The muscle activity paradox during circular rhythmic leg movements

A cyclist's legs make a simple 360 degrees circular and rhythmic movement, activated by a simple flexion-extension function in a sagittal plane. However, because of the simultaneous combination of leg rotation in the hip, knee and ankle joint with translation of the upper body, the general motion becomes quite complex. This complexity is increased by the anatomical interpretations of EMG readings taken during the pedalling cycle, indicating a high activity of 'flexor' muscles during the downward 'extension' of the leg (0-90 degrees propulsion phase of the pedalling cycle). This calls for an anatomical paradox. In order to verify these interpretations, the activity of six lower limb muscles was measured under field circumstances on nine elite cyclists using a portable EMG data acquisition system and active surface electrodes allowing remote (non-telemetric) monitoring of the cyclists' muscle activity patterns. Measurements were made during a 1000 m submaximal but constant effort and during a 200 m sprint. Confirmation of the anatomical paradox was found in both test circumstances. Analyses of the normalized EMG in combination with torque values of both hip and knee during the pedalling cycle indicate a zero torque at 135 degrees for the knee, while at this same angle the overall extensor activity ends in one leg and starts simultaneously in the other leg (at 315 degrees). Since the propulsion does not continue until 180 degrees, the flexor muscles have to be activated before the extension activity ends in order to generate the continuation of the circular motion until (and beyond) the bottom dead centre (180 degrees).(ABSTRACT TRUNCATED AT 250 WORDS)     

The psychophysics of remembered duration

Pigeons were trained on a psychophysical choice task to make one response after a 2-sec signal and a different response after a 10-sec signal. Delayed dimensional control was assessed by presenting durations intermediate to the short and long signals and by introducing delays between the signals and choice opportunities. In Experiment 1, choices after intermediate durations were not reinforced; in Experiment 2, one choice was reinforced after the three shortest durations and another was reinforced after the three longest durations. In Experiment 1, the slopes of the psychophysical functions decreased with increases in delays, but the decrease in stimulus control was not unbiased; choice probabilities decreased for longer durations, but did not increase for shorter durations. Experiment 2 revealed the same generalized loss of stimulus control on the temporal dimension, but not the same pattern of bias; temporal control was relinquished equally for shorter and longer durations. These results are evaluated in the context of the subjective shortening model of remembered duration (Spetch & Wilkie, 1983) and Staddon’s theory of timing and remembering (Staddon, 1984).     

3. Christoph Butterwegge/Gudrun Hentges (Hrsg.): Politische Bildung und Globalisierung

Ohne Zusammenfassung     

The processes involved in the solid-to-liquid phase transition

The detailed study of crystals and the dynamics of the melting process in two and three dimensions is a fundamental and interesting research topic, which is important for increasing our knowledge of solid state physics. In natural crystals, structure information can be obtained principally by Bragg-scattering of neutrons, electrons or photons on the crystal, followed by an analysis in Fourier space. Dynamical aspects cannot be investigated in these systems. Recently, a new crystalline system was discovered whose properties are such that the melting transition can be investigated in great detail — the ‘plasma crystal’. This article presents the results of such an investigation and shows evidence for the existence of intermediate phases between the solid, liquid and gaseous phases. The observed ‘structured’ phase transition may be specific for plasma crystals but, alternatively, it may indicate the existence of intermediate stages in the melting transition more generally.     

How reinforcement context affects temporal production and categorization

The behavioral theory of timing assumes that timing is governed by a pacemaker whose pulses move organisms from one state to the next, and that the speed of the pacemaker covaries with the rate of reinforcement in the experimental context. The goal of the present experiments was to clarify just what constitutes that context. In Experiment 1, pigeons responded on signaled fixed-interval 20-sec and 40-sec schedules of food reinforcement that were presented randomly within sessions (alternating condition) or between sessions (isolated condition). In Experiment 2, pigeons categorized the duration of a short or a long set of intervals in the alternating or the isolated condition. Performance in both experiments was under strong control by the signals, with scalar timing between long and short sets, but no significant differences between the alternating and isolated conditions. The context of reinforcement that determines pacemaker period can thus be specific to a particular timing task and signal.     

On discriminating temporal relations: Is it relational?

Pigeons were presented on each trial with a pair of keylight stimuli that varied in duration. One of two subsequent choices was reinforced, depending on which of the two stimuli was longer. For some pairs, the duration of one stimulus was predictive of relative duration, but for other pairs, absolute duration was unpredictive. Choice responses depended on relative differences between the stimuli, but were also controlled to some degree by absolute duration of the second member of the pair. Individual differences in control by absolute and relative duration were evident. Those pigeons whose behavior was most influenced by absolute duration showed poorer transfer to a different set of duration pairs.     

Same-different comparison of duration

Pigeons were trained on a task in which a red light of durationt ₁ was followed by a green light of durationt ₂ and then responses to different keys were reinforced according to whether the durations of the stimuli were the same or different. For Experiment 1, duration pairs consisted of all combinations of 1, 2, 4, and 8 sec. In Experiment 2,different-duration pairs included only combinations witht ₁ >t ₂ and, in addition, 2 subjects with extended training involving lesser-greater duration comparisons were transferred to the same-different task. Two of 3 subjects learned the task in Experiment 1 and analyses suggested that choices were based on specific instances, not on a temporal same-different rule. All 5 birds acquired the discrimination in Experiment 2, where it appeared that choices were controlled by a combination of relative and absolute rules. Accuracy decreased following transfer from a lesser-greater to a same-different discrimination, but performance was above chance on the first transfer session. In both experiments, however, accuracy was below that found in earlier work with lesser-greater comparisons of duration. These findings are discussed in relation to prior research with lesser-greater comparisons of duration and same-different tasks involving nontemporal stimuli.     

Toddlers Prefer Adults as Informants: 2- and 3-Year-Olds’ Use of and Attention to Pointing Gestures From Peer and Adult Partners

Two- and 3-year-old children (N = 96) were tested in an object-choice task with video presentations of peer and adult partners. An immersive, semi-interactive procedure enabled both the close matching of adult and peer conditions and the combination of participants’ choice behavior with looking time measures. Children were more likely to use information provided by adults. As the effect was more pronounced in the younger age-group, the observed bias may fade during toddlerhood. As there were no differences in children’s propensity to follow peer and adult gestures with their gaze, these findings provide some of the earliest evidence to date that young children take an interlocutor’s age into account when judging ostensively communicated testimony.     

The elusive experience of ‘flow’: Qualitative and quantitative indicators

In this investigation mixed methods were used to examine flow within the context of a writing task. Forty-four secondary students aged between 15 and 18 years completed a computer-based writing task requiring them to examine information and then write on one of two topical issues. We examined the level of agreement between end-of-task ratings based on two approaches to measuring flow, the four-channel model ratio of challenge and skill, and a classification based on ratings of absorption, timelessness and effort. Flow groups that emerged were significantly associated but not entirely consistent. A range of additional data sources concerning students’ on-task experiences illustrated different patterns of task engagement. The variability between indicators suggests that flow cannot be reliably identified through any narrow set of measures. Just as the flow experience is complex so a range of indicators is needed to identify when and how students experience flow in their learning.