Different temporal bases for body and arm movements in volleyball serve reception.

In many sports, successfully intercepting a ball requires players to move both their body and their arms. Yet, studies of interception typically focus on one or the other. We performed an analysis of the moments of first foot and arm movements of elite-level volleyball players during serve reception. Video footage of five international matches of the Netherlands men's national volleyball team allowed the systematic coding and analysis of 347 different serve reception events. For each event, we identified the time of serve (TS) and time of contact (TC). Ball flight time (from TS to TC) varied between and within types of serve (power jump serves, n = 193, and jumping float serves, n = 154). Correlation analyses revealed that foot movement was initiated with respect to time from TS, while arm movement was initiated with respect to time until TC. These results suggest that whole-body and arm movements rely on different control processes.

A robust solution for dealing with environmental changes in intercepting moving balls.

The authors tested whether a simple model based on the cancellation of the rate of change in bearing angle could account for the behavioral adaptations produced when individuals intercept moving balls while walking. In Experiment 1, the place of arrival of the ball and the angle of approach were varied. In accord with the model, velocity regulations were earlier and more pronounced the larger the angle of approach. In Experiment 2, ball speed unexpectedly changed during a trial, once again highlighting participants' functional velocity adaptations. A direct test of the model on the basis of each individual trial (N = 256) revealed that, on average, 70% of the total variance could be explained. Together, those results confirm the usefulness of such a robust strategy in the control of interceptive tasks.

The dynamics of human isometric pointing movements under varying accuracy requirements.

The goal of the present study was to explore the relation between speed-accuracy trade-off phenomena and action kinematics in the case of pointing under isometric conditions. Increasing task difficulty resulted in a linear increase in movement time (as predicted by Fitts' law) and in systematic changes in the spatio-temporal patterning of force production. The observed changes in motion topology were similar to those reported for isotonic tasks and adequately captured by a limit cycle model derived for the latter type of task. These results indicate that, for isometric force control as for isotonic position control, the reasons underlying the emergence of Fitts' law might be sought in dynamic trajectory formation processes.

Visual information about time-to-collision between two objects.

In a forced-choice paradigm, human observers' sensitivity to visual information specifying a moving object's future time of arrival at a designated position in the field of view was evaluated. A geometrical analysis demonstrated that information specifying a first-order temporal relationship (i.e., without taking changes in velocity into account) is available in the combination of the relative rate of dilation of the optical contour of the moving object and the relative rate of constriction of the optical gap separating the moving object from the target position. Observers were sensitive to information contained in the relative rate of constriction of the optical gap if no contour dilation component was present and to the combination of information contained in the relative rates of dilation of the optical contour of the moving object and constriction of the optical gap if both were present albeit with a differential weighting of the 2 components.

Grasping tau.

In the present study a direct manipulation of the optical expansion pattern was carried out. What happens to the timing of the grasp movements involved in catching a ball when optical expansion information is not veridically provided? By using 2 luminescent balls of constant size and a luminescent ball that could change its diameter during flight, it was possible to manipulate the rate of optical expansion directly. The results of 2 experiments (binocular vision in Experiment 1 and monocular vision in Experiment 2) showed that the time of the maximal closing velocity of the hand--which conforms to the prediction if Ss use retinal expansion information--was later for the deflating ball than for the balls of constant size. Adjustments to the aperture of the hand in response to the different ball sizes, especially the adjustment of the hand to the deflating ball (even though Ss were not aware that the ball was deflating during its approach), point to a finely attuned perception-action coupling.

Two-handed performance of a rhythmical fitts task by individuals and dyads.

The authors investigated 4 variants of a reciprocal Fitts task in which the pointer was moved to a stationary target, the target was moved to a stationary pointer, or both the pointer and the target were moved to each other bimanually; the bimanual task was carried out either by a single person or by a dyad. Fitts's law held in all 4 conditions, with only minor parametric changes. The kinematic organization varied with task difficulty but remained invariant in task space (i.e., in the mutual frame of reference of the pointer-target system) whatever the pointing condition. In the bimanual conditions, the 2 effectors were coordinated in antiphase with compensatory variability. The authors suggest that the observed chronometric and kinematic patterns emerge from an interplay between simple harmonic motion and the stabilizing influence of the informational flow generated by the closing of the gap between the pointer and the target interval.

Catching balls: how to get the hand to the right place at the right time.

Information specifying the future passing distance of an approaching object is available (in units of object size) in the ratio of optical displacement velocity and optical expansion velocity. Despite empirical support for the assumption that object size can serve as a metric in the perception of passing distance, the present series of experiments reveals that in catching a ball subjects do not rely on such "point-predictive" information. The angle at which (real and simulated) balls approached the subject systematically affected verbal and manual estimates of future passing distance, as well as the kinematic characteristics of catching movements. To catch a ball, the actor uses momentary action-related information instead of spatiotemporal estimates. The hand velocity is geared to information specifying the currently required velocity. This secures ending up at the right place in the right time, regardless of where this may be.

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.

The dynamics of rhythmical aiming in 2D task space: relation between geometry and kinematics under examination.

We explored a two-dimensional task space variant of the classical rhythmical Fitts' task in which participants were asked to sequentially cross four targets arranged around the extreme points of the major axes of an ellipse. Fitts' law was found to adequately describe the changes in movement time with the variations in task difficulty (ID), but the 1/3 power-law relating curvature and tangential velocity of the trajectory did not resist the increase in ID. Kinematic analyses showed that the behavioral adaptation to the ID resulted in an increase in the contribution of non-linear terms to the kinematics along the two axes of task space. Moreover, a limit cycle model (combining Rayleigh damping and Duffing stiffness, as in one-dimensional Fitts' task) captured such a behavior. In such a context, Fitts' law and the 1/3 power law appear as surface relations that emerge from parametric changes in a dynamical structure that captures the nature of Fitts' task.

Accuracy demands, environmental structure and viewing conditions in interception tasks.

30 subjects performed an interception task in which the accuracy demands were varied under binocular vs monocular viewing conditions and under different conditions of environmental structure. The pattern of results suggests that the advantage of binocular viewing is due to the concordance of information detected by both eyes rather than to binocular vision per se. The presence of static environmental structure enhances performance because it provides a stable external basis against which optical changes can be evaluated.

Timing in relay running.

Skillful performance in relay running requires that the athlete who is to take possession of the baton meets the baton carrier before reaching the end of the transmission zone while running at the highest speed possible. In the present study we examined the effects of variations in approach speed on the running behavior of the receiver. Kinematic analyses indicated that receivers regulated both the moment of initiation as well as the accelerative characteristics of their run. Contrary to the instructions given, baton carriers adapted their running speed as well to ensure a safe passage, making it difficult for the receiver to evaluate the appropriateness of their behavior.

Accuracy of perceptual processes subserving different perception-action systems.

The accuracy of perceptual processes subserving different perception-action systems was evaluated by presenting subjects (N = 17) with similar optic flow patterns, while requiring different actions from them. Squash balls were dropped along a fixed trajectory, and subjects were asked to (a) hit the ball using their own arm, (b) release an artificial arm to hit the ball, and (c) indicate when the ball was at the point of contact of conditions (a) and (b). The variability of the temporal initiation point of the actions, which served as an operationalization of perceptual accuracy, was compared under all three conditions. The results indicated that the variability of the temporal initiation point was smallest under the natural arm condition, even though movement time was more variable here than under the artificial arm condition. It is argued that, because perception and action are intimately interwoven components of a perceiving/acting system, it is not an extrinsically (experimenter-) determined simplicity of perceptual and/or motor aspects of the task, but the intrinsic make-up of this overarching system that determines which couplings lead to a more accurate performance.

The dynamics of goal-directed rhythmical aiming.

Based on the kinematics of goal-directed aiming movements in a reciprocal Fitts' task, a minimal limit cycle model is proposed that is capable of producing the behavior observed at levels of task difficulty ranging from 3 to 7. From graphical and statistical analyses of the phase planes, Hooke's planes and velocity profiles, we concluded that the minimal terms to be included in the model were (i) a nonlinear damping in the form of a self-sustaining, velocity-driven Rayleigh oscillator and (ii) a nonlinear stiffness in the form of a softening spring Duffing term. The model reproduced the kinematic patterns experimentally observed in rhythmical precision aiming, accounting for 95% of the variance. The coefficients in the model changed in a systematic way when distance and precision constraints were varied, and the meaning of these changes is discussed in the framework of the dynamical patterns approach.

The perceptual control of goal-directed locomotion: a common control architecture for interception and navigation?

Intercepting a moving object while locomoting is a highly complex and demanding ability. Notwithstanding the identification of several informational candidates, the role of perceptual variables in the control process underlying such skills remains an open question. In this study we used a virtual reality set-up for studying locomotor interception of a moving ball. The subject had to walk along a straight path and could freely modify forward velocity, if necessary, in order to intercept-with the head-a ball moving along a straight path that led it to cross the agent's displacement axis. In a series of experiments we manipulated a local (ball size) and a global (focus of expansion) component of the visual flow but also the egocentric orientation of the ball. The experimental observations are well captured by a dynamic model linking the locomotor acceleration to properties of both global flow and egocentric direction. More precisely the changes in locomotor velocity depend on a linear combination of the change in bearing angle and the change in egocentric orientation, allowing the emergence of adaptive behavior under a variety of circumstances. We conclude that the mechanisms underlying the control of different goal-directed locomotion tasks (i.e. steering and interceptive tasks) could share a common architecture.

Trajectory formation and speed-accuracy trade-off in aiming movements.

The trade-off between speed and accuracy and the patterning of movement kinematics have been central issues for theories of human movement for almost a century. In the present contribution experimentally obtained kinematics of reciprocal aiming movements, performed under different task conditions, are modelled as resulting from a single non-linear dynamical system whose parameters vary so as to respond to the task demands. Providing a unified account of speed-accuracy trade-off and trajectory formation phenomena, the model offers a theoretical framework in which both discrete and continuous movements, performed along one or more dimensions can be understood.

The Topology of Limb Deceleration in Prehension Tasks.

Whether a subject uses a constant tau-dot strategy to decelerate a reach to grasp an object is typically determined by performing a linear regression of tau onto real time to contact. This method has the disadvantage of requiring an assumption (on the part of the experimenter) concerning the endpoint targeted by the subject. In the present contribution, an alternative, topological method is proposed that makes use of a phase-plane representation of the movement. In two experiments, experimentally obtained movement trajectories were evaluated, using this new method, and the results were found to support the constant tau-dot strategy. At the same time, however, an alternative model, associated with dynamical systems theory, was demonstrated to fit the data even better. It is argued that the topology of the phase plane suggests a softening spring description for a large part of the decelerative phase, followed by a final adjustment. Deterioration of the functionality of the adjustment made in this latter phase was observed when visual information was not available during movement execution.

Fitts' law in two-dimensional task space.

A traditional continuous Fitts' task may be described as a one-dimensional oscillation between two targets. The combination of two such oscillations along intersecting axes gives rise to a two-dimensional aiming task, allowing the study of the speed-accuracy trade-off in two-dimensional task space. In two experiments subjects were asked to draw as many ellipses as possible while passing through four targets, arranged around the extreme points of the two major axes of a model ellipse. In the first experiment, task difficulty was manipulated simultaneously along the two axes of the ellipse. Regardless of ellipse eccentricity and orientation, movement time (MT) was found to depend linearly on Fitts' index of difficulty (ID), which combines between-target distance and target width. In the second experiment, ID was manipulated independently for the short and the long axes of the ellipse. There was a strong linear relation between MT and ID averaged over the two axes, with the two independent measures of task difficulty exerting interactive effects on MT: the higher the ID on one axis, the smaller the effect of the ID on the other. The present results demonstrate that Fitts' law, only examinated so far in one-dimensional aiming tasks, generalises to two-dimensional task space.

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.

The underarm throw for accuracy in children.

Forty boys and girls of two age groups (mean ages 52 and 70 months, respectively) performed 35 trials on a throwing task demanding accuracy. As might be expected, the older children were significantly more accurate than the younger ones but there were no sex differences. Film analysis of the throws showed that, although both age groups attempted to employ the adult strategy of varying velocity while keeping ball-release angle constant, the younger group was less successful in the latter aspect. The manner in which consistent release angles were arrived at by the older group was through the use of a movement pattern in which the elbow angle did not change during the forward swing. Such a movement pattern appeared less frequently in the younger age group. The results of this study provide support for a mode of research identifying functionally relevant kinematic variables that can subsequently be shown to follow from distinctive execution categories. In such a way, changes in outcome measures with increasing age and/or skill level can be functionally understood and identified.

The effects of task constraints on the organization of interception movements.

In the light of the intensity coupling effects reported in the literature, subjects' capacities for independently controlling the absolute velocity of their movement at the point of interception was evaluated by asking them to learn to propel orthogonally approaching balls, varying in their speed of approach, into target boxes placed at various distances from the interception point. As required for successful accomplishment of the task, movement velocity was found to be adapted to the distance to be covered, with the near target giving rise to lower velocities and the far target giving rise to higher velocities even when the conditions were presented in random order. Nevertheless, even though target distance accounted for a substantial proportion of the total variance, a small but significant effect of ball approach speed on movement velocity was found, suggesting that intensity coupling is a persistent but modulable phenomenon in interception tasks.