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Search Results: 1 - 10 of 219441 matches for " John D. Sorkin "
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Impaired Economy of Gait and Decreased Six-Minute Walk Distance in Parkinson's Disease
Leslie I. Katzel,Frederick M. Ivey,John D. Sorkin,Richard F. Macko,Barbara Smith,Lisa M. Shulman
Parkinson's Disease , 2012, DOI: 10.1155/2012/241754
Abstract: Changes in the biomechanics of gait may alter the energy requirements of walking in Parkinson's Disease (PD). This study investigated economy of gait during submaximal treadmill walking in 79 subjects with mild to moderate PD and the relationship between gait economy and 6-minute walk distance (6?MW). Oxygen consumption (VO2) at the self-selected treadmill walking speed averaged 64% of peak oxygen consumption (VO2 peak). Submaximal VO2 levels exceeded 70% of VO2 peak in 30% of the subjects. Overall the mean submaximal VO2 was 51% higher than VO2 levels expected for the speed and grade consistent with severe impairment in economy of gait. There was an inverse relationship between economy of gait and 6MW ( , ) and with the self-selected walking speed ( , ). Thus, the impairment in economy of gait and decreased physiologic reserve result in routine walking being performed at a high percentage of VO2 peak. 1. Introduction Walking capacity is central to the performance of many activities of daily living. Difficulty with walking is one of the cardinal symptoms of Parkinson’s Disease (PD). Alterations in the biomechanics of gait, such as decreased stride length, increased stride length variability, and reduced gait speed, are common even in early stages of PD [1–3]. Most often, PD patients attempt to compensate for short steps by increasing gait cadence, thereby potentially altering energy requirements. This higher energy cost of movement is often referred to as a lower economy of gait and is a function of abnormal gait patterns that accompany aging and neurological disability. Reduced economy of gait has been associated with impaired function and fatigue in non-PD populations [4–9], but there is currently scant information on how parkinsonian gait affects energy expenditure or economy of gait using direct measures of oxygen consumption [10]. Further, little is known about the relationship between economy of gait and mobility. Hence, the purpose of this study was to investigate economy of gait during submaximal treadmill walking in mild to moderate PD, and the relationship between economy of gait and the distance covered during the 6-minute walk (6?MW). 2. Methods 2.1. Subjects Participants for this study were recruited from the University of Maryland Parkinson’s Disease Center and the Baltimore VA Medical Center neurology clinics as part of an exercise intervention trial in PD [11]. Inclusion criteria were (1) diagnosis of levodopa-responsive PD characterized by 2 of 3 cardinal signs (resting tremor, bradykinesia, rigidity), (2) Hoehn and Yahr (HY) [12] stage
The Unified Parkinson’s Disease Rating Scale as a predictor of peak aerobic capacity and ambulatory function
Frederick M. Ivey, PhD,Leslie I. Katzel, MD, PhD,John D. Sorkin, MD, PhD,Richard F. Macko, MD
Journal of Rehabilitation Research and Development , 2012, DOI: 10.1682/jrrd.2011.06.0103
Abstract: The Unified Parkinson’s Disease Rating Scale (UPDRS) is a widely applied index of disease severity. Our objective was to assess the utility of UPDRS for predicting peak aerobic capacity (VO2 peak) and ambulatory function. Participants (n = 70) underwent evaluation for UPDRS (Total and Motor ratings), VO2 peak, 6-minute walk distance (6MW), and 30-foot self-selected walking speed (SSWS). Using regression, we determined the extent to which the Total and Motor UPDRS scores predicted each functional capacity measure after adjusting for age and sex. We also tested whether adding the Hoehn and Yahr scale (H-Y) to the model changed predictive power of the UPDRS. Adjusted for age and sex, both the Total UPDRS and Motor UPDRS subscale failed to predict VO2 peak. The Total UPDRS did weakly predict 6MW and SSWS (both p < 0.05), but the Motor UPDRS subscale did not predict these ambulatory function tests. After adding H-Y to the model, Total UPDRS was no longer an independent predictor of 6MW but remained a predictor of SSWS. We conclude that Total and Motor UPDRS rating scales do not predict VO2 peak, but that a weak relationship exists between Total UPDRS and measures of ambulatory function.
A Parallelizable Implicit Evolution Scheme for Regge Calculus
John W. Barrett,Mark Galassi,Warner A. Miller,Rafael D. Sorkin,Philip A. Tuckey,Ruth M. Williams
Physics , 1994, DOI: 10.1007/BF02435787
Abstract: The role of Regge calculus as a tool for numerical relativity is discussed, and a parallelizable implicit evolution scheme described. Because of the structure of the Regge equations, it is possible to advance the vertices of a triangulated spacelike hypersurface in isolation, solving at each vertex a purely local system of implicit equations for the new edge-lengths involved. (In particular, equations of global ``elliptic-type'' do not arise.) Consequently, there exists a parallel evolution scheme which divides the vertices into families of non-adjacent elements and advances all the vertices of a family simultaneously. The relation between the structure of the equations of motion and the Bianchi identities is also considered. The method is illustrated by a preliminary application to a 600--cell Friedmann cosmology. The parallelizable evolution algorithm described in this paper should enable Regge calculus to be a viable discretization technique in numerical relativity.
Big extra dimensions make lambda too small
Sorkin, Rafael D.;
Brazilian Journal of Physics , 2005, DOI: 10.1590/S0103-97332005000200012
Abstract: i argue that the true quantum gravity scale cannot be much larger than the planck length, because if it were then the quantum gravity-induced fluctuations in l would be insufficient to produce the observed cosmic "dark energy". if one accepts this argument, it rules out scenarios of the "large extra dimensions" type. i also point out that the relation between the lower and higher dimensional gravitational constants in a kaluza-klein theory is precisely what is needed in order that a black hole's entropy admit a consistent higher dimensional interpretation in terms of an underlying spatio-temporal discreteness.
Is the spacetime metric Euclidean rather than Lorentzian?
Rafael D. Sorkin
Physics , 2009,
Abstract: My answer to the question in the title is "No". In support of this point of view, we analyze some examples of saddle-point methods, especially as applied to quantum "tunneling" in nonrelativistic particle mechanics and in cosmology. Along the way we explore some of the interrelationships among different ways of thinking about path-integrals and saddle-point approximations to them.
Two Topics concerning Black Holes: Extremality of the Energy, Fractality of the Horizon
Rafael D. Sorkin
Physics , 1995,
Abstract: We treat two aspects of the physics of stationary black holes. First we prove that the proportionality, d(energy) ~ d(area) for arbitrary perturbations (``extended first law''), follows directly from an extremality theorem drawn from earlier work. Second we consider quantum fluctuations in the shape of the horizon, concluding heuristically that they exhibit a fractal character, with order lambda fluctuations occurring on all scales lambda below M^{1/3} in natural units.
How Wrinkled is the Surface of a Black Hole?
Rafael D. Sorkin
Physics , 1997,
Abstract: We present evidence that, below a certain threshold scale, the horizon of a black hole is strongly wrinkled, with its shape manifesting a self-similar (``fractal'') spectrum of fluctuations on all scales below the threshold. This threshold scale is small compared to the radius of the black hole, but still much larger than the Planck scale. If present, such fluctuations might account for a large part of the horizon entropy.
Logic is to the quantum as geometry is to gravity
Rafael D. Sorkin
Physics , 2010,
Abstract: I will propose that the reality to which the quantum formalism implicitly refers is a kind of generalized history, the word history having here the same meaning as in the phrase sum-over-histories. This proposal confers a certain independence on the concept of event, and it modifies the rules of inference concerning events in order to resolve a contradiction between the idea of reality as a single history and the principle that events of zero measure cannot happen (the Kochen-Specker paradox being a classic expression of this contradiction). The so-called measurement problem is then solved if macroscopic events satisfy classical rules of inference, and this can in principle be decided by a calculation. The resulting conception of reality involves neither multiple worlds nor external observers. It is therefore suitable for quantum gravity in general and causal sets in particular.
Is the cosmological "constant" a nonlocal quantum residue of discreteness of the causal set type?
Rafael D. Sorkin
Physics , 2007, DOI: 10.1063/1.2823750
Abstract: The evidence for an accelerating Hubble expansion appears to have confirmed the heuristic prediction, from causal set theory, of a fluctuating and ``ever-present'' cosmological term in the Einstein equations. A more concrete phenomenological model incorporating this prediction has been devised and tested, but it remains incomplete. I will review these developments and also mention a possible consequence for the dimensionality of spacetime.
Impossible Measurements on Quantum Fields
Rafael D. Sorkin
Physics , 1993,
Abstract: It is shown that the attempt to extend the notion of ideal measurement to quantum field theory leads to a conflict with locality, because (for most observables) the state vector reduction associated with an ideal measurement acts to transmit information faster than light. Two examples of such information-transfer are given, first in the quantum mechanics of a pair of coupled subsystems, and then for the free scalar field in flat spacetime. It is argued that this problem leaves the Hilbert space formulation of quantum field theory with no definite measurement theory, removing whatever advantages it may have seemed to possess vis a vis the sum-over-histories approach, and reinforcing the view that a sum-over-histories framework is the most promising one for quantum gravity.
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