waltzing rats held their heads tilted down: the horizontal semicircular-canal's plane was near the earth-horizontal's plane, instead of being tilted up as in Long Evans control rats. In addition, their cervical column was pitched more forward (33.6 degrees) than in the control group (6.9 degrees). The circling behavior was observed frequently, and the rats had episodes of circling in both directions. The episodes of circling amounted to an average of 17 turns, and the average angular velocity of the circling was 645 degrees/s. Unilateral labyrinthectomy induced the same postural and oculomotor syndromes in the waltzing and control groups. This indicates that the mutant vestibular nerve had a significant resting discharge before the lesion. Eye movements were recorded using acutely implanted search coils. Although waltzing rats were able to perform normal spontaneous eye movements, they showed a complete deficit of the horizontal vestibulo-ocular reflex (HVOR) and an impairment of the maculo-ocular reflex (MOR) during constant velocity, off-vertical axis rotation (OVAR). These results show, for the first time, that deficient transduction and/or processing of the horizontal-canal- and macular-related information can be causally related to the circling behavior and abnormal posture, respectively.

Animals↗
PubMed2000

Posture-related changes in heteronymous recurrent inhibition from quadriceps to ankle muscles in humans.

The possibility was investigated that changes in heteronymous recurrent inhibition (RI) from quadriceps (Q) to soleus (Sol) and tibialis anterior (TA) motoneurons (MNs) occur during postural tasks requiring cocontraction of Q with one of these muscles. Stimulation of the femoral nerve (FN), which elicited a Q H-reflex discharge, was used to activate Renshaw cells. The resulting inhibition of TA and Sol MNs was assessed using three test responses: (1) the rectified and averaged ongoing electromyogram (EMG) activity in TA or Sol; (2) the motor-evoked potential (MEP) elicited by cortical stimulation in these muscles; and (3) the Sol H reflex. The characteristics of the depression (appearance and increase with the conditioning reflex discharge, short central delay and long duration) are consistent with a Renshaw origin. In addition, results obtained in control experiments (no change in the EMG suppression after an ischaemic blockade of group-I afferents from the leg, time course of the FN-induced depression of the MEP similar to that of the ongoing EMG) made a significant contribution from other pathways activated by FN stimulation unlikely. Posture-related heteronymous RI from Q was compared in different postural tasks at matched levels of background EMG activity: voluntary co-contraction of Q and of the relevant ankle muscle while sitting (control situation), postural co-contraction of Q and TA (while leaning backwards during stance), or contraction of Sol with (preparation for hopping) and without (standing on tip of toes and leaning forwards during stance) associated contraction of the Q. During stance, heteronymous RI from Q was reduced to TA (but not to Sol) while leaning backwards and to Sol in preparation for hopping, but not in the other situations. Thus, RI from Q to TA or Sol was specifically decreased when a co-contraction of the Q and of the relevant muscle operating at the ankle was required to maintain bipedal stance. It is argued that this control of Renshaw cells is descending in origin and contributes to selection of the appropriate synergism in various postural tasks.

Adult↗
PubMedDate not supplied

Involvement of a midbrain vocal nucleus in the production of both the acoustic and postural components of crowing behavior in Japanese quail.

Many bird species produce vocalizations which are accompanied by distinctive postural displays, but the neural mechanisms that allow such integrated production of vocal and postural motor patterns are not understood. In the crowing behavior of Japanese quail, a characteristic vocal pattern is accompanied by and coordinated with a postural display that consists of a sequence of rapid, patterned head movements. The purpose of the present study was to investigate the role of a midbrain vocal nucleus, the nucleus intercollicularis, in the production of the acoustic and postural components of crowing in quail. Brief electrical stimuli were applied to the nucleus intercollicularis during spontaneously emitted crows in quail with chronically implanted electrodes, to determine if perturbing neural activity in the nucleus intercollicularis resulted in a disruption of ongoing crowing behavior. The most common effect of such stimuli was a concurrent, premature termination of both the acoustic and head movement components of the crow. These results imply that the nucleus intercollicularis plays a role in the production of both the acoustic and postural components of crowing in quail.

Animals↗
PubMed2004

The effect of postural restrictions in the treatment of benign paroxysmal positional vertigo.

The treatment of benign paroxysmal positional vertigo (BPPV) consists of a repositioning maneuver in order to remove otoliths from the posterior semicircular canal and subsequent postural restrictions to prevent debris from reentering the canal. However, the extent to which postural restrictions affect the final outcomes of BPPV is still uncertain. The purpose of this study is to determine the effects of postural restrictions in the treatment of BPPV, which is caused by otoliths in the posterior semicircular canal, and to evaluate its usefulness as a therapy. Seventy cases diagnosed as BPPV of the posterior semicircular canal were studied. All patients were treated utilizing the modified Epley maneuver. They were divided into two groups. The first group (group A, 35 patients) was instructed to sleep in a semi-sitting position and to avoid moving their heads forward or backward, whereas no instructions were given to the second group (group B, 35 patients). The information gathered from the patients was analyzed by age of onset, gender, duration of the disease and cure rates or recurrence rates. In group A, the average age was 54.5 years and the mean duration of symptoms was 116.5 days, whereas in group B, these measurements were 54.8 years and 86.7 days. The cure rates were 91.4 and 94.3%, respectively. Symptoms recurred in 12.5% of group A patients, while in group B, there was a 9.1% recurrence rate. Our study did not show any significant difference between the two groups. Postural restriction therapy, practiced after the modified Epley repositioning maneuver, did not have a significant effect on the final outcomes of BPPV. Based on our results, we do not recommend this therapy since there was no significant benefit for the patients who utilized postural restrictions.

Adult↗
PubMed2002

Postural variation in intraocular pressure in primary chronic autonomic failure.

Patients with syndromes of generalised autonomic failure often have extreme posture-related lability of blood pressure, with both orthostatic hypotension and recumbent hypertension. Whether these changes influence intraocular pressure (IOP) is not known. Mean arterial pressure (MAP) and IOP were measured in response to variations in posture between +45 degrees and -20 degrees in 8 normal subjects and 9 subjects with primary generalised chronic autonomic failure (AF). With postural change normal subjects showed minimal change in MAP (p=0.6) and small but significant changes in IOP (p < 0.001). Subjects with AF showed large and significant changes in both MAP (p < 0.001) and IOP (p < 0.001). Two AF subjects had raised IOP when recumbent, despite normal IOP at +45 degrees. There was significant covariance of MAP and IOP (p < 0.001 overall, p=0.004 in normal subjects, p=0.006 in AF subjects). However, individually, those patients with large changes in IOP could not be predicted from changes in MAP. These data show that patients with autonomic failure are subject to large posture-related changes in IOP. These appear to be related to the large posture-induced changes in systemic blood pressure which occur in these patients.

Aged↗
PubMed2006

The postural reduction in middle cerebral artery blood velocity is not explained by PaCO2.

In the normocapnic range, middle cerebral artery mean velocity (MCA Vmean) changes approximately 3.5% per mmHg carbon-dioxide tension in arterial blood (PaCO2) and a decrease in PaCO2 will reduce the cerebral blood flow by vasoconstriction (the CO2 reactivity of the brain). When standing up MCA Vmean and the end-tidal carbon-dioxide tension (PETCO2) decrease, suggesting that PaCO2 contributes to the reduction in MCA Vmean. In a fixed body position, PETCO2 tracks changes in the PaCO2 but when assuming the upright position, cardiac output (Q) decreases and its distribution over the lung changes, while ventilation (VE) increases suggesting that PETCO2 decreases more than PaCO2. This study evaluated whether the postural reduction in PaCO2 accounts for the postural decline in MCA Vmean). From the supine to the upright position, VE, Q, PETCO2, PaCO2, MCA Vmean, and the near-infrared spectrophotometry determined cerebral tissue oxygenation (CO2Hb) were followed in seven subjects. When standing up, MCA Vmean (from 65.3+/-3.8 to 54.6+/-3.3 cm s(-1) ; mean +/- SEM; P<0.05) and cO2Hb (-7.2+/-2.2 micromol l(-1) ; P<0.05) decreased. At the same time, the VE/Q ratio increased 49+/-14% (P<0.05) with the postural reduction in PETCO2 overestimating the decline in PaCO2 (-4.8+/-0.9 mmHg vs. -3.0+/-1.1 mmHg; P<0.05). When assuming the upright position, the postural decrease in MCA Vmean seems to be explained by the reduction in PETCO2 but the small decrease in PaCO2 makes it unlikely that the postural decrease in MCA Vmean can be accounted for by the cerebral CO2 reactivity alone.

Adult↗
PubMed2005

How attentional focus on body sway affects postural control during quiet standing.

The purpose of this study was to investigate how attentional focus on body sway affects postural control during quiet standing. To address this issue, sixteen young healthy adults were asked to stand upright as immobile as possible on a force platform in both Control and Attention conditions. In the latter condition, participants were instructed to deliberately focus their attention on their body sways and to increase their active intervention into postural control. The critical analysis was focused on elementary motions computed from the centre of pressure (CoP) trajectories: (1) the vertical projection of the centre of gravity (CoG(v)) and (2) the difference between CoP and CoG(v) (CoP-CoG(v)). The former is recognised as an index of performance in this postural task, whilst the latter constitutes a fair expression of the ankle joint stiffness and is linked to the level of neuromuscular activity of the lower limb muscles required for controlling posture. A frequency-domain analysis showed increased amplitudes and frequencies of CoP-CoG(v) motions in the Attention relative to the Control condition, whereas non-significant changes were observed for the CoG(v) motions. Altogether, the present findings suggest that attentional focus on body sway, induced by the instructions, promoted the use of less automatic control process and hampered the efficiency for controlling posture during quiet standing.

Adult↗
PubMed2002

Muscle force evaluation and the role of posture in human lumbar spine under compression.

Using two nonlinear finite element models of the lumbar spine, the concept of optimal posture is explored by minimizing the segmental sagittal moments required for the equilibrium of the passive lumbar spine under a total of 2800 N axial compression while varying the pelvic tilt and lumbar lordosis. The redundant active-passive system is subsequently solved for this posture using a novel kinematics-based muscle calculation algorithm along with minimization approach. Some flattening in the lumbar spine substantially reduces the required moments and internal passive shear forces under 2800 N axial compression force. Small muscle forces are calculated for this optimal posture. The role of flattening in the lumbar lordosis and posterior pelvic tilt in diminishing the lumbar muscle activities in neutral postures is demonstrated. Without such changes in posture, the required moments probably exceed the moment-generating capability of local lumbar muscles. Consideration of such active-passive synergy and lack of its restriction may prove crucial in many activities. Moreover, a kinematics-based algorithm is proposed for the solution of spinal redundancy that fully accounts for the existing passive-active synergy while simultaneously satisfying all kinematics and equilibrium conditions along the length of the spine.

Biomechanical Phenomena↗