How is body orientation controlled during somersaulting?

How body orientation is controlled during somersaulting was investigated in 2 experiments that analyzed the kinematics of 223 backward standing somersaults. In Experiment 1, open-loop, initial-condition (flight duration), and prospective (time to contact, or TC1) control strategies were tested as candidates for the regulation of body moment of inertia during the jump. Decreasing between-trials variability of body orientation over time as well as a negative correlation between body angular velocity and TC1 suggested that the moment of inertia was regulated prospectively. In Experiment 2, the visual basis for this regulation was examined by asking experts and novices to execute somersaults either with eyes closed or open. Results showed that the prospective regulation observed in the vision condition disappeared in the no-vision condition with the experts, arguing in favor of a visual control during the jump. Such a coupling was absent with the novices, thus illustrating the role played by the perception-action cycle in the learning process.

Postural coordination modes considered as emergent phenomena.

The coordination of multiple body segments (torso and legs) in the control of standing posture during a suprapostural task was studied. The analysis was motivated by dynamical theories of motor coordination. In 2 experiments it was found that multisegment postural coordination could be described by the relative phase of rotations around the hip and ankle joints. The effective length of the feet, the height of the center of mass, and the amplitude of head motions in a visual tracking task were varied. Across these variations, 2 modes of hip-ankle coordination were observed: in-phase and anti-phase. The emergence of these modes was influenced by constraints imposed by the suprapostural tracking task, supporting the idea that such tasks influence postural control in an adaptive manner. Results are interpreted in terms of a dynamical approach to coordination in which postural coordination modes can be viewed as emergent phenomena.

On the role of global and local visual information in goal-directed walking.

The aim of the present study was to determine what interactions occur between the visual information available to walking subjects in the global optical flow and those of a more local nature relating to the dilation rate of a target on the retina. A goal-directed walking task was used in which thirteen subjects were asked to stop spontaneously as near as possible to a stationary target. The experiment was carried out in a special room, by means of a texture flow generator with which the velocity and direction of the optical flow arising from the ground were varied. Twelve experimental conditions were tested, involving various combinations of target size and texture velocity. The results show that with both of the targets, modifications to the global flow significantly affected the subjects' performances (walking speed and time-to-contact with the target upon braking) in the fast-approaching texture situation, but not in the receding or slowly-approaching situations. The results are discussed as to what they reveal about the visual strategies used by an actively moving observer to anticipate a collision with a stationary target.

A note on data smoothing for movement analysis: the relevance of a nonlinear method.

The goal of this experiment was to test a potentially useful nonlinear method for smoothing noisy position data, which often is encountered in the analysis of data. This algorithm (7RY) uses a nonlinear smoothing function and behaves like a low-pass filter, automatically removing aberrant points; it is used prior to differentiation of time series so that usable acceleration information can be obtained. The experimental procedure comprises position data collection along with direct accelerometric data recording. From the position-time data, (a) 7RY and (b) Butterworth algorithms have been used to compute twice-differentiated acceleration curves. The directly recorded acceleration measurements were then compared with the acceleration computed from the original position data. Although the results indicated an overall good fit between the recorded and the calculated acceleration curves, only the nonlinear method led to reliable acceleration curves when aberrant points were present in the position data.

Visual cues and attention demand in locomotor positioning.

The aim of this study was to determine what visual cues are used to assess the time-to-contact (Tc) with a stationary target during locomotor positioning. For this purpose, we performed an experiment with restricted central visual cues and analysed the attention demand involved in use of visual cues. 11 subjects were asked to walk towards a target surface and stop in front of it. Two target sizes were used: the tau strategy, consisting of assessing Tc from the relative expansion rate of the target during locomotion, was possible only with the larger one. The same task was also performed in a dual-task situation wherein reaction time to auditory stimuli was recorded. Analysis showed that braking was initiated earlier only in the dual-task situation involving the small target. The secondary task reaction time also increased considerably and actually even sooner when the positioning was done on the small target. These results suggest that a strategy other than tau may be used to assess Tc: with this strategy, the braking distance and walking speed are taken into account. Furthermore, different processing levels must have been involved in these two different methods, since the attention demand differed from one positioning task to the other.

Principal component structure and sport-specific differences in the running one-leg vertical jump.

The aim of this study is to identify the kinetic principal components involved in one-leg running vertical jumps, as well as the potential differences between specialists from different sports. The sample was composed of 25 regional skilled athletes who play different jumping sports (volleyball players, handball players, basketball players, high jumpers and novices), who performed a running one-leg jump. A principal component analysis was performed on the data obtained from the 200 tested jumps in order to identify the principal components summarizing the six variables extracted from the force-time curve. Two principal components including six variables accounted for 78 % of the variance in jump height. Running one-leg vertical jump performance was predicted by a temporal component (that brings together impulse time, eccentric time and vertical displacement of the center of mass) and a force component (who brings together relative peak of force and power, and rate of force development). A comparison made among athletes revealed a temporal-prevailing profile for volleyball players, and a force-dominant profile for Fosbury high jumpers. Novices showed an ineffective utilization of the force component, while handball and basketball players showed heterogeneous and neutral component profiles. Participants will use a jumping strategy in which variables related to either the magnitude or timing of force production will be closely coupled; athletes from different sporting backgrounds will use a jumping strategy that reflects the inherent demands of their chosen sport.

On specification and the senses.

In this target article we question the assumption that perception is divided into separate domains of vision, hearing, touch, taste, and smell. We review implications of this assumption for theories of perception and for our understanding of ambient energy arrays (e.g., the optic and acoustic arrays) that are available to perceptual systems. We analyze three hypotheses about relations between ambient arrays and physical reality: (1) that there is an ambiguous relation between ambient energy arrays and physical reality, (2) that there is a unique relation between individual energy arrays and physical reality, and (3) that there is a redundant but unambiguous relation, within or across arrays, between energy arrays and physical reality. This is followed by a review of the physics of motion, focusing on the existence and status of referents for physical motion. Our review indicates that it is not possible, in principle, for there to be a unique relation between physical motion and the structure of individual energy arrays. We argue that physical motion relative to different referents is specified only in the global array, which consists of higher-order relations across different forms of energy. The existence of specificity in the global array is consistent with the idea of direct perception, and so poses a challenge to traditional, inference-based theories of perception and cognition. However, it also presents a challenge to much of the ecological approach to perception and action, which has accepted the assumption of separate senses.

Visual control of braking in goal-directed action and sport.

Reaching a moving object, avoiding an obstacle, or controlling a rotation are common requirements of experts in sport and goal-directed action. Since the original analysis of optic flow, a large number of studies have addressed the problem of perception and control of braking. In this paper, the perception-action strategies described for deceleration control are reviewed; driving, docking, landing, somersaulting, running and reaching are analysed. The role played by 'tau dot', the first temporal derivative of tau, is shown to be critical. However, the so-called constant tau-dot strategy proposed to explain how we regulate our deceleration in such circumstances is critically examined and rejected. New directions in the problem of braking control are proposed that emphasize: (1) the advantage of tau-dot over other kinematic variables; (2) the task specificity of tau-dot; (3) the need to consider tau-dot as a control variable; and (4) the role played by the controller dynamics in the perception-action loop. Several directions for future research are suggested.

The role of central and peripheral vision in postural control during walking.

Three hypotheses have been proposed for the roles of central and peripheral vision in the perception and control of self-motion: (1) peripheral dominance, (2) retinal invariance, and (3) differential sensitivity to radial flow. We investigated postural responses to optic flow patterns presented at different retinal eccentricities during walking in two experiments. Oscillating displays of radial flow (0 degree driver direction), lamellar flow (90 degrees), and intermediate flow (30 degrees, 45 degrees) patterns were presented at retinal eccentricities of 0 degree, 30 degrees, 45 degrees, 60 degrees, or 90 degrees to participants walking on a treadmill, while compensatory body sway was measured. In general, postural responses were directionally specific, of comparable amplitude, and strongly coupled to the display for all flow patterns at all retinal eccentricities. One intermediate flow pattern (45 degrees) yielded a bias in sway direction that was consistent with triangulation errors in locating the focus of expansion from visible flow vectors. The results demonstrate functionally specific postural responses of both central and peripheral vision, contrary to the peripheral dominance and differential sensitivity hypotheses, but consistent with retinal invariance. This finding emphasizes the importance of optic flow structure for postural control regardless of the retinal locus of stimulation.

Level of gymnastic skill as an intrinsic constraint on postural coordination.

In this study, we considered the interacting effects of expertise in gymnastics, the type of support surface and the required frequency of head movement on the emergence of postural modes of coordination. A group of elite female gymnasts and a control group of non-gymnasts were asked to track the fore-aft motion of a target with their heads. Two support surface conditions (a balance beam vs the floor) were crossed with four frequencies of target motion. The relative phase between the angular motion of the ankles and hips was analysed. Two stable patterns emerged: an in-phase mode and an anti-phase mode, with the hip-ankle relative phase close to 0 degrees and 180 degrees, respectively. Increasing target frequency produced a change from in-phase to anti-phase coordination, in conditions where no instructions were given to the participants (Experiment 1) as well as in those where participants were instructed to maintain an in-phase mode for as long as possible (Experiment 2). This change, however, occurred earlier for the non-gymnasts than for the gymnasts. We conclude that 0 degrees and 180 degrees are two stable postural coordination modes, that expertise in gymnastics leads to a functional modification of existing patterns of coordination, and that expertise in general can be considered an intrinsic constraint on coordination.

Motion parallax is used to control postural sway during walking.

Three experiments tested the hypothesis that postural sway during locomotion is visually regulated by motion parallax as well as optical expansion. Oscillating displays of three-dimensional scenes were presented to participants walking on a treadmill, while postural sway was recorded. Displays simulated: (a) a cloud, in which parallax and expansion are congruent, (b) a hallway, (c) the side walls of the hallway, (d) a ground surface, (e) a wall, (f) the wall with a central hole, (g) a hall farther from the observer, and (h) a wall farther from the observer. In contrast to previous results with a hallway, responses with the cloud were isotropic and directionally specific. The other displays demonstrated that motion parallax was more effective than simple horizontal flow in eliciting lateral sway. These results are consistent with the hypothesis that adaptive control of sway during walking is based on congruent expansion and parallax in natural environments.

Effect of swimming velocity on arm coordination in the front crawl: a dynamic analysis.

We examined the preferred mode of arm coordination in 14 elite male front-crawl swimmers. Each swimmer performed eight successive swim trials in which target velocity increased from the swimmer's usual 3000-m velocity to his maximal velocity. Actual swim velocity, stroke rate, stroke length and the different arm stroke phases were then calculated from video analysis. Arm coordination was quantified by an index of coordination based on the lag time between the propulsive phases of each arm. The index expressed the three coordination modes in the front crawl: opposition, catch-up and superposition. First, in line with the dynamic approach to movement coordination, the index of coordination could be considered as an order parameter that qualitatively captured arm coordination. Second, two coordination modes were observed: a catch-up pattern (index of coordination= -8.43%) consisting of a lag time between the propulsive phases of each arm, and a relative opposition pattern (index of coordination= 0.89%) in which the propulsive phase of one arm ended when the propulsive phase of the other arm began. An abrupt change in the coordination pattern occurred at the critical velocity of 1.8 m. s(-1), which corresponded to the 100-m pace: the swimmers switched from catch-up to relative opposition. This change in coordination resulted in a reorganization of the arm phases: the duration of the entry and catch phase decreased, while the duration of the pull and push phases increased in relation to the whole stroke. Third, these changes were coupled to increased stroke rate and decreased stroke length, indicating that stroke rate, stroke length, the stroke rate/stroke length ratio, as well as velocity, could be considered as control parameters. The control parameters can be manipulated to facilitate the emergence of specific coordination modes, which is highly relevant to training and learning. By adjusting the control and order parameters within the context of a specific race distance, both coach and swimmer will be able to detect the best adapted pattern for a given race pace and follow how arm coordination changes over the course of training.