AN EMG OPTIMIZATION MODEL OF THE KINETIC DEMANDS ON THE LOWER BACK DURING ASYMMETRICAL GAIT AND LOAD CARRIAGE

Gait asymmetries are associated with a high incidence of lower back pain (LBP). Although there are several causes of gait asymmetry (i.e. amputation, injury, or deformities), lower back kinetic demands have not been quantified and suitably compared due to experimental limitations in these clinical populations. Further, the impact of gait asymmetry on lower back demands during carrying tasks has not been established. This dissertation addressed these issues by artificially and safely inducing gait asymmetry in healthy able-bodied participants during walking and carrying tasks. LBP risk was assessed by L5/S1 vertebral joint force levels estimated with an OpenSim musculoskeletal model of the lower back adapted to incorporate participant-specific responses using an EMG optimization approach. The model was evaluated systematically for force estimate efficacy and sensitivity to input parameters prior to gait asymmetry assessments. Twelve participants performed walking and carrying tasks on a treadmill at individually scaled speeds while kinematics, external kinetics, and muscle activities (EMG) were recorded. Walking conditions consisted of unperturbed symmetrical gait, and asymmetrical gait induced by perturbing the right leg with a 2.54 cm shoe leveler, ~1 kg ankle weight, combined weight and shoe leveler, or a clinical walking boot that restricted ankle joint motion and added mass. Load carrying was performed while holding 7.5% and 15% bodyweight dumbbells in one or two hands during symmetric gait and asymmetric gait induced by the walking boot. The perturbations were successful in producing different degrees of gait asymmetry. However, L5/S1 joint forces were not significantly different between conditions despite unique spatiotemporal asymmetries. This indicates that LBP in those with gait asymmetry may not be due solely to level planar walking. During carrying tasks, gait asymmetry induced by the walking boot increased some metrics of lower back loading. Further, carrying a load in the hand contralateral to the walking boot produced larger forces than when carried on the same side. These results emphasize the importance of evaluating specific sources of gait asymmetry during daily activities other than walking when assessing LBP risk and would encourage more inclusive ergonomic carrying guidelines

How Fast Does Information Leak out from a Black Hole?

Hawking's radiance, even as computed without account of backreaction, departs from blackbody form due to the mode dependence of the barrier penetration factor. Thus the radiation is not the maximal entropy radiation for given energy. By comparing estimates of the actual entropy emission rate with the maximal entropy rate for the given power, and using standard ideas from communication theory, we set an upper bound on the permitted information outflow rate. This is several times the rates of black hole entropy decrease or radiation entropy production. Thus, if subtle quantum effects not heretofore accounted for code information in the radiance, the information that was thought to be irreparably lost down the black hole may gradually leak back out from the black hole environs over the full duration of the hole's evaporation.Comment: 8 pages, plain TeX, UCSBTH-93-0

Supergravity and The Large N Limit of Theories With Sixteen Supercharges

We consider field theories with sixteen supersymmetries, which includes U(N) Yang-Mills theories in various dimensions, and argue that their large N limit is related to certain supergravity solutions. We study this by considering a system of D-branes in string theory and then taking a limit where the brane worldvolume theory decouples from gravity. At the same time we study the corresponding D-brane supergravity solution and argue that we can trust it in certain regions where the curvature (and the effective string coupling, where appropriate) are small. The supergravity solutions typically have several weakly coupled regions and interpolate between different limits of string-M-theory.Comment: 24 pages, latex. v2: reference added, v3: typos correcte

Entropy Bounds and Black Hole Remnants

We rederive the universal bound on entropy with the help of black holes while allowing for Unruh--Wald buoyancy. We consider a box full of entropy lowered towards and then dropped into a Reissner--Nordstr\"om black hole in equilibrium with thermal radiation. We avoid the approximation that the buoyant pressure varies slowly across the box, and compute the buoyant force exactly. We find, in agreement with independent investigations, that the neutral point generically lies very near the horizon. A consequence is that in the generic case, the Unruh--Wald entropy restriction is neither necessary nor sufficient for enforcement of the generalized second law. Another consequence is that generically the buoyancy makes only a negligible contribution to the energy bookeeping, so that the original entropy bound is recovered if the generalized second law is assumed to hold. The number of particle species does not figure in the entropy bound, a point that has caused some perplexity. We demonstrate by explicit calculation that, for arbitrarily large number of particle species, the bound is indeed satisfied by cavity thermal radiation in the thermodynamic regime, provided vacuum energies are included. We also show directly that thermal radiation in a cavity in $D$ dimensional space also respects the bound regardless of the value of $D$. As an application of the bound we show that it strongly restricts the information capacity of the posited black hole remnants, so that they cannot serve to resolve the information paradox.Comment: 12 pages, UCSBTH-93-2

Nanofabrication technologies: high-throughput for tomorrow's metadevices

Fabrication fundamentals1. Serial versus parallel? Most are currently fabricated by serial writing….2. Additive or subtractive?3. Feature size required.4. One-off demonstration (journal paper) or volume production (in the shops by next Christmas…)5. What material?6. Cost….(+ normalise to 150mm diameter wafer)7. Time to fabricat

Exotic Non-Supersymmetric Gauge Dynamics from Supersymmetric QCD

We extend Seiberg's qualitative picture of the behavior of supersymmetric QCD to nonsupersymmetric models by adding soft supersymmetry breaking terms. In this way, we recover the standard vacuum of QCD with $N_f$ flavors and $N_c$ colors when $N_f < N_c$. However, for $N_f \geq N_c$, we find new exotic states---new vacua with spontaneously broken baryon number for $N_f = N_c$, and a vacuum state with unbroken chiral symmetry for $N_f > N_c$. These exotic vacua contain massless composite fermions and, in some cases, dynamically generated gauge bosons. In particular Seiberg's electric-magnetic duality seems to persist also in the presence of (small) soft supersymmetry breaking. We argue that certain, specially tailored, lattice simulations may be able to detect the novel phenomena. Most of the exotic behavior does not survive the decoupling limit of large SUSY breaking parameters.Comment: 36 pages, latex + 2 figures (uuencoded ps

Fine-structure constant variability, equivalence principle and cosmology

It has been widely believed that variability of the fine-structure constant alpha would imply detectable violations of the weak equivalence principle. This belief is not justified in general. It is put to rest here in the context of the general framework for alpha variability [J. D. Bekenstein, Phys. Rev. D 25, 1527 (1982)] in which the exponent of a scalar field plays the role of the permittivity and inverse permeability of the vacuum. The coupling of particles to the scalar field is necessarily such that the anomalous force acting on a charged particle by virtue of its mass's dependence on the scalar field is cancelled by terms modifying the usual Coulomb force. As a consequence a particle's acceleration in external fields depends only on its charge to mass ratio, in accordance with the principle. And the center of mass acceleration of a composite object can be proved to be independent of the object's internal constitution, as the weak equivalence principle requires. Likewise the widely employed assumption that the Coulomb energy of matter is the principal source of the scalar field proves wrong; Coulomb energy effectively cancels out in the continuum description of the scalar field's dynamics. This cancellation resolves a cosmological conundrum: with Coulomb energy as source of the scalar field, the framework would predict a decrease of alpha with cosmological expansion, whereas an increase is claimed to be observed. Because of the said cancellation, magnetic energy of cosmological baryonic matter is the main source of the scalar field. Consequently the expansion is accompanied by an increase in alpha; for reasonable values of the framework's sole parameter, this occurs at a rate consistent with the observers' claims.Comment: RevTeX-4, 22 pages, no figures, added a section on caveats as well as several new references with discussion of them in body. To appear in Phys. Rev.

Smart Swarms of Bacteria-Inspired Agents with Performance Adaptable Interactions

Collective navigation and swarming have been studied in animal groups, such as fish schools, bird flocks, bacteria, and slime molds. Computer modeling has shown that collective behavior of simple agents can result from simple interactions between the agents, which include short range repulsion, intermediate range alignment, and long range attraction. Here we study collective navigation of bacteria-inspired smart agents in complex terrains, with adaptive interactions that depend on performance. More specifically, each agent adjusts its interactions with the other agents according to its local environment – by decreasing the peers' influence while navigating in a beneficial direction, and increasing it otherwise. We show that inclusion of such performance dependent adaptable interactions significantly improves the collective swarming performance, leading to highly efficient navigation, especially in complex terrains. Notably, to afford such adaptable interactions, each modeled agent requires only simple computational capabilities with short-term memory, which can easily be implemented in simple swarming robots

Baryonic Popcorn

In the large N limit cold dense nuclear matter must be in a lattice phase. This applies also to holographic models of hadron physics. In a class of such models, like the generalized Sakai-Sugimoto model, baryons take the form of instantons of the effective flavor gauge theory that resides on probe flavor branes. In this paper we study the phase structure of baryonic crystals by analyzing discrete periodic configurations of such instantons. We find that instanton configurations exhibit a series of "popcorn" transitions upon increasing the density. Through these transitions normal (3D) lattices expand into the transverse dimension, eventually becoming a higher dimensional (4D) multi-layer lattice at large densities. We consider 3D lattices of zero size instantons as well as 1D periodic chains of finite size instantons, which serve as toy models of the full holographic systems. In particular, for the finite-size case we determine solutions of the corresponding ADHM equations for both a straight chain and for a 2D zigzag configuration where instantons pop up into the holographic dimension. At low density the system takes the form of an "abelian anti-ferromagnetic" straight periodic chain. Above a critical density there is a second order phase transition into a zigzag structure. An even higher density yields a rich phase space characterized by the formation of multi-layer zigzag structures. The finite size of the lattices in the transverse dimension is a signal of an emerging Fermi sea of quarks. We thus propose that the popcorn transitions indicate the onset of the "quarkyonic" phase of the cold dense nuclear matter.Comment: v3, 80 pages, 18 figures, footnotes 5 and 7 added, version to appear in the JHE

Family social support, community “social capital” and adolescents’ mental health and educational outcomes: a longitudinal study in England

Purpose To examine the associations between family social support, community “social capital” and mental health and educational outcomes. Methods The data come from the Longitudinal Study of Young People in England, a multi-stage stratified nationally representative random sample. Family social support (parental relationships, evening meal with family, parental surveillance) and community social capital (parental involvement at school, sociability, involvement in activities outside the home) were measured at baseline (age 13–14), using a variety of instruments. Mental health was measured at age 14–15 (GHQ-12). Educational achievement was measured at age 15–16 by achievement at the General Certificate of Secondary Education. Results After adjustments, good paternal (OR = 0.70, 95% CI 0.56–0.86) and maternal (OR = 0.65, 95% CI 0.53–0.81) relationships, high parental surveillance (OR = 0.81, 95% CI 0.69–0.94) and frequency of evening meal with family (6 or 7 times a week: OR = 0.77, 95% CI 0.61–0.96) were associated with lower odds of poor mental health. A good paternal relationship (OR = 1.27, 95% CI 1.06–1.51), high parental surveillance (OR = 1.37, 95% CI 1.20–1.58), high frequency of evening meal with family (OR = 1.64, 95% CI 1.33–2.03) high involvement in extra-curricular activities (OR = 2.57, 95% CI 2.11–3.13) and parental involvement at school (OR = 1.60, 95% CI 1.37–1.87) were associated with higher odds of reaching the educational benchmark. Participating in non-directed activities was associated with lower odds of reaching the benchmark (OR = 0.79, 95% CI 0.70–0.89). Conclusions Building social capital in deprived communities may be one way in which both mental health and educational outcomes could be improved. In particular, there is a need to focus on the family as a provider of support