Biomedical subjects

Karl M Newell

Publications and source records attributed to Karl M Newell.

At least 19 recordsLinked to original sources

PubMed2007

Dynamical origins of stereotypy: relation of postural movements during sitting to stereotyped movements during body-rocking.

The relation between the movement dynamic properties of sitting still and of seated body-rocking in adults with stereotyped movement disorder and mental retardation and a contrast group of typically developing age-matched adults was examined. Continuous measurement of sequential displacements in center-of-pressure was made using a force platform while subjects were engaged in seated body-rocking and quiet sitting. Properties of movement were compared across conditions (rocking, sitting) and groups (stereotyped movement disorder, contrast). The contrast group had the same modal frequency for both movement properties. The intrinsic dynamics of the stereotyped movement disorder group were similar to those of the contrast group for body-rocking but very different for quiet sitting. Findings support the suggestion that body-rocking in stereotyped movement disorder originates partly as an adaptation to an inability to control posture in a seated position.

Adult↗

PubMed2006

Speed influences on the scaling behavior of gait cycle fluctuations during treadmill running.

The current study examined the temporal structure of gait cycle fluctuations in running. Participants ran at 80%, 90%, 100%, 110% and 120% of preferred running speed for 8min trials. Kinematic and kinetic gait cycle variables were generated from ground reaction force data. Mean, SD and CV of the kinematic and kinetic variables changed linearly with speed, whereas U-shaped functions were found for the scaling exponent alpha in 5 of the 8 variables investigated. Our findings reveal that long range correlations are present in both kinetic and kinematic variables of the gait cycle. The dependent structure of the stride interval is reduced at preferred running speed and this is hypothesized to be related to the enhanced stability and flexibility of this gait speed.

Adult↗

PubMed2006

On No-KR tests in motor learning, retention and transfer.

In this paper we examine the role of no-knowledge of results (KR) tests in motor learning, retention and transfer. A distinction is drawn between studies of KR and studies of other issues in motor learning that use a No-KR test. Arguments are presented against the currently received position that a No-KR test is an essential protocol to examine motor learning, although a No-KR test is most relevant in the few contexts where the outcome information is not naturally available. Perspectives are provided on the limited though proper role of No-KR tests in motor learning, retention and transfer and these are illustrated through experimental examples. It is proposed that the current restrictive interpretation of KR as only augmented outcome information is narrowing not only the context of KR research but also the study of motor learning in general.

Conditioning, Classical↗

PubMed2006

Virtual time-to-contact of postural stability boundaries as a function of support surface compliance.

This study investigated the hypothesis that virtual time-to-contact, which specifies the time to reach the functional stability boundary, is a variable controlled in the maintenance of upright posture. Three different levels of support surface compliance were used on a force platform (no foam, 5 cm of foam, and 15 cm of foam). The participant's task was to stand still under each surface support condition both with and without vision. The stability boundary was determined for each set of conditions where the participant was required to lean as far as possible in all directions of the horizontal plane without losing stability. The results showed that the no vision conditions had a significantly larger center of pressure displacement than the vision conditions. No vision and increasing support surface compliance also increased the velocity of the center of pressure trajectory. The distribution of the radial displacement of the center of pressure showed relatively equal frequency over spatial location with no central tendency. The virtual time-to-contact with the stability boundary decreased as platform surface support became more compliant. Furthermore, the distribution of virtual time over the effective scaling range was a power law with a larger exponent in the more unstable no vision and increasing surface foam conditions. The findings provide additional evidence for the hypothesis that virtual time-to-contact with stability boundaries is a postural control variable that is regulated rather than the preservation of minimal motion around the center of the stability region as proposed in pendulum models of posture.

Adolescent↗

PubMed2006

Arm constraint and walking in healthy adults.

The aim of the present study was to examine the functional adaptations in inter-segmental coordination when constraining one arm in healthy adults during treadmill walking at different velocities. Subjects were instructed to walk on a motorized treadmill at different walking velocities (range: 0.22-1.52 m/s) during three experimental conditions, i.e.: (1) no arm constraint, (2) dominant arm constrained, and (3) non-dominant arm constrained. Movements of body segments were recorded with a 3D motion analysis system. A comparison between walking with one arm constrained and normal walking revealed decreased, transverse pelvic, thoracic, and trunk rotation, however there were slight increases in non-constrained arm movement amplitude. Reduced arm movement amplitude did result in altered frequency and phase relations between the arm and leg. Persons with upper extremity movement dysfunction may walk slower due to atypical coordination between upper and lower body movement at higher walking velocities. Future studies should focus on examining the underlying dynamics of adaptations in inter-limb and trunk coordination during walking in both healthy adults and persons with upper extremity movement disorders.

Adult↗

PubMed2006

The effects of auditory rhythms and instruction on walking patterns in individuals post stroke.

The objective was to investigate the effects of auditory rhythms and arm movement on inter-segmental coordination during walking in persons who have suffered a stroke. Eleven subjects walked on a treadmill: (1) during systematic increases in velocity (0.22-1.52 m/s), (2) with instructions to 'step to the beat' during systematic increases in metronome frequency (1-2.2 Hz), and (3) with instructions: 'move the arms and legs to the beat' during systematic increases in metronome frequency (1-2.2 Hz). Movement amplitude of upper and lower body segments, frequency coordination between arm and leg movements, phase relation between upper and lower body segments were measured. Moving the arms and legs to the beat resulted in increased arm swing along with 1:1 frequency coordination between the arm and leg, and a more out-of-phase relation between transverse pelvic and thoracic rotation was observed with larger pelvic and thoracic rotations. Verbal instructions to move the arms to the beat of a metronome leads to increased arm swing, increased stride length, but further study is needed to examine the dynamics of the changes in arm movement, to enhance understanding of how upper extremity movement dysfunction affects inter-segmental coordination during walking.

Adolescent↗

PubMed2006

Walking speed influences on gait cycle variability.

The purpose of this study was to investigate the influence of walking speed on the amount and structure of the stride-to-stride fluctuations of the gait cycle. Based on previous findings for both walking [Hausdorff JM, Purdon PL, Peng CK, Ladin Z, Wei JY, Goldberger AL. Fractal dynamics of human gait: stability of long-range correlations in stride interval fluctuations. J Appl Physiol 1996;80:1448-57], and running [Jordan K, Challis JH, Newell KM. Long range correlations in the stride interval of running. Gait Posture 2006;24:120-5] it was hypothesized that the fractal nature of human locomotion is a reflection of the attractor dynamics of human locomotion. Female participants walked for 12min trials at 80%, 90%, 100%, 110% and 120% of their preferred walking speed. Eight gait cycle variables were investigated: stride interval and length, step interval and length, and from the vertical ground reaction force profile the impulse, first and second peak forces, and the trough force. Detrended fluctuation analysis (DFA) revealed the presence of long range correlations in all gait cycle variables investigated. Speed related U-shaped functions occurred in five of the eight variables, with the minima of these curves falling between 100% and 110% of the preferred walking speed. These findings are consistent with those previously shown in running studies and support the hypothesis that reduced strength of long range correlations at preferred locomotion speeds is reflective of enhanced stability and adaptability at these speeds.

Adult↗

PubMed2006

Longitudinal expressions of infant's prehension as a function of object properties.

This longitudinal study examined the prehensile development of infants (9-37 weeks) under different task constraints (object shape, size and texture). At 9 weeks of age, the infants did not reach or make contact with the objects, but all 10 infants showed goal directed prehensile movement by about 17 weeks. As they continued to age the infants further differentiated an adaptive prehensile grip configuration to the object constraints. The pattern of longitudinal findings provides further evidence that: (1) the constraints of object properties play an important role in channeling the expressions of infant's prehensile functioning and (2) the classic observation by Halverson [Halverson, H. M. (1931). An experimental study of prehension in infants by means of systematic cinema records. Genetic Psychology Monographs, 10, 107-283] of an apparent order to the development of the fundamental prehensile sequence is task dependent.

Child Development↗

PubMed2006

Age-related changes in the frequency profile of children's finger tremor.

Little consensus exists as to the age-related pattern of change in the frequency characteristics of postural tremor through childhood. We investigated postural finger tremor of children (6 and 10 years) and adults (18-22 years) using accelerometers under dual and single limb conditions (10s trials). The postural tremor of the children exhibited proportionally more power below 10 Hz and less power above 20 Hz than that of the adults. It also showed a significantly lower peak frequency and lower proportion of power at the peak frequency than the adults in the 15-30 Hz frequency band but did not differ significantly from the adults in peak frequency or proportion of power at the peak frequency in the 5-15 Hz frequency band. The greater relative contribution of fast time scales over the 1-30 Hz frequency band in the organization of the postural tremor of the adults in comparison to the children may be a contributing factor to adult's typically observed reduced motor skill performance variability.

Adolescent↗

PubMed2006

Visual angle is the critical variable mediating gain-related effects in manual control.

Theoretically visual gain has been identified as a control variable in models of isometric force. However, visual gain is typically confounded with visual angle and distance, and the relative contribution of visual gain, distance, and angle to the control of force remains unclear. This study manipulated visual gain, distance, and angle in three experiments to examine the visual information properties used to regulate the control of a constant level of isometric force. Young adults performed a flexion motion of the index finger of the dominant hand in 20 s trials under a range of parameter values of the three visual variables. The findings demonstrate that the amount and structure of the force fluctuations were organized around the variable of visual angle, rather than gain or distance. Furthermore, the amount and structure of the force fluctuations changed considerably up to 1 degrees , with little change higher than a 1 degrees visual angle. Visual angle is the critical informational variable for the visuomotor system during the control of isometric force.

Accommodation, Ocular↗

PubMed2006

Are age-related increases in force variability due to decrements in strength?

The purpose of this investigation was to examine the relationship between strength and the magnitude and time sequential structure of force variability. Young and old adults produced isometric force via index finger abduction to a visually presented target corresponding to a constant force level of 5 or 25% maximal voluntary contraction (MVC). Cluster analysis was used to divide subjects into groups based on age and strength. The variability of older adults was greater and showed more time dependent structure than their younger counterparts. The force output of weaker subjects was also more variable and had a stronger sequential structure. Indeed, when MVC was controlled for there was no significant age effect on force variability. The relationship between strength and variability remained significant, however, when chronological age was controlled for. The findings revealed that the established age-related changes in force variability are more fundamentally due to the association between strength and force variability and provide a further challenge to using chronological age as a marker of the biological aging process in studies of motor control.

Adult↗

PubMed2006

Qualitative and quantitative change in the dynamics of motor learning.

The experiments examined qualitative and quantitative changes in the dynamics of learning a novel motor skill (roller ball task) as a function of the manipulation of a control parameter (initial ball speed). The focus was on the relation between the rates of change in performance over practice time and the changing time scales of the evolving attractor dynamic. Results showed 3 different learning patterns to the changes in the dynamics as a function of practice that were mediated by the initial ball speed. Only participants who learned the task showed a bifurcation in coordination mode that was preceded by enhanced performance variability. The observed multiple time scales to motor learning are interpreted as the products of the dynamical stability and instability realized from (a) the continually evolving landscape dynamics due to bifurcations between attractor organization and (b) the transient phenomena associated with moving toward and away from fixed-point dynamics.

Adult↗

PubMed2006

Power-law scaling for macroscopic entropy and microscopic complexity: evidence from human movement and posture.

We investigated the relationship between macroscopic entropy and microscopic complexity of the dynamics of body rocking and sitting still across adults with stereotyped movement disorder and mental retardation (profound and severe) against controls matched for age, height, and weight. This analysis was performed through the examination of center of pressure (COP) motion on the mediolateral (side-to-side) and anteroposterior (fore-aft) dimensions and the entropy of the relative phase between the two dimensions of motion. Intentional body rocking and stereotypical body rocking possessed similar slopes for their respective frequency spectra, but differences were revealed during maintenance of sitting postures. The dynamics of sitting in the control group produced lower spectral slopes and higher complexity (approximate entropy). In the controls, the higher complexity found on each dimension of motion was related to a weaker coupling between dimensions. Information entropy of the relative phase between the two dimensions of COP motion and irregularity (complexity) of their respective motions fitted a power-law function, revealing a relationship between macroscopic entropy and microscopic complexity across both groups and behaviors. This power-law relation affords the postulation that the organization of movement and posture dynamics occurs as a fractal process.

Adult↗

PubMed2006

Aging, visual intermittency, and variability in isometric force output.

We tested the hypothesis that increases in minimal visual motor processing (VMP) time in older adults contribute to age-related increases in force variability. We manipulated the intermittency rate of visual information feedback over a 100-fold range as young (20-29 years old) and old (60-79 years old) participants produced isometric force output to a visually presented target. The force output of the old adults was more variable and more structured, and the old adults had an increase in minimal VMP time compared with the young adults. However, there was no significant relation between VMP time and force variability. We propose that the age-related changes in variability are a reflection of information-processing capacity limitations and not a decrement in minimal VMP time.

Adult↗

PubMed2006

The generalization of perceptual-motor intra-individual variability in young and old adults.

We examined the generalizability of intra-individual perceptual-motor variability in young and old adults. Participants completed four motor tasks that all required the maintenance of a steady state: finger tremor, and single-digit, two-digit, and three-digit isometric force production. Older adults had a greater amount of variability and an increased time and frequency structure of variability, except in the tremor task. There was a moderate association between the magnitudes of variability across tasks in the oldest age group, but the structural measures of variability were more highly related across tasks and the strength of this relationship increased with age. These findings support the hypothesis that older adults are less able than younger adults to adapt the structure of their perceptual-motor variability to task demands.

Adaptation, Psychological↗

PubMed2006

Information entropy and the variability of space-time movement error.

The authors investigated the effects of movement time and movement distance on the information entropy and variability of spatial and temporal error in a discrete aiming movement. In Experiment 1, the authors held movement distance (100 mm) constant and manipulated 11 movement times (300-800 ms) of 8 participants. In Experiment 2, the authors tested 6 movement distances at 2 given movement times (15-60 mm at 300 ms; 40-240 mm at 800 ms) in 8 participants. The variability and entropy for spatial error increased with average movement velocity, whereas the variability and entropy for temporal error decreased as a function of average movement velocity. The common variance between variable error and entropy averaged about 84% and 72% for spatial and temporal errors, respectively, suggesting that the probabilistic approach of entropy reveals features that are not present in the standard deviation index of variability. The findings provide further evidence that information entropy may be a useful single-index representation of variability in the movement speed-accuracy relation.

Adult↗

PubMed2005

Information processing limitations with aging in the visual scaling of isometric force.

The experiment examined if age-related increases in force variability were due to decreases in visual acuity and/or visual-motor information processing deficits. Visual information scale was manipulated over a 250-fold range as young (20-29 years old) and old (60-79 years old) participants produced isometric force output to a visually presented target. Older adults were found to have a very small decrement in visual acuity, but there was no relation between visual acuity and force variability. Force variability exhibited a U-shaped trend as a function of visual information scale. Young adults had less relative variability and higher visual information transfer than the oldest old and these age differences increased with visual information scale. It is concluded that the age-related declines in visual-motor information processing influence changes in neuromuscular function and the emergent differences in force variability at the behavioral level.

Adult↗

PubMed2005

Independence between the amount and structure of variability at low force levels.

The purpose of the current experiment was to investigate the amount (standard deviation (S.D.) and coefficient of variation (CV)) and structure (approximate entropy (ApEn)) of force variability at very low force levels. Participants produced isometric force output of index finger abduction at five levels (0.4, 0.8, 1.0, 2.0, and 4.0 N) with high and low visual feedback gain. The findings showed that: subjects scaled their force output to the targets; S.D. increased non-linearly with force level and decreased with visual gain; and CV decreased with force level as well as visual gain. ApEn of the force output did not change as a function of force level, although the high gain increased ApEn in contrast to low gain. It is proposed that the recruitment of additional motor units at very low force levels does not significantly alter the structure of the force output, although it does increase the magnitude of force and its amount of variability. Overall, the findings provide evidence that the amount and structure of motor variability can be influenced by separate control processes at low force levels.

Adult↗