46 research outputs found
The effects of different speed skating push-off techniques on the mechanical power, power distribution and energy expenditure
No full textThe technique of speed skating is unique in comparison to other sports that require human propulsion. Skaters generate a forward velocity by pushing off sideways. The ideal push-off technique is a trade-off: a more sideward directed push-off facilitates power production, but a more forward directed push-off increases the transfers rate of push-off power into a forward velocity. The exact trade-off for the ideal push-off technique is unknown. Insight in the distribution of power in more than one direction and into the energy expenditure during different push-off techniques helps a speed skater in improving his or her push-off technique to the ideal push-off technique. This study quantifies the effect of different push-off techniques on the mechanical power, power distribution and energy expenditure. The study was limited to a group analysis of three push-off techniques: the small, self-chosen and wide push-off technique. A three-dimensional power balance model was used to calculate the mechanical power in forward, sideward and upward components during speed skating. This model was driven by velocity and acceleration data, obtained by two synchronized measurement systems, and estimations of air and ice friction coefficients. The acceleration was measured by an Xsens MTI (Xsens) device and the velocity was calculated by fusing position (Local Position Measurement (LPM), Inmotio) and acceleration measurements (Xsens). The subject specific air and ice coefficients were estimated with position measurements (LPM) during gliding experiments. The mechanical power results were limited to the average mechanical powers of one representative stroke-cycle of skater and technique. The energy expenditure was estimated with steady state heart rate measurements (Polar) during speed skating. This study proved a significant difference in forward power component, sideward power component and the total mechanical power between the push-off techniques studied, as well as the energy expenditure between the push-off techniques. The sideward power increased from small to self-chosen to wide push-off technique. In addition, this study showed that the change in total mechanical power was mainly due to the change in the sideward power component and that the energy expenditure in the self-chosen technique was the lowest. The relative mechanical efficiency, the ratio between total mechanical power and steady state heart rate, was significantly different for the three different push-off techniques. In addition, the relative mechanical efficiency increased from small to selfchosen to wide push-off technique. In summary, of the three push-off technique was the self-chosen push-off technique the most energy efficient push-off technique; this technique required the lowest energy expenditure for the required forward velocity. However, this technique was not the most mechanical efficient. The total mechanical power of the wide push-off technique was generated most mechanical efficient. This is because the more sideward push-off will make the push-off velocity less independent of the moving velocity and can be freely chosen to the optimal leg extension velocity, but introduces more ’wasted power’ in non-effective movements for the performance. The selected measurement method of this study can improve the current determination of the ideal push-off technique: analyzing the important trade-off between the mechanical efficiency and push-off orientation. This method be used to identify small changes between push-off technique in the mechanical efficiency and push-off orientation, and thus be used to improve speed skating performance. However, the measurement accuracy of this method requires to be improved further.BMEBioMechanical EngineeringMechanical, Maritime and Materials Engineerin
Continuously distributed sensing via two photon excited fluorescence in doped optical fibre
No full textThe design and operating parameters of a new class of continuously distributed optical fibre sensor are described. Using counter-propagating pulses, two-photon excitation of fluorescence from ions doped into the fibre enables any position to be monitored. By this means temperature or strain may be sensed with high spatial and temporal resolution. As the doped fibre is transparent for single photon absorption at the wavelength of the light pulses, attenuation does not set an upper limit to its length
Growth and spectroscopic properties of Cr3+-doped LaSc3(BO3)(4)
No full textThis paper reports the growth and optical properties of Cr3+-doped LaSc3(BO3)(4) (Cr3+ :LSB). The Cr3+ -doped LaSc3(BO3)(4) crystal with dimension null set 25 x 35 mm(3) was obtained by Czochralski method. The absorption and photoluminescence spectra have been measured. The dominant features of absorption spectrum are two broadbands at ca. 400-700 nm. A broad emission band extending from 740 to 1280 nm with a peak at near 960 nm was observed. The lifetime of Cr3+ :LSB was 15 mus. (C) 2002 Elsevier Science B.V. All rights reserved
Crystal Growth and Optical Properties of Potassium Lithium Niobate for Frequency Doubling of Diode Lasers
No full text
Optical study of Cr3+-doped LaSc3(BO3)(4) crystal
No full textThis paper reports on the optical properties of Cr3+-doped LaSc3(BO3)(4) (Cr3+:LSB). Based on measurement of the absorption spectrum the crystal field strength Dq, the Racah parameters B and C were calculated. The photoluminescence spectrum of Cr3+:LSB via T-4(2) -->(4)A(2) transition is a broadband emission from 740 to 1280 nm at room temperature. The measurements of absorption and photoluminescence spectra show that in Cr3+:LSB the Cr3+ ions occupy weak crystal field sites
The Growth and Characterisation of Stoichiometric LiNbO/sub 3/ Crystals for Second Harmonic Generation of 1.06 μm Radiation
No full textNear-infrared laser crystals based on 3d2 ions Spectroscopic studies of 3d2 ions in oxide, melilite and apatite crystals
No full textThe role of the Mg2+ ions in Cr3+ spectroscopy for near-stoichiometric LiNbO3 crystals
No full textThe optical spectroscopy of Cr3+ ions doped into near-stoichiometric LiNbO3 crystals, pure and co-doped with MgO, has been investigated. In the near-stoichiometric LiNbO3:Cr(0.2 mol%):Mg(2 mol%) crystal, the optical spectra resemble those previously observed for congruent LiNbO3:Cr:MgO samples when the total MgO content exceeds the 4.6 mol% threshold. The coexistence of two types of Cr3+ centre ([Cr]Li and [Cr]Nb) characterized the optical and luminescence spectra of this sample. The concentration equilibrium between the two types of centre is strongly displaced towards the [Cr3+]Nb centre, permitting us to obtain with accuracy the parameters of the broad bands. The R-line associated with the [Cr]Nb centre is only observable in the low-temperature emission spectrum. The Fano antiresonance lines present have been observed to be more pronounced for the near-stoichiometric samples than for congruent ones
