BIOMECHANICAL TESTING OF A COMPLIANT BENCH FOR STEP AEROBICS

INTRODUCTION The purpose of this study was to determine if adding compliance to the bench used in step aerobics routines would decrease the stress on the musculoskeletal system. The stresses are imposed by the shock of the ground reaction forces (external) and due to joint compression caused by muscles attempting to absorb and generate forces (internal). Injury has been related to the amount of force and the rate of force application. Repetitive strain was not a primary consideration in this study. METHODS The present investigation involved four step aerobics instructors and 11 recreational calibre subjects. Each subject performed two types of stepping exercises at a cadence of 120 beats per minute. A compliant bench was constructed with similar dimensions to the traditional rigid bench. The major difference was that the compliant bench had an arched top of plywood that would deform by becoming more flat as it was loaded. The stiffness of the top was about 8 KN*m-l. Two force plates were used to measure the ground and bench reaction forces. The data were sampled at 100 samples per second and ensemble averaged over 15 steps for each subject and each task to obtain a representative record and a measure of the variability. The representative record was analysed for peak forces and each individual step was analysed for the rate of rise of force. The rate of rise of force was processed by filtering out the high frequency noise of the force channel and differentiating the signal with respect to time. EMG of the knee extensors of the right leg was also examined. Four two-way analyses of variance with repeated measures were applied to the peak force and rate of rise of force data. The two factors were the type of bench and state of fatigue. The significance level was chosen to be p < 0.05. RESULTS The step-to-step variability within each subject was quite low. The peak ground reaction forces of the compliant bench were not significantly different for the instructors on each bench and the EMG of the vastus lateralis did not show a significant increase in muscle activity to absorb shock on the rigid bench. The 11 recreational calibre subjects showed a main effect for three of the four analyses. The compliant bench was found to have significantly lower rates of rise of force in both the step and propulsive moves and a lower peak force in the propulsive move. The peak forces were lower in the step move but not enough for statistical significance. There was no main effect for fatigue or interaction between fatigue and bench type. CONCLUSIONS Risk of injury by either high peak force or muscle force was neither increased nor decreased when the instructors used the compliant bench. It appears that professionals are able to provide musculoskeletal shock absorption without significantly increasing the muscle activation when performing on a rigid surface. Recreational athletes, however, were not as capable and would benefit more by exercising on a compliant surface

Cold Atom Physics Using Ultra-Thin Optical Fibers: Light-Induced Dipole Forces and Surface Interactions

The strong evanescent field around ultra-thin unclad optical fibers bears a high potential for detecting, trapping, and manipulating cold atoms. Introducing such a fiber into a cold atom cloud, we investigate the interaction of a small number of cold Caesium atoms with the guided fiber mode and with the fiber surface. Using high resolution spectroscopy, we observe and analyze light-induced dipole forces, van der Waals interaction, and a significant enhancement of the spontaneous emission rate of the atoms. The latter can be assigned to the modification of the vacuum modes by the fiber.Comment: 4 pages, 4 figure

Design of C-sections Against Deformational Lip Buckling

Deformational lip buckling is one of the considerations affecting the choice of cross-section dimensions which were adopted for the C-sectlon chord members of a space frame system. No guidance on lip buckling of channel sections is given in the current British design specification. A method was therefore required for determining the lip buckling resistance of the members when subjected to axial compression or bending. A design procedure was formulated in which the lips are represented as a strut on an elastic foundation, the stiffness of which depends on the deformational stiffness of the section. The critical buckling stress and the wavelength of buckling can then be determined. By assigning imperfections to the nominally straight lips, the critical lip buckling stress can be introduced into the Perry-Roberston formulation for the strength of columns, and the lip buckling strength can be estimated. Comparative resulis from the design formulation, experiments, and non-linear elasto-plasttc finite element analysis are given. It is shown that the proposed design model is satisfactory but conservative

Heralded Two-Photon Entanglement from Probabilistic Quantum Logic Operations on Multiple Parametric Down-Conversion Sources

An ideal controlled-NOT gate followed by projective measurements can be used to identify specific Bell states of its two input qubits. When the input qubits are each members of independent Bell states, these projective measurements can be used to swap the post-selected entanglement onto the remaining two qubits. Here we apply this strategy to produce heralded two-photon polarization entanglement using Bell states that originate from independent parametric down-conversion sources, and a particular probabilistic controlled-NOT gate that is constructed from linear optical elements. The resulting implementation is closely related to an earlier proposal by Sliwa and Banaszek [quant-ph/0207117], and can be intuitively understood in terms of familiar quantum information protocols. The possibility of producing a ``pseudo-demand'' source of two-photon entanglement by storing and releasing these heralded pairs from independent cyclical quantum memory devices is also discussed.Comment: 5 pages, 4 figures; submitted to IEEE Journal of Selected Topics in Quantum Electronics, special issue on "Quantum Internet Technologies

The time-reversal test for stochastic quantum dynamics

The calculation of quantum dynamics is currently a central issue in theoretical physics, with diverse applications ranging from ultra-cold atomic Bose-Einstein condensates (BEC) to condensed matter, biology, and even astrophysics. Here we demonstrate a conceptually simple method of determining the regime of validity of stochastic simulations of unitary quantum dynamics by employing a time-reversal test. We apply this test to a simulation of the evolution of a quantum anharmonic oscillator with up to $6.022\times10^{23}$ (Avogadro's number) of particles. This system is realisable as a Bose-Einstein condensate in an optical lattice, for which the time-reversal procedure could be implemented experimentally.Comment: revtex4, two figures, four page

Microcavities coupled to multilevel atoms

A three-level atom in the $\Lambda$-configuration coupled to a microcavity is studied. The two transitions of the atom are assumed couple to different counterpropagating mode pairs in the cavity. We analyze the dynamics both, in the strong-coupling and the bad cavity limit. We find that compared to a two-level setup, the third atomic state and the additional control field modes crucially modify the system dynamics and enable more advanced control schemes. All results are explained using appropriate dressed state and eigenmode representations. As potential applications, we discuss optical switching and turnstile operations and detection of particles close to the resonator surface.Comment: 14 pages, 9 figure

Quantum reflection of atoms from a solid surface at normal incidence

We observed quantum reflection of ultracold atoms from the attractive potential of a solid surface. Extremely dilute Bose-Einstein condensates of ^{23}Na, with peak density 10^{11}-10^{12}atoms/cm^3, confined in a weak gravito-magnetic trap were normally incident on a silicon surface. Reflection probabilities of up to 20 % were observed for incident velocities of 1-8 mm/s. The velocity dependence agrees qualitatively with the prediction for quantum reflection from the attractive Casimir-Polder potential. Atoms confined in a harmonic trap divided in half by a solid surface exhibited extended lifetime due to quantum reflection from the surface, implying a reflection probability above 50 %.Comment: To appear in Phys. Rev. Lett. (December 2004)5 pages, 4 figure

Husimi's Q(α)Q(\alpha) function and quantum interference in phase space

We discuss a phase space description of the photon number distribution of non classical states which is based on Husimi's $Q(\alpha)$ function and does not rely in the WKB approximation. We illustrate this approach using the examples of displaced number states and two photon coherent states and show it to provide an efficient method for computing and interpreting the photon number distribution . This result is interesting in particular for the two photon coherent states which, for high squeezing, have the probabilities of even and odd photon numbers oscillating independently.Comment: 15 pages, 12 figures, typos correcte

Optical interface created by laser-cooled atoms trapped in the evanescent field surrounding an optical nanofiber

Trapping and optically interfacing laser-cooled neutral atoms is an essential requirement for their use in advanced quantum technologies. Here we simultaneously realize both of these tasks with cesium atoms interacting with a multi-color evanescent field surrounding an optical nanofiber. The atoms are localized in a one-dimensional optical lattice about 200 nm above the nanofiber surface and can be efficiently interrogated with a resonant light field sent through the nanofiber. Our technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes. Moreover, it is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices