Philip W. Fink

Local and global stabilization of coordination by sensory information.

Abstract. In studies of rhythmic coordination, where sensory information is often generated by an auditory stimulus, spatial and temporal variability are known to decrease at points in the movement cycle coincident with the stimulus, a phenomenon known as anchoring (Byblow et al. 1994). Here we hypothesize that the role of anchoring may be to globally stabilize coordination under conditions in which it would otherwise undergo a global coordinative change such as a phase transition. To test this hypothesis, anchoring was studied in a bimanual coordination paradigm in which either inphase or antiphase coordination was produced as auditory pacing stimuli (and hence movement frequency) were scaled over a wide range of frequencies. Two different anchoring conditions were used: a single-metronome condition, in which peak amplitude of right finger flexion coincided with the auditory stimulus; and a double-metronome condition, in which each finger reversal (flexion and extension) occurred simultaneously with the auditory stimuli. Anchored reversal points displayed lower spatial variation than unanchored reversal points, resulting in more symmetric phase plane trajectories in the double- than the single-metronome condition. The global coordination dynamics of the double-metronome condition was also more stable, with transitions from antiphase to inphase occurring less often and at higher movement frequencies than in the single-metronome condition. An extension of the Haken-Kelso-Bunz model of bimanual coordination is presented briefly which includes specific coupling of sensory information to movement through a process we call parametric stabilization. The parametric stabilization model provides a theoretical account of both local effects on the individual movement trajectories (anchoring) and global stabilization of observed coordination patterns, including the delay of phase transitions.

Haptic information stabilizes and destabilizes coordination dynamics

Goal–directed, coordinated movements in humans emerge from a variety of constraints that range from ‘high–level’ cognitive strategies based on perception of the task to ‘low–level’ neuromuscular–skeletal factors such as differential contributions to coordination from flexor and extensor muscles. There has been a tendency in the literature to dichotomize these sources of constraint, favouring one or the other rather than recognizing and understanding their mutual interplay. In this experiment, subjects were required to coordinate rhythmic flexion and extension movements with an auditory metronome, the rate of which was systematically increased. When subjects started in extension on the beat of the metronome, there was a small tendency to switch to flexion at higher rates, but not vice versa. When subjects were asked to contact a physical stop, the location of which was either coincident with or counterphase to the auditory stimulus, two effects occurred. When haptic contact was coincident with sound, coordination was stabilized for both flexion and extension. When haptic contact was counterphase to the metronome, coordination was actually destabilized, with transitions occurring from both extension to flexion on the beat and from flexion to extension on the beat. These results reveal the complementary nature of strategic and neuromuscular factors in sensorimotor coordination. They also suggest the presence of a multimodal neural integration process—which is parametrizable by rate and context—in which intentional movement, touch and sound are bound into a single, coherent unit.

Parametric stabilization of biological coordination: a theoretical model

In human coordination studies information from the environment may not only pace rhythmic behavior, but also contribute to the observed dynamics, e.g. aphenomenon known as anchoring in the literature. For the paradigmatic caseof bimanual coordination we study these contributions mathematically and develop a model of the interaction between the limbs intrinsic dynamics and environmental signals from a metronome in terms of oscillator equations. We discuss additive versus multiplicative metronomeimpact and show the latter to be more appropriate.Our model describes single limb-metronome interaction, as well as multilimb-metronome interaction. We establish a parametricstabilization term which preserves the characteristicsof bimanual coordination and additionally explains the varyingstability of movement under different metronome conditions, the frequency dependence of the amplitudes of finger movements, anchoring phenomena andgeometries of phase space trajectories. Predictions of our model are tested against experimental observations.

Obstacle avoidance during walking in real and virtual environments

Immersive virtual environments are a promising research tool for the study of perception and action, on the assumption that visual--motor behavior in virtual and real environments is essentially similar. We investigated this issue for locomotor behavior and tested the generality of Fajen and Warrens [2003] steering dynamics model. Participants walked to a stationary goal while avoiding a stationary obstacle in matched physical and virtual environments. There were small, but reliable, differences in locomotor paths, with a larger maximum deviation (Δ = 0.16 m), larger obstacle clearance (Δ = 0.16 m), and slower walking speed (Δ = 0.13 m/s) in the virtual environment. Separate model fits closely captured the mean virtual and physical paths (R2 > 0.98). Simulations implied that the path differences are not because of walking speed or a 50% distance compression in virtual environments, but might be a result of greater uncertainty about the egocentric location of virtual obstacles. On the other hand, paths had similar shapes in the two environments with no difference in median curvature and could be modeled with a single set of parameter values (R2 > 0.95). Fajen and Warrens original parameters successfully generalized to new virtual and physical object configurations (R2 > 0.95). These results justify the use of virtual environments to study locomotor behavior.

Recruitment of degrees of freedom stabilizes coordination.

By showing that transitions may be obviated by recruiting degrees of freedom in the coupled pendulum paradigm, the authors reveal a novel mechanism for coordinative flexibility. In Experiment 1, participants swung pairs of unconstrained pendulums in 2 planes of motion (sagittal and frontal) at 8 movement frequencies starting from either an in-phase or antiphase mode. Few transitions were observed. Measures of spatial trajectory showed recruitment effects tied to the stability of the initial coordinative pattern. When the motion of the pendulums was physically restricted to a single plane in Experiment 2, transitions were more common, indicating that recruitment delays--or even eliminates--transitions. Such recruitment complements transitions as a source of coordinative flexibility and is incorporated in a simple extension of the Haken-Kelso-Bunz (1985) model.

Transference of 3D accelerations during cross country mountain biking

Investigations into the work demands of Olympic format cross country mountain biking suggest an incongruent relationship between work done and physiological strain experienced by participants. A likely but unsubstantiated cause is the extra work demand of muscle damping of terrain/surface induced vibrations. The purpose of this study was to describe the relationship between vibration mechanics and their interaction with terrain, bicycle and rider during a race pace effort on a cross country mountain bike track, on both 26″ and 29″ wheels. Participants completed one lap of a cross country track using 26″ and 29″ wheels, at race pace. Power, cadence, speed, heart rate and geographical position were sampled and logged every second for control purposes. Tri-axial accelerometers located on the bicycle and rider, recorded accelerations (128Hz) and were used to quantify vibrations experienced during the whole lap and over terrain sections (uphill and downhill). While there were no differences in power output (p=0.3062) and heart rate (p=0.8423), time to complete the lap was significantly (p=0.0061) faster on the 29″ wheels despite increased vibrations in the larger wheels (p=0.0020). Overall accelerometer data (RMS) showed location differences (p<0.0001), specifically between the point of interface of bike-body compared to those experienced at the lower back and head. The reduction in accelerations at both the lower back and head are imperative for injury prevention and demonstrates an additional non-propulsive, muscular, challenge to riding. Stress was greatest during downhill sections as acceleration differences between locations were greater when compared to uphill sections, and thus possibly prevent the recovery processes that may occur during non-propulsive load.

The effects of pediatric obesity on dynamic joint malalignment during gait

BACKGROUND There is a greater prevalence of lower extremity malalignment in obese children during static posture; however, there has been less examination of dynamic joint function in this cohort. Therefore, the purpose of this study was to determine kinematic differences that exist between obese and non-obese children that would support previously reported static joint malalignment. METHODS Forty children were classified as obese (n=20) or non-obese (n=20). Lower extremity joint kinematics were collected during five walking trials at a self-selected pace. Peak joint displacement and amount of joint motion throughout the gait cycle (calculated as the integrated displacement curve) were analyzed for group differences. FINDINGS Non-obese children had greater peak knee and hip extension during gait; however, there were no group differences in the integrated sagittal displacement curve. Obese children had greater peak angular displacement and integrals of angular displacement for peak hip adduction, hip internal rotation, and foot abduction (toe-out) than non-obese children. Obese children also had greater peak knee external rotation than non-obese children. INTERPRETATION Non-obese children showed greater range of motion in the sagittal plane, particularly at the hip and knee. Frontal and transverse plane differences suggest that obese children function in a more genu valgum position than non-obese children. Static measures of genu valgum have been previously associated with pediatric obesity; the findings indicate that there are also dynamic implications of said malalignment in obese children. Genu valgum presents increased risk of osteoarthritis for obese children and should be considered when prescribing weight bearing exercise to this cohort.

Mass affects lower extremity muscle activity patterns in children's gait

Overweight children demonstrate biomechanical differences during gait; however it is not known if these differences occur within active or passive tissue. The purpose of this study was to examine differences in lower extremity muscle activation patterns of children with different body mass during three walking speeds. Twenty children (8-12 years) were recruited and classified as overweight (OW), normal-weight (NW), or underweight (UW). Electromyography was recorded for vastus lateralis, semitendinosus, gastrocnemius, and tibialis anterior while participants walked on a treadmill at slow (SP), self-selected (SSP), and fast (FP) speeds. Differences in group and walking speed were analyzed for duration of muscle activation (presented as a percentage of stride, stance, or swing phases). Compared to OW, UW experienced greater duration of vastus lateralis and tibialis anterior activation during the swing phase. OW had greater duration of gastrocnemius activation during stride than UW. Increased walking speed resulted in greater duration of vastus lateralis activation for all groups. NW also exhibited greater duration of tibialis anterior activation at faster walking speeds. During FP, OW had greater duration of gastrocnemius activity during stance, but lower duration during swing. These findings are consistent with the idea that children with greater mass adopt a more passive gait strategy during swing to maximize energy recovery. Increased duration of gastrocnemius activity during stance also provides greater stability and stronger propulsion, which corroborates previous research. These findings help to understand the neuromuscular mechanisms associated with previous biomechanical findings in childrens gait.

Perturbation-Induced False Starts as a Test of the Jirsa–Kelso Excitator Model

One difference between the excitator model and other theoretical models of coordination is the mechanism of discrete movement initiation. In addition to an imperative signal common to all discrete movement initiation, the excitator model proposes that movements are initiated when a threshold element in state space, the so-called separatrix, is crossed as a consequence of stimulation or random fluctuations. The existence of a separatrix predicts that false starts will be caused by mechanical perturbations and that they depend on the perturbations direction. The authors tested this prediction in a reaction-time task to an auditory stimulus. Participants applied perturbations in the direction of motion (i.e., index finger flexion) or opposed to the motion prior to the stimulus on 1/4 of the trials. The authors found false starts in 34% and 9% of trials following flexion perturbations and extension perturbations, respectively, as compared with only 2% of trials without perturbations, confirming a unique prediction of the excitator model.

The influence of tyre characteristics on measures of rolling performance during cross-country mountain biking

Abstract This investigation sets out to assess the effect of five different models of mountain bike tyre on rolling performance over hard-pack mud. Independent characteristics included total weight, volume, tread surface area and tread depth. One male cyclist performed multiple (30) trials of a deceleration field test to assess reliability. Further tests performed on a separate occasion included multiple (15) trials of the deceleration test and six fixed power output hill climb tests for each tyre. The deceleration test proved to be reliable as a means of assessing rolling performance via differences in initial and final speed (coefficient of variation (CV) = 4.52%). Overall differences between tyre performance for both deceleration test (P = 0.014) and hill climb (P = 0.032) were found, enabling significant (P < 0.0001 and P = 0.049) models to be generated, allowing tyre performance prediction based on tyre characteristics. The ideal tyre for rolling and climbing performance on hard-pack surfaces would be to decrease tyre weight by way of reductions in tread surface area and tread depth while keeping volume high.