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At least 505 records · Page 28Linked to original sources

PubMed2003

End posture selection in manual positioning: evidence for feedforward modeling based on a movement choice method.

Although it is well known that the postures adopted at the ends of movements depend on where one starts and how one moves, it is not yet clear whether those differing end postures are selected before movements begin. Two experiments were designed to test the hypothesis that end postures for positioning movements are chosen before movements commence. The experiments were further designed to check whether movements are internally simulated before overt movements occur and end-postures are still being selected. To address these questions we used a movement choice method. Participants were presented with two possible end postures and were asked to choose between them by moving to the one that seemed easier to adopt. End-posture choices were affected by starting positions and also by the movements that would have to be made, as affected by having obstacles in the way. The results suggest that participants relied on feedforward modeling of prospective movements as they selected end postures prior to overt movement production. The fact that the movement choice method could confirm this suggests that the method holds considerable promise as a tool for investigating motor planning.

Analysis of Variance↗

PubMed2006

Fatigue-induced adaptive changes of anticipatory postural adjustments.

To examine the fatigue-induced adaptive changes (e.g., timing) of anticipatory postural adjustments (APAs), APAs of 30 research participants were recorded before (baseline) and after (post-test) conditions of either rest (control group, n=15) or fatigue (fatigue group, n=15). Muscle fatigue was generated using a dead-lift exercise performed to exhaustion. Self-initiated postural perturbations were induced using a rapid unilateral arm-raising maneuver (focal movement), and APAs were obtained using electromyography (EMG) recorded bilaterally in the lumbar and thoracic paraspinal muscles as well as the hamstring muscles. Postural stability during the focal movement was assessed using a force plate. Results showed that fatigue had no effect on postural stability during the focal movement, and yet caused earlier APA onsets in three of the six muscles evaluated. In spite of early APA activation, the APA EMG integrals of two of the three postural control muscles which exhibited fatigue-induced early APA onsets (T9 and L4 contralateral paraspinals) did not differ between baseline and post-test measures. The findings suggest that early APA onset may enhance postural stability by permitting a longer duration APA which can counteract fatigue-induced decreases in the force-producing capability of muscles that contribute to postural stability.

Adaptation, Physiological↗

PubMed1998

Anticipatory postural adjustments during self-paced and reaction-time movements.

We studied the changes in the anticipatory postural adjustments (APAs), associated with dropping a load from extended arms and during fast bilateral shoulder flexion movements, when movements were performed in a self-paced manner and under a simple reaction-time instruction. The latter instruction applied time pressure and did not allow the regular pattern of APAs to be used. In particular, the following questions were asked: (1) are there changes in the relative timing of APAs under the reaction time condition; (2) are changes in the relative timing of APAs associated with changes in APAs themselves; (3) can different postural strategies be used to maintain stability under self-paced and reaction time conditions; and (4) are changes in APAs related to actual reaction time or to a change in the instruction? In particular, under reaction-time conditions, APAs occurred later in time, typically simultaneously with the initiation of the focal movement. Additional changes in electromyographic (EMG) patterns in postural muscles included an increase in the amplitude of EMG bursts and "speeding-up" some of the tri-phasic patterns in postural dorsal-ventral muscle pairs. This was accompanied by a smaller early shift of the center of pressure followed by its more rapid delayed displacement. There was considerable variability in the changes of EMG and dynamic characteristics across subjects. Some of the changes in the EMG patterns in postural muscles depended on actual reaction time, while others were related to a change in the instruction and occurred even if actual reaction times were long enough to allow for the typical self-paced APA patterns to occur. These findings can be interpreted as supporting the parallel control hypothesis for the focal movement and postural adjustments. Alternatively, they can be interpreted within a framework that implies the generation of a single control function, which is transformed into two components, one directed at the focal muscles/joints and the other directed at postural muscles/joints.

Adult↗

PubMed1999

Remembered positions: stored locations or stored postures?

Many recent studies indicate that memory for final position is superior to memory for movement. There is ambiguity about what is meant by the term final position, however. Is it final spatial location or final posture? According to a recently proposed theory by Rosenbaum et al., which maintains that stored postures form the basis for movement planning, when people try to return to recently reached positions, they should try to adopt the postures they just occupied. An alternative view, which holds that movements are primarily planned with respect to spatial locations, predicts that subjects should tend to return to places in external space. We describe an experiment that tested these opposing predictions. The experiment relied on the notion that if people store and use postures, they should "copy" the posture adopted with one arm to the other arm when possible. The results support this hypothesis. In this article, we review previous work that bears on the question of what is learned when people move repeatedly to a given position. Then we present two theoretical perspectives which make diverging predictions about what should be learned in repositioning tasks. One perspective predicts that final positions are remembered as postures; the other predicts that final positions are remembered as locations. We describe an experiment designed to distinguish between these two predictions. The experiment indicates that final postures are remembered and are "copied" from one arm to the other when subjects try to reach repeatedly to the same location in the midsagittal plane with alternating arms or when subjects try to reach repeatedly to the same location anywhere in the workspace with the same arm. In the last section of the article, we discuss the implications of our findings.

Adult↗

PubMed2001

Development of postural adaptation to arm raising.

We studied the development of the coordination between posture and movement by analyzing the shifts of the center of pressure (CoP) associated with arm raising. Three groups of children aged 3-5 years, 6-8 years, and 9-10 years and an adult group were tested. The subjects were required to raise their arms to the horizontal position while standing still, with their hands free or loaded (5% of the body weight). The arm movements were recorded by a TV-image processor, and the changes in position of the CoP were measured by a force platform and analyzed before, during, and after the arm movement. The data show that the CoP moved forward during arm raising, that additional load induced a greater shift in all age groups, and that the relative amplitude of the shift decreased with age. The greatest changes occurred between ages 3-5 years and 6-8 years. The pre- and postmovement CoP shift suggests qualitative changes in the postural adaptation to movement between these two age groups: the anticipatory postural adjustments moved from a supporting function to a compensatory function, yielding an increasing functional convergence between the feedforward and the feedback modes of postural control, and an increasing rapidness in recovering postural stability after arm movement. The postural behavior shown by the 9- to 10-year-old children and by the adults in the arms-free condition suggests an increased tolerance to unbalance when postural oscillations do not jeopardize static equilibrium.

Adaptation, Physiological↗

PubMed2005

Effect of posture on high-intensity constant-load cycling performance in men and women.

The time sustained during a graded cycle exercise is approximately 10% longer in an upright compared with a supine posture. However, during constant-load cycling this effect is unknown. Therefore, we tested the postural effect on the performance of high-intensity constant-load cycling. Twenty-two active subjects (11 men, 11 women) performed two graded tests (one upright, one supine), and of those 22, 10 subjects (5 men, 5 women) performed three high-intensity constant-load tests (one upright, two supine). To test the postural effect on performance at the same absolute intensity, during the upright and one of the supine constant-load tests subjects cycled at 80% of the peak power output achieved during the upright graded test. To test the postural effect on performance at the same relative intensities, during the second supine test subjects cycled at 80% of the peak power output achieved during the supine graded test. Exercise time on the graded and absolute intensity constant-load tests for all subjects was greater (P<0.05) in the upright compared with supine posture (17.9+/-3.5 vs. 16.1+/-3.1 min for graded; 13.2+/-8.7 vs. 5.2+/-1.9 min for constant-load). This postural effect at the same absolute intensity was larger in men (19.4+/-8.5 upright vs. 6.6+/-1.6 supine, P<0.001) than women (7.1+/-2 upright vs. 3.9+/-1.4 supine, P>0.05) and it was correlated (P<0.05) with both the difference in VO2 between positions during the first minute of exercise (r=0.67) and the height of the subjects (r=0.72). In conclusion, there is a very large postural effect on performance during constant-load cycling exercise and this effect is significantly larger in men than women.

Adult↗

PubMed2007

Postural differences in hemodynamics and diastolic function in healthy older men.

The shift from upright to supine posture increases stroke volume in healthy young adults, primarily through increased end-diastolic volume. Aging is associated with increased ventricular stiffness and impaired diastolic function. The purpose of this study was to determine whether the stroke volume change between the upright and supine posture was blunted in healthy older men and whether the early mitral inflow response to this postural change was reduced by aging. Pulsed Doppler echocardiography and tissue Doppler imaging were used to assess stroke volume and left ventricular function during upright and supine posture in 10 young and 12 older healthy men. Cardiac output was greater in the supine versus upright condition in both groups. In young men supine posture was associated with a large increment in stroke volume (63.1 +/- 7.2 upright to 98.6 +/- 11.7 ml supine; P < 0.05) and a decrease in heart rate. In older men, the stroke volume increment was smaller (63.9 +/- 3.6 upright to 82.0 +/- 5.7 ml supine; P < 0.05) and heart rate decreased less than young men when comparing upright to supine posture. Increased stroke volume was associated with higher peak early diastolic filling velocity (E) in young men and higher peak late diastolic filling velocity (A) in older men. These findings indicate that healthy aging attenuates the increase in early filling associated with moving from an upright to a supine posture. Furthermore, healthy older men are more dependent upon atrial diastolic filling to augment end-diastolic and stroke volume, which may be due to age-associated increases in left ventricular filling pressure and impaired compliance.

Adult↗

PubMed1999

Acute systematic and variable postural adaptations induced by an orthopaedic shoe lift in control subjects.

A small leg length inequality, either true or functional, can be implicated in the pathogenesis of numerous spinal disorders. The correction of a leg length inequality with the goal of treating a spinal pathology is often achieved with the use of a shoe lift. Little research has focused on the impact of this correction on the three-dimensional (3D) postural organisation. The goal of this study is to quantify in control subjects the 3D postural changes to the pelvis, trunk, scapular belt and head, induced by a shoe lift. The postural geometry of 20 female subjects (X = 22, sigma = 1.2) was evaluated using a motion analysis system for three randomised conditions: control, and right and left shoe lift. Acute postural adaptations were noted for all subjects, principally manifested through the tilt of the pelvis, asymmetric version of the left and right iliac bones, and a lateral shift of the pelvis and scapular belt. The difference in the version of the right and left iliac bones was positively associated with the pelvic tilt. Postural adaptations were noted to vary between subjects for rotation and postero-anterior shift of the pelvis and scapular belt. No notable differences between conditions were noted in the estimation of kyphosis and lordosis. The observed systematic and variable postural adaptations noted in the presence of a shoe lift reflects the unique constraints of the musculoskeletal system. This suggests that the global impact of a shoe lift on a patient's posture should also be considered during treatment. This study provides a basis for comparison of future research involving pathological populations.

Adaptation, Physiological↗

PubMed2005

Evaluation of dynamic postural balance using the Biodex Stability System in rheumatoid arthritis patients.

The aim of this study was to investigate dynamic postural balance in patients with rheumatoid arthritis (RA) in relation to the disease characteristics. Seventy-four patients with RA and 42 controls of the same age group were tested using the Biodex Stability System (Biodex Medical Systems, Shirley, NY, USA). Anterior/posterior (AP), medial/lateral (ML), and overall (OA) indices were obtained with bilateral stance at platform stabilities of 2 and 8. Subjects were tested with "eyes open" at all times. At the same time, Disease Activity Score, functional disability [Health Assessment Questionnaire, (HAQ)], and Steinbrocker Functional Class (SFC) were assessed. Both the AP and OA indices in the RA group were significantly higher than in the control group for level 8. For OA index, the results were 2.7+/-0.9 in RA and 2.2+/-0.7 in the control group (p=0.006), and for AP index, the results were 2.1+/-0.7 in RA and 1.7+/-0.6 in the control group (p=0.002). Eleven patients (15.9%) and three controls (7.1%) could not complete the test at level 2. When the patients and controls who completed the test were compared, a significant difference was found only in the ML index. The results were 4.6+/-2.4 in RA and 3.8+/-1.6 in the control group (p=0.047). A positive correlation between HAQ and postural balance for all three stability indices at level 8 was detected. A positive correlation between SFC and postural balance for OA and ML at level 8 was also found. Multiple linear regression analyses revealed age and body mass index (BMI) to be the most important factors influencing postural dynamic balance at both levels in the RA group and in healthy controls. RA has a negative effect on dynamic postural stability. The functional status affects dynamic balance more than disease activity. Age and BMI were the most important factors influencing postural dynamic balance in the RA group and in healthy controls. Level 2 does not appear to be an appropriate level for evaluating postural stability in RA.

Adult↗

PubMed1989

Site-specific postural and locomotor changes evoked in awake, freely moving intact cats by stimulating the brainstem.

Locomotor behaviors evoked by stimulating the hypothalamus and the brainstem were studied in freely moving, awake cats. To do this, stimulating microelectrodes were chronically implanted into the subthalamic locomotor region (SLR) in the lateral hypothalamic area (LHA), the mesencephalic locomotor region (MLR) corresponding to the nucleus cuneiformis, the dorsal tegmental field (DTF) and the ventral tegmental field (VTF) of caudal pons along its midline. After recovery from surgery (2-3 days), open field tests were performed to study stimulus effects upon posture and locomotor movements. The stimuli consisted of pulses of 0.2 ms duration of less than 80 microA delivered at 50 pulses/s for 5-20 s. DTF stimulation resulted in suppression of postural support by the hindlimbs. When the cat was in a standing posture, DTF stimulation simply resulted in a sequential alteration of posture to a squatting and then to a final lying posture. In contrast, VTF stimulation evoked an almost opposite series of postural changes to those induced by DTF stimulation. With VTF stimulation, the cat changed from a lying or a squatting position, and then started to walk during continuation of the stimulation. With MLR stimulation, the cat invariably exhibited fast walking and then running movements. It ran straight forward, avoiding collision with walls or other obstacles, and even tried to jump over a fence placed in front of it. With LHA stimulation, the cat started to walk slowly extending its head forward and looking around repeatedly. It tended to walk with a stoop and stealthy steps along the corners of the room. Induced postural and locomotor changes were always accompanied by behavioral arousal reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Postural kinematics of trumpet playing.

This paper examines the effects of anthropometry on body posture of trumpeters playing in standing position. Sixteen virtuosi trumpeters were photographed while hitting three notes (low C, high F and high F sustained) during performance of musical tasks. Initial standing posture and anthropometric data were recorded. Six body segment angles were computed and a vectorial sum was obtained to describe whole body posture in neutral and playing conditions. Horn angle and dental overbite were also computed. Earlier results showed that the musical task has no effect on playing posture. One-way ANOVA showed notable differences between the neutral posture and the note-related playing postures. A multiple regression model showed that in addition to the note effect, anthropometric variables, mainly neck length, explain the changes in playing posture. Horn angle is determined by the dental overbite. The importance of the anthropometric variables in playing the more demanding notes indicate that anthropometry may act to constrain trumpeters' performance.

Adult↗

PubMed1996

Posture prediction for static sagittal-plane lifting.

Three separate models are presented to predict the postures of humans performing static sagittal lifting tasks. The models use a common inverse-kinematics characterization to represent mathematically feasible postures, but explore different criteria functions for selecting a final posture. The first criterion assumes that subjects minimize the overall effort associated with a posture. The second criterion expresses effort locally as opposed to globally, and minimizes this value. The third criterion maximizes body stability. The postures predicted by these three models were compared to the postures assumed by 16 subjects performing 4 static sagittal lifting tasks. The results showed that all of the prediction errors were significantly greater than zero, but that the first objective function (minimum total torque) was more accurate than the other two criteria. The models were in general less accurate for postures that had lower hand positions than for those with higher hand positions.

Adult↗

PubMed1993

Impaired scaling of long latency postural reflexes in patients with Parkinson's disease.

Young normal subjects adapt the size of posturally stabilizing reflexes in the lower extremity to predictable and unpredictable perturbations through shifts in cognitive set. It is unknown whether limitations in this ability to shift cognitive set may contribute to impaired scaling of postural reflexes in patients with Parkinson's disease. In this study, we have addressed this issue in 12 posturally unstable Parkinson patients and 13 age- and sex-matched controls. Postural stability was disturbed by sudden toe-up rotations of a supporting platform upon which subjects were standing. Subjects' cognitive set was altered by varying the perturbation amplitude either predictably (serial 4 degrees versus serial 10 degrees) or unpredictably (random mixture of 4 degrees and 10 degrees). Posturally stabilizing long latency (LL) reflexes were recorded from the shortened tibialis anterior muscle of both legs. We found that Parkinson patients, unlike some control subjects, were unable to scale the size of their LL reflex in response to variations in perturbation amplitude during predictable conditions. In addition, we observed that Parkinson patients could not modify the amplitude of the LL reflex through alterations in cognitive set during random conditions. We conclude that Parkinson patients have a fundamental difficulty in modifying the size of posturally stabilizing LL reflexes, as reflected by both problems with amplitude scaling and difficulties with changes in cognitive set. It is possible that this inability to modify LL reflexes may be a factor contributing to postural instability in Parkinson's disease.

Adult↗

PubMed2006

An evaluation of backpack harness systems in non-neutral torso postures.

Much of the research on backpack design has been focused on spinal loading/biomechanics while the wearer is in a neutral/upright trunk posture, such as those employed by outdoor enthusiasts and schoolchildren. This research has led to some important harness design improvements that reduce trunk muscle exertions, fatigue and improve overall comfort. There are number of occupations, however, wherein workers wear back-mounted packs/devices (e.g. air tanks) while working in non-neutral trunk postures. The objective of the current study was to evaluate the effects of these non-neutral postures on biomechanical loading and then reconsider the backpack system design recommendations. Fifteen participants were asked to support a 18.2 kg load on their back while assuming static forward flexed postures of the torso (15 degrees , 30 degrees , 45 degrees , and 60 degrees of sagittal bend). The mass on the back was attached to the participant through two different harness mechanisms: a basic harness design (as seen on college student backpacks) and a more advanced design containing lateral stiffness rods and a weight-bearing hip belt (as seen on backpacks for hikers). While performing these static, posture maintenance tasks, the activation levels of the bilateral trapezius, erector spinae, and rectus abdominis were collected. Participants also provided subjective ratings of comfort. The results showed that there was a significant interaction between harness type and forward flexion angle for the trapezius and the erector spinae muscles. The normalized EMG for the trapezius muscles showed a 14% and 11% reduction in muscle activity at 15 degrees and 30 degrees , respectively, with the advanced design but these positive effects of the advanced design were not found at the greater flexion angles. Likewise the erector spinae muscles showed a 24% and 14% reduction in muscle activity at 15 degrees and 30 degrees , respectively, with the advanced design harness but these effects of the advanced design were not found at the greater forward flexion angles. The level of forward flexion angle affected the rectus abdominis muscle activity, but neither the harness type main effect nor the interaction of harness type and forward flexion angle was significant. The subjective survey results agreed with the EMG results and showed the advanced design harness was generally more comfortable with respect to the shoulder and low back areas. Collectively, the subjective and objective results show a significant improvement with the advanced harness system but also note an interesting interaction with degree of sagittal flexion, indicating a diminished effectiveness of the design improvements at forward flexed postures. Design criteria for harness systems in these forward flexed postures are discussed.

Adult↗

PubMed2004

Attentional demand for regulating postural sway: the effect of expertise in gymnastics.

A dual-task paradigm was used to investigate whether the expertise in motor skills requiring a fine postural control can modify the attentional demand necessary for regulating postural sway. Seven expert gymnasts and seven experts in other non-gymnastic sports were asked to respond as rapidly as possible to an unpredictable auditory stimulus while maintaining stable seated and in three upright postures of increasing difficulty: bipedal, unipedal, and unipedal on an unstable support (i.e. a 7 cm thick foam surface). RT values were used as an index of the attentional demand necessary for performing the postural tasks. Results showed that the attentional demand necessary for regulating postural sway increased as the postural task increased in difficulty. Interestingly, this effect was smaller for the gymnasts during unipedal stance. These findings suggest a decreased dependency on attentional processes for regulating postural sway during unipedal stance in gymnasts with respect to non-gymnasts.

Adolescent↗

PubMed2006

Dual-tasking postural control: aging and the effects of cognitive demand in conjunction with focus of attention.

Postural control in everyday life is generally accompanied by posture-unrelated cognitive activity. Thus, mild forms of dual-tasking postural control are the norm rather than the exception. Based on this consideration and available evidence, we propose and empirically examined, in young and old adults, a non-monotonic, U-shaped relation between the efficacy of postural control and concurrent cognitive demands that reflect opposing trends of the effects of attention focus and attentional resource competition. When instructed to perform an easy cognitive task that presumably shifted the focus of attention away from posture control, the center of body pressure (COP) excursions decreased both in young and older adults relative to a single-task baseline where the focus of attention was explicitly directed towards the postural control task itself. However, when performing more demanding cognitive tasks, older adults showed increased COP displacements, in line with the predicted U-shape function, whereas young adults did not. We outline mechanisms linking postural control to cognitive demand and suggest routes for future investigation.

Adult↗

PubMed2004

Effects of keyswitch design and finger posture on finger joint kinematics and dynamics during tapping on computer keyswitches.

OBJECTIVE: To examine the effects of postural and keyswitch characteristics on musculoskeletal tissue loading during tapping on computer keyswitches. DESIGN: We hypothesized that joint torques, stiffness and work parameters differ across keyswitch designs and finger postures typical of those observed during computer keyboard typing. We experimentally measured joint kinematics and calculated joint torques while tapping on different keyswitches in different postures, and analyzed the data using mechanical impedance models. METHODS: Sixteen human subjects tapped with the index finger on computer keyswitches mounted on a sensor which measured vertical and horizontal forces. Miniature electro-optical goniometers mounted dorsally across each finger joint measured joint kinematics. Joint torques were calculated from endpoint forces and joint kinematics using an inverse dynamics algorithm. A linear spring-damper impedance model was fitted to joint torque, position, and velocity during the contact period of each tap. Subjects tapped in three postures approximating those employed during tapping on three rows of a computer keyboard, on four different keyswitches, resulting in 12 conditions. RESULTS: More extended finger posture was associated with greater joint torques, energies, and stiffnesses, despite minimal differences in endpoint forces across posture. Greater keyswitch make forces were associated with increased forces, joint torques and joint stiffnesses, however this relationship was not monotonic. CONCLUSIONS: Joint torques and stiffness parameters differed across keyswitch designs and finger postures. Estimates of joint impedance and work provided a unique perspective into finger dynamics. RELEVANCE: Determining the causes of work-related musculoskeletal disorders is facilitated by characterizing workplace task biomechanics, which can be linked to specific injury mechanisms.

Adult↗

PubMed2006

Modification of human postural responses to soleus muscle vibration by rotation of visual scene.

Sensory interaction in posture control in 20 healthy subjects was investigated by postural responses to differently timed proprioceptive and visual stimulation. As proprioceptive stimulation was used both soleus muscles vibration. Visual stimulation was rotating disc moving in forward and backward direction. Centre of foot pressure (CoP) and trunk tilts in antero-posterior (AP) direction were measured during stance by two accelerometers on the upper and the lower trunk level. Subjects performed four series of eight trials lasted 20s: vibration with eyes open and closed, visual scene motion forward or backward, vibration together with scene motion forward or backward, scene motion forward or backward 3s before vibration. The results showed that early velocities and final angles of body tilt induced by soleus muscle vibration were modified by motion of visual scene. Early part of postural responses was changed slightly if sensory stimulation starts together. When visual stimulation started 3s before muscle vibration, the induced early CoP and trunk tilt records of postural response to vibration occurred with the faster slope similar for both directions of scene motion and for condition of vibration alone with eyes closed. Significantly different final CoP shifts and trunk angles were measured between condition with visual scene motion forward and backward. The effect of visual input on posture control was two times enhanced in paired sensory stimulation. The results indicated an important influence of unstable visual field on posture response to somatosensory stimulation where the early part of postural reaction indicated absence of visual influence.

Adult↗