Excitated state properties of 20-chloro-chlorophyll a

The excited-state and lasing properties of 20-chloro-chlorophyll a in ether solution were compared to those of chlorophyll a. Desactivation parameters and cross-sections were obtained from non-linear absorption spectroscopy in combination with a physico-mathematical methods package. The Cl substituent at C-20 (1) increases both intersystem crossing and internal conversion, (2) produces a blue-shift of the S1 absorption spectrum, and (3) leads to pronounced photochemistry

Loading rate and contraction type effects on the human Achilles tendon force-elongation relationship

IntroductionWhile it is accepted that tendons are viscoelastic, the loading rate of contractions is often not strictly controlled when assessing human tendon mechanical properties in vivo. Given the potential benefits of sustained constant load isometric contractions for in vivo tendon property assessment, we aimed to determine if sustained submaximal isometric plantarflexion contractions result in a similar force-elongation relationshipand stiffness of the Achilles tendon (AT) to other loading methods.MethodsThe AT mechanical properties (elongation and stiffness) of the dominant leg inten male adults (26.5±5.5y) were assessed during isometric plantarflexion contractions by integrating dynamometry and ultrasonography (Aloka α7, Tokyo, Japan).Measurements were taken on two consecutive days and the results from all participants on day one and seven participants on day two (three excluded due to measurement problems) were pooled for the analysis. Maximum voluntary contractions (MVC; high loading rate), ramp maximum force contractions with three seconds loading (RAMP; lower loading rate), and sustained contractions (held for three seconds) at forces of 25%, 50% and 80% of the maximal tendon force with the lower loading rate (SUS) were conducted.ResultsA two way repeated measures ANOVA with method and tendon force level as factors revealed a significant method (P<0.001) effecton tendon elongation. Post hoc tests with Bonferroni corrections revealed significantlygreater tendon elongation in SUS compared withMVC (P=0.001) and RAMP(P=0.002),but no differences in tendon elongationbetween MVC and RAMP(P=0.077).A one way ANOVA with method as a factor did not reveal a significant method effect on tendon stiffness(P=0.079; MVC: 653.6±220.9 N/mm; RAMP: 694.8±190.3 N/mm; SUS: 564.2±148.1 N/mm).ConclusionSustained plantarflexion contractions appear to lead to an increased AT elongation for a given force, presumably due to the reduced influence of the loading rate on the viscoelastic behaviour of the tendon during the sustained contractions. However,AT stiffness was not significantly different between methods, suggesting that the differences in the rate of elongation occurred prior to the linear region of the force elongation relationship. Sustained isometric contractions appear to be appropriate for assessing AT stiffness in vivo, although potential differences in tendon elongation should be considered when comparing results with other methods

Loading rate and contraction duration effects on in vivo human Achilles tendon mechanical properties.

Tendons are viscoelastic, which implies loading rate dependency, but loading rates of contractions are often not controlled during assessment of human tendon mechanical properties in vivo. We investigated the effects of sustained submaximal isometric plantarflexion contractions, which potentially negate loading rate dependency, on the stiffness of the human Achilles tendon in vivo using dynamometry and ultrasonography. Maximum voluntary contractions (high loading rate), ramp maximum force contractions with 3 s loading (lower loading rate) and sustained contractions (held for 3 s) at 25%, 50% and 80% of maximal tendon force were conducted. No loading rate effect on stiffness (25-80% max. tendon force) was found. However, loading rate effects were seen up to 25% of maximum tendon force, which were reduced by the sustained method. Sustained plantarflexion contractions may negate loading rate effects on tendon mechanical properties and appear suitable for assessing human Achilles tendon stiffness in vivo

Aging and the effects of a half marathon on Achilles tendon force-elongation relationship.

PURPOSE: We aimed to determine whether there are different changes in Achilles tendon (AT) mechanical properties in middle-aged, compared to younger runners that might indicate that tendon fatigue, induced by long-distance running, is age-dependent. METHODS: 27 middle-aged (50-67 years) and 22 younger (21-29 years) participants ran a 21 km route at their own pace (mean and SD: old: 3.1 ± 0.3 m s(-1); young: 3.6 ± 0.5 m s(-1)). We tested for changes in the AT force-elongation relationship using dynamometry and ultrasonography during isometric voluntary ankle plantarflexion ramp contractions, conducted 20-28 h pre-run, immediately pre-run, immediately post-run and 20-28 h post-run. Stride frequency and number were examined to estimate cyclic tensile loading characteristics of the tendon during running. RESULTS: Muscle strength decreased significantly (P < 0.05) in both groups immediately post-run (old: 17 %; young: 11 %) and recovered to baseline within 20-28 h post-run. AT stiffness did not change for the younger adults, whereas the middle-aged adults showed a significant (P < 0.05) decrease in AT stiffness (22 %). However, tendon stiffness recovered to baseline 20-28 h post-run. Middle-aged, compared to young adults, demonstrated significantly (P < 0.05) greater stride frequency and number, but no correlations with tendon fatigue changes were determined (R (2) ≤ 0.038). CONCLUSIONS: The results suggest that the plasticity of the AT in response to short-term mechanical loading may be age dependent and that the AT length-tension properties of middle-aged runners may be more vulnerable to change following running compared to younger athletes. However, the observed AT changes in the middle-aged runners dissipated within 20-28 h post-run, suggesting that a tendon viscoelastic recovery mechanism may occur in vivo

The influence of spectral solar irradiance data on stratospheric heating rates during the 11 year solar cycle

Heating rate calculations with the FUBRad shortwave (SW) radiation parameterization have been performed to examine the effect of prescribed spectral solar fluxes from the NRLSSI, MPS and IUP data sets on SW heating rates over the 11 year solar cycle 22. The corresponding temperature response is derived from perpetual January General Circulation Model (GCM) simulations with prescribed ozone concentrations. The different solar flux input data sets induce clear differences in SW heating rates at solar minimum, with the established NRLSSI data set showing the smallest solar heating rates. The stronger SW heating in the middle and upper stratosphere in the MPS data warms the summer upper stratosphere by 2 K. Over the solar cycle, SW heating rate differences vary up to 40% between the irradiance data sets, but do not result in a significant change of the solar temperature signal. Lower solar fluxes in the newer SIM data lead to a significantly cooler stratosphere and mesosphere when compared to NRLSSI data for 2007. Changes in SW heating from 2004 to 2007 are however up to six times stronger than for the NRLSSI data. Key Points: - Solar minimum and solar cycle differences in SW heating rates and temperature - Comparison of three spectral solar input data sets for solar cycle 22 - Comparison of the newly compiled SORCE-data with the commonly used NRLSSI-dat

Variability of Sun-like stars: reproducing observed photometric trends

The Sun and stars with low magnetic activity levels, become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behaviour and get fainter when their activity increases. We reproduce the observed photometric trends in stellar variations with a model that treats stars as hypothetical Suns with coverage by magnetic features different from that of the Sun. The presented model attributes the variability of stellar spectra to the imbalance between the contributions from different components of the solar atmosphere, such as dark starspots and bright faculae. A stellar spectrum is calculated from spectra of the individual components, by weighting them with corresponding disc area coverages. The latter are obtained by extrapolating the solar dependences of spot and facular disc area coverages on chromospheric activity to stars with different levels of mean chromospheric activity. We have found that the contribution by starspots to the variability increases faster with chromospheric activity than the facular contribution. This causes the transition from faculae-dominated variability and direct activity--brightness correlation to spot-dominated variability and inverse activity--brightness correlation with increasing chromospheric activity level. We have shown that the regime of the variability also depends on the angle between the stellar rotation axis and the line-of-sight and on the latitudinal distribution of active regions on the stellar surface. Our model can be used as a tool to extrapolate the observed photometric variability of the Sun to Sun-like stars at different activity levels, which makes possible the direct comparison between solar and stellar irradiance data.Comment: 20 pages, 16 figures, accepted for publication in Astronomy&Astrophysic