Prevalence and Incidence of Microhemorrhages in Adolescent Football Players.

BACKGROUND AND PURPOSE: SWI is an advanced imaging modality that is especially useful in cerebral microhemorrhage detection. Such microhemorrhages have been identified in adult contact sport athletes, and the sequelae of these focal bleeds are thought to contribute to neurodegeneration. The purpose of this study was to utilize SWI to determine whether the prevalence and incidence of microhemorrhages in adolescent football players are significantly greater than those of adolescent noncontact athletes. MATERIALS AND METHODS: Preseason and postseason SWI was performed and evaluated on 78 adolescent football players. SWI was also performed on 27 adolescent athletes who reported no contact sport history. Two separate one-tailed Fisher exact tests were performed to determine whether the prevalence and incidence of microhemorrhages in adolescent football players are greater than those of noncontact athlete controls. RESULTS: Microhemorrhages were observed in 12 football players. No microhemorrhages were observed in any controls. Adolescent football players demonstrated a significantly greater prevalence of microhemorrhages than adolescent noncontact controls (P = .02). Although 2 football players developed new microhemorrhages during the season, microhemorrhage incidence during 1 football season was not statistically greater in the football population than in noncontact control athletes (P = .55). CONCLUSIONS: Adolescent football players have a greater prevalence of microhemorrhages compared with adolescent athletes who have never engaged in contact sports. While microhemorrhage incidence during 1 season is not significantly greater in adolescent football players compared to adolescent controls, there is a temporal association between playing football and the appearance of new microhemorrhages.

Injury risk due to collisions in Major League Baseball.

Purpose is to determine if Major League Baseball plays at risk for collisions have higher injury rates than typical base running plays. 2002-2011 Major League Baseball play data was obtained: non-force putouts by catcher at home plate (Catcher Tag Out), groundball force outs at 2(nd) base with less than 2 outs (Double Play Attempt), and the control play, outfield assisted non-force putouts of runners attempting to advance to 2(nd) or 3(rd) base (Outfield Assist 2(nd)/3(rd)). This list was cross-referenced with 2002-2011 disabled lists to see if an involved player went on the disabled list the day of or day after the play. An on-line search for each match determined if the injury was attributable to that play. Rate calculated per 1 000 plays, severity in days on disabled list. Injury rate and severity for Catcher Tag Out was 6.98 and 45.6 respectively, Double Play Attempt 0.42 and 41.3, Outfield Assist 2(nd/)3(rd) 1.56 and 47.0. Injury rate for Catcher Tag Out was higher (P = 0.03) than the control while Double Play Attempt trended lower (P = 0.05). There was no difference in severity. Catcher Tag Outs carry greater injury risk than typical base running plays. Major League Baseball should consider prohibiting base path collisions.

Estimation of injury simulation in international men's soccer.

The aim of this study was to determine the frequency of apparent injury incidents in men's international soccer and attempt to estimate what proportion of these incidents are authentic. Broadcast recordings of 89 group stage games from 4 tournaments were reviewed to identify incidents in which a player behaved as if injured. Apparent injuries were considered definite if a player withdrew from participation within 5 min or if bleeding was visible. The remaining incidents were considered questionable. A total of 980 apparent injuries were observed at a rate of 5.63/team game. The definite injury rate was only 0.41/team game vs. 5.22/team game for questionable injuries. Definite injuries were associated with on-field treatment (P<0.001) and stretcher (P<0.001) while questionable injuries were associated with fouls (P<0.001), yellow cards (P=0.013), and the first half (P=0.001). Questionable injuries were more associated with the final 1/3 of the second half than the initial 2/3 (P=0.039). For the 24 games of the 2007 AFC Asian Cup there was an association between questionable injuries and a contact mechanism (P<0.001). The range of the rate of questionable injury was 4.17-5.4 by confederation. This information may assist soccer governing bodies in developing plans to prevent injury simulation behavior.

Variation in injury risk over the course of a two-day youth club soccer tournament.

Athletic injury risk in youth is known to be greater in organised sport, higher levels of competition, games, and tournaments, yet young athletes are often expected to participate in tournaments that involve multiple matches per day on consecutive days. In 2006 and 2008 injury rates were observed during a club youth soccer tournament that involved athletes aged 9-18 years playing three to four matches over two consecutive days, to determine if injury risk varied over the course of the event. The injury rate for day 1 was 7.91/1000 hours with a mean severity of 3.5 days missed versus a day 2 rate of 6.75 and severity of 2.3. The rate and severity for the first game in a day were 7.13 and 3.5 respectively compared to 7.9 and 2.5 for game 2. Further research is needed to determine if a high density match schedule increases injury risk for youth soccer players.

Posture-based motion planning: applications to grasping.

This article describes a model of motion planning instantiated for grasping. According to the model, one of the most important aspects of motion planning is establishing a constraint hierarchy--a set of prioritized requirements defining the task to be performed. For grasping, constraints include avoiding collisions with to-be-grasped objects and minimizing movement-related effort. These and other constraints are combined with instance retrieval (recall of stored postures) and instance generation (generation of new postures and movements to them) to simulate flexible prehension. Dynamic deadline setting is used to regulate termination of instance generation, and performance of more than one movement at a time with a single effector is used to permit obstacle avoidance. Old and new data are accounted for with the model.

Frames of reference in perceptual-motor learning: evidence from a blind manual positioning task.

Participants moved a joystick to bring a computer-displayed cursor to each of six on-screen target locations arrayed around the center of the screen. At the start of each trial, the stick rested vertically, with a cursor occupying the center of the screen. A target appeared at another location and as soon as the stick was moved away from its rest position the cursor disappeared until the participant pressed a trigger on the stick to indicate when s/he thought the stick-controlled cursor was at the target site. With training, participants improved on the blind positioning task, but when conditions changed their performance suffered. Changing the hand used in the task or the location of the stick caused approximately equal disruptions, but changing both hand and location was significantly more disruptive than changing just one feature. The results support the hypothesis that perceptual-motor learning entails coding of extrinsic (spatial coordinates) as well as intrinsic (postural or body movement) information.

Timing and reaction time.

Because reaction time (RT) tasks are generally repetitive and temporally regular, participants may use timing strategies that affect response speed and accuracy. This hypothesis was tested in 3 serial choice RT experiments in which participants were presented with stimuli that sometimes arrived earlier or later than normal. RTs increased and errors decreased when stimuli came earlier than normal, and RTs decreased and errors increased when stimuli came later than normal. The results were consistent with an elaboration of R. Ratcliff's diffusion model (R. Ratcliff, 1978; R. Ratcliff & J. N. Rouder, 1998; R. Ratcliff, T. Van Zandt, & G. McKoon, 1999), supplemented by a hypothesis developed by D. Laming (1979a, 1979b), according to which participants initiate stimulus sampling before the onset of the stimulus at a time governed by an internal timekeeper. The success of this model suggests that timing is used in the service of decision making.

Multijoint grasping movements. Simulated and observed effects of object location, object size, and initial aperture.

Studies of human prehension have revealed characteristic patterns of grasping kinematics. We sought to gain insight into the determinants of those patterns by means of a computer simulation and accompanying behavioral experiment concerning multijoint, planar grasping behavior. The simulation was based on a recent theory of posture-based motion planning which hypothesizes that movement preparation entails time-limited, multiple task-constraint satisfaction. Prehension was modeled with a stick-figure animation involving 12 series of 81 grasping movements. Circular objects to be grasped were located at three angles (45 degrees, 90 degrees, and 135 degrees) and at three distances (20 cm, 30 cm, and 40 cm) relative to the initial location of the hand in the workplane. Additionally, three object sizes (2 cm, 4 cm, and 6 cm in diameter) and three initial aperture sizes (0.3 cm, 3.3 cm, and 7.0 cm) were used. Analyses of the simulated grasping movements focused on the time course of the hand opening, the tangential velocity of the wrist, and the rotations of the joints in the arm, hand, and fingers. The results showed that the model accurately mimicked detailed kinematics of prehension observed in earlier studies. With respect to the frequently reported relationship between object size and hand opening, the simulations further revealed an effect of initial aperture. This predicted effect was confirmed in an experiment in which four participants performed analogous planar grasping tasks. An analysis of the time course of the opening of the hand showed that maximum aperture covaried with initial aperture. A conclusion of this work is that a major determinant of grasping kinematics is avoidance of collisions with objects that are to be grasped.

Planning reaching and grasping movements: theoretical premises and practical implications.

This paper presents the background, premises, and results of a model of movement planning. The model's central claims are fourfold: (a) A task is defined by a set of prioritized requirements, or what we call a constraint hierarchy; (b) movement planning works first by specifying a goal posture and then by specifying a movement to that goal posture; (c) movements have characteristic forms; and (d) movements can be shaped through simultaneous performance of different movements, even by the same effector. We review the model and then speculate on its implications for clinical concerns, especially spasticity

Planning reaching and grasping movements: simulating reduced movement capabilities in spastic hemiparesis.

In this paper we describe how a theory of posture-based motion planning recently applied to human grasping may contribute to the understanding of grasping pathology. The theory is implemented as a computer model rendered as a stick-figure animation capable of generating realistic multi-joint grasping movements. As shown here, the model can also be used to simulate grasping movements whose kinematics resemble those of grasps performed by people with spastic hemiparesis. The simulations demonstrate effects of: (a) reduced ranges of motion of arm joints on the size of the reachable workspace, (b) awkward starting postures on the time course of the hand closing around an object, (c) increased costs of joint rotations on movement time, and (d) addition of noise to biphasic joint rotations on the low-velocity phase of wrist transport.

Planning reaching and grasping movements: the problem of obstacle avoidance.

In this article, we review a model of the movement-planning processes that people use for direct reaching, reaching around obstacles, and grasping, and we present observations of subjects' repeated movements of the hand to touch 2 target locations, circumventing an intervening obstacle. The model defines an obstacle as a posture that, if adopted, would intersect with any part of the environment (including the actor himself or herself). The model finds a trajectory that is likely to bring the end-effector to the target by means of a one-or two- stage planning process. Each stage exploits the principles of instance retrieval and instance generation. In the first stage, a goal posture is identified, and the trajectory of a direct transition to that posture is tested for collision. If the direct movement has no collision, the movement to the target is immediately executed in joint space. If, however, the direct movement is foreseen to result in a collision, a second planning stage is invoked. The second planing stage identifies a via posture, movement through which will probably avoid the collision. Movement to and from the via posture is then superimposed on the main movement to the target so that the combined movement reaches the target without colliding with intervening obstacles. We describe the details of instance retrieval and instance generation for each of these planning stages and compare the model's performance with the observed kinematics of direct movements as well as movements around an obstacle. Then we suggest how the model might contribute to the study of movements in people with motor disorders such as spastic hemiparesis.

Acquisition of intellectual and perceptual-motor skills.

Recent evidence indicates that intellectual and perceptual-motor skills are acquired in fundamentally similar ways. Transfer specificity, generativity, and the use of abstract rules and reflexlike productions are similar in the two skill domains; brain sites subserving thought processes and perceptual-motor processes are not as distinct as once thought; explicit and implicit knowledge characterize both kinds of skill; learning rates, training effects, and learning stages are remarkably similar for the two skill classes; and imagery, long thought to play a distinctive role in high-level thought, also plays a role in perceptual-motor learning and control. The conclusion that intellectual skills and perceptual-motor skills are psychologically more alike than different accords with the view that all knowledge is performatory.

Frames of reference for human perceptual-motor coordination: space-based versus joint-based adaptation.

In this study, the authors addressed the issue of whether space-based motor planning occurs at a higher, equal, or lower level of central nervous system control than joint-based motor planning by using a computerized adaptation paradigm. Visual displays of participants' (N = 32) reaching movements to spatial targets were distorted either with respect to spatial hand displacements (space-based distortion) or with respect to joint angle displacements (joint-based distortion). Participants adapted more easily to space-based distortion than to joint-based distortion. The results suggest that when the participants were confronted with new visuomotor mappings, they aimed for virtual spatial targets whose positions were adjusted to compensate for the distortions associated with the new mappings. That strategy was preferred over a joint- or posture-based strategy, in which a posture is selected for the displayed spatial target and is then modified so that the new mapping between adopted and seen positions can be accommodated. The results support the widely held view that space-based planning occurs at a higher level than joint-based planning.

Coordination of reaching and grasping by capitalizing on obstacle avoidance and other constraints.

Reaching and grasping an object can be viewed as the solution of a multiple-constraint satisfaction problem. The constraints include contact with the object with the appropriate effectors in the correct positions as well as generation of a collision-free trajectory. We have developed a computational model that simulates reaching and grasping based on these notions. The model, rendered as an animation program, reproduces many basic features of the kinematics of human reaching and grasping behavior. The core assumptions of the model are: (1) tasks are defined by flexibly organized constraint hierarchies; (2) manual positioning acts, including prehension acts, are first specified with respect to goal postures and then are specified with respect to movements towards those goal postures; (3) goal postures are found by identifying the stored posture that is most promising for the task, as determined by the constraint hierarchy, and then by generating postures that are more and more dissimilar to the most-promising stored posture until a deadline is reached, at which time the best posture that was found during the search is defined as the goal posture; (4) depending on when the best posture was encountered in the search, the deadline for the search in the next trial is either increased or decreased; (5) specification of a movement to the goal posture begins with straight-line interpolation in joint space between the starting posture and goal posture; (6) if an internal simulation of this default movement suggests that it will result in collision with an obstacle, the movement can be reshaped until an acceptable movement is found or until time runs out; (7) movement reshaping occurs by identifying a via posture that serves as a body position to which the actor moves from the starting posture and then back to the starting posture, while simultaneously making the main movement from the starting posture to the goal posture; (8) the via posture is identified using the same posture-generating algorithm as used to identify the goal posture. These processes are used both for arm positioning and, with some elaboration, for prehension. The model solves a number of problems with an earlier model, although it leaves some other problems unresolved.

Remembered positions: stored locations or stored postures?

Many recent studies indicate that memory for final position is superior to memory for movement. There is ambiguity about what is meant by the term final position, however. Is it final spatial location or final posture? According to a recently proposed theory by Rosenbaum et al., which maintains that stored postures form the basis for movement planning, when people try to return to recently reached positions, they should try to adopt the postures they just occupied. An alternative view, which holds that movements are primarily planned with respect to spatial locations, predicts that subjects should tend to return to places in external space. We describe an experiment that tested these opposing predictions. The experiment relied on the notion that if people store and use postures, they should "copy" the posture adopted with one arm to the other arm when possible. The results support this hypothesis. In this article, we review previous work that bears on the question of what is learned when people move repeatedly to a given position. Then we present two theoretical perspectives which make diverging predictions about what should be learned in repositioning tasks. One perspective predicts that final positions are remembered as postures; the other predicts that final positions are remembered as locations. We describe an experiment designed to distinguish between these two predictions. The experiment indicates that final postures are remembered and are "copied" from one arm to the other when subjects try to reach repeatedly to the same location in the midsagittal plane with alternating arms or when subjects try to reach repeatedly to the same location anywhere in the workspace with the same arm. In the last section of the article, we discuss the implications of our findings.

Is dynamical systems modeling just curve fitting?

The development of mathematical tools for describing dynamical systems has made it possible to characterize forms of behavior that could not be characterized before. This represents progress, but the enterprise runs the risk of being nothing more than curve fitting if investigators fail to identify the physical, biological, or psychological mechanisms which are common to systems that follow the same dynamical regime and which are not common to systems that do not follow the same dynamical regime.

Finding final postures.

In this study, a model of movement planning (Rosenbaum, Engelbrecht, Bushe, & Loukopoulos, 1993a, 1993b; Rosenbaum, Loukopoulos, Meulenbroek, Vaughan, & Engelbrecln, 1995), in which movements are generated on the basis of the efficacy of different possible goal postures, was tested. The model predicts which limb segments will be used and how the segments will be combined in reaching. The model's predictions were compared with observations from a study in which seated participants reached for targets in a sagittal plane, using the hip, shoulder, and elbow. Estimates of 4 free parameters-an expense factor for each of the 2 contributing joints, and a 4th parameter that specified the relative weight of spatial accuracy versus effort minimization, were used for fitting the model to the observed joint angles. The model accounted for 96% of the variance of the observed joint angles and did a better job accounting for the data than several alternative models. A key new finding was that balance constraints play a more important role in determining joint contributions than was previously recognized.

Speed and sequential effects in reaching.

To investigate the impact of future task demands on reaching, participants performed repetitive sagittal-plane reaches at low and high speeds. In a control condition, they reached from a start location to a target and back. In the experimental conditions, they reached from the start to the target, then to a second target (the location of which varied between trials), then back to the first target, and finally back to the start. Contributions of the hip, shoulder, and elbow to reaches made to the first target depended on the second target's location, on movement speed, and on repetition. Participants combined sustained and transient postural adjustments to minimize effort. The results support the knowledge model of movement selection (D. A. Rosenbaum, L. D. Loukopoulos, R. G. M. Meulenbroek, J. Vaughan, & S. E. Engelbrecht, 1995) but also call for its elaboration. Variants of the model are explored through simulations of the above study.

From cognition to biomechanics and back: the end-state comfort effect and the middle-is-faster effect.

Consistent preferences for particular types of movement suggest criteria for movement selection. These can be important when, as is usually the case, infinitely many movements allow a task to be achieved. The experiments reported here were designed to identify the source of a strong preference observed in earlier object-manipulation studies. In those earlier studies, subjects usually grabbed objects to be moved from one location to another in a way that afforded a comfortable final posture rather than a comfortable initial posture (the end-state comfort effect). The comfortable final state usually allowed the forearm to be at or near the middle of its range of motion on the pronation-supination dimension. The hypothesis tested here was that the end-state comfort effect stemmed from an expectation that movements can be made more quickly in the middle of the pronation-supination range than at either extreme. To test this hypothesis, we asked subjects, in the first experiment, to perform a handle rotation task that demanded little or no precision and so no need to make rapid to-and-fro homing-in movements near the end of the rotation. Half the subjects did not show the end-state comfort effect, in contrast to all previous studies, where all subjects showed the effect. An incidental finding of the first experiment was that handle rotations that ended at or near the end of the range of motion took longer than handle rotations that ended at or near the middle of the range of motion. To test the latter result more carefully, we asked subjects, in Experiments 2 and 3, to oscillate the forearm as quickly as possible, either in the supination part of the forearm rotation range, in the middle part of the range, or in the pronation part of the range. As predicted, oscillation frequencies were highest in midrange, and this was true for both hands. The results as a whole have implications for the relation between cognitive psychology and biomechanics, and for human factors.