Search Results

You are looking at 101 - 110 of 3,147 items for :

Clear All
Restricted access

Todd L. Allinger and Robert W. Motl

This study used a vertical jump model to simulate the push-off phase for a skater using klap speed skates and evaluated die effects of pivot location and shoe base flexion on energy production. Boards of different lengths and one board with a hinge under the metatarsal heads were attached to the running shoes of volunteers. Six skaters performed 3 maximal effort vertical jumps across 5 different base conditions: running shoe, board that hinged under metatarsal heads, and rigid boards that pivoted with the ground al -25 mm (typical pivot location for klapskales), 0 mm, and +25 mm from the toes. There were no significant differences in total energy at take-off among the 3 rigid base conditions, but there were differences in potential and kinetic energy production. The total and kinetic energy produced at take-off was 9% greater in the hinged base condition than the corresponding rigid base condition. If differences in energy measures from the vertical jump reflect those for skating, a hinged boot base could increase skating speeds by about 3% over the current klap-skales, which have a rigid boot base.

Restricted access

Marcin Baranowski, Jan Górski, Barbara Klapcinska, Zbigniew Waskiewicz and Ewa Sadowska-Krepa

We have previously shown that acute exercise increases the level of sphingosine-1-phosphate (S1P) in plasma and ceramide in erythrocytes of untrained subjects. The aim of the current study was to examine the effect of ultramarathon run on the plasma and erythrocyte levels of the following bioactive sphingolipids: S1P, sphinganine-1-phosphate (SA1P), sphingosine, sphinganine, and ceramide. Blood samples were collected from seven male amateur runners participating in a 48-hr ultramarathon race before the run, after 24 and 48 hr of running, and following 24 and 48 hr of recovery. The sphingolipids were quantified by means of HPLC. Sustained running for 48 hr resulted in a progressive decline in plasma S1P to a level significantly lower than at prerace, and then remained stable over the next 48 hr of recovery. In erythrocytes, S1P content was stable until 24 hr of recovery, then rose abruptly to reach peak values after 48 hr of recovery. The plasma level of SA1P decreased progressively during the competition and remained unchanged over the recovery. In erythrocytes, the level of SA1P increased after 24 hr running and normalized thereafter. The level of ceramide, both in plasma and erythrocytes, was not significantly affected by the ultraendurance run. We speculate that reduction in plasma level of S1P during and after the run reduces its biological actions and might be responsible for some negative side-effects of the ultraendurance effort.

Restricted access

Jinsung Wang and George E. Stelmach

Smeets and Brenner propose a model that attempts to account for the action patterns involved in prehensile behaviors. However, the model does not provide a full account of the available data on temporal and spatial relationships between the transport and grasp components. Predictions from the model in its current form appear to correspond only to experimental results in a very general way.

Restricted access

Slobodan Jaric, Charli Tortoza, Ismael F.C. Fatarelli and Gil L. Almeida

A number of studies have analyzed various indices of the final position variability in order to provide insight into different levels of neuromotor processing during reaching movements. Yet the possible effects of movement kinematics on variability have often been neglected. The present study was designed to test the effects of movement direction and curvature on the pattern of movement variable errors. Subjects performed series of reaching movements over the same distance and into the same target. However, due either to changes in starting position or to applied obstacles, the movements were performed in different directions or along the trajectories of different curvatures. The pattern of movement variable errors was assessed by means of the principal component analysis applied on the 2-D scatter of movement final positions. The orientation of these ellipses demonstrated changes associated with changes in both movement direction and curvature. However, neither movement direction nor movement curvature affected movement variable errors assessed by area of the ellipses. Therefore it was concluded that the end-point variability depends partly, but not exclusively, on movement kinematics.

Restricted access

Alexander S. Aruin

A number of factors are likely to play a major role in the process of generation of anticipatory postural adjustments (APAs). Among them are the magnitude and direction of an expected perturbation, properties of a voluntary action associated with the perturbation, and features of the postural task such as a body's configuration prior to the action. The aim of this study was to analyze the effect of body configuration on APAs. Experiments were performed on 8 healthy subjects performing fast bilateral shoulder extension movements while standing. Body configuration was modified by instructions to the subjects to stand vertically or with a forward upper body bend varying from 15 to 60°. The electrical activity of postural muscles and displacements of the center of pressure were recorded. Results indicated that APAs were modified with changes in the angular position of the upper body. Decreased anticipatory activation was seen in rectus abdominis and rectus femoris, while increased anticipatory inhibition was observed in erecteor spinae and biceps femoris across conditions with forward bend. As a result, the total anticipatory activity of muscles in a muscle pair in series with a forward bend showed only slight modulation as compared to vertical posture. These results suggest that the CNS uses reorganization of the anticipatory activity of postural muscles by compensating for the changes in APAs of individual muscles in a muscle pair in such a way that the overall anticipatory activity of the muscle pair stays unchanged. Such compensation in counteracting the expected mechanical effects of the perturbation is used to accommodate both changes in the length of postural muscles and diminished stability of the body due to forward bend.

Restricted access

Halla B. Olafsdottir, Sun Wook Kim, Vladimir M. Zatsiorsky and Mark L. Latash

We tested the ability of healthy elderly persons to use anticipatory synergy adjustments (ASAs) prior to a self-triggered perturbation of one of the fingers during a multifinger force production task. An index of a force-stabilizing synergy was computed reflecting covariation of commands to fingers. The subjects produced constant force by pressing with the four fingers of the dominant hand on force sensors against constant upwardly directed forces. The middle finger could be unloaded either by the subject pressing the trigger or unexpectedly by the experimenter. In the former condition, the synergy index showed a drop (interpreted as ASA) prior to the time of unloading. This drop started later and was smaller in magnitude as compared with ASAs reported in an earlier study of younger subjects. At the new steady state, a new sharing pattern of the force was reached. We conclude that aging is associated with a preserved ability to explore the flexibility of the mechanically redundant multifinger system but a decreased ability to use feed-forward adjustments to self-triggered perturbations. These changes may contribute to the documented drop in manual dexterity with age.

Restricted access

Anthony J. Lisi, Conor W. O’Neill, Derek P. Lindsey, Robert Cooperstein, Elaine Cooperstein and James F. Zucherman

This paper presents the first reported measurements of lumbar intervertebral disc pressure in vivo during spinal manipulation. A pressure transducer was inserted into the nucleus pulposus of one normal-appearing lumbar disc in an asymptomatic adult volunteer. Pressures were recorded during several body positions and maneuvers, then during spinal manipulation, and lastly during a repetition of the preintervention body positions. Baseline pressures in the prone and side-lying positions measured 110 kPa and 150 kPa, respectively. During the manipulation, pressure rose to a peak of 890 kPa over 250 ms. Immediately following, pressures in the prone and side-lying positions measured 150 kPa and 165 kPa, respectively. These data do not support the hypotheses that manipulation can reduce a herniation by decreasing intradiscal pressure, or cause a herniation by raising pressure to failure levels. Further work may lead to a better understanding of this treatment method.

Restricted access

Adriana M. Degani, Alessander Danna-Dos-Santos and Mark L. Latash

We tested the hypothesis that a sequence of mechanical events occurs preceding a step that scales in time and magnitude as a whole in a task-specific manner, and is a reflection of a “motor program.” Young subjects made a step under three speed instructions and four tasks: stepping straight ahead, down a stair, up a stair, and over an obstacle. Larger center-of-pressure (COP) and force adjustments in the anteriorposterior direction and smaller COP and force adjustments in the mediolateral direction were seen during stepping forward and down a stair, as compared with the tasks of stepping up a stair and over an obstacle. These differences were accentuated during stepping under the simple reaction time instruction. These results speak against the hypothesis of a single motor program that would underlie postural preparation to stepping. They are more compatible with the reference configuration hypothesis of whole-body actions.

Restricted access

Veena Iyengar, Marcio J. Santos and Alexander S. Aruin

We investigated whether slower velocity of arm movement affects grip-force generation in conditions with the finger touch provided to the wrist of the target arm. Nine subjects performed the task of lifting and transporting an object at slow, intermediate, and fast velocities with a light finger touch from the contralateral arm and without it. There was an effect of velocity of arm movement on grip-force generation in both conditions. However, when the no touch and touch trials performed with similar velocity were matched, the effect of touch on grip-force reduction was statistically significant (p < .001). The observed decrease in grip force could not be explained by slower movement execution in the touch conditions and underlines the importance of using a contralateral touch in the performance of activities of daily living. It also points to a possibility of the development of therapeutic advances for the enhancement of grip-force control in patients with neurological impairments.

Restricted access

Robert K. Jensen, Tina Treitz and Sylvie Doucet

The purpose of this study was to develop prediction equations to estimate mass, radius to the center of mass (CM), and principal moments of the segments during pregnancy. Nonlinear regression equations were determined for the lower trunk, upper trunk, and thigh. The third sampling month of a longitudinal study was used (Sample 1, n = 15). The nonlinear regressions were then used to predict segment inertias above and below the third sampling month (Sample 2, the remaining 74 measurements). For the remaining segments, body mass and segment lengths were used as predictor variables for mass, radius to CM, and radius of gyration about the centroidal axes. The remaining seven segments did not change substantially during pregnancy, and the means of the repeated measures were used for the simple linear regressions. Eighteen of the 28 regressions and all of the CM regressions were significant. With pregnant subjects it is recommended that these regressions be used if application of the elliptical cylinder model is not possible.