Volume 7 (2018): Issue 2 (May 2018)
Abundant Degrees of Freedom Are Not a Problem
Mark L. Latash
The problem of motor redundancy has been one of the fundamental, albeit elusive, problems in motor control. Traditionally, it has been viewed as a computational problem for the brain, solved with either optimization methods or by introducing additional constraints to motor tasks. This review suggests that the problem was wrongly formulated, and that the abundant degrees of freedom are not to be eliminated but used to ensure dynamic stability of motor performance, which is vital given the unpredictable intrinsic states and external forces. The idea of synergies as mechanisms ensuring action stability is introduced based on the uncontrolled manifold hypothesis and the theory of control with spatial referent coordinates. The importance of controlled stability is illustrated with the phenomena of anticipatory synergy adjustments. This approach is productive for both basic and applied fields as illustrated, in particular, by changes in motor synergies with neurological disorder and exercise.
Action, Movement, and Culture: Does Culture Shape Movement?
What any traveler can definitely notice is the incredible diversity of everyday skills due to the cultural diversity of tools, raw materials, physical environments, or local postural habits that set up the conditions for performing tasks. Do cultural environments influence motor skills? Are there “motor styles” common to members of a given cultural group? Focusing on instrumental everyday actions from a functional perspective, we propose four cases to illustrate in detail cultural variations in motor behavior. The first example explores the movement repertoire of expert potters from two cultural backgrounds when asked to produce pots of the same shape. A second example analyzes how a dance figure based on the same mechanical principles gives rise to different cultural aesthetics. The third example questions the adaptation of metabolic processes while performing the same load-carrying task in various physical environments. The last example brings up the issue of cultural choices of working and resting postures. Each case refers to a critical dimension of what generates the cultural diversity of motor skills: operational equivalence of movements, variation in the “weighing” of the parameters of the action, adaptation of metabolic processes, and adaptive benefit of specific posture. We conclude that if the countless diversity of cultural contexts and tasks give rise to an enormous diversity of movements and postures, this diversity is anchored in the many degrees of freedom of the organism. It is this profusion of degrees of freedom that sustains the endless variations of cultural motor skills giving ways to infinite manners of using one’s own body.
Body Size and Movement
John H. Challis
Humans of different sizes move in very similar ways despite the size difference. The principles of geometric scaling provide insight into the reasons for the similar movement patterns observed. In human locomotion, body size influences endurance running performance, with shorter body sizes being an advantage due to better heat exchange compared with their taller counterparts. Scaling can also show the equivalence of child gait with that of adults in terms of stride length and walking velocity. In humans, maximum jump height is independent of standing height, a scaling result which has been validated by examining jumps with mass added to the body. Finally, strength scales in proportion to body mass to the two-thirds power, which explains why shorter people have greater relative body strength compared with taller individuals. Geometric scaling reveals the underlying principles of many human movement forms.
Brain Dynamics and Motor Behavior: A Case for Efficiency and Refinement for Superior Performance
Bradley D. Hatfield
The paper presents a theoretical perspective on brain activity that characterizes expert cognitive-motor performance grounded in neural and psychomotor efficiency. Evidence for the position is derived from several different measurement tools (EEG, ERPs, fMRI, EEG coherence) based on empirical studies of (1) expert-novice contrasts, (2) changes in the brain after practice, and (3) motor performance under conditions of mental stress. The impact of mental stress on brain processes during motor performance is then discussed followed by a model of the hypothesized central neural responses to emotion-eliciting events to explain resilience to stress and the ability to “perform under pressure” as observed in high-performing athletes. An overall explanation is offered of the cascade of events that link the perception of the environment in which the performance occurs to the peripheral process of motor unit recruitment and the resultant quality of movement. This integrative perspective on human performance considers multiple levels of explanation including the psychology of sport performance, cognitive-motor neuroscience, and basic biomechanics to understand the kinematic qualities of movement and the effort cost involved.
Concussion: A Window Into Brain–Movement Relations in Motor Control
Michael Gay and Semyon Slobounov
Research into sports-related concussion (SRC) or brain injury has vastly expanded our knowledge of the connection between brain activity and behavioral outcomes. Historical examination of concussion reveals components of structural changes in the brain resulting from injury. A constellation of clinical symptoms is typically present following concussion for several days and weeks. However, the intersection of structural changes and clinical examination still remains elusive to medical professionals. With emerging technologies and modalities such as quantitative electroencephalography (EEG), functional magnetic resonance imaging (fMRI), virtual reality (VR), and the study of movement, we can better understand the brain–behavior relationship on clinical findings post-injury. Our advancement in SRC study using athletics provides a unique window into the advances in our ability to study this public health crisis. SRC also allows us to understand how athletics and exercise influence brain health. The evolution of SRC diagnosis, treatment, and management informs our current abilities in the study of the brain.
Ecological Physics and the Perceptual Information That Supports Motor Control
Thomas A. Stoffregen
Ordinary behavior, such as walking, reading, and throwing, depends on real-time perceptual guidance. In this article, I discuss the nature of perceptual information that, in principle, might be sufficient for the guidance of movement to achieve behavioral goals. I argue that we achieve behavioral goals by controlling movements relative to multiple physical referents. Movement relative to different physical referents causes changes in the structure of different forms of ambient energy (e.g., light, sound) and, therefore, to changes in sensory stimulation. I claim that movement always is controlled simultaneously relative to multiple referents, such that no single form of ambient energy can, in principle, contain information that is sufficient for successful control. The needed perceptual information exists, I claim, solely in the global array, that is, in emergent, higher-order patterns that extend across different forms of ambient energy. I review formal and empirical examples, and discuss implications for kinesiology.
The Embodied Origins of Infant Reaching: Implications for the Emergence of Eye-Hand Coordination
Daniela Corbetta, Rebecca F. Wiener, Sabrina L. Thurman, and Emalie McMahon
This article reviews the literature on infant reaching, from past to present, to recount how our understanding of the emergence and development of this early goal-directed behavior has changed over the decades. We show that the still widely-accepted view, which considers the emergence and development of infant reaching as occurring primarily under the control of vision, is no longer sustainable. Increasing evidence suggests that the developmental origins of infant reaching is embodied. We discuss the implications of this alternative view for the development of eye-hand coordination and we propose a new scenario stressing the importance of the infant body-centered sensorimotor experiences in the months prior to the emergence of reaching as a possible critical step for the formation of eye-hand coordination.
Fractal Dynamics, Variability, and Coordination in Human Locomotion
Scott W. Ducharme and Richard E.A. van Emmerik
In human locomotion, the magnitude of gait variability is a strong predictor of fall risk and frailty due to aging and disease. Beyond variability magnitude, the past two decades have provided emerging alternative methodologies for studying biological variability. Specifically, coordination variability has been found to be critically important within a healthy, adaptive system. While many activities aim to minimize end-point variability, greater coordination variability indicates a more flexible system, and is greater in experts compared to novices, or healthy compared to diseased individuals. Finally, variability structure (i.e., fractal dynamics) may describe the overall adaptive capacity of the locomotor system. We provide empirical support that fractal dynamics are associated with step length symmetry during challenging split-belt treadmill walking. Individuals whose fractal scaling approached 1/f fractal scaling during constrained walking also exhibited the best gait adaptability performance. Importantly, this relation between fractality and gait adaptability was not observed in unperturbed preferred speed walking.