Generation of forward angular impulse with different initial conditions

Generation of angular impulse during foot contact is regulated by controlling the relative orientation between the total body center of mass (CoM) and the reaction force (RF) applied to the feet. Between-task differences in initial CoM horizontal momentum were hypothesized to alter how forward angular impulse was generated during two forward translating tasks. Five skilled athletes performed standing (SFS) and running (RFS) forward somersaulting dives. Sagittal plane kinematics and RFs were obtained during the take-off phase of both tasks. The initial CoM momentum differences resulted in significant differences in control of the CoM relative to the RF, RF generation mechanisms, and knee and hip net joint moments (NJMs). During the RFS, angular impulse was generated by positioning the feet anterior to the CoM at initial contact so that the RF passed posterior to the CoM throughout the take-off phase. During the SFS, angular impulse was generated by positioning the CoM anterior to the feet prior to the push interval so that the RF passed posterior to the CoM. Task-specific differences in segment kinematics and RF direction contributed to the redistribution of knee and hip NJMs. These results suggest that initial conditions influence strategies the nervous system uses to satisfy task objectives.     

Effect of stride length on overarm throwing delivery: Part II: An angular momentum response

This is the second component of a two-part series investigating 3D momentum profiles specific to overhand throwing, where altering stride reportedly influences throwing mechanics resulting in significantly different physiologic outcomes and linear momentum profiles. Using a randomized cross-over design, nineteen pitchers (15 collegiate and 4 high school) were assigned to pitch two simulated 80-pitch games at ±25% of their desired stride length. An 8-camera motion capture system (240 Hz) integrated with two force plates (960 Hz) and radar gun tracked each overhand throw. Segmental angular momentums were summed yielding throwing arm and total body momentums, from which compensation ratio’s (relative contribution between the two) were derived. Pairwise comparisons at hallmark events and phases identified significantly different angular momentum profiles, in particular total body, throwing arm, and momentum compensation ratios (P ⩽ 0.05) as a result of manipulating stride length. Sagittal, frontal, and transverse angular momentums were affected by stride length changes. Transverse magnitudes showed greatest effects for total body, throwing arm, and momentum compensation ratios. Since the trunk is the main contributor to linear and angular momentum, longer strides appear to better regulate transverse trunk momentum in double support, whereas shorter strides show increased momentum prior to throwing arm acceleration.     

Intra-limb coordination in karate kicking: Effect of impacting or not impacting a target

This study aimed to investigate the kicking limb coordinative patterns adopted by karate practitioners (karateka) when impacting (IRK), or not impacting (NIRK) a target during a roundhouse kick. Six karateka performed three repetitions of both kicks while kicking limb kinematics were recorded using a stereophotogrammetric system. Intra-limb coordination was quantified for hip and knee flexion–extension from toe-off to kick completion, using the Continuous relative phase (CRP). Across the same time interval, thigh and shank angular momentum about the vertical axis of the body was calculated. For all trials, across all participants, CRP curve peaks and maximum and minimum angular momentum were determined. A RM-ANOVA was performed to test for differences between kicking conditions.The CRP analysis highlighted, during the central portion of both kicks, a delayed flexion of the hip with respect to the knee. Conversely, during the terminal portion of the CRP curves, the NIRK is performed with a more in-phase action, caused by a higher hip angular displacement. The NIRK is characterized by a lower angular momentum which may enhance control of the striking limb. It would seem that the issue of no impact appears to be solved through the control of all segments of the kicking limb, in contrast to the primary control of the lower leg only observed during the IRK.     

Specificity of Postural Control: Comparing Expert and Intermediate Dancers

The expert–novice approach is inappropriate for studying postural control in sport and dance when novices are completely unable to perform relevant postural tasks and experts cannot demonstrate specific skills on everyday postural tasks. We tested expertise-specific differences on 6 static everyday and 5 dynamic dance-like postural tasks of varying difficulty in 13 professional and 12 intermediate nonprofessional dancers. Results showed a clear expert advantage on sway area for dance-like postural tasks, but not for static everyday tasks. This effect was also found for the control parameter of root mean square (RMS) velocity and partly for RMS amplitude of the difference signal between CoP and CoG line location. Results indicate that the expert advantage is task-specific and deliver new insights into the specificity of experts' postural performance.     

The Stationary-Gaze Task Should Not Be Systematically Used as the Control Task in Studies of Postural Control

In studies of postural control, a control task is often used to understand significant effects obtained with experimental manipulations. This task should be the easiest task and (therefore) engage the lowest behavioral variability and cognitive workload. Since 1983, the stationary-gaze task is considered as the most relevant control task. Instead, the authors expected that free looking at small targets (white paper or images; visual angle: 12°) could be an easier task. To verify this assumption, 16 young individuals performed stationary-gaze, white-panel, and free-viewing 12° tasks in steady and relaxed stances. The stationary-gaze task led to significantly higher cognitive workload (mean score in the National Aeronotics and Space Administration Task Load Index questionnaire), higher interindividual body (head, neck, and lower back) linear variability, and higher interindividual body angular variability—not systematically yet—than both other tasks. There was more cognitive workload in steady than relaxed stances. The authors also tested if a free-viewing 24° task could lead to greater angular displacement, and hence greater body sway, than could the other tasks in relaxed stance. Unexpectedly, the participants mostly moved their eyes and not their body in this task. In the discussion, the authors explain why the stationary-gaze task may not be an ideal control task and how to choose this neutral task.     

Distinguishing clockwise from counterclockwise: Does it require mental rotation?

In three experiments, subjects were timed as they judged whether stimuli, presented in different angular orientations, represented clockwise or counterclockwise directions. In each experiment, there was also a control condition in which the subjects were required to make mirror image judgments relative to some canonical orientation. Analysis of reaction times suggested that, in the control tasks, the subjects generally rotated the stimuli mentally to the canonical orientation before making their decision. Mental rotation was invoked less frequently in the case of the experimental tasks, suggesting at least limited access to an orientation-free code representing the difference between clockwise and counterclockwise. This was most evident in Experiment 1, where the stimuli represented 1-h jumps of a hand on a clock face. It was less so in Experiment 2, where direction of motion was indicated in a static display. In Experiment 3, only a few subjects proved able to use an orientation-free, clockwise versus counterclockwise rubric in order to discriminate normal from backward letters.     

The reproduction of vertical and oblique orientations in the visual, haptic, and somatovestibular systems

This study investigates whether the vertical orientation may be predominantly used as an amodal reference norm by the visual, haptic, and somato-vestibular perceptual systems to define oblique orientations. We examined this question by asking the same sighted adult subjects to reproduce, in the frontal (roll) plane, the vertical (0°) and six oblique orientations in three tasks involving different perceptual systems. In the visual task, the subjects adjusted a moveable rod so that it reproduced the orientation of a visual rod seen previously in a dark room. In the haptic task, the blindfolded sighted subjects scanned an oriented rod with one hand and reproduced its orientation, with the same hand, on a moveable response rod. In the somato-vestibular task, the blindfolded sighted subjects, sitting in a rotating chair, adjusted this chair in order to reproduce the tested orientation of their own body. The results showed that similar oblique effects (unsigned angular error difference between six oblique orientations and vertical orientation) were observed across the three tasks. However, there were no positive correlations between the visual, haptic,     

Dynamic Visual Acuity

Forty subjects took part in a one-handed catching task in which the period for which the mechanically projected tennis ball was illuminated in flight was varied systematically. Additionally, they were tested for (a) static visual acuity and (b) dynamic visual acuity, in which angular velocity was varied. As expected, both viewing period in the catching task and angular velocity in the acuity task were significant variables in performance. Correlation and principal-components analyses confirmed the findings of a previous experiment in that the correlated static visual acuity tasks were unrelated to both dynamic visual acuity (even when angular velocity was only 75°/sec) and catching performance. Further, dynamic acuity and catching were related under the majority of the combinations, and most frequently at the highest angular velocity, a fact which suggested that the dynamic element in both tasks is the common factor.     

An fMRI study of imagined self-rotation

In the present study, functional magnetic resonance imaging was used to examine the neural mechanisms involved in the imagined spatial transformation of one’s body. The task required subjects to update the position of one of four external objects from memory after they had performed an imagined self-rotation to a new position. Activation in the rotation condition was compared with that in a control condition in which subjects located the positions of objects without imagining a change in selfposition. The results indicated similar networks of activation to other egocentric transformation tasks involving decisions about body parts. The most significant area of activation was in the left posterior parietal cortex. Other regions of activation common among several of the subjects were secondary visual, premotor, and frontal lobe regions. These results are discussed relative to motor and visual imagery processes as well as to the distinctions between the present task and other imagined egocentric transformation tasks.     

SELF-STIMULATION AND LEARNING IN AUTISTIC CHILDREN: PHYSICAL OR FUNCTIONAL INCOMPATIBILITY?1

This study investigated whether an observed inverse relationship between self-stimulation and learning in autistic children is due to a physical inability of the subject to use the same body part for self-stimulation and task responding, or whether self-stimulation “distracts” the subject from task responding. Four actively self-stimulating autistic children were taught two discrimination tasks; one required a response that physically interferred with their self-stimulation; the other did not. Results were: (a) all subjects responded similarly across both tasks; (b) the three subjects with higher mental age scores learned both tasks without external suppression of self-stimulation; and (c) none of the subjects showed an inverse relationship between self-stimulation and learning. The study demonstrated that elimination of self-stimulation is not a necessary prerequisite for the acquisition of a new behavior in all autistic children.     

Effect of a terminal constraint on control of balance during sit-to-stand

The speed at which sit-to-stand (STS) motions are performed and the subsequent terminal constraint upon upright stance can present subjects with contradictory goals. Previous findings suggested that subjects might adopt a strategy of limiting the peak horizontal momentum of the center of mass (CM), and perhaps of body segments as well, regardless of the speed of ascent. The primary purpose of this study was to test the hypothesis that the limitation in CM momentum is related to the constraint on upright stance at the termination of the task. The secondary purpose was to describe the contribution of the shank, thigh, and upper body (head-arm-trunk) to the peak horizontal momentum of the CM under each test condition. Nine healthy adult males rose from a seated position under the following three conditions: (a) at natural speeds (natural STS); (b) as fast as possible (fast STS); and (c) as fast as possible, followed by falling forward while keeping the feet fixed and using the arms on a support bar to stop the fall (fast STS+fall). The results showed that the peak horizontal momentum of the CM did not change substantially from the natural to fast STS, but increased significantly from the fast STS to the fast STS+fall. These findings are consistent with the hypothesis that limiting peak horizontal momentum of the CM may reflect a movement control strategy related to maintaining equilibrium at the termination of the voluntary task of rising from a chair. The momentum profile of the upper body, but not of the thigh or shank, remained constant across all experimental conditions, suggesting that the motion of the upper body may be tightly controlled in STS, regardless of the altered constraint on balance.     

Dynamic visual acuity: a possible factor in catching performance

Forty subjects took part in a one-handed catching ask in which the period for which the mechanically projected tennis ball was illuminated in flight was varied systematically. Additionally, they were tested for (a) static visual acuity and (b) dynamic visual acuity, in which angular velocity was varied. As expected, both viewing period in the catching task and angular velocity in the acuity task were significant variables in performance. Correlation and principal-components analyses confirmed the findings of a previous experiment in that the correlated static visual acuity tasks were unrelated to both dynamic visual acuity (even when angular velocity was only 75 degrees /sec) and catching performance. Further, dynamic acuity and catching were related under the majority of the combinations, and most frequently at the highest angular velocity, a fact which suggested that the dynamic element in both tasks is the common factor.     

Modeling of human posturokinetic movements by a linear feedback system: relations among feedback coefficients

This study describes a method of modeling human trunk and whole body backward bending and suggests a possible neural control strategy. The hypothesis was that the control system can be modeled as a linear feedback system, in which the torque acting at a given joint is a function of the state variables (angular positions and angular velocities). The linear system enabled representation of the feedback system by a gain matrix. The matrix was computed from the kinematics recorded by a movement analysis system and from the joint torques calculated by inverse dynamics. To validate the control model, a comparison was made between the angular kinematics yielded by the model and the experimental data. Moreover, for all subjects, the same relationships between feedback coefficients were found although gain values were different. The study showed that the feedback system is an appropriate model of the strategy from performing an accurate controlled trunk or whole body backward bending in the sagittal plane.     

Whole-body angular momentum in a complex dance sequence: Differences across skill levels

Due to the redundant degrees of freedom (DOF) and nonlinearity of reactional kinetic elements within the human motor apparatus, controlling the complex dynamics of the human musculoskeletal system presents considerable difficulties. Based on this challenge, Bernstein (1967) viewed skill development as the process whereby the central nervous system (CNS) gains mastery of kinematic DOF and kinetic reactional elements (passive forces, moments etc.), with the highest level of skill characterised by optimal exploitation of reactional elements in the achievement of movement goals. A previous kinematic investigation into coordination differences in a complex multidirectional dance sequence demonstrated that general unfreezing of kinematic DOF occurred as dance skill progressed (Chang et al., submitted for publication). To gain insight into the role of angular reactional elements in skill, the present kinetic study investigated angular momentum and associated variables across three skill levels (beginners, intermediates, experts) within this same complex dance sequence. The results showed that the angular momenta of segments and accompanying angular reactional elements generally increased with skill level. More specifically, the findings suggested that while improvements in movement economy from cancellation of angular momentum between body segments occur early in skill progression, later in skill progression, experts utilise increased whole-body angular momentum. Although this is energetically expensive, it may enhance the aesthetic value of dance movements, and/or have mechanical advantages. Overall, the findings here provide support for Bernstein’s (1967) model of skill development. Future research should quantify the relations between energy expenditure, key biomechanical variables that reflect skill and dance aesthetics as perceived by audiences.     

Effect of stride length on overarm throwing delivery: A linear momentum response

Changing stride length during overhand throwing delivery is thought to alter total body and throwing arm linear momentums, thereby altering the proportion of throwing arm momentum relative to the total body. Using a randomized cross-over design, nineteen pitchers (15 collegiate and 4 high school) were assigned to pitch two simulated 80-pitch games at ±25% of their desired stride length. An 8-camera motion capture system (240 Hz) integrated with two force plates (960 Hz) and radar gun tracked each throw. Segmental linear momentums in each plane of motion were summed yielding throwing arm and total body momentums, from which compensation ratio’s (relative contribution between the two) were derived. Pairwise comparisons at hallmark events and phases identified significantly different linear momentum profiles, in particular, anteriorly directed total body, throwing arm, and momentum compensation ratios (P ⩽ .05) as a result of manipulating stride length. Pitchers with shorter strides generated lower forward (anterior) momentum before stride foot contact, whereas greater upward and lateral momentum (toward third base) were evident during the acceleration phase. The evidence suggests insufficient total body momentum in the intended throwing direction may potentially influence performance (velocity and accuracy) and perhaps precipitate throwing arm injuries.     

The effect of simulation exercises on the control of aggressive behaviour in children

Abstract.— The subjects consisted of two. matched, extremely aggressive (experimental = ExG and control = CoG) groups of twelve 8–year-old boys. and of one criterion group (CrG) of extrovert, well-controlled boys. Video-tape recording of behaviour was used both in pretest (T₁) and post-test (T₂). Between T₁ and T₂ the ExG was submitted to simulation exercises of 8 lessons given in a period of four weeks. The exercises consisted in social problem solutions on the purely cognitive (imaginary and symbolic) level and in role-playing (behavioral level). The hypothesis was that the combination of cognitive training and observational learning with the aim of making children realise alternatives to aggression in coping with thwarting situations and their after-effects. would influence individuals with strong aggressive habits to abandon theiraggressive reactions in favour of more constructive behaviour. As to aggression, the results supported the hypothesis. For constructive behaviour, (1) the ExG maintained the samelevel of control of social behaviour in T₁ and T₂, while a significant drop occurred in the GoC, and (2) the strategies of problem solution improved significantly in the ExG. In T₂, the behaviour of the ExG resembled more closely that of the CoG than the behaviour of the CrG, as hypothesized.     

儿童空间观点采择的自动性及其发展

使用空间观点采择任务,探究7~11岁儿童的空间观点采择的自动性和发展趋势。结果发现：（1）在他人任务中,正确率随着角度的增长而减小;（2）在自我任务中,相比于空间关系不一致时,一致时的正确率更高,且反向的一致性效应出现在150°~180°间。本研究结果显示：本项空间观点采择任务对儿童有效;7~9岁的儿童可以自动进行空间转换;11岁的儿童可以自动进行需要判断左右位置的空间观点采择。本研究首次证明,11岁是儿童自动进行需要判断左右位置的空间观点采择的相对年龄转折点。     

Fluency versus conscious recollection in category-production performance: the performance of schizophrenic patients

The purpose of this study was to investigate the relative contribution in schizophrenics of automatic processes (fluency) and conscious processes (conscious recollection) for the control of preencoded material in category production tasks. In one condition (Exclusion condition), subjects were told specifically not to produce previously presented words during the category-production task. This condition was compared with a standard category-production task in which subjects were told to produce the six first words that came to mind for a semantic category (Inclusion condition). In the inclusion condition, the effects of conscious control and automatic processes operated in the same direction, whereas in the exclusion condition automatic influences and conscious control were opposed. A recognition task followed the category-production tasks. Since the exclusion condition required conscious control of encoded items, we hypothesized that schizophrenic patients would be less able than control subjects to avoid producing study list items. These results indicated that schizophrenics' performance differed from these of control subjects in the exclusion condition but not in inclusion condition. Recognition performance was similar in both the schizophrenic and the control groups. These results suggest a defective conscious control in schizophrenic patients and confirm the data from the literature on explicit memory in these patients.