Development of anticipatory orienting strategies during locomotor tasks in children

Postural stability in strabismus and amblyopia

BACKGROUND

Postural control is a complex skill based on the collaboration of dynamic sensory mechanisms, namely the visual, vestibular, and somatosensory systems.

METHODS

A literature survey regarding postural stability in strabismus and amblyopia was conducted using databases in order to collect data for a narrative review of published reports and available literature.

RESULTS

The results of the literature survey were analyzed to provide an overview of the current knowledge of postural stability in strabismus and amblyopia. The results revealed that although postural control depends on the fundamental integration of three essential components (the visual, vestibular, and somatosensory systems), the role of vision is critical in postural stability. Once normal binocular vision is undesirably disrupted in childhood by some reason, especially in strabismus and/or amblyopia, balance is also affected. Abnormal balance affects coordination in gross and fine motor controls in school-age children and results in weakened academic performance and delayed social progress. It also impacts a child's general health, self-esteem, and safety.

CONCLUSIONS

Binocular vision is imperative for the maturation and preservation of balance control in children, as balance performance is reduced in strabismus and/or amblyopia.

Head control and head-trunk coordination as a function of anticipation in sidestepping

Head reorientation precedes body reorientation during direction change to facilitate gaze realignment, thus enhancing perceptual awareness. Whole body kinematics are dependent on the available planning time. The purpose of this study was to assess the role of anticipation on head control and head-trunk coordination during sidestepping tasks. Fourteen male collegiate athletes performed anticipated and unanticipated sidestepping tasks. Transverse plane head, trunk and heading direction, as well as head-trunk coordination were assessed. During change of direction tasks, we observed greater head orientation towards the new travel direction, followed by heading direction and then trunk direction during both anticipated and unanticipated tasks. With reduced planning time, heading in the preparatory phase and trunk rotation in the preparatory and stance phases were significantly less oriented towards the new travel direction, with no differences in head rotation. During anticipated sidestepping, significantly greater in-phase coordination was observed during the preparatory phase compared to unanticipated sidestepping. Head reorientation facilitates gaze realignment and may be prioritized irrespective of planning time during sidestepping tasks. During anticipated trials, the head and trunk move more synchronously compared to unanticipated sidestepping, highlighting the potential benefits of aligning the degrees of freedom earlier in the change of direction stride and optimizing perceptual awareness.

Immersive virtual reality interferes with default head-trunk coordination strategies in young children

The acquisition of postural control is an elaborate process, which relies on the balanced integration of multisensory inputs. Current models suggest that young children rely on an 'en-block' control of their upper body before sequentially acquiring a segmental control around the age of 7, and that they resort to the former strategy under challenging conditions. While recent works suggest that a virtual sensory environment alters visuomotor integration in healthy adults, little is known about the effects on younger individuals. Here we show that this default coordination pattern is disrupted by an immersive virtual reality framework where a steering role is assigned to the trunk, which causes 6- to 8-year-olds to employ an ill-adapted segmental strategy. These results provide an alternate trajectory of motor development and emphasize the immaturity of postural control at these ages.

Eye-Head-Trunk Coordination While Walking and Turning in a Simulated Grocery Shopping Task

Previous studies argued that body turns are executed in an ordered sequence: the eyes turn first, followed by the head and then by the trunk. The purpose of this study was to find out whether this sequence holds even if body turns are not explicitly instructed, but nevertheless are necessary to reach an instructed distal goal. We asked participants to shop for grocery products in a simulated supermarket. To retrieve each product, they had to walk down and aisle, and then turn left or right into a corridor that led towards the target shelf. The need to make a turn was never mentioned by the experimenter, but it nevertheless was required in order to approach the target shelf. Main variables of interest were the delay between eye and head turns towards the target shelf, as well as the delay between head and trunk turns towards the target shelf. We found that both delays were consistently positive, and that their magnitude was near the top of the range reported in literature. We conclude that the ordered sequence of eye - then head - then trunk turns can be observed not only with a proximal, but also with a distal goal.

Effect of Kinesio taping on Y-balance test performance and the associated leg muscle activation patterns in children with developmental coordination disorder: A randomized controlled trial

BACKGROUND

Children with developmental coordination disorder (DCD) have leg muscular deficits which negatively affects their dynamic postural stability. Kinesio tape (KT) could enhance muscle activation, postural control and functional activities in healthy individuals. Therefore, we hypothesized that the usage of KT may address the postural instability problem of children with DCD.

RESEARCH QUESTION

To investigate the immediate effect of KT on dynamic postural stability and the associated lower limb muscle activity in children with DCD.

METHODS

Forty-nine children with DCD were recruited where twenty-five children were randomly assigned to the KT group (mean age = 8.18 ± 1.16 years) and twenty-four to the control group (mean age = 8.06 ± 0.93 years). KT group received KT application to the rectus femoris and gastrocnemius muscles whereas the control group received no intervention. Measurements were taken before and after the application of KT. Dynamic balance performance was measured using a lower quartile Y-balance test (YBT-LQ). Leg muscle peak activation and time-to-peak muscle activation of the dominant lower limb during YBT-LQ were measured by surface electromyography.

RESULTS

YBT-LQ composite score increased by 6.3% in the KT group at posttest (95% CI: -7.308, -2.480). In addition, a higher rectus femoris peak activation was illustrated for YBT-LQ anterior (32.5%; 95% CI: -48.619, -16.395) and posteromedial (24.6%; 95% CI: -42.631, -6.591) reach directions from pretest values in the KT group. Moreover, KT group exhibited a 38% (95% CI: 0.015, 2.983) longer gastrocnemius medialis time-to-peak duration for YBT-LQ posteromedial reach direction when compared to the control group.

SIGNIFICANCE

KT revealed an immediate beneficial effect on YBT-LQ performance. Application of KT also increased rectus femoris peak activation and lengthened the muscle time-to-peak duration for specific reach directions. Incorporating KT as an adjunct with dynamic balance training programme could be beneficial for children with DCD.

Reducing gait speed affects axial coordination of walking turns

Turning is a common feature of daily life and dynamic coordination of the axial body segments is a cornerstone for safe and efficient turning. Although slow walking speed is a common trait of old age and neurological disorders, little is known about the effect of walking speed on axial coordination during walking turns. The aim of this study was to investigate the influence of walking speed on axial coordination during walking turns in healthy elderly adults. Seventeen healthy elderly adults randomly performed 180° left and right turns while walking in their self-selected comfortable pace and in a slow pace speed. Turning velocity, spatiotemporal gait parameters (step length and step time), angular rotations and angular velocity of the head and pelvis, head-pelvis separation (i.e. the angular difference in degrees between the rotation of the head and pelvis) and head-pelvis velocity were analyzed using Wilcoxon signed-rank tests. During slow walking, turning velocity was 15% lower accompanied by shorter step length and longer step time compared to comfortable walking. Reducing walking speed also led to a decrease in the amplitude and velocity of the axial rotation of the head and pelvis as well as a reduced head-pelvis separation and angular velocity. This study demonstrates that axial coordination during turning is speed dependent as evidenced by a more 'en bloc' movement pattern (i.e. less separation between axial segments) at reduced speeds in healthy older adults. This emphasizes the need for matching speed when comparing groups with diverse walking speeds to differentiate changes due to speed from changes due to disease.

The role of environmental constraints in walking: Effects of steering and sharp turns on gait dynamics

Stride durations in gait exhibit long-range correlation (LRC) which tends to disappear with certain movement disorders. The loss of LRC has been hypothesized to result from a reduction of functional degrees of freedom of the neuromuscular apparatus. A consequence of this theory is that environmental constraints such as the ones induced during constant steering may also reduce LRC. Furthermore, obstacles may perturb control of the gait cycle and also reduce LRC. To test these predictions, seven healthy participants walked freely overground in three conditions: unconstrained, constrained (constant steering), and perturbed (frequent 90° turns). Both steering and sharp turning reduced LRC with the latter having a stronger effect. Competing theories explain LRC in gait by positing fractal CPGs or a biomechanical process of kinetic energy reuse. Mediation analysis showed that the effect of the experimental manipulation in the current experiment depends partly on a reduction in walking speed. This supports the biomechanical theory. We also found that the local Hurst exponent did not reflect the frequent changes of heading direction. This suggests that the recovery from the sharp turn perturbation, a kind of relaxation time, takes longer than the four to seven meters between successive turns in the present study.

Anticipatory control and spatial cognition in locomotion and navigation through typical development and in cerebral palsy

Behavioural evidence, summarized in this narrative review, supports a developmental model of locomotor control based on increasing neural integration of spatial reference frames. Two consistent adult locomotor behaviours are head stabilization and head anticipation: the head is stabilized to gravity and leads walking direction. This cephalocaudal orienting organization aligns gaze and vestibula with a reference frame centred on the upcoming walking direction, allowing anticipatory control on body kinematics, but is not fully developed until adolescence. Walking trajectories and those of hand movements share many aspects, including power laws coupling velocity to curvature, and minimized spatial variability. In fact, the adult brain can code trajectory geometry in an allocentric reference frame, irrespective of the end effector, regulating body kinematics thereafter. Locomotor trajectory formation, like head anticipation, matures in early adolescence, indicating common neurocomputational substrates. These late-developing control mechanisms can be distinguished from biomechanical problems in children with cerebral palsy (CP). Children's performance on a novel navigation test, the Magic Carpet, indicates that typical navigation development consists of the increasing integration of egocentric and allocentric reference frames. In CP, right-brain impairment seems to reduce navigation performance due to a maladaptive left-brain sequential egocentric strategy. Spatial integration should be considered more in rehabilitation.

The effects of medication on turning in people with Parkinson disease with and without freezing of gait

Turning difficulty is prevalent in Parkinson disease (PD) and may lead to falls or freezing. Medication improves motor symptoms of PD, but its effects on turning in people with PD with (PD+FOG) and without (PD-FOG) freezing of gait are unclear. This study evaluated the effects of medication on turning in PD compared to healthy older adults (controls), and in PD+FOG compared to PD-FOG. We assessed timed-up-and-go (TUG), and in-place turns in 16 controls and 20 people with PD (10 PD+FOG, 10 PD-FOG) OFF and ON medication. PD+FOG performed worse than PD-FOG (p < 0.05) in TUG, turn duration, step count, and had earlier head rotation onset (HTO). These measures improved ON medication in PD+FOG and PD-FOG (p < 0.05). Turn duration and step count improved more with medication in PD+FOG than PD-FOG (p < 0.005). There were subtle differences in gastrocnemius and sternocleidomastoid onsets, with PD OFF or ON activating muscles earlier than controls. Tibialis anterior, gastrocnemius, and sternocleidomastoid initial onset times were similar between PD+FOG and PD-FOG. Though medication improved turning, turn duration and step count impairments still existed in PD ON, compared to controls. Relative to PD-FOG, PD+FOG turned worse, but improved more with medication, potentially because PD+FOG were initially more impaired than PD-FOG or were taking higher medication dosages. Further treatment options may be needed to address ON medication turning deficits.

Gaze anticipation during human locomotion

During locomotion, a top-down organization has been previously demonstrated with the head as a stabilized platform and gaze anticipating the horizontal direction of the trajectory. However, the quantitative assessment of the anticipatory sequence from gaze to trajectory and body segments has not been documented. The present paper provides a detailed investigation into the spatial and temporal anticipatory relationships among the direction of gaze and body segments during locomotion. Participants had to walk along several mentally simulated complex trajectories, without any visual cues indicating the trajectory to follow. The trajectory shapes were presented to the participants on a sheet of paper. Our study includes an analysis of the relationships between horizontal gaze anticipatory behavior direction and the upcoming changes in the trajectory. Our findings confirm the following: 1) The hierarchical ordered organization of gaze and body segment orientations during complex trajectories and free locomotion. Gaze direction anticipates the head orientation, and head orientation anticipates reorientation of the other body segments. 2) The influence of the curvature of the trajectory and constraints of the tasks on the temporal and spatial relationships between gaze and the body segments: Increased curvature resulted in increased time and spatial anticipation. 3) A different sequence of gaze movements at inflection points where gaze plans a much later segment of the trajectory.

Differences in axial segment reorientation during standing turns predict multiple falls in older adults

BACKGROUND

The assessment of standing turning performance is proposed to predict fall risk in older adults. This study investigated differences in segmental coordination during a 360° standing turn task between older community-dwelling fallers and non-fallers.

METHODS

Thirty-five older adults age mean (SD) of 71 (5.4) years performed 360° standing turns. Head, trunk and pelvis position relative to the laboratory and each other were recorded using a Vicon motion analysis system. Fall incidence was monitored by monthly questionnaire over the following 12 months and used to identify non-faller, single faller and multiple faller groups.

RESULTS

Multiple fallers were found to have significantly different values, when compared to non-fallers, for pelvis onset (p=0.002); mean angular separation in the transverse plane between the head and trunk (p=0.018); peak angular separation in the transverse plane between the trunk and pelvis (p=0.013); and mean angular separation between the trunk and pelvis (p<0.001).

CONCLUSIONS

Older adults who subsequently experience multiple falls show a simplified turning pattern to assist in balance control. This may be a predictor for those at increased risk of falling.

The influence of gaze behaviour on postural control from early childhood into adulthood

In the present study we aimed to track the influence of natural gaze behaviour on postural control from early childhood into adulthood. We measured time series of centre of pressure (COP) as well as head movement in three children groups aged around five (n=16), eight (n=15), and eleven (n=14) and in one group of young adults (n=15) during quiet stance with eyes closed, gaze fixed on a dot, and with gaze shifts between two dots. We adopted magnitude and irregularity of COP displacement as indexes of postural control and cross correlation between COP displacement and target oscillation as an index of the dynamical coupling between the postural and visual systems. Magnitude and irregularity of COP displacement decreased with age, which suggests a steady improvement of postural control from five to beyond eleven years of age. Cross correlations were weak and relative phases highly variable across age groups. Across conditions, and most prominently in the gaze shift conditions, 5-year-olds showed both more head movement and lower postural stability than other age groups. Finally, only in 5-year-olds did we find a marked deterioration of postural stability with gaze shifts. We thus conclude that excessive head movement, particularly during gaze shifts, may be a primary cause of lower postural stability in young children compared to older children and adults.

Lack of Short-Term Effectiveness of Rotating Treadmill Training on Turning in People with Mild-to-Moderate Parkinson's Disease and Healthy Older Adults: A Randomized, Controlled Study

Since turning is often impaired in Parkinson's disease (PD) and may lead to falls, it is important to develop targeted treatment strategies for turning. We determined the effects of rotating treadmill training on turning in individuals with PD. This randomized controlled study evaluated 180° in-place turns, functional turning (timed-up-and-go), and gait velocity before and after 15 minutes of rotating treadmill training or stepping in place in 26 people with PD and 27 age-matched controls. A subset of participants with PD (n = 3) completed five consecutive days of rotating treadmill training. Fast as possible gait velocity, timed-up-and-go time, 180° turn duration, and steps to turn 180° were impaired in PD compared to controls (P < 0.05) and did not improve following either intervention (P > 0.05). Preferred pace gait velocity and timing of yaw rotation onset of body segments (head, trunk, pelvis) during 180° turns were not different in PD (P > 0.05) and did not change following either intervention. No improvements in gait or turning occurred after five days of rotating treadmill training, compared to one day. The rotating treadmill is not recommended for short-term rehabilitation of impaired in-place turning in the general PD population.

Coordination of segments reorientation during on-the-spot turns in healthy older adults in eyes-open and eyes-closed conditions

Turning has frequent occurrence in everyday activities. Despite the prevalence of turning in everyday life and the challenge it poses to older adults, there is far less known about the multisegmental control of turning than the control of standing and straight walking, especially in elderly individuals. The purpose of this study was to examine the timing and sequence of segments reorientation in healthy older adults during 90° on-the-spot turns. The role of vision on segments coordination was also examined by testing the participants in eyes-open and eyes-closed conditions. When turning on-the-spot, healthy elderly reoriented their head, shoulder and pelvis simultaneously, followed by foot displacement. This was a robust behavior not affected by visual condition. Axial segments turned slower and more synchronously when vision was not available. While all segments started to turn together in both visual conditions, head turned faster and reached its peak velocity earlier than shoulder and pelvis. However, the difference in segmental velocity and the time to reach the peak velocity was smaller in eyes-closed than eyes-open condition. Without vision, the functional importance of a faster head turn is diminished. Participants may have adopted a tighter control of segments to simplify the control of movement by reducing the degrees of freedom.

Anticipatory postural adjustments in children with typical motor development

Anticipatory postural adjustments (APAs) play an important role in the performance of many activities requiring the maintenance of vertical posture. However, little is known about how children utilize APAs during self-induced postural perturbations. A group of children, aged 7-16 years, with typical motor development, performed various arm movements while standing on a force platform. APAs were measured by recording the electromyographic activity of six trunk and leg muscles on both sides of the body and displacement of center of pressure (COP). Anticipatory bursts of activity in the dorsal muscle groups of the trunk and legs and suppression in the ventral muscle groups as well as posterior COP displacement were found during the performance of bilateral shoulder flexion. Conversely, during bilateral shoulder extension, the COP displacement was anterior, and APAs were reversed showing bursts of activity in the ventral muscle groups and suppression in the dorsal muscles. During right and left reciprocal arm movements, COP displacement was minimal and APAs were generated in the dorsal muscle groups on the side of the forward moving arm and in the ventral muscle groups on the side of the arm moving into extension. However this pattern reversed for lower leg muscles, where APAs were generated in the ventral muscles on the side of forward moving arm and in the dorsal muscle on the side of the arm moving into extension. The results of this study indicate that children with typical motor development are able to generate APAs, produce task-specific sequencing of muscle activity and differentiate between perturbations in the sagittal and transverse planes. The results of this study indicate that by at least age 7, children who are typically developing demonstrate the ability to generate patterns of anticipatory muscle activation and suppression, along with center of pressure changes, similar to those reported in healthy adults.

Effect of walking velocity on segment coordination during pre-planned turns in healthy older adults

BACKGROUND

Despite the prevalence of turning in daily activities and the challenges it poses to mobility-impaired individuals, far less is known about the multi-segmental control of turning than the control of straight walking. Gait slows with aging and neurological disorders such as Parkinson's disease and falls in these populations frequently occur when turning. Nevertheless, the influence of walking velocity on the complex inter-segmental coordination of the head, trunk and lower limbs during turning has not been examined. The purpose of this study was to examine the effect of walking velocity on the coordination of segment reorientation during turns embedded in locomotion in healthy older adults.

METHODS

Nineteen healthy older adults volunteered to participate. Participants made a 45 degrees or 90 degrees turn to their right while walking either at their natural self-selected speed or slower or faster than their natural speed. We quantified the timing and sequence of segments reorientation during the turns.

RESULTS

There was a top-down temporal sequence in initiation of segments reorientation during turning, i.e., head turned first, followed by shoulder, pelvis, and mediolateral foot displacement. Furthermore, results indicate that the top-down temporal sequence in segments reorientation during turning was a robust behavior which was not affected by the walking velocity or magnitude of the turn.

CONCLUSIONS

Walking velocity does not affect segment coordination during pre-planned turns in healthy elderly. Therefore, we conclude that changes in coordination of segments reorientation during pre-planned turns in individuals with neurological disorders such as Parkinson's disease is not due to their slower gait.

Head movements produced during linear translations in unexpected directions

Passive translation of the body in space elicits a complex combination of directionally-specific torques that are exerted on the neck. The inertial torques that are produced by linear translation are counteracted by linear vestibular and proprioceptive reflexes that maintain head stability. A novel experimental apparatus was used in this study to translate human subjects in a random and unpredictable direction in order to quantify the head's 3-D movement with respect to the direction of translation. Head movements were found to be produced in systematic patterns as a function of stimulus direction. Roll and yaw head movements were produced in proportion to the magnitude of the lateral component of the translation. Pitch head movements were proportionate to the magnitude of the fore-aft component of the translation. One surprising observation was that head movements produced during lateral translations were, on average, 17% smaller than those produced during fore-aft translations. This suggests that linear vestibular reflexes that stabilize the head may be directionally-specific and more active during lateral whole body translations.

Obstacle stepping in children: task acquisition and performance

The aim of this study was to investigate the locomotor capacity of children during the performance of different lower extremity tasks with increasing difficulty. Two subject groups of children (aged 6-8 and 9-12 years) and adult controls performed several motor tasks from the Zürich Neuromotor Assessment (ZNA) test, as well as a unilateral and bilateral obstacle stepping test during treadmill walking. Performance of ZNA items, changes in foot clearance, and obstacle hits were assessed. Correlations between children's age, ZNA and obstacle measures were calculated. Performance of all motor tasks improved with increasing age. All three groups improved foot clearance during unilateral obstacle stepping, while the younger children achieved a poorer performance level. During bilateral obstacle stepping, only the adult controls and the 9-12 years old children's group further improved foot clearance, while no further improvement occurred in the 6-8 years old children's group. A relationship between items of ZNA and bilateral obstacle stepping was found. It is concluded that children in the mid-childhood range are able to significantly improve performance of a high-precision locomotor task. However, children below 9 years of age have a poorer motor performance compared to older children and adults that becomes more pronounced with increasing complexity of the task. Finally, ZNA tests can improve the prediction of complex adaptive locomotor behaviour compared to calendar age alone.

[Developmental and locomotor disorders in children]

From one year of age, gait and balance disorders are frequent complaints in neurology. During the first year following the acquisition of independent walking, most of the gait, such as transition from digitigrade to plantigrade locomotion, parameters are well-established in children. Nevertheless, bipedal locomotion means solving a large number of balance problems. Locomotor balance needs many years to mature in the course of ontogenesis. From various gait analysis, it was possible to establish a repertoire of locomotor strategies used in typical and pathological development. The last part of this paper is devoted to the possible responses that can be proposed for gait and balance disorders occurring during childhood.

Maneuvers during legged locomotion

Maneuverability is essential for locomotion. For animals in the environment, maneuverability is directly related to survival. For humans, maneuvers such as turning are associated with increased risk for injury, either directly through tissue loading or indirectly through destabilization. Consequently, understanding the mechanics and motor control of maneuverability is a critical part of locomotion research. We briefly review the literature on maneuvering during locomotion with a focus on turning in bipeds. Walking turns can use one of several different strategies. Anticipation can be important to adjust kinematics and dynamics for smooth and stable maneuvers. During running, turns may be substantially constrained by the requirement for body orientation to match movement direction at the end of a turn. A simple mathematical model based on the requirement for rotation to match direction can describe leg forces used by bipeds (humans and ostriches). During running turns, both humans and ostriches control body rotation by generating fore-aft forces. However, whereas humans must generate large braking forces to prevent body over-rotation, ostriches do not. For ostriches, generating the lateral forces necessary to change movement direction results in appropriate body rotation. Although ostriches required smaller braking forces due in part to increased rotational inertia relative to body mass, other movement parameters also played a role. Turning performance resulted from the coordinated behavior of an integrated biomechanical system. Results from preliminary experiments on horizontal-plane stabilization support the hypothesis that controlling body rotation is an important aspect of stable maneuvers. In humans, body orientation relative to movement direction is rapidly stabilized during running turns within the minimum of two steps theoretically required to complete analogous maneuvers. During straight running and cutting turns, humans exhibit spring-mass behavior in the horizontal plane. Changes in the horizontal projection of leg length were linearly related to changes in horizontal-plane leg forces. Consequently, the passive dynamic stabilization associated with spring-mass behavior may contribute to stability during maneuvers in bipeds. Understanding the mechanics of maneuverability will be important for understanding the motor control of maneuvers and also potentially be useful for understanding stability.

Locomotor skills and balance strategies in children with internal rotations of the lower limbs

The purpose of this study was to investigate the functional effects of a structural deformation, internal rotations (IR) of the lower limbs, on upper body balance strategies used during locomotion in 5-6 year-old and 7-10 year-old children. Balance control was examined in terms of rotation around the longitudinal axis in horizontal plane (yaw) and around the sagittal axis in a frontal plane (roll). Kinematics of foot, pelvis, shoulder, and head rotations were measured with an automatic optical TV image processor and used to calculate angular dispersions and segmental stabilizations. Older children with IR showed a lower gait velocity, particularly in difficult balance conditions than typically developing (TD) children. In younger children, the effect of the local biomechanical deficit remained limited to the lower limbs and did not affect upper body coordination. By contrast, in older children with IR, the development of head stabilization in space was affected. This was demonstrated by an "en bloc" instead of an articulated mode of head-trunk unit systematically adopted by the control group. As pelvic stabilization remains the main reference frame to organize balance control in older children with IR, we conclude that the structural deformity of the legs affect and possibly delay the acquisition of the head stabilization in space strategy.

Proprioceptive deficits of the lower limb following anterior cruciate ligament deficiency affect whole body steering control

The role of lower limb proprioception in the steering control of locomotion is still unclear. The purpose of the current study was to determine whether steering control is altered in individuals with reduced lower limb proprioception. Anterior cruciate ligament deficiency (ACLD) results in a decrease in proprioceptive information from the injured knee joint (Barrack et al. 1989). Therefore the whole body kinematics were recorded for eight unilateral ACLD individuals and eight CONTROL individuals during the descent of a 20 degrees incline ramp followed by either a redirection using a side or cross cutting maneuver or a continuation straight ahead. Onset of head and trunk yaw, mediolateral displacement of a weighted center of mass (COM(HT)) and mediolateral displacement of the swing foot were analyzed to evaluate differences in the steering control. Timing analyses revealed that ACLD individuals delayed the reorientation of body segments compared to CONTROL individuals. In addition, ACLD did not use a typical steering synergy where the head leads whole body reorientation; rather ACLD individuals reoriented the head, trunk and COM(HT) in the new direction at the same time. These results suggest that when lower limb proprioceptive information is reduced, the central nervous system (CNS) may delay whole body reorientation to the new travel direction, perhaps in order to integrate existing sensory information (vision, vestibular and proprioception) with the reduced information from the injured knee joint. This control strategy is maintained when visual information is present or reduced in a low light environment. Additionally, the CNS may move the head and trunk segments as, effectively, one segment to decrease the number of degrees of freedom that must be controlled and increase whole body stability during the turning task.

Locomotor skills and balance strategies in adolescents idiopathic scoliosis

STUDY DESIGN

Locomotor balance control assessment was performed to study the effect of idiopathic scoliosis on head-trunk coordination in 17 patients with adolescent idiopathic scoliosis (AIS) and 16 control subjects.

OBJECTIVE

The aim of this study was to explore the functional effects of structural spinal deformations like idiopathic scoliosis on the balance strategies used during locomotion.

SUMMARY OF BACKGROUND DATA

Up to now, the repercussion of the idiopathic scoliosis on head-trunk coordination and balance strategies during locomotion is relatively unknown.

METHODS

Seventeen patients with AIS (mean age 14 years 3 months, 10 degrees < Cobb angle > 30 degrees) and 16 control subjects (mean age 14 years 1 month) were tested during various locomotor tasks: walking on the ground, walking on a line, and walking on a beam. Balance control was examined in terms of rotation about the vertical axis (yaw) and on a frontal plane (roll). Kinematics of foot, pelvis, trunk, shoulder, and head rotations were measured with an automatic optical TV image processor in order to calculate angular dispersions and segmental stabilizations.

RESULTS

Decreasing the walking speed is the main adaptive strategy used in response to balance problems in control subjects as well as patients with AIS. However, patients with AIS performed walking tasks more slowly than normal subjects (around 15%). Moreover, the pelvic stabilization is preserved, despite the structural changes affecting the spine. Lastly, the biomechanical defect resulting from idiopathic scoliosis mainly affects the yaw head stabilization during locomotion.

CONCLUSIONS

Patients with AIS show substantial similarities with control subjects in adaptive strategies relative to locomotor velocity as well as balance control based on segmental stabilization. In contrast, the loss of the yaw head stabilization strategies, mainly based on the use of vestibular information, probably reflects the presence of vestibular deficits in the patients with AIS.

Vestibular assessment with Balance Quest Normative data for children and young adults

OBJECTIVE

The purpose of the present study was to compare equilibrium pattern in 12-year-old children with 20-year-old young adults and to obtain normative data for the BQ in both groups.

METHODS

Mean stability percentages and synthesis ratios of 29 healthy children aged 12 years were compared to those of 68 young adults aged 20 years, using BQ.

RESULTS

The mean stability percentages for children were significantly lower than for young adults. Vestibular ratios were lower in children compared to young adults, whereas somesthesic ratios were similar for the two groups. Visual dependence was significant higher in children.

CONCLUSIONS

Children unlike young adults had lower stability percentages when visual information was not available or was incorrect. Ratio synthesis pattern was different in the two groups. Our results on BQ partially confirms previous results obtained in children assessed with Equitest CDP. This study also provides BQ normative data for these two age groups.

Characteristics of single and double obstacle avoidance strategies: a comparison between adults and children

Activities of daily living often require us to negotiate several obstacles in the travel path. To date, there is little work investigating how adults accomplish such tasks, and there is even less known about multiple obstacle avoidance strategies used by children. The current work will expand our knowledge about the role of vision in adults and children when avoiding two obstacles placed in their travel path under altered ambient lighting. Healthy 7-year old children (n=10; aged 7.51+/-0.2 years) and adults (n=10; aged 22.76+/-1.7 years) were instrumented with infrared markers (Optotrak, NDI) placed on anatomical landmarks and asked to walk along a ten meter path under three conditions: unobstructed, single obstacle, or double obstacle. These trials were performed under two lighting conditions: Full (simulating standard office lighting) and Low (simulating a dark hallway lit by nightlights). Data analyses included lead and trail clearance values, step length, step width and step velocity, take-off distance and Horizontal toe Displacement at Apex (HDA) which was defined as the distance between the horizontal position of the toe to the leading edge of the obstacle when the toe reaches its peak height. Adults were able to maintain consistent behaviour regardless of the number of obstacles in the travel path. Children, however, adjusted their foot placement for the second obstacle. This indicates that having multiple obstacles in the travel path is a more challenging task for 7-year old, and suggests that children at this age may not have fully developed anticipatory locomotor strategies. Children had larger clearance values than adults for the lead foot crossing the obstacle under all obstacle and lighting conditions, and consistently used larger HDA values than adults. Together, these findings suggest that children adopt more cautious strategies than adults in complex environments. Additionally, children decreased walking velocity, increased step width and decreased their step length in a Low light environment. These changes are all indicators of a more careful avoidance strategy, which implies that children at this age rely heavily on visual information to guide foot placements in a complex environment.

Children use different anticipatory control strategies than adults to circumvent an obstacle in the travel path

Carrying out the daily activities of work and play requires the ability to integrate available sensory information in order to navigate complex, potentially cluttered, environments. The expression of locomotor adjustment behaviour is still maturing during mid- to late-childhood (Grasso et al. in Neurosci Biobehav Rev 22(4): 533-539, 1998a; McFadyen et al. in Gait Posture 13:7-16, 2001), which raises the question, do children coordinate their body segments differently than adults when circumventing an obstacle in their travel path? Healthy young children (n=5; age 10.3+/-1.5 years) and adults (n=6; age 26.3+/-2.9 years) were asked to walk at their natural pace during unobstructed walking, as well as during the avoidance to the right or left of a cylindrical obstacle located in the travel path 3 m from the initial starting position. Fourteen infrared markers were fixed to participants and tracked using the Optotrak motion analysis system (60 Hz; Northern Digital Inc, Canada). Data analyses included center of mass (COM) clearance from the obstacle, gait speed, angular movement of the head and trunk (yaw, pitch and roll) and medial-lateral (M-L) COM displacement. Onset of change in these variables from unobstructed walking was also calculated as the time from OBS crossing. Although there were no differences in when adults or children altered their M-L COM trajectory, adults reoriented their head and trunk segments at the same time as their COM while children reoriented their head and trunk prior to changing COM direction. A comparison of foot placement data for this task indicated that while adults changed their gait patterns well in advance of obstacle crossing, children initiated M-L adjustments to gait patterns just prior to OBS crossing. Vallis and McFadyen (Exp Brain Res 152 (3):409-414, 2003) indicated that during circumvention of an obstacle, adults coordinate body segments for a single transient change in COM trajectory while maintaining the underlying travel direction. The present data suggest, however, that children partition obstacle avoidance into two tasks, initially steering with proactive movement of the head and trunk segments and finally making adjustments to their gait trajectory, via stride and step width changes, to ensure adequate obstacle clearance just prior to obstacle crossing. This study demonstrates different anticipatory control strategies used by children as compared to adults to circumvent obstacles in the travel path. The different head and trunk anticipatory segmental coordination suggests that children gather visual information differently when circumventing an obstacle in their travel path and are more dependent on visual input to guide their circumvention strategy.

Head motion in humans alternating between straight and curved walking path: combination of stabilizing and anticipatory orienting mechanisms

Anticipatory head orientation relative to walking direction was investigated in humans. Subjects were asked to walk along a 20 m perimeter, figure of eight. The geometry of this path required subjects to steer their body according to both curvature variations (alternate straight with curved walking) and walking direction (clock wise and counter clock wise). In agreement with previous results obtained during different locomotor tasks [R. Grasso, S. Glasauer, Y. Takei, A. Berthoz, The predictive brain: anticipatory control of head direction for the steering of locomotion, NeuroReport 7 (1996) 1170-1174; R. Grasso, P. Prevost, Y.P. Ivanenko, A. Berthoz, Eye-head coordination for the steering of locomotion in humans: an anticipatory synergy, Neurosci. Lett. 253 (2) (1998) 115-118; T. Imai, S.T. Moore, T. Raphan, B. Cohen, Interaction of body, head, and eyes during walking and turning, Exp. Brain Res. 136 (2001) 1-18; P. Prevost, Y. Ivanenko, R. Grasso, A. Berthoz, Spatial invariance in anticipatory orienting behaviour during human navigation, Neurosci. Lett. 339 (2002) 243-247; G. Courtine, M. Schieppati, Human walking along a curved path. I. Body trajectory, segment orientation and the effect of vision, Eur. J. Neurosci. 18 (2003) 177-190], the head turned toward the future walking direction. This anticipatory head behaviour was continuously modulated by the geometrical variations of the path. Two main components were observed in the anticipatory head behaviour. One was related to the geometrical form of the path, the other to the transfer of body mass from one foot to the other during stepping. A clear modulation of the head deviation pattern was observed between walking on curved versus straight parts of the path: head orientation was influenced to a lesser extent by step alternation for curved path where a transient head fixation was observed. We also observed good symmetry in the head deviation profile, i.e. the head tended to anticipate the future walking direction with the same amplitude when turning to the left (29.75 +/- 7.41 degrees of maximum head deviation) or to the right (30.86 +/- 9.92 degrees ). These findings suggest a combination of motor strategies underlying head stabilization in space and more global orienting mechanisms for steering the whole body in the desired direction.

Expected and unexpected head yaw movements result in different modifications of gait and whole body coordination strategies

During locomotion we routinely make voluntary head movements, similar to those made during steering tasks, in order to scan our environment and obtain information about objects in the environment and our proximity to these objects. Given the importance that head segment orientation during locomotion has received in the recent literature, two studies were designed to investigate responses following a voluntarily generated and an unexpected, externally applied head turn. During a voluntary head turn, an efferent copy of the head movement could cancel the sensory effects of the head turn, effectively isolating the movement response to that segment. Alternatively, if the steering synergy is a part of our motor repertoire, as has been suggested, movement of the head could automatically release a steering "synergy" of segmental control and coordination. A unique head mounted air-jet apparatus, designed and developed at the University of Waterloo, was used for both studies to ensure that auditory stimuli and the physical presence of the apparatus on the head were similar for participants of the two experiments. During certain points in the gait cycle, this device was triggered and a short burst of compressed air (350 ms) was released to cue participants to make a voluntary head turn (Experiment 1). The same device was triggered in Experiment 2; however, in this experiment compressed air was released for a longer duration (1,500 ms) which resulted in an unexpected and quick turn of the participants' head to either the left or right. In these experiments, vision was also manipulated in certain trials with liquid crystal display glasses that occluded vision for the duration of the head turn. Data from the first experiment indicates that a subset of the steering synergy previously observed is released following the voluntary head movement; however, the travel trajectory path is preserved, suggesting that sensory input resulting from the head movement is partially nullified by the central nervous system. Overall safety is ensured by maintaining the same travel path. In the second experiment, an unexpected perturbation was applied to the head during locomotion to determine how the absence of an efferent copy of the movement pattern influences the level of control over body segments during locomotion. Whole body responses similar to those observed during steering tasks were observed following application of this unexpected head perturbation. It is proposed that the CNS interprets an unexpected yaw movement of the head as a change in the frame of reference, and global modifications of the walking trajectory, similar to that observed during steering tasks, are made in the perceived new direction of travel. Collectively this work extends our understanding of how the CNS establishes a head based orientation frame for locomotion. The CNS interprets and integrates anticipated and unexpected changes in sensory information from the head segment and subsequently modifies locomotion patterns according to the perceived whole body orientation in space. The sequence of control following these head movements appears to be part of a movement repertoire that is not immutable; maintaining whole body stability during locomotion is paramount.

Stabilization and mobility of the head and trunk in wild monkeys during terrestrial and flat-surface walks and gallops

This study investigated the patterns of rotational mobility (> or =20 degrees ) and stability (< or =20 degrees ) of the head and trunk in wild Indian monkeys during natural locomotion on the ground and on the flat-topped surfaces of walls. Adult hanuman langurs (Semnopithecus entellus) and bonnet macaques (Macaca radiata) of either gender were cine filmed in lateral view. Whole-body horizontal linear displacement, head and trunk pitch displacement relative to space (earth horizontal), and vertical head displacement were measured from the cine films. Head-to-trunk pitch angle was calculated from the head-to-space and trunk-to-space measurements. Locomotor velocities, cycle durations, angular segmental velocities, mean segmental positions and mean peak frequencies of vertical and angular head displacements were then calculated from the displacement data. Yaw rotations were observed qualitatively. During quadrupedal walks by both species, the head was free to rotate in the pitch and yaw planes on a stabilized trunk. By contrast, during quadrupedal gallops by both species, the trunk pitched on a stabilized head. During both gaits in both species, head and trunk pitch rotations were symmetrical about comparable mean positions in both gaits, with mean head position aligning the horizontal semicircular canals near earth horizontal. Head pitch direction countered head vertical displacement direction to varying degrees during walks and only intermittently during gallops, providing evidence that correctional head pitch rotations are not essential for gaze stabilization. Head-to-space pitch velocities were below 350 deg. s(-1), the threshold above which, at least among humans, the vestibulo-ocular reflex (VOR) becomes saturated. Mean peak frequencies of vertical translations and pitch rotations of the head ranged from 1 Hz to 2 Hz, a lower frequency range than that in which inertia is predicted to be the major stabilizer of the head in these species. Some variables, which were common to both walks and gallops in both species, are likely to reflect constraints in sensorimotor control. Other variables, which differed between the two gaits in both species, are likely to reflect kinematic differences, whereas variables that differed between the two species are attributed primarily to morphological and behavioural differences. It is concluded that either the head or the trunk can provide the nervous system with a reference frame for spatial orientation and that the segment providing that reference can change, depending upon the kinematic characteristics of the chosen gait.

Human walking along a curved path. II. Gait features and EMG patterns

We recorded basic gait features and associated patterns of leg muscle activity, occurring during continuous body progression when humans walked along a curved trajectory, in order to gain insight into the nervous mechanisms underlying the control of the asymmetric movements of the two legs. The same rhythm was propagated to both legs, in spite of inner and outer strides diverging in length (P < 0.001). There was a phase lag in limb displacement between the inner and outer leg of 7% of the total cycle duration (P = 0.0001). Swing velocity was greater for outer than inner foot (P < 0.001). The duration of the stance phase diminished and increased in the outer and inner leg (P < 0.01), respectively, and was associated with trunk leaning toward the inside of the path. Muscle activity was not dramatically altered during curved walking. The amplitude of soleus burst during stance increased in the outer (P < 0.05) and decreased in the inner leg (P < 0.05), without changes in timing. Tibialis anterior activity increased in both legs during the swing phase (P < 0.05); it was advanced on the outer and delayed on the inner side (P < 0.01; 2% of the cycle). The peroneus longus burst decreased in both legs, but more in the inner than the outer leg, and lasted longer in the inner leg at the onset of swing. Closing the eyes did not affect the gait pattern and muscle activity during turning. The command to walk along a curved path may exploit the basic mechanisms of the spinal locomotor generator, thereby limiting the computational cost of turning.

Human walking along a curved path. I. Body trajectory, segment orientation and the effect of vision

Task-related characteristics of gait and segment orientation during natural locomotion along a curved path have been described in order to gain insight into the neural organization of walking. The locomotor task implied continuous deviation from straight-ahead, thereby requiring continuous adjustment of body movement to produce and assist turn-related torques. Performance was compared to straight-ahead locomotion. Subjects easily reproduced both trajectories with eyes open (EO). The actual-to-required trajectory difference increased blindfolded (BF), more so during turning. Stride length was unchanged for the outer but decreased for the inner leg. The feet anticipated subsequent body rotation by pivoting toward the inner side of the curve at heel strike. A shift of body centre of mass and trunk roll toward the inner side accompanied turning. The head turned more than dictated by the heading change, and the absolute range of yaw oscillation increased. Head yaw anticipated body yaw by approximately 200 ms. Despite the minor effect of vision on the behaviour of all other segments, a difference in head pitch occurred between EO and BF; with EO, the head was flexed (P < 0.01), as to look at the path, while pitch was negligible with BF. In general, the changes in the amplitude of head, trunk and feet movements proved to be well related to the kinematics of the steering body, and constituted a sort of basic library of motor synergies.

Perceptual-motor contributions to static and dynamic balance control in children

The authors addressed balance control in children from the perspective of skill development and examined the relationship between specific perceptual and motor skills and static and dynamic balance performance. Fifty 11- to 13-year-old children performed a series of 1-legged balance tasks while standing on a force platform. Postural control was reflected in the maximum displacement of the center of mass in anterior-posterior and mediolateral directions. Simple visual, discrimination, and choice reaction times; sustained attention; visuomotor coordination; kinesthesis; and depth perception were also assessed in a series of perceptual and motor tests. The correlation analysis revealed that balancing under static conditions was strongly associated with the ability to perceive and process visual information, which is important for feedback-based control of balance. On the other hand, when greater task demands were imposed on the system under dynamic balancing conditions, the ability to respond to the destabilizing hip abductions-adductions in order to maintain equilibrium was associated with motor response speed, suggesting the use of a descending, feedforward control strategy. Therefore, like adults, 11- to 13-year-old children have the ability to select varying balance strategies (feedback, feedforward, or both), depending on the constraints of a particular task.

Anticipatory locomotor control for obstacle avoidance in mid-childhood aged children

The present work explored the anticipatory locomotor adjustments during obstacle avoidance by eight children aged 7--9 years. Analyses involved kinematic, kinetic and muscle mechanical power patterns at each lower limb joint, as well as electromyographic data from five muscles. The children demonstrated adult-like limb displacements and general dynamic strategies for obstacle clearance. However, when normalized to body mass, amplitudes of certain muscle power bursts related to anticipatory locomotor adjustments were decreased from those reported in the literature for adults, and an absence of the usual antagonistic knee extensor power burst at the end of the stance phase was frequently observed. The data suggest that the expression of anticipatory locomotor adjustments is still maturing during mid-childhood.

Multisensory integration in spatial orientation

Recent psychophysical studies on normal subjects, as well as brain imaging studies, have revised the concepts concerning the mechanisms underlying spatial orientation during navigation tasks. The emphasis has been put on internal models that allow the prediction of a planned trajectory and are essential in the steering of locomotion. Cognitive factors such as strategies and emotional parameters are now starting to be included in the research on spatial orientation. It is obvious that important individual and gender differences exist in the brain operations underlying spatial orientation in humans, which may help to understand the construction of a coherent perception and the organic neural disorders related to the internal representation of space.

Eye-head coordination for the steering of locomotion in humans: an anticipatory synergy

We investigated head and gaze orientation in six healthy volunteers walking along 90 degrees corner trajectories, both at light and with eyes closed. We found that head and eyes systematically deviated toward the future direction of the curved trajectory. Anticipation lead was about 1 s. Strikingly, the same behaviour was observed in darkness. In backward (BW) locomotion along the trajectory (from end- to start-point), gaze deviated toward the opposite direction, such that the forward locomotor pattern did not appear time-reversed. Orienting movements displayed higher amplitude, reproducibility and time lead in the forward (FW) direction at light. We suggest that anticipatory orienting synergies belong to the behavioural repertoire of human navigation and may reflect the need to prepare a stable reference frame for intended action.