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Karl M. Newell

In this paper I discuss briefly some traditional and contemporary issues that challenge the academic structure of the field of Kinesiology. These include the long-standing polemics of the profession-discipline debate and the fragmentation of the academic content knowledge, together with the more recent challenges of education or health as the umbrella construct and the relation of kinesiology to physical and occupational therapy. It appears that the essence of our persistent problems remains, but it is augmented with related and more contemporary issues. Thus, these continue to be challenging times in kinesiology, as they are for higher education in general, reinforcing the long-held notion that change is the one constant.

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Karl M. Newell

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Xiaogang Hu and Karl M. Newell

The purpose of this study was to investigate the mechanisms contributing to the different scaling functions between force and force variability in continuous and discrete isometric forces. Muscle forces were simulated with the Fuglevand et al. (1993) model of motor unit recruitment and rate coding, and a range of recruitment and firing properties were manipulated. The influence of time-to-peak force on the discrete force variability was also examined. The results revealed that the peak firing rate, the synchrony between motoneurons, and the recruitment range contributed to the different variability functions in continuous and discrete forces. The shorter time-to-peak force led to higher variability in the peak force. The findings show that the model can produce the distinct properties of the force variability scaling functions in continuous and discrete forces. The simulation results provide preliminary insight into the neuromuscular mechanisms of the different force variability functions in continuous and discrete isometric forces.

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Xiaogang Hu and Karl M. Newell

This study investigated the asymmetry of bilateral interference in relation to the relative difference of force amplitude between hands and the hand dominance. In Experiment 1, one hand produced a fixed constant force of 5% maximum voluntary contraction (MVC) while the other hand produced different constant forces of 5%, 20%, and 50% MVC in blocked conditions. Asymmetric interference in force amplitude alone was evident in that the hand producing the fixed low force showed a stronger interference than the hand performing the higher force. Asymmetric interference in hand dominance was also found in that more interference was observed when the nondominant left hand produced the higher force, a finding that does not support the hemisphere specialization hypothesis. Experiment 2 was performed to rule out the fixed force level interpretation compared with the low force level account and the fixed force was set at 50% MVC. The results were consistent with the findings in Experiment 1 showing asymmetric interference with force amplitude rather than with fixed force level. The findings revealed that without a timing constraint the task demand associated with force amplitude alone can induce the asymmetric bilateral interference. The external task asymmetry and intrinsic asymmetry of the organism interact and influence the bimanual force coordination and control patterns.

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Karl M. Newell and Steven Morrison

This paper presents a framework for an evolving dynamical landscape of movement forms and their stability over the lifespan. It is proposed that the complexity and dimensionality of movement forms can expand and contract on a number of growth/decay time scales of change including those of adaptation, development, and learning. The expansion and contraction is reflected in: (1) the range of potential movement forms of the individual in developmental time; and (2) the dimensionality and complexity of any single movement form at a moment of observation given the confluence of individual, environmental, and task constraints. It is postulated that practice, exercise, and fatigue also coalesce to change the time scales of complexity and dimension of movement forms.

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Steven Morrison and Karl M. Newell

The relation between limb stiffness and postural tremor in the upper arm was investigated during a pointing task. The task goal was to minimize the amount of motion (tremor) at the index finger under levels of increasing limb stiffness. This study investigated the influence of increasing limb stiffness on the pattern of intra- and interlimb dynamics. The frequency profile of the tremor for all limb segments across all conditions displayed two peaks, one between 2-4 Hz and another between 8-12 Hz. A third, higher frequency component (20-22 Hz) was present in the index finger. Increasing limb stiffness through voluntary co-contraction of antagonistic muscle pairs effectively constrained the segments of the upper limb to increasingly operate as a single biomechanical degree of freedom. Higher levels of limb stiffness typically led to an increase in the frequency and power of the 2-4 and 8-12 Hz peaks. There was also a decrease in the frequency of the 20-22 Hz component of finger tremor. The act of reducing the effective degrees of freedom in joint space through voluntarily stiffening of the upper limbs also resulted in decreased performance as determined by an increase in finger tremor. In the preferred, natural level of limb stiffness, specific intralimb segment relations were observed but there was no significant interlimb coupling. The intralimb segment correlations were characterized by compensatory (out of phase) coupling between the upper arm/forearm and hand/index finger segment pairs of each limb that were organized about the action of the wrist joint. Increasing the degree of limb stiffness led to a decrease in the level of intralimb coupling. The findings suggest that the most efficient mechanism for reducing tremor at the periphery is that of compensatory coupling between relevant intralimb segments with a low level of limb stiffness.

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Paola Cesari and Karl M. Newell

The experiment reported examined: (a) the role of the geometrical body scaled informational invariant for the transition of human grip configurations; (b) whether the same invariant can be scaled considering also the force applied during the grasp phase; and (c) how the temporal duration of the grasp and displacement phases of prehension are scaled to the object properties of size and mass. Adult subjects performed a series of trials in reaching, grasping, and displacing spheres that varied in size and mass. The grip transitions were described by the body scaled relation:

K=logLs+logMsa+bMh+cLh
where Ls and Ms are, respectively, the diameter and the mass of the spheres grasped and Lh and Mh are the length and the mass of the hand. The impulse during the grasp phase was linearly related with the mass of the spheres within each density. The temporal durations of the grasp and displacement components were scaled coherently to the object properties. These findings provide support to the hypothesis that the grasp and displacement components of prehension are organized coherently within a single action.

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Karl M. Newell and Paola Cesari

Smeets and Brenner provide a very clear and useful statement of the work that has been stimulated by Jeannerod's 1984 paper but seem more concerned about the viability of model fitting than model assumptions. The theoretical and practical limitations of viewing “grasping as nothing more than pointing” are noted. We reemphasize the importance in prehension of the union of the hand with the object in the act of realizing a task goal.

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Semyon M. Slobounov and Karl M. Newell

This study provides a comparative analysis of certain features of upright and inverted stance in collegiate-level competitive gymnastic and diving athletes. A particular focus was the compensatory movement strategies used to maintain inverted stance. The analyses revealed that the motion of the center of pressure was significantly greater in the hand stance as opposed to the upright stance condition. Instability increased over the duration of a 15-s hand stance trial, and it was paralleled by the introduction of a small set of compensatory movement strategies that included enhanced motion at the distal segments of the legs and at the elbow joint. The compensatory movement strategies appeared to be in support of minimizing variability of motion in the head and trunk. The relative contribution of the principal sources of this instability in the hand stance remains to be determined.

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Joseph P. Stitt and Karl M. Newell

This paper presents the stochastic modeling of isometric force variability in the steady-state time series recorded from the index finger of young adults in the act of attempting to hold different levels of constant force. The isometric force time series were examined by assuming that the stochastic (random) models were linear. System identification techniques were employed to estimate the parameters of each linear model. Once the models were parameterized, the values of the estimated parameters were compared to determine if a single linear time-invariant model was applicable across the entire isometric force range. Although the overall random models were found to be nonlinear functions of the target force level, within a fixed target level, linear modeling provided adequate estimates of the underlying processes thus enabling the use of well-known linear system identification algorithms.