The influence of femoral structure, hip capsular constraints, and gluteal muscle strength and activation on temporal patterns of functional valgus collapse

Functional valgus collapse (a combination of knee abduction and internal rotation and hip adduction and internal rotation) is a modifiable lower extremity movement pattern commonly associated with anterior cruciate ligament (ACL) injuries in females. Though the gluteus maximus and gluteus medius have frequently been named contributors to functional valgus collapse, evidence supporting their role in lower extremity movement has been inconsistent, and could in part be due to methodological differences between studies and the accepted practice of analyzing discrete variables instead of overall movement patterns. Better elucidation of gluteal muscle influence on lower extremity biomechanics may be a critical step for the reduction of ACL injury rates, as neuromuscular dysfunction is likely more responsive to injury prevention efforts than are other risk factors such as bony anatomy, ligament quality, or hormonal influences, that are more difficult to modify. Therefore, the purpose of this study was to 1) describe the neuromechanical profiles throughout the landing phase of single-leg and double-leg forward landings in males and females, 2) quantify the contributions of gluteal muscle strength and activation to peak angles and moments of functional valgus collapse after controlling for one’s femoral alignment, and 3) explore the association between gluteal muscle function and overall functional valgus collapse throughout the landing phase. To accomplish this, 45 females and 45 males with no history of knee surgery were measured for femoral anteversion, hip ROM, and hip strength and then underwent biomechanical testing during single-leg and double-leg forward landings to examine muscle activation and 3-dimensional biomechanics. Data were analyzed using conventional group and correlative analyses and also with statistical parametric mapping (SPM), which allowed for a more comprehensive examination of the entire biomechanical time series. Biomechanical variables of interest included joint angles and moments comprising functional valgus collapse: hip adduction and internal rotation and knee abduction and internal rotation. In the comparison between single-leg and double-leg landings by sex, sex differences in the frontal plane were task dependent, though females maintained greater absolute knee abduction and hip adduction throughout the landing phases. Sex by task interactions revealed that females landed with smaller knee adduction angles than males, particularly during the single-leg landing (p=.03), while females’ knee abduction excursion was greater than males’, particularly during the double-leg landing (p=.01). Across task, females displayed 4.1° greater peak knee abduction than males (p=.002), and this was specific to 37-46% of the landing phase (p=.05). Females went through 1.0° more hip abduction than males (p=.05), and used a smaller proportion of their gluteus maximus (p=.01) in both tasks. Examination of gluteal muscle contribution to individual and overall levels of functional valgus collapse in females revealed that at the 18% and 20% time points during the landing phase, less hip abduction strength and greater gluteus medius activation predicted greater peak hip adduction angles (R2 change = .10; p = .02) and higher external hip adduction moments (R2 change = .14, p = .06). Greater hip extension strength predicted greater peak hip abduction angles (R2 change = .08; p = .05), while greater gluteus maximus activation strengthened the prediction of greater initial (R2 change = .10, p = .03) and peak (R2 change = .14, p = .01) knee internal rotation angles. From 7% - 8% of the landing phase, greater external rotation ROM was associated with greater external hip adduction moment (R2 change = .18, p = .01). In males, less hip abduction strength strengthened the prediction of greater initial (R2 change = .12, p = .01) and peak knee internal rotation angles (R2 change = .14, p = .01), lesser peak knee external rotation angles (R2 change = .07, p =.09), and lesser peak knee abduction moments (R2 change = .06, p =.11). Less hip extension strength with greater gluteus maximus activation predicted greater peak hip external rotation moments (R2 change = .14, p = .01). Specifically from the 3% - 9% time points of the landing phase, greater hip extension strength was associated with greater knee abduction moment (R2 change = .17, p = .01) and less hip adduction moment (R2 change = .24, p = .001). At 0% and from 2% - 3% of the landing phase, greater internal and external rotation ROM were associated with greater knee abduction angle (R2 change = .27, p = .01) and greater hip adduction angle (R2 change = .23, p = .02). These results indicate that lower extremity biomechanics during a single-leg landing task are appreciably different than those observed during a double-leg landing task, and that a single-leg landing task elicits more profound sex differences, particularly during the early stage of single-leg load acceptance when ACL injuries are thought to occur (30-40ms post initial ground contact). As such, a single-leg landing task may be more appropriate for biomechanical screening of ACL injury risk. Gluteal strength and activation explained a unique proportion of variance in lower extremity biomechanics beyond what was explained by femoral alignment. In females, weaker gluteal muscles predicted riskier frontal plane hip kinematics. In males, gluteal function was more associated with kinetics. This implies that our male cohort used their musculature to create torque about a joint, whereas our female cohort was unable to create torque. Though femoral alignment (total ROM) explained considerably greater proportions of biomechanical variance than did gluteal function, observed associations between gluteal muscle function and biomechanics occurred 10-20ms after associations between femoral alignment and biomechanics. While the gluteal muscles may act mechanically independent of femoral alignment, it is possible that gluteal muscle function could be temporally linked to one’s femoral alignment. With these findings in mind, it may be beneficial for clinicians to implement gluteal strengthening programs and to encourage gluteal muscle pre-activation in individuals with excessive hip ROM to lessen their propensity for functional valgus collapse

Life-Expectancy Disparities Among Adults With HIV in the United States and Canada: The Impact of a Reduction in Drug- and Alcohol-Related Deaths Using the Lives Saved Simulation Model.

Improvements in life expectancy among people living with human immunodeficiency virus (PLWH) receiving antiretroviral treatment in the United States and Canada might differ among key populations. Given the difference in substance use among key populations and the current opioid epidemic, drug- and alcohol-related deaths might be contributing to the disparities in life expectancy. We sought to estimate life expectancy at age 20 years in key populations (and their comparison groups) in 3 time periods (2004-2007, 2008-2011, and 2012-2015) and the potential increase in expected life expectancy with a simulated 20% reduction in drug- and alcohol-related deaths using the novel Lives Saved Simulation model. Among 92,289 PLWH, life expectancy increased in all key populations and comparison groups from 2004-2007 to 2012-2015. Disparities in survival of approximately a decade persisted among black versus white men who have sex with men and people with (vs. without) a history of injection drug use. A 20% reduction in drug- and alcohol-related mortality would have the greatest life-expectancy benefit for black men who have sex with men, white women, and people with a history of injection drug use. Our findings suggest that preventing drug- and alcohol-related deaths among PLWH could narrow disparities in life expectancy among some key populations, but other causes of death must be addressed to further narrow the disparities

SEGUE-2 Limits on Metal-Rich Old-Population Hypervelocity Stars In the Galactic Halo

We present new limits on the ejection of metal-rich old-population hypervelocity stars from the Galactic center (GC) as probed by the SEGUE-2 survey. Our limits are a factor of 3-10 more stringent than previously reported, depending on stellar type. Compared to the known population of B-star ejectees, there can be no more than 30 times more metal-rich old-population F/G stars ejected from the GC. Because B stars comprise a tiny fraction of a normal stellar population, this places significant limits on a combination of the GC mass function and the ejection mechanism for hypervelocity stars. In the presence of a normal GC mass function, our results require an ejection mechanism that is about 5.5 times more efficient at ejecting B-stars compared to low-mass F/G stars.Comment: 18 pages including 5 figures; Submitted to Ap

Binary Contamination in the SEGUE sample: Effects on SSPP Determinations of Stellar Atmospheric Parameters

Using numerical modeling and a grid of synthetic spectra, we examine the effects that unresolved binaries have on the determination of various stellar atmospheric parameters for SEGUE targets measured using the SEGUE Stellar Parameter Pipeline (SSPP). To model undetected binaries that may be in the SEGUE sample, we use a variety of mass distributions for the primary and secondary stars in conjunction with empirically determined relationships for orbital parameters to determine the fraction of G-K dwarf stars, as defined by SDSS color cuts, that will be blended with a secondary companion. We focus on the G-K dwarf sample in SEGUE as it records the history of chemical enrichment in our galaxy. To determine the effect of the secondary on the spectroscopic parameters, we synthesize a grid of model spectra from 3275 to 7850 K (~0.1 to 1.0 \msun) and [Fe/H]=-0.5 to -2.5 from MARCS model atmospheres using TurboSpectrum. We analyze both "infinite" signal-to-noise ratio (S/N) models and degraded versions, at median S/N of 50, 25 and 10. By running individual and combined spectra (representing the binaries) through the SSPP, we determine that ~10% of the blended G-K dwarf pairs with S/N>=25 will have their atmospheric parameter determinations, in particular temperature and metallicity, noticeably affected by the presence of an undetected secondary. To account for the additional uncertainty from binary contamination at a S/N~10, uncertainties of ~140 K and ~0.17 dex in [Fe/H] must be added in quadrature to the published uncertainties of the SSPP. (Abridged)Comment: 68 pages, 20 figures, 9 table

A Photometric Metallicity Estimate of the Virgo Stellar Overdensity

We determine photometric metal abundance estimates for individual main-sequence stars in the Virgo Overdensity (VOD), which covers almost 1000 deg^2 on the sky, based on a calibration of the metallicity sensitivity of stellar isochrones in the gri filter passbands using field stars with well-determined spectroscopic metal abundances. Despite the low precision of the method for individual stars, we derive [Fe/H] = -2.0 +/-0.1 (internal) +/-0.5 (systematic) for the metal abundance of the VOD from photometric measurements of 0.7 million stars in the Northern Galactic hemisphere with heliocentric distances from ~10 kpc to ~20 kpc. The metallicity of the VOD is indistinguishable, within Delta [Fe/H] < 0.2, from that of field halo stars covering the same distance range. This initial application suggests that the SDSS gri passbands can be used to probe the properties of main-sequence stars beyond ~10 kpc, complementing studies of nearby stars from more metallicity-sensitive color indices that involve the u passband.Comment: 5 pages, 3 figures, Accepted for publication in ApJ Letter

The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters

We validate the performance and accuracy of the current SEGUE (Sloan Extension for Galactic Understanding and Exploration) Stellar Parameter Pipeline (SSPP), which determines stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) by comparing derived overall metallicities and radial velocities from selected likely members of three globular clusters (M 13, M 15, and M 2) and two open clusters (NGC 2420 and M 67) to the literature values. Spectroscopic and photometric data obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its first extension (SDSS-II/SEGUE) are used to determine stellar radial velocities and atmospheric parameter estimates for stars in these clusters. Based on the scatter in the metallicities derived for the members of each cluster, we quantify the typical uncertainty of the SSPP values, sigma([Fe/H]) = 0.13 dex for stars in the range of 4500 K < Teff < 7500 K and 2.0 < log g < 5.0, at least over the metallicity interval spanned by the clusters studied (-2.3 < [Fe/H] < 0). The surface gravities and effective temperatures derived by the SSPP are also compared with those estimated from the comparison of the color-magnitude diagrams with stellar evolution models; we find satisfactory agreement. At present, the SSPP underestimates [Fe/H] for near-solar-metallicity stars, represented by members of M 67 in this study, by about 0.3 dex.Comment: 56 pages, 8 Tables, 15 figures, submitted to the Astronomical Journa

The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars

We report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which we compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R = 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/N), we empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km/s, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. We estimate random errors for lower S/N spectra based on numerical simulations.Comment: 37 pages, 6 tables, 6 figures, submitted to the Astronomical Journa

Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. II. Test of Theoretical Stellar Isochrones

We perform an extensive test of theoretical stellar models for main-sequence stars in ugriz, using cluster fiducial sequences obtained in the previous paper of this series. We generate a set of isochrones using the Yale Rotating Evolutionary Code (YREC) with updated input physics, and derive magnitudes and colors in ugriz from MARCS model atmospheres. These models match cluster main sequences over a wide range of metallicity within the errors of the adopted cluster parameters. However, we find a large discrepancy of model colors at the lower main sequence (Teff < ~4500 K) for clusters at and above solar metallicity. We also reach similar conclusions using the theoretical isochrones of Girardi et al. and Dotter et al., but our new models are generally in better agreement with the data. Using our theoretical isochrones, we also derive main-sequence fitting distances and turn-off ages for five key globular clusters, and demonstrate the ability to derive these quantities from photometric data in the Sloan Digital Sky Survey. In particular, we exploit multiple color indices (g - r, g - i, and g - z) in the parameter estimation, which allows us to evaluate internal systematic errors. Our distance estimates, with an error of sigma(m - M) = 0.03-0.11 mag for individual clusters, are consistent with Hipparcos-based subdwarf fitting distances derived in the Johnson-Cousins or Stromgren photometric systems.Comment: 26 pages, 28 figures. Accepted for publication in ApJ. Version with high resolution figures available at http://spider.ipac.caltech.edu/~deokkeun/sdss_iso.pd

The Metallicity Distribution Functions of SEGUE G and K dwarfs: Constraints for Disk Chemical Evolution and Formation

We present the metallicity distribution function (MDF) for 24,270 G and 16,847 K dwarfs at distances from 0.2 to 2.3 kpc from the Galactic plane, based on spectroscopy from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. This stellar sample is significantly larger in both number and volume than previous spectroscopic analyses, which were limited to the solar vicinity, making it ideal for comparison with local volume-limited samples and Galactic models. For the first time, we have corrected the MDF for the various observational biases introduced by the SEGUE target selection strategy. The SEGUE sample is particularly notable for K dwarfs, which are too faint to examine spectroscopically far from the solar neighborhood. The MDF of both spectral types becomes more metal-poor with increasing |Z|, which reflects the transition from a sample with small [alpha/Fe] values at small heights to one with enhanced [alpha/Fe] above 1 kpc. Comparison of our SEGUE distributions to those of two different Milky Way models reveals that both are more metal-rich than our observed distributions at all heights above the plane. Our unbiased observations of G and K dwarfs provide valuable constraints over the |Z|-height range of the Milky Way disk for chemical and dynamical Galaxy evolution models, previously only calibrated to the solar neighborhood, with particular utility for thin- and thick-disk formation models.Comment: 70 pages, 25 figures, 7 tables. Accepted by The Astrophysical Journa