Kinematic Markers Research Articles

A new look at an old problem: Defining weight acceptance in human walking

BackgroundWeight acceptance (WA) is an important phase of bipedal gait that has received relatively little study to date. This study tested the hypothesis that the first peak knee flexion would better demarcate the end of WA power absorption activity across varying gait speeds than would the more commonly used event of contralateral toe off (CTO) or the peak hip adduction angle. MethodsEight control subjects (4F/4M) walked on a treadmill at slow, self-selected, and fast speeds. Kinematic and kinetic data were recorded. Joint angles and power absorption were analyzed about the, lower extremity joints (sagittal ankle, knee, hip and frontal hip). Differences in event timings and, magnitudes of negative work were analyzed (ANOVA). ResultsKnee sagittal power absorption continued after the CTO event at self-selected (p=0.009) and fast speeds (p=0.001), while hip frontal power absorption continued after the CTO event at slow (p=0.019), self-selected (p=0.001), and fast speeds (p=0.001). The contribution of frontal hip to overall power absorption increased as speed decreased. DiscussionPeak hip adduction angle is the best kinematic marker of the end of the WA phase, and peak knee flexion angle is the best alternative marker across speeds. CTO is only appropriate to use when gait speeds are slow. In addition, the relative contribution of power absorbed in the frontal hip during WA highlights the importance of frontal plane pelvic control in locomotion, especially when gait speed is slow.

Kinematic markers dissociate error correction from sensorimotor realignment during prism adaptation

This study investigated the motor control mechanisms that enable healthy individuals to adapt their pointing movements during prism exposure to a rightward optical shift. In the prism adaptation literature, two processes are typically distinguished. Strategic motor adjustments are thought to drive the pattern of rapid endpoint error correction typically observed during the early stage of prism exposure. This is distinguished from so-called ‘true sensorimotor realignment’, normally measured with a different pointing task, at the end of prism exposure, which reveals a compensatory leftward ‘prism after-effect’. Here, we tested whether each mode of motor compensation – strategic adjustments versus ‘true sensorimotor realignment’ – could be distinguished, by analyzing patterns of kinematic change during prism exposure. We hypothesized that fast feedforward versus slower feedback error corrective processes would map onto two distinct phases of the reach trajectory. Specifically, we predicted that feedforward adjustments would drive rapid compensation of the initial (acceleration) phase of the reach, resulting in the rapid reduction of endpoint errors typically observed early during prism exposure. By contrast, we expected visual-proprioceptive realignment to unfold more slowly and to reflect feedback influences during the terminal (deceleration) phase of the reach. The results confirmed these hypotheses. Rapid error reduction during the early stage of prism exposure was achieved by trial-by-trial adjustments of the motor plan, which were proportional to the endpoint error feedback from the previous trial. By contrast, compensation of the terminal reach phase unfolded slowly across the duration of prism exposure. Even after 100 trials of pointing through prisms, adaptation was incomplete, with participants continuing to exhibit a small rightward shift in both the reach endpoints and in the terminal phase of reach trajectories. Individual differences in the degree of adaptation of the terminal reach phase predicted the magnitude of prism after-effects. In summary, this study identifies distinct kinematic signatures of fast strategic versus slow sensorimotor realignment processes, which combine to adjust motor performance to compensate for a prismatic shift.

Influence of fault rock foliation on fault zone permeability: The case of deeply buried arkosic sandstones (Grès d'Annot, southeastern France)

We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195C to 268C. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sandstone (10–2 md for Y against 10–3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions.

Distal hindlimb kinematics of gallopingThoroughbred racehorses on dirt and synthetic racetrack surfaces

The effect of racetrack surface (dirt or synthetic) on distal hindlimb kinematics of racehorses running at competition speeds is not known. To compare distal hindlimb and hoof kinematics during stance of breezing (unrestrained gallop) racehorses between dirt and synthetic surfaces. Two-dimensional kinematic video analysis of 5 Thoroughbred racehorses galloping at high speeds (12-17 m/s) on a dirt racetrack and a synthetic racetrack. The positions of kinematic markers applied to the left hindlimb were recorded at 500 Hz. Position, velocity and acceleration of joint angles and hoof translation during stance were calculated in the sagittal plane. Peak translational and angular kinematic values were compared between the dirt and synthetic race surfaces using mixed model analyses of covariance. Maximum and heel-strike metatarsophalangeal (fetlock) angles were greater (P<0.05) on the dirt surface than on the synthetic surface. Maximum fetlock angle occurred earlier during stance on the dirt surface (P<0.05). Greater horizontal displacement of the heel during slide occurred on the dirt surface (P<0.05). During high-speed gallop, hindlimb fetlock hyperextension and horizontal hoof slide are greater on a dirt surface than on a synthetic surface. Synthetic race surfaces may mitigate risk of injury to hindlimb fetlock structures by reducing fetlock hyperextension and associated strains in fetlock support structures. Differences in hoof slide may contribute to different distal hindlimb kinematics between surfaces.

Biomechanical evaluation of the helica femoral implant system using traditional and modified techniques

To determine the effect of implant placement on proximal femoral axial bone strains, implant subsidence, implant motion, and failure mechanical properties of Helica implants. In vitro biomechanical study. Cadaveric canine femora (n = 8 pairs). Femora instrumented with strain gauges and kinematic markers were cyclically loaded in axial compression before (intact femora) and after implantation with a Helica prosthesis that engaged only cancellous bone (traditional technique) or cancellous bone and lateral cortex (modified technique) to evaluate bone strains, subsidence, and motion; femora were then loaded to failure to evaluate failure mechanical properties. After implantation, modified femoral prosthesis angle was 5% less than intact femora and 5.7% less than traditional implanted femora. Medial femoral bone strain was lower (P ≤ .05) for intact (-570 µ strain) than modified (-790), but not (P = .08) traditional (-700) implanted femora. High-load implant subsidence was present but small (-0.087 mm) for the modified technique. Motion (traditional and modified) increased (P = .05) during cyclic loading (-0.17 and -0.328 mm) and failure (P = .04) (-2.121 and -3.390 mm); remaining yield and failure properties revealed no significant findings (P ≤ .05). The modified technique resulted in a smaller neck angle and minimal subsidence. Bone strain was minimally altered so stress shielding may be less compared to findings with traditional implants. Motion detected during cyclic and failure testing may lead to implant loosening in vivo.

The Pan-African quartz-syenite of Guider (north-Cameroon): Magnetic fabric and U–Pb dating of a late-orogenic emplacement

The small Guider pluton (70km2) in-between the cities of Maroua and Garoua (north Cameroon) is one of the quartz-syenite bodies that intrude Pan-African orthogneisses, and that are aligned along the western side of the NNE-directed Poli-Léré volcano-sedimentary corridor. The Guider syenite is here studied for its internal structures, using field and anisotropy of magnetic susceptibility measurements, and for its emplacement age of 593±4 Ma (U–Pb zircon), ∼20Ma younger than the beginning of gneissification of the country-rocks, mainly made of deformed granodiorites and diorites. In these orthogneisses, constantly NNE-trending vertical foliations, variously plunging stretching lineations and fold axes, as well as kinematic markers attest to a transpressive and possibly dextral regime. The Guider syenite, rich in K-feldspar and magnetite, displays typical magmatic microstructures with incipent solid-state features such as chess-board subgrains in quartz. Its magnetic fabric displays a well-defined feeding-zone in its centre and western side, attesting to its unrooted nature. However, N- to NE-trending lineations at its northern and southern ends, parallel to the overall linear structure of the country-rocks, point to its late-orogenic emplacement. Our results reveal that the NE-trending and dextral transpressive shear zone that acted in the Guider area ended after the emplacement of the syenite body; they specify the age of collage of the NW-Cameroon domain with the Mayo-Kébbi domain. They also show that the tectonic evolution of Guider area is closer to the E-Nigerian domain, to the west, than to its eastern neighbouring Mayo-Kébbi and Adamawa-Yadé domains. The connection of our sector with E-Brazil is briefly discussed.

Nonword repetition and nonword reading abilities in adults who do and do not stutter

PurposeIn the present study a nonword repetition and a nonword reading task were used to investigate the behavioral (speech accuracy) and speech kinematic (movement variability measured as lip aperture variability index; speech duration) profiles of groups of young adults who do (AWS) and do not stutter (control). MethodParticipants were 9 AWS (8 males, Mean age=32.2, SD=14.7) and 9 age- and sex-matched control participants (Mean age=31.8, SD=14.6). For the nonword repetition task, participants were administered the Nonword Repetition Test (Dollaghan & Campbell, 1998). For the reading task, participants were required to read out target nonwords varying in length (6 vs. 11 syllables). Repeated measures analyses of variance were conducted to compare the groups in percent speech accuracy for both tasks; only for the nonword reading task, the groups were compared in movement variability and speech duration. ResultsThe groups were comparable in percent accuracy in nonword repetition. Findings from nonword reading revealed a trend for the AWS to show a lower percent of accurate productions compared to the control group. AWS also showed significantly higher movement variability and longer speech durations compared to the control group in nonword reading. Some preliminary evidence for group differences in practice effect (seen as differences between the early vs. later 5 trials) was evident in speech duration. ConclusionsFindings suggest differences between AWS and control groups in phonemic encoding and/or speech motor planning and production. Findings from nonword repetition vs. reading highlight the need for careful consideration of nonword properties.Educational objectives At the end of this activity the reader will be able to: (a) summarize the literature on nonword repetition skills in adults who stutter, (b) describe processes underlying nonword repetition and nonword reading, (c) summarize whether or not adults who stutter differ from those who do not in the behavioral and kinematic markers of nonword reading performance, (d) discuss future directions for research.

Equine hoof slip distance during trot at training speed: Comparison between kinematic and accelerometric measurement techniques

Longitudinal sliding of horse’s hooves at the beginning of stance can affect both performance and orthopaedic health. The objective of this study was to compare two measurement methods for quantifying hoof slip distances at training trot. The right front hoof of four French Trotters was equipped with an accelerometer (10kHz) and kinematic markers. A firm wet sand track was equipped with a 50m calibration corridor. A high-frequency camera (600Hz) was mounted in a vehicle following each horse trotting at about 7m/s. One of the horses was also trotted on raw dirt and harrowed dirt tracks. Longitudinal slip distance was calculated both from kinematic data, applying 2D direct linear transformation (2D-DLT) to the markers image coordinates, and from the double integration of the accelerometer signal. For each stride, both values were compared. The angle of the hoof with respect to the track was also measured.There was ‘middling/satisfactory’ agreement between accelerometric and 2D-DLT measurements for total slip and ‘fairly good’ agreement for hoof–flat slip. The influence of hoof rotation on total slip distance represented <6% of accelerometric measures. The differences between accelerometric and kinematic measures (from −0.5cm to 2.1cm for total slip and from −0.2cm to 1.4cm for hoof–flat slip) were independent of slip distance magnitude. The accelerometric method was a simple method to measure hoof slip distances at a moderate training speed trot which may be useful to compare slip distances on various track surfaces.

The tectonic significance of dikes of irregular fold-like shape

The effects of postemplacement deformation on originally nonplanar dikes were tested experimentally and compared to field data from the Rainy Lake zone (Ontario, Canada) where irregular fold-like structures are observed. The analyses reveal that two processes are responsible for the irregular dike geometries. First, dikes intrude with varying orientations as they propagate across layers with rheology contrasts, thus developing nonplanar profiles. Second, with postemplacement ductile deformation, dikes that are more competent than the host rocks are folded and refracted in variable amounts, with the amount depending on the initial dike geometry, the heterogeneities in the host, and the regional strain. Deformed dikes and veins are often used as kinematic markers to evaluate regional tectonics, using methods that generally assume that veins had planar walls prior to deformation. The experimental results indicate that caution should be taken when using this approach, and they provide criteria to distinguish between intrusive and deformation structures in intensely deformed areas where distinction between the two is not readily apparent.

Kinematic Determinants of Anterior Knee Pain in Cerebral Palsy: A Case-Control Study

Sheehan FT, Babushkina A, Alter KE. Kinematic determinants of anterior knee pain in cerebral palsy: a case-control study. ObjectiveTo quantify the role patellofemoral and tibiofemoral kinematics may play in development of anterior knee pain (AKP) in individuals with cerebral palsy (CP). DesignCase-control. SettingClinical research center. ParticipantsKnees from individuals with diagnosed CP (n=20) and control knees (n=40) were evaluated. Controls were matched for sex and age based on the group average. Matching by height and weight was a secondary priority. Subjects in the control cohort were asymptomatic with no history of lower leg abnormalities, surgery, or major injury. Only individuals who were physically capable of sustaining slow, cyclic knee flexion-extension for 2.5 minutes and had no contraindications to magnetic resonance imaging were enrolled. Both groups were samples of convenience. InterventionsNot applicable. Main Outcome MeasuresThree-dimensional patellofemoral and tibiofemoral joint kinematics, acquired during active leg extension, under volitional control. ResultsParticipants with CP and AKP (n=8) demonstrated significantly greater patellofemoral extension, valgus rotation, and superior and posterior displacement relative to controls and to the subgroup of participants with CP and no AKP (n=12). Patellofemoral extension discriminated AKP in individuals with CP with 100% accuracy. ConclusionsIn quantifying the 3-dimensional, in vivo knee joint kinematics during a volitional extension task, kinematic markers that discriminate AKP in individuals with CP were identified. This provides an ability to predict which individuals with CP are most likely to develop AKP and could enable aggressive conservative treatment, aimed at reducing patella alta and excessive patellofemoral extension, to be prescribed before considering surgical options. The current findings will likely lead to improved clinical diagnostics and interventions for individuals with CP, with the ultimate goal of helping maintain, if not improve, functional mobility throughout the lifespan.

Wrist and arm movements of varying difficulties

Although a great deal of experimental attention has been directed at understanding Fitts' law, only a limited number of experiments have attempted to determine if performance differs across effectors for a given movement difficulty. In three experiments reciprocal wrist and arm movements were compared at IDs of 1.5, 3, 4.5 and 6. When absolute movement requirements and visual display were constant, participants' movement times and response characteristics for the arm and wrist were remarkably similar (Experiment 1). However, when amplitude for wrist movements was reduced to 8° and the gain (4×) for the visual display increased participants' movement time, defined on the basis of kinematic markers (movement onset − movement termination), was increasingly shorter relative to arm movements as movement difficulty was increased (Experiment 2). Experiment 3 where the arm was tested at 32° and 8° with the 8° movements provided the same gain (4×) that was used for the 8° wrist movements in Experiment 2, no advantage was observed for the arm at the shorter amplitude. The results are interpreted in terms of the advantages afforded by the increased gain of the visual display, which permitted the wrist, but not the arm, to more effectively preplan and/or correct ongoing movements to achieve the required accuracy demands. It was also noted that while the wrist was more effective during the actual movement production this was accompanied by an offsetting increase in dwell time which presumably is utilized to dissipate the forces accrued during movement production and plan the subsequent movement segment.

Kinematic Markers of Distance-Specific Control in Linear Hand Movements

ABSTRACT In this article, the authors analyze kinematic characteristics of reaching movements to memorized visual target locations. An increase in target distance was associated with a decrease in correlation between peak acceleration and movement distance and with a simultaneous increase in correlation between peak acceleration and movement time. According to the previous work on motor control in isometric force responses and in reaching movements these results seem to indicate a continuous transition from a rather preplanned to a more corrective mode of movement control, which may be associated with an adaptive mechanism serving to counteract an increase in signal-dependent noise of the motor system.

Dissecting online control in Developmental Coordination Disorder: A kinematic analysis of double-step reaching

In a recent study, children with movement clumsiness (or Developmental Coordination Disorder—DCD) were shown to have difficulties making rapid online corrections when reaching, demonstrated by slower and less accurate movements to double-step targets (Hyde & Wilson, 2011). These results suggest that children with DCD have difficulty using predictive estimates of limb position when making rapid adjustments to movement, in-flight. However, chronometric data alone does not provide strong evidence for this hypothesis: it remains unclear whether early (and rapid) control parameters or post-correction stages of the movement trajectory are affected. Thus, the overarching aim of this study was to conduct a kinematic analysis of double-step reaching in order to isolate the different control parameters that might explain the slower and less accurate double-step reaching performance of children with DCD. Participants were a new sample of 13 children with DCD aged between 8–12years and 13 age-matched controls. Children were required to reach and touch one of three possible targets presented at the coordinates −20°, 0° and 20° on a 17in. LCD touch-screen. For most trials (80%) the target remained stationary for the duration of movement (non-jump trials), while for the remainder (20%), the target jumped randomly to one of two peripheral locations at movement onset (jump trials). Consistent with earlier work, children with DCD were slower to initiate reaching compared to controls and showed longer MT and more errors on jump trials. Kinematic data showed that while the two groups did not differ on time to peak velocity or acceleration, children with DCD were slower to correct reach trajectory on jump trials. No group differences were observed on late kinematic markers, e.g., post-correction time. The pattern of results support and extend earlier work showing deficits in ROC in DCD. From a computational perspective, delayed corrections to the reach trajectory suggests some difficulty integrating information about the target perturbation with a predictive (or forward) estimate of limb position relative to the initial target. These conclusions are discussed, along with directions for future research.

Kinematics of the Forcarei Synform (NW Iberian Variscan belt)

Abstract The Forcarei Synform is a kilometric fold developed in the hinterland of the NW Iberian Variscan belt. A detailed analysis of the synform, based on quartz fabrics and kinematic markers, shows pervasively deformed rocks that have been continuously deformed during the last two main Variscan deformation phases (D 2 and D 3 ). Variscan D 2 minor structures related to the synform were formed under general non-coaxial flow and have a sub-horizontal maximum finite stretching toward the SSE. Later a coaxial D 3 produced a sub-vertical crenulation cleavage, and type 2 and 3 refolded folds figures at the meso- and micro-scale. Consequently, the strike of the foliations (S 2 and S 3 ) become sub-parallel and lineation (L 2 and L 3 ) are sub-horizontal. The geometry of the synform and the related structures can be interpreted in the context of D 3 . However the dominantly sinistral shear sense indicators, observed in both limbs of the synform (in map view), seem to be most probably developed during D 2 . A model is proposed involving the progressive clockwise rotation of a sub-horizontal shear, with general top-to-the-south sense, during the simultaneous development of shear zone and foliation in agreement with the kinematic indicators.

The effect of body size on the wing movements of pteropodid bats, with insights into thrust and lift production

In this study we compared the wing kinematics of 27 bats representing six pteropodid species ranging more than 40 times in body mass (M(b)=0.0278-1.152 kg), to determine whether wing posture and overall wing kinematics scaled as predicted according to theory. The smallest species flew in a wind tunnel and the other five species in a flight corridor. Seventeen kinematic markers on the midline and left side of the body were tracked in three dimensions. We used phylogenetically informed reduced major axis regression to test for allometry. We found that maximum wingspan (b(max)) and maximum wing area (S(max)) scaled with more positive allometry, and wing loading (Q(s)) with more negative allometry (b(max)∝M(b)(0.423); S(max)∝M(b)(0.768); Q(s)∝M(b)(0.233)) than has been reported in previous studies that were based on measurements from specimens stretched out flat on a horizontal surface. Our results suggest that larger bats open their wings more fully than small bats do in flight, and that for bats, body measurements alone cannot be used to predict the conformation of the wings in flight. Several kinematic variables, including downstroke ratio, wing stroke amplitude, stroke plane angle, wing camber and Strouhal number, did not change significantly with body size, demonstrating that many aspects of wing kinematics are similar across this range of body sizes. Whereas aerodynamic theory suggests that preferred flight speed should increase with mass, we did not observe an increase in preferred flight speed with mass. Instead, larger bats had higher lift coefficients (C(L)) than did small bats (C(L)∝M(b)(0.170)). Also, the slope of the wingbeat period (T) to body mass regression was significantly more shallow than expected under isometry (T∝M(b)(0.180)), and angle of attack (α) increased significantly with body mass [α∝log(M(b))7.738]. None of the bats in our study flew at constant speed, so we used multiple regression to isolate the changes in wing kinematics that correlated with changes in flight speed, horizontal acceleration and vertical acceleration. We uncovered several significant trends that were consistent among species. Our results demonstrate that for medium- to large-sized bats, the ways that bats modulate their wing kinematics to produce thrust and lift over the course of a wingbeat cycle are independent of body size.

A dynamic method for in vitro multisegment spine testing

Robotics recently spread to spine biomechanical research. The aim of the present work is to describe and validate a new method for in vitro studying of a multisegmental spinal specimen under dynamic conditions. This method relies on the use of a simulator with six degrees of freedom (to impose movements in all directions), an optoelectric apparatus (for collecting kinematics data) and an original system for attaching kinematic markers, allowing their precise removal and replacement under different examination conditions. The accuracy of measurements as well as their reproducibility under static and dynamic conditions is reported here in the study of a human lumbar spinal specimen (L1-sacrum). The method appears to be reliable and reproducible, and should therefore enable future studies of variations in mobility between healthy and pathological spines, to better understand the influence of different implants on spinal kinematics.

Quantifying the complexity of bat wing kinematics

Body motions (kinematics) of animals can be dimensionally complex, especially when flexible parts of the body interact with a surrounding fluid. In these systems, tracking motion completely can be difficult, and result in a large number of correlated measurements, with unclear contributions of each parameter to performance. Workers typically get around this by deciding a priori which variables are important (wing camber, stroke amplitude, etc.), and focusing only on those variables, but this constrains the ability of a study to uncover variables of influence.Here, we describe an application of proper orthogonal decomposition (POD) for assigning importances to kinematic variables, using dimensional complexity as a metric. We apply this method to bat flight kinematics, addressing three questions: (1) Does dimensional complexity of motion change with speed? (2) What body markers are optimal for capturing dimensional complexity? (3) What variables should a simplified reconstruction of bat flight include in order to maximally reconstruct actual dimensional complexity?We measured the motions of 17 kinematic markers (20 joint angles) on a bat (Cynopterus brachyotis) flying in a wind tunnel at nine speeds. Dimensional complexity did not change with flight speed, despite changes in the kinematics themselves, suggesting that the relative efficacy of a given number of dimensions for reconstructing kinematics is conserved across speeds.By looking at subsets of the full 17-marker set, we found that using more markers improved resolution of kinematic dimensional complexity, but that the benefit of adding markers diminished as the total number of markers increased. Dimensional complexity was highest when the hindlimb and several points along digits III and IV were tracked.Also, we uncovered three groups of joints that move together during flight by using POD to quantify correlations of motion. These groups describe 14/20 joint angles, and provide a framework for models of bat flight for experimental and modeling purposes.

A model of plate kinematics in Gondwana breakup

SUMMARY An accurate model of relative plate motions in Gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones (FZ) from the Riiser-Larsen Sea and Mozambique Basin. Used predictively, the model precisely locates kinematic markers in the West Somali Basin, which allows the conclusion that the spreading centres in the West Somali and Mozambique basins and the Riiser-Larsen Sea formed parts of the boundary between the same two plates. The locations of FZ and less well-defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the West Somali Basin and northern Natal Valley to the West Enderby Basin and Lazarev Sea during chron M10n. Small independently moving plates thus played no role in the breakup of this core part of Gondwana. In an inversion procedure, the data from these areas yield more precise finite rotations that describe the history of the two plates’ separation. Breakup is most simply interpreted to have occurred in coincidence with Karoo volcanism, and a reconstruction based on the rotations shows the Lebombo and Mateke-Sabi monoclines and the Mozambique and Astrid ridges as two sets of conjugate volcanic margins. Madagascar’s pre-drift position can be used as a constraint to reassess the positions of India and Sri Lanka in the supercontinent.

Visual control of action in step descent

Visual guidance of forwards, sideways, and upwards stepping has been investigated, but there is little knowledge about the visuomotor processes underlying stepping down actions. In this study we investigated the visual control of a single vertical step. We measured which aspects of the stepping down movement scaled with visual information about step height, and how this visual control varied with binocular versus monocular vision. Subjects stepped down a single step of variable and unpredictable height. Several kinematic measures were extracted including a new measure, "kneedrop". This describes a transition in the movement of the lower leg, which occurs at a point proportional to step height. In a within-subjects design, measurements were made with either full vision, monocular vision, or no vision. Subjects scaled kneedrop relative to step height with vision, but this scaling was significantly impaired in monocular and no vision conditions. The study establishes a kinematic marker of visually controlled scaling in single-step locomotion which will allow further study of the visuomotor control processes involved in stepping down.

ANÁLISE ESTRUTURAL E DEFORMACIONAL EM ZONAS DE CISALHAMENTO DE BAIXO MERGULHO E TRANSCORRENTES ASSOCIADAS A ZONA DE SUTURA ALTEROSA, NA REGIÃO DE CRISTINA E ITAJUBÁ - MG

The region or Cristina and Itajuba-MG, located south of Sao Francisco Craton, comprises paragneisses of the Piracaia Complex, granulites and ortogneisses of the Paraisopolis Complex which are overthrusted on metassediments of the Andrelândia Group and its basement (Amparo/Sao Goncalo do Sapucai Complex), metamorphosed in the amphibolite facies. Ductile fabrics record two main tectonic events. A thrusting event (Dn) generated the regional Sn, which shows foliation with NW-SE strike and low dips to SW, in association with down dip stretching lineations. Dn is attributed to the collision of the Sao Paulo Block (Socorro-Guaxupe Terrane) to ENE onto the Sao Francisco Plate during the Brasilia Orogeny (630-610 Ma), whose limit, the Alterosa Suture Zone (ASZ), is marked in that area by the Lourenco Velho SZ. Sn also shows a progressive sinistral inflection to a almost vertical NE-SW orientation along the Maria da Fe and Cristina strike slip shear zones. These amphibolite to granulite facies shear zones are consistent with the SW-NE compressive tectonic regime and are interpreted as lateral ramps of the Dn event. A Strike-slip/transpressive deformation (Dn + 1) formed, in lower amphibolite facies, the mylonitic foliation Sn + 1, that occur along NE-SW trending dextral shear zones within the transpressive Rio Paraiba do Sul Belt, with is related to the younger Ribeira collision (590-560 Ma). The strain analysis using Fry and Rf/phy methods revealed oblate strain elipsoids, with predominance of pure shear component of over simple shear. Simple shear predominates only in samples close to the SLSZ, where kinematic markers indicate top movement to NE and constitutes an important zone of mass transport . The obliquity between the largest axis of the strain e1lipse in relation to the foliation in planes XZ and YZ indicates non-coaxial deformation in both kinematic planes. The analyses of the strain ratio on these sections indicates that the quartz exerted strong rheologic importance for shear zone development, better accomodating the regional strains in relation to feldspars, that does not show substantial strain variation among samples with low and high deformation.