1,720,981 research outputs found
Trajectories of arm pointing movements on the sagittal plane vary with both direction and speed.
No full textFive subjects performed arm upward and
downward movements at different speeds (movement
duration ranged from 0.26 to 1.2 s). Fingertip paths,
velocity profiles and muscle activation patterns of arm
and forearm were computed. Inspection of the electromyograph
(EMG) revealed that for relatively slow speeds
(>0.7 s) and for both directions, only the flexor muscles
were active, mainly the anterior deltoid, for motor
(upward) and braking action (downward) respectively.
However, where gravity was no longer sufficient to
accelerate downward and decelerate upward movements
(<0.7 s), both flexors and extensors muscles were active.
Path curvature and position of maximum deviation from
straightness were lower for downward than for upward
movements. In addition, the position of maximum
deviation from straightness became progressively higher
with increase in duration for both upward and downward
movements. The ratio of acceleration duration to total
movement duration was greater for downward than
upward directions for all the range of speeds. The ratio
of maximum to mean velocity was similar for upward and
downward movements but decreased with decrease in
speed. The results indicate that the brain accomplishes
arm movements in the vertical plane with different
planning processes for movements with or against gravity.
Furthermore, they provide evidence that both gravitational
and inertial forces are determinant for arm trajectory
generation in the vertical plan
Coordinated modulation of locomotor muscle synergies constructs straight-ahead and curvilinear walking in humans.
No full textWe describe the muscle synergies accompanying
steering of walking along curved trajectories, in order
to analyze the simultaneous control of progression
and balance-threatening emerging forces. For this purpose,
we bilaterally recorded in ten subjects the electromyograms
(EMGs) of a representative sample of leg
and trunk muscles (n=16) during continuous walking
along one straight and two curved trajectories at natural
speed. Curvilinear locomotion involved a graded, limbdependent
modulation of amplitude and timing of
activity of the muscles of the legs and trunk. The turnrelated
modulation of the motor pattern was highly
coordinated amongst muscles and body sides. For all
muscles, linear relationships were detected between the
spatial and temporal features of muscle EMG activity.
The largest modulation of EMG was observed in gastrocnemius
medialis and lateralis muscles, which showed
opposite changes in timing and amplitude during curvewalking.
Moreover, amplitude and timing characteristics
of muscle activities were significantly correlated with the
spatial and temporal gait adaptations that are associated
with curvilinear locomotion. The present results reveal
that fine-modulation of the muscle synergies underlying
straight-ahead locomotion is enough to generate the
adequate propulsive forces to steer walking and maintain
balance. These findings suggest that the turn-related
command operates by modulation of the phase relationships
between the tightly coupled neuronal assemblies
that drive motor neuron activity during walking.
This would produce the invariant templates for locomotion
kinematics that are at the base of human navigation
in space
Imagined and actual arm movements have similar durations when performed under different conditions of direction and mass.
No full textSeveral experiments have suggested that similar
physiological substrates are involved in movement
execution and motor imagery, and that the same laws of
movement control apply to both processes. Using a mental
chronometry paradigm, we examined the effects of
movement direction and added mass on the duration of
actual and imagined movements. Six subjects executed
or imagined arm movements in the sagittal and horizontal
plane, in three different loading conditions: without added mass, and with an added mass of 1 and 1.5 kg. The duration of both actual and imagined movements was measured by an electronic stopwatch. The actual movements were significantly increased in duration as a
function of mass, for both movement directions. However,
direction per se had no effect on duration. The duration
of imagined movements was very similar to that of actual movements whatever the subject and mass and direction
condition. These results show that both inertial and gravitational constraints are accurately incorporated
in the timing of the motor imagery process, which appears
therefore to be functionally very close to the process
of planning and performing the actual movement
Does order and timing in performance of imagined and actual movements affect the motor imagery process? The duration of walking and writing task
No full textThe purpose of the present study was to investigate the effects on the duration of imagined movements of changes in timing and
order of performance of actual and imagined movement. Two groups of subjects had to actually execute and imagine a walking and
a writing task. The first group first executed 10 trials of the actual movements (block A) and then imagined the same movements at
different intervals: immediately after actual movements (block I-1) and after 25 min (I-2), 50 min (I-3) and 75 min (I-4) interval. The
second group first imagined and then actually executed the tasks. The duration of actual and imagined movements, recorded by
means of an electronic stopwatch operated by the subjects, was analysed. The duration of imagined movements was very similar to
those of actual movements, for both tasks, regardless of either the interval elapsed from the actual movements (first group) or the
order of performance (second group). However, the variability of imagined movement duration was significantly increased
compared to variability of the actual movements, for both motor tasks and groups. The findings give evidence of similar cognitive
processes underlying both imagination and actual performance of movement
Weight discrimination ability during an action observation task is dependent on the type of muscle contraction
No full textConcentric and eccentric contractions show different patterns of neural activity at both peripheral and cortical levels, which are thought to influence the perception of action properties such as the weight of objects moved by others. The aim of this study was to investigate how the type of muscle contraction influences weight estimation during action observation. Forty-eight volunteers completed the Main experiment and the Control experiment. In the Main experiment, they performed a weight discrimination video task in which they watched videos of an actor moving two objects, a comparison, and a reference box, executing concentric or eccentric contractions and they had to indicate which box was the heaviest. Sensitivity analysis and psychometric functions were used to analyse the data. In the Control experiment, observers judged the actor's effort in moving the boxes. The results of the Main experiment showed that the weight discrimination sensitivity was higher in the eccentric condition for the light boxes. Conversely, for the heaviest boxes, discrimination sensitivity was higher in the concentric condition. These results were confirmed by the psychometric function analysis. The control experiment showed that the perceived difference in effort between the comparison and reference stimuli was greater in the eccentric than in the concentric condition for light stimuli. These results showed that the ability to evaluate the weight of the object involved in the observed action was influenced by the type of contraction and the amount of weight. The effort attributed to the actor influenced the observer's perception
Motor Cortical Excitability Changes in Preparation to Concentric and Eccentric Movements
No full textSpecific neural mechanisms operate at corticospinal levels during eccentric and concentric contractions. Here, we investigated the difference in corticospinal excitability (CSE) when preparing these two types of contraction. In this study we enrolled 16 healthy participants. They were asked to perform an instructed-delay reaction time (RT) task involving a concentric or an eccentric contraction of the right first dorsal interosseus muscle, as a response to a proprioceptive cue (Go signal) presented 1 s after a warning signal. We tested CSE at different time points ranging from 300 ms before up to 40 ms after a Go signal. CSE increased 300–150 ms before the Go signal for both contractions. Interestingly, significant changes in CSE in the time interval around the Go signal (from −150 ms to +40 ms) were only revealed in eccentric contraction. We observed a significant decrease in excitability immediately before the Go cue (Pre_50) and a significant increase 40 ms after it (Post_40) with respect to the MEPs recorded at Pre_150. Finally, CSE in eccentric contraction was lower before the Go cue (Pre_50) and greater after it (Post_40) compared to the concentric contraction. A similar result was also found in NoMov paradigm, used to disentangle the effects induced by movement preparation from those induced by the movement preparation linked to the proprioceptive cue. We could conclude that different neural mechanisms observed during concentric and eccentric contractions are mirrored with a different time-specific modulation of CSE in the preparatory phase to the movement
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
