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Effect of fatigue on the precision of a whole-body pointing task.
No full textWe addressed the issue of the possible degradation
of the aiming precision of a whole-body pointing task,
when movement coordination is deranged by selective fatigue
of the postural task component. The protocol involved
continuous repetition (0.1 Hz frequency) of rapid whole-body
pointing trials toward a target located beyond arm length,
starting from stance and requiring knee flexion. Six healthy
human subjects repeated the trials until exhaustion. Such
repetition led to electromyography signs of fatigue in rectus
femoris (active in body lowering and raising), but not in
deltoid (prime mover for arm reaching component). Rectus
femoris fatigue affected the equilibrium control strategy,
since the anteroposterior displacement of the center of foot
pressure was reduced during the fatigued compared with the
initial trials. Conversely, the precision of the aiming movement
was unaffected by the rectus femoris fatigue in spite of
changes in finger trajectory. Trunk inclination at the end of
whole-body pointing task and hip and shoulder marker trajectories
were unaffected by rectus femoris fatigue. Control
experiments were made, whereby fatiguing repetitions of the
postural component of the task were performed without finger
pointing, except in the first and last five complete wholebody
pointing trials. The results were not different from those
of the main protocol, except for a transient change in finger
trajectory in the very first trial after fatigue. The CNS takes
into account the state of postural muscles’ fatigue and the
concurrently ensuing equilibrium constraints in order to appropriately
modify whole-body pointing strategy and keep
pointing precision at the target
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
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
Continuous leg muscle vibration is not detrimental to human walk.
No full textSensory feedback from the moving limbs contributes to the regulation of animal and human locomotion. However, the question of the specific role of the various modalities is still open. Further, functional loss of leg afferent fibres due to peripheral neuropathy does not always lead to major alteration in the gait pattern. In order to gain further insight on proprioceptive control of human gait, we applied vibratory tendon stimulation, known to recruit spindle primary afferent fibres, to both triceps surae muscles during normal floor walk. This procedure would disturb organisation and execution of walking, especially if spindles fire continuously and subjects are blindfolded. Vibration induced significant, though minor, changes in duration and length of stance and swing phase, and on speed of walking and kinematics of lower limb segments. No effect was induced on angular displacement of the ankle joint or trunk and
head kinematics. This paucity of effects was at variance with the perception of the subjects, who reported illusion of leg stiffness and gait imbalance. These findings would speak for a selective gating of Ia input during locomotion and emphasise the notion that the central nervous system can cope with an unusual continuous input along the Ia fibres from a key muscle like the soleus
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
Training the Motor Cortex by Observing the Actions of Others During Immobilization.
No full textLimb immobilization and nonuse are well-known causes of corticomotor depression. While physical training can drive the recovery from nonuse-dependent corticomotor effects, it remains unclear if it is possible to gain access to motor cortex in alternative ways, such as through motor imagery (MI) or action observation (AO). Transcranial magnetic stimulation was used to study the excitability of the hand left motor cortex in normal subjects immediately before and after 10 h of right arm immobilization. During immobilization, subjects were requested either to imagine to act with their constrained limb or to observe hand actions performed by other individuals. A third group of control subjects watched a nature documentary presented on a computer screen. Hand corticomotor maps and recruitment curves reliably showed that AO, but not MI, prevented the corticomotor depression induced by immobilization. Our results demonstrate the existence of a visuomotor mechanism in humans that links AO and execution which is able to effect cortical plasticity in a beneficial way. This facilitation was not related to the action simulation, because it was not induced by explicit MI
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