Contribution of the Upper-Body in Skate Cross-Country Skiing

The skate technique in cross-country skiing has a unique gait transition. Typically, skiers will use the two-skate technique at low speeds, transition to the one-skate technique at intermediate speeds, then return to the two-skate technique at high speeds. We hypothesize that this unique gait transition can be explained by differences in the contribution of the arms to propulsion and the associated metabolic cost of upper-body and arm work. In one-skate, poles are planted simultaneously with every skate stride, while in two-skate, poles are planted with every second skate stride (Smith, 2000). Using four trained cross-country ski racers, two separate tests were performed for each technique of skate skiing. First, subjects skied at 6, 15, and 30 km/h on a rollerski treadmill. During the entire test VO2, pole force, lactate, and video were recorded for one technique and repeated on another day using the second technique. In the second phase of testing, the poling motion only was simulated on a pole ergometer with subjects matching their stroke rate and poling forces using video and force feedback. Upper-body VO2 and lactate were measured and compared to the treadmill test values. The average metabolic cost associated with the upper-body work was 60% of the total metabolic cost when skiing on the treadmill. The upper-body metabolic cost was always higher for the one skate compared to the two skate technique. At slow speeds the difference between the two techniques was small (3%), but this difference increased at higher speeds from 10% at 15km/h to 14% at 30km/h. The poling motion associated with one-skate becomes more metabolically costly than two-skate as speed increases. A skier’s regressive transition from one-skate to two-skate at high speeds may be explained by a need to transfer impulse generation to the legs, since the sliding limbs remain effective at high speeds while the fixed limbs (poles) become less effective

Mechanical Properties of the Poling Motion in Cross Country Skiing

Skate cross-country skiing is a unique gait with the skis acting as sliding limbs and poles acting as fixed limbs. As skiers increase their speed, the sliding limbs (skis) remain relatively unrestricted in their ability to generate forward impulse. The poles, however, are fixed and thus the poling action depends on the skier’s speed. Since muscles generate less force at higher shortening velocity (Hill, 1938), the arms become limited in their ability to generate force through the poles. Also, muscles ability to generate force depends on their length, or the configuration of joints the muscles cross (Gordon et al., 1966). Therefore, it might be expected that the forces of arm and trunk muscles contributing to the poling action change as a function of the poling cycle. The purpose of this study was to relate maximum isometric force of the muscles contributing to poling as a function of the poling cycle, and quantify the dynamic force of these muscles as a function of skiing speed. Maximal isometric force was measured at 11 points in the poling stride of ten nationally ranked skiers. Five of these subjects were also tested for their maximal dynamic poling force at skiing speeds ranging from 6 to 36 km/h, increasing by 6km/h increments. Maximal isometric poling force was maximal (223N) at 20% of the poling cycle. The component of the pole force in the direction of travel was highest (117N) at 30% of the poling cycle. Toward the end of the poling cycle, the propulsive force approaches the total force and the total force decreases to 50 N. The dynamic poling force was maximal for the two slowest speeds tested (236 N at 6km/h and 254 N at 12km/h), and then decreased force almost linearly with increasing speeds and reached 102 N at 36 km/h. The results of this study suggest that the propulsive forces in poling depend greatly on the position of arms and trunk and the speed of skiing

Determination of the trap-assisted recombination strength in polymer light emitting diodes

The recombination processes in poly(p-phenylene vinylene) based polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs, which has not been considered until now. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate.

Thickness dependence of the efficiency of polymer:fullerene bulk heterojunction solar cells

We study the thickness dependence of the performance of bulk heterojunction solar cells based on poly[2-methoxy-5-(3('),7(')-dimethyloctyloxy)-1,4-phenylenevinylene] as electron donor and [6,6]-phenyl C-61 butyric acid methyl ester as electron acceptor. Typically, these devices have an active layer thickness of 100 nm at which only 60% of the incoming light is absorbed. Increasing device thickness results in a lower overall power conversion efficiency, mainly due to a lowering of the fill factor. We demonstrate that the decrease in fill factor and hence device efficiency is due to a combination of charge recombination and space-charge effects

Why do early mathematics skills predict later mathematics and reading achievement? The role of executive function

A robust association between young children’s early mathematical proficiency and later academic achievement is well established. Less is known about the mechanisms through which early mathematics skills may contribute to later mathematics and especially reading achievement. Using a parallel multiple mediator model, the current study investigated whether executive function (integration of working memory, inhibition, and cognitive flexibility) can explain the relations between early mathematics skills and elementary school mathematics and reading achievement. Data in this longitudinal study were collected from 243 children during the last year of early childhood education and care (kindergarten ages 5 and 6 years), 1 year later in first grade, and 5 years later when the children were in fifth grade. Background variables (maternal education, age, sex, and immigrant status), kindergarten baseline skills, and mediating effects of first-grade mathematics, phonological awareness, vocabulary, and possible omitted variables were controlled. Results showed that first-grade executive function mediated the effects of kindergarten mathematics on fifth-grade mathematics and on reading achievement. These findings suggest that executive function may work as a mechanism that may help to explain the frequently found strong association between children’s early mathematics skills and later mathematics and reading achievement.publishedVersio

Creencias de los Hombres y Mujeres Americanos sobre la Discriminación de Género: Para los Hombres, Esto No Es Precisamente un Juego de Suma Zero

We surveyed Americans regarding their beliefs about gender discrimination over the past several decades. Men and women agreed that women faced much more discrimination than men in the past, and they agreed that the discrimination gap between men and women has narrowed in recent years. However, men perceived the gap as narrower than women did at all time periods, and reported that there is little difference today in the amount of gender discrimination women and men face. Political ideology moderated these beliefs such that conservative men were most likely to report that anti-Man bias now equals or exceeds anti-Woman bias. Similar to recent research on beliefs about racism, these findings suggest that groups which differ historically in status and power exhibit perceptual differences regarding the changing nature of discrimination.Hemos realizado encuestas a personas americanas sobre sus creencias respecto a la discriminación de género durante las últimas décadas. Tanto hombres como mujeres estaban de acuerdo en que en el pasado las mujeres han sufrido mucha más discriminación que los hombres, y también en que esta diferencia se ha ido estrechando en los últimos años. Sin embargo, a los hombres esta diferencia siempre les ha parecido más estrecha que a las mujeres durante todos los periodos, y perciben que ahora hay pocas diferencias entre la discriminación que afrontan hombres y mujeres. Las ideologías políticas moderan estas creencias, de manera que los hombres conservadores son más propensos a afirmar que el sesgo anti masculino es igual o mayor al sesgo anti femenino. Al igual que algunas investigaciones recientes sobre las creencias respecto al racismo, nuestros hallazgos sugieren que grupos que difieren históricamente en estatus y poder perciben de forma diferente la naturaleza cambiante de la discriminación

Red Tides In the Gulf of Mexico: Where, When, and Why?

Independent data from the Gulf of Mexico are used to develop and test the hypothesis that the same sequence of physical and ecological events each year allows the toxic dinoflagellate Karenia brevis to become dominant. A phosphorus-rich nutrient supply initiates phytoplankton succession, once deposition events of Saharan iron-rich dust allow Trichodesmium blooms to utilize ubiquitous dissolved nitrogen gas within otherwise nitrogen-poor sea water. They and the co-occurring K. brevis are positioned within the bottom Ekman layers, as a consequence of their similar diel vertical migration patterns on the middle shelf. Upon onshore upwelling of these near-bottom seed populations to CDOM-rich surface waters of coastal regions, light-inhibition of the small red tide of similar to 1 ug chl l(-1) of ichthytoxic K. brevis is alleviated. Thence, dead fish serve as a supplementary nutrient source, yielding large, self-shaded red tides of similar to 10 ug chl l(-1). The source of phosphorus is mainly of fossil origin off west Florida, where past nutrient additions from the eutrophied Lake Okeechobee had minimal impact. In contrast, the P-sources are of mainly anthropogenic origin off Texas, since both the nutrient loadings of Mississippi River and the spatial extent of the downstream red tides have increased over the last 100 years. During the past century and particularly within the last decade, previously cryptic Karenia spp. have caused toxic red tides in similar coastal habitats of other western boundary currents off Japan, China, New Zealand, Australia, and South Africa, downstream of the Gobi, Simpson, Great Western, and Kalahari Deserts, in a global response to both desertification and eutrophication