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How Spreading the Forefeet Apart Influences Upright Standing Control

Patrice R. Rougier

Resultant center-of-pressure (CP) displacements result along mediolateral (ML) and anteroposterior (AP) axes from strategies mobilizing hips and ankles, respectively, and thus, should be largely influenced by the angles between the feet. To assess this relation and the effects of foot position on postural performance, 9 healthy young adults were tested. The main results, as the forefeet are spread farther apart (from 30° in endorotation to 120° in exorotation), indicate (1) a larger contribution of the estimated ankle mechanisms in the generation of the CP trajectories along the ML axis, (2) increased variances along the longitudinal axis of the feet, (3) a constant longitudinal pattern of the CP trajectories under each foot whose main axis displays a progressively increased angle with the inner borders of the feet, and (4) increased variances for CP displacements along both ML and AP axes. These data emphasize the importance of foot positioning in stance control, especially along the ML axis where spreading the forefeet apart progressively increases the contribution of the mechanisms mobilizing the ankles.

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Kinematic Analysis of Early Online Control of Goal-Directed Reaches: A Novel Movement Perturbation Study

Lawrence E. M. Grierson, Claudia Gonzalez, and Digby Elliott

This study was designed to examine the importance of vision to corrective processes associated with a mechanical perturbation to the limb during goal-directed aiming. With a hand held stylus, under vision and no vision conditions, performers reached to a target represented by the intersection of perpendicular lines. The stylus was connected to an air compressor and engineered such that 80 ms following movement initiation reaches were perturbed by a short air burst either in the direction of, or opposite to, the movement. Spatial position analysis of the limb at early kinematic landmarks revealed that the single direction bursts were successful in advancing and hindering the movement progress. Furthermore, within subject trial-to-trial variability analysis indicated that performers adopted different control strategies for dealing with the perturbations depending on the availability of vision. The present findings suggest that a continuous form of online control is exercised during the early portions of the aiming trajectories. This form of control may be mediated by visual or proprioceptive information.

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Development of Force Control and Timing in a Finger-Tapping Sequence with an Attenuated-Force Tap

Nobuyuki Inui and Yumi Katsura

We conducted an experiment to examine age-related differences in the control of force and timing in a finger-tapping sequence with an attenuated-force tap. Participants between 7 and 20 years old tapped on a load cell with feedback on practice trials. They were required to recall the force pattern (300 g, 300 g, 300 g, 100 g) and the intertap interval (400 ms) without feedback on test trials. Analysis indicated that the last attenuated tap affected the first three taps of the tapping sequence in adults and adolescents but not in children. Adults and adolescents appeared to respond with four taps as a chunk, resulting in a contextual effect on the timing of force control, but younger children had difficulty with such chunking. Further, adults and adolescents were able to more accurately produce individual force magnitudes to match target magnitudes than younger children. For the ratio of force in serial positions 1:4, 2:4, and 3:4, consequently, 7- to 8-year-old children had lower ratios than the other age groups. Although there was no difference among age groups for timing control of peak force to press duration as a control strategy of force, 7- to 8-year-old children spent more time to produce force than the other age groups. Peak force with a decreased force was more variable in the attenuated force serial position (4) than in the other serial positions in all five age groups. Peak force variability was particularly robust in younger children. These findings suggest that younger children have difficulty with both temporal and spatial (i.e., magnitude) components of force control.

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Direction of Foot Force for Pushes against a Fixed Pedal: Variation with Pedal Position

Kreg G. Gruben, Lynn M. Rogers, Matthew W. Schmidt, and Liming Tan

The force that healthy humans generated against a fixed pedal was measured and compared with that predicted by four models. The participants (n = 11) were seated on a stationary bicycle and performed brief pushing efforts against an instrumented pedal with the crank fixed. Pushes were performed to 10 force magnitude targets and at 12 crank angles. The increasing magnitude portion of the sagittal-plane force path for each push effort was fitted with a line to determine the direction of the muscle component of the foot force. Those directions varied systematically with the position of the pedal (crank angle) such that the force path lines intersected a common region superior and slightly anterior to the hip. The ability of four models to predict force path direction was tested. All four models captured the general variation of direction with pedal position. Two of the models provided the best performance. One was a musculoskeletal model consisting of nine muscles with parameters adjusted to provide the best possible ft. The other model was derived from (a) observations that the lines-of-action of the muscle component of foot force tended to intersect in a common region near the hip, and (b) the corresponding need for foot force to intersect the center-of-mass during walking. Thus, this model predicted force direction at each pedal position as that of a line intersecting the pedal pivot and a common point located near the hip (divergent point). The results suggest that the control strategy employed in this seated pushing task reflects the extensive experience of the leg in directing force appropriately to maintain upright posture and that relative muscle strengths have adapted to that pattern of typical activation.

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Goal-Equivalent Joint Coordination in Pointing: Affect of Vision and Arm Dominance

Yaweng Tseng, John P. Scholz, and Gregor Schöner

This study used the uncontrolled manifold (UCM) approach to study joint coordination underlying the control of task-related variables important for success at reaching and pointing to targets. More combinations of joint motions are available to the control system to achieve task success than are strictly necessary. How this abundance of motor solutions is managed by the nervous system and whether and how the availability of visual information affects the solution to joint coordination was investigated in this study. The variability of joint angle combinations was partitioned into 2 components with respect to control of either the hand's path or the path of the arm's center of mass (CM). The goal-equivalent variability (GEV) component represents trial-to-trial fluctuations of the joint configuration consistent with a stable value of the hand or CM path. The other component, non-goal-equivalent variability (NGEV), led to deviations away from the desired hand or CM path. We hypothesized a style of control in which the NGEV component is selectively restricted while allowing a range of goal-equivalent joint combinations to be used to achieve stability of the hand and CM paths. Twelve healthy right-handed subjects reached across their body to the center of a circular target with both the right and left arms and with their eyes open or closed on different trials. When repeating the task with the same arm under identical task conditions, subjects used a range of goal-equivalent joint configurations to control the entire trajectory of both the hand's and the arm's CM motion, as well as the terminal position of the pointer-tip. Overall joint configuration variability was consistently larger in the middle of the movement, near the time of peak velocity. The style of joint coordination was qualitatively similar regardless of the arm used to point or the visual condition. Quantitative differences in the structure of joint coordination were present for the non-dominant arm, however, when pointing in the absence of vision of the hand and target. The results of this study suggest that the nervous system uses a control strategy that provides for a range of goal-equivalent, rather than unique, joint combinations to stabilize the values of important task-related variables, while selectively restricting joint configurations that change these values. The possible advantage of this style of control is discussed. Absence of vision during reaching affected joint coordination only quantitatively and only for the less skilled left arm, suggesting that the role of visual information may be greater when organizing the motor components of this arm.

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Improvements in Obstacle Clearance Parameters and Reaction Time Over a Series of Obstacles Revealed After Five Repeated Testing Sessions in Older Adults

Deborah A. Jehu, Yves Lajoie, and Nicole Paquet

tend to adopt a risky obstacle crossing strategy when crossing one or two obstacles by reducing step velocity, step length, and step width, coupled with placing the heel closer to the trailing edge of the obstacle compared with young adults ( Lowrey, Watson, & Vallis, 2007 ; McFadyen & Prince 2002

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Effect of Different Evasion Maneuvers on Anticipation and Visual Behavior in Elite Rugby League Players

Jonathan D. Connor, Robert G. Crowther, and Wade H. Sinclair

-region/torso, and this may have been enough to optimize information and process relevant cues pertaining to future running direction, perhaps suggesting that there is a sequence in which postural cue information may become available and influence anticipatory skill and search strategies. Muller and Abernethy ( 2012

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The Effects of Obstacle Type and Locomotion Form on Path Selection in Rugby Players

Lana M. Pfaff and Michael E. Cinelli

; Gérin-Lajoie, Richards, & McFadyen, 2005 ). The action strategies used by an individual to avoid an obstacle(s) are dependent on characteristics of both the individual (body size and action capabilities) and the physical properties of the environment, all of which determine the opportunities for action

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Balance and Mobility Training With or Without Simultaneous Cognitive Training Reduces Attention Demand But Does Not Improve Obstacle Clearance in Older Adults

Deborah A.M. Jehu, Nicole Paquet, and Yves Lajoie

cognitive resources, motor planning, and visually dependent gait regulation ( Brown, McKenzie, & Doan, 2005 ). In order to successfully navigate these environments, accommodation strategies are employed and involve an adaptation of the gait pattern for a change in terrain that bring the lower limb to a new

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Should Ballet Dancers Vary Postures and Underfoot Surfaces When Practicing Postural Balance?

Nili Steinberg, Gordon Waddington, Roger Adams, Janet Karin, and Oren Tirosh

challenging movement, ballet dancers require superior control strategies with decent balance ability. While trying to stand in their “best” posture, the dancers might have restricted their ability to use their joints optimally in order to adjust to the postural sway ( Lin et al., 2014 ; Lobo da Costa et