Development of Postural Adjustment During Gait Initiation: Kinematic and EMG Analysis

The authors studied the development of postural adjustments associated with the initiation of gait in children by using kinematic and electromyographic (EMG) analysis. Participants (N = 28) included infants with 1-4 and 9-17 months of walking experience, children 4-5 years of age, and adults. Anticipatory postural adjustments (APA) were present in the youngest age groups, including a clear anticipatory lateral tilt of the pelvis and the stance leg, which enabled the child to unload the opposite leg shortly before its swing phase. An anticipatory activation of the hip abductor of the leg in stance phase prior to heel-off was found, suggesting pelvis stabilization. APA did not appear consistently until 4-5 years of age. A decrease in segmental oscillations occurred across the ages, indicating better control of intersegmental coordination in the frontal and sagittal planes during the postural phase of gait initiation. Young walkers presented APA involving movements of both the upper and the lower parts of the body, whereas, like adults, 4- to 5-year-olds were able to laterally shift only the pelvis and the stance leg. The oldest children and the adults also showed lower activation levels of hip and knee muscles but higher activation at the ankle level. Those kinematic and EMG results taken together suggest a clear developmental sequence from an en bloc operation of the body through an articulated operation with maturation, walking experience, or both.     

Effects of methylphenidate on kinematic aspects of handwriting in hyperactive boys

Poor handwriting in hyperactive children often contributes to academic failure. Beneficial effects of methylphenidate on the quality of handwriting have been shown. Using a digitizing tablet, the handwriting of 21 hyperactive boys was examined both during methylphenidate treatment and following withdrawal of the drug. Half of the hyperactive boys were tested first on methylphenidate and then following withdrawal of the drug and the remaining hyperactive boys were examined in the reverse order. Twenty-one control boys underwent the same examination. Velocity and acceleration of handwriting movements were measured. Furthermore, every writing specimen was independently rated by four examiners regarding the quality of handwriting. Following withdrawal of the drug, the quality of handwriting specimens of hyperactive boys was poorer than during treatment with methylphenidate. Statistical comparison of writing movements of hyperactive boys on and off methylphenidate revealed that the medication resulted in a deterioration in handwriting fluency. The results showed that following withdrawal of medication, hyperactive children did not differ from control boys in handwriting movements. The improvement in hyperactive behavior through methylphenidate was associated with increased legibility and greater accuracy of handwriting. The intention to write neatly may interfere with the fluent writing process.     

What COP and Kinematic Parameters Better Characterize Postural Control in Standing Balance Tasks?

The authors’ aim was to determine which variables allow for the characterization of motor balance behavior. Traditional measures and nonlinear measures of center of pressure (COP; n = 30) and kinematics (n = 10) were tested in their absolute and relative consistency in a 30-s standing balance task protocol under stable and unstable conditions. Regarding COP variables, mean velocity (mVel), permutation entropy (PE) and detrended fluctuation analysis (DFA) exhibited high consistency between trials and ranked individuals more accurately compare with other metrics. In the kinematic signal mVel, PE and DFA had good intrasession reliability values in unstable conditions. Overall, the intrasession reliability values were better in the unstable condition than in the stable condition and the measures calculated using derived data had better intrasession reliability values. In conclusion, mVel, PE, and DFA allow for the good characterization of motor balance behavior in a simplified protocol where velocity time series are analyzed.     

Changes in Movement Variables Associated With Transient Overshoot of the Final Position

Transient overshoot (TO), which is assessed as the distance between the movement amplitude and the final position, was measured in a series of rapid, discrete elbow flexion movements performed under different distance and loading conditions by 7 participants. A positive relationship was found between kinematic variables (peak velocity, peak acceleration and deceleration, and the symmetry ratio) and the magnitude of TO, particularly in short movements performed against a light load. The relationships between TO and electromyographic (EMG) variables were low and mainly insignificant. Thus, TO contributes to the variability of rapid, discrete movements and therefore should be taken into account as an additional parameter in studies of the scaling of movement variables with movement mechanical conditions. TO could also represent a consequence of mechanical properties of the single-joint system rather than an independently programmed primary submovement.     

Characteristics of the Force Applied to a Pedal During Human Pushing Efforts: Emergent Linearity

The force seated humans exert on a translationally fixed pedal (foot force) may be directed at any angle because the fixed distance between the seat and the pedal axis kinematically constrains the lower limb. The authors' objective in the present work was to characterize such force. Participants (N = 7) generated force with their lower limb by pushing against the pedal in the most comfortable manner. Pushing efforts were repeated randomly 3 times at each of 97 sagittal-plane pedal axis positions and 10 additional times in 9 of those positions (2,895 total pushes). In 87% of the pushes, the measured sagittal-plane force exerted on the pedal by the foot changed magnitude and direction through time, such that the path of the head of the force vector traced a straight line. The linearity of the foot force paths reflected directional invariance in the changes of the foot force vector as the magnitude of the vector increased. The orientation of those linear force paths varied with limb posture in a similar manner across participants. The authors conclude mat the emergent linearity of the force path is consistent with minimization of path length in foot force space. Alternatively, the linearity of the force paths suggests a motor control strategy that simplifies the control to a monoparametric form.     

An Outline of a Theory of Action Systems

It is argued that the traditional psychological construal of the distinction between central (“open loop”) and peripheral (“closed loop”) neural processes is no longer tenable. A review of the relevant psychobiological research on the control of movement shows that both the central-peripheral dichotomy and the distinction between sensory and motor systems derived from that dichotomy are incompatible with what is known about the processes underlying action. Based on experimental and comparative data, a new theory of action is proposed. The basic concept of this new theory is the action system: Unlike motor systems, action systems involve sensory as well as motor processes; action systems are not organized into response hierarchies, but rather in coalitional structures of adjustable movements and postures. The phylogeny of action systems is discussed, and the eight most important action systems are distinguished in terms of their evolved distinctive functions. The difficulties of developing a taxonomy of actions for functionally specific types of behavior made up of units that can be controlled is resolved by showing how kinds of action are differentiated by the kinds of affordances their components help to realize.     

The role of kinematic mental simulation in creating false memories

Our investigation focuses on memory for scenes depicted in photos. According to the mental model theory, the observation of a static scene depicted in a photo which portrays an actor near to perform an action can trigger a kinematic mental simulation of that action unfolding in time. The deriving prediction is that such a kinematic mental model supports the creation of a false memory of the actor performing an advanced phase of the action. We test this prediction in three experiments in which participants are presented with static scenes of actions, and after three days they perform a recognition task in which they assess recognition of old and novel static scenes depicting actions. The results confirm that false memories occur for actions that represent the unfolding over time of the static action initially observed. Our theoretical framework can accommodate also several previous findings in the literature on false memories.     

Yielding and strain hardening in aluminium single-crystal foils subjected to tension and bending

A series of tensile and microbend tests were conducted on aluminium single-crystal foil specimens with different thicknesses ranging from 20 to 90?µm. Two different crystal orientations relative to the tensile direction were considered in the tests: one is an orientation that was excepted to activate at least four slip systems on four different slip planes, and the other is an orientation that was expected to activate only a single-slip system. In the tensile tests, typical size dependence of initial yield strength was observed. The microbend test method was extended to include a reversal of bending direction. Using the curves of bending moment normalized by the square of foil thickness versus surface strain, which were obtained from the reversed bending tests, the total amounts of strain hardening were divided into isotropic and kinematic hardening components. In the microbend tests, a pronounced size-dependent kinematic hardening behaviour was clearly observed; meanwhile, the amounts of isotropic hardening were very small, particularly for the single-slip orientation.     

Children's kinematic false memories

ABSTRACT

Individuals misrecognise as seen the never-presented natural continuation of an action. These false memories derive from the running of kinematic mental models of the actions seen, which rest on motor inferences from implicit knowledge. We verified an implied prediction: kinematic false memories should be detectable even in children. The participants in our experiments first observed photos in which actors were about to perform actions on objects. At recognition they were presented with the original photos, plus (a) distractors representing the unseen natural continuation of the original actions, (b) distractors representing the beginning of other actions on the same objects and (c) distractors representing completed different actions on the same objects. In contrast to the original studies in which participants expressed their confidence in recognition, in our experiments the participants catgorirzed the action as seen or not seen. After replicating the original results with the dichotomous recognition task (Experiment 1), we detected spontaneous false memories also in children (Experiment 2).