Humans Jumping on Flexible Structures: Effect of Structural Properties

Volume 2 ◽  
2004 ◽  
Author(s):  
ShiPing Yao ◽  
Robert E. Harrison ◽  
Jan R. Wright ◽  
Aleksandar Pavic ◽  
Paul Reynolds
Keyword(s):  
Structural Integrity ◽  
Damping Ratio ◽  
Flexible Structures ◽  
Vertical Motion ◽  
Drop Out ◽  
Mass Ratio ◽  
Test Rig ◽  
Near Resonance ◽  
The Subject ◽  
Force Drop

The behaviour of humans jumping on flexible structures has become a matter of some importance for both structural integrity and also human tolerance. The issue is of great interest for stadia, footbridge and floor structures. A test rig has been developed for exploring the forces, accelerations and displacements that occur when a human subject jumps on a flexible structure where motion can be perceived. In tests reported earlier, it was found that the human is able to generate near resonant response of the structure but it was extremely difficult, if not impossible, to jump at or very near to the natural frequency of the structure when the structural vertical motion is significant. Also, the force developed by the subject was found to drop significantly near resonance. In this paper, the effect of the subject-to-structure mass ratio and the damping ratio of the structure on the ability of the subject to jump near resonance, and on the force drop out, is presented. It is shown that as the structure becomes more massive and more highly damped it moves less for nominally the same jumping excitation. In this situation, it becomes easier to jump near resonance and the degree of force drop out reduces, though it is still significant.

scholarly journals On the Force Drop Off Phenomenon in Shaker Testing in Experimental Modal Analysis

2002 ◽  
Vol 9 (4-5) ◽  
pp. 165-175 ◽  
Author(s):  
Paulo Sergio Varoto ◽  
Leopoldo Pisanelli Rodrigues de Oliveira
Keyword(s):  
Input Signal ◽  
Modal Testing ◽  
Analytical Models ◽  
Vibration Exciter ◽  
Electromechanical Device ◽  
Vibration Tests ◽  
Operational Modes ◽  
Electrodynamic Vibration ◽  
Force Drop

The Electrodynamic Vibration Exciter (shakers) has been one of the most employed excitation sources in modal tests. The shaker is an electromechanical device that provides a mechanical motion due to the input signal sent to its coil. Despite being widely used, it is well known that the shaker interacts with the structure under test. In particular, when the structure passes through a given resonance, the force delivered by the shaker abruptly decreases, causing the so called drop off phenomenon. This paper aims to study this force drop off phenomenon in the single shaker modal testing. Analytical models are developed to help in understanding the physical principles involved in the interaction between the shaker and the structure under test. Experimental analyses are performed using different shakers as well as excitation signals, in order to evaluate the effects of the input signal, as well as the power amplifier operational modes, on the structure dynamics. Preliminary tests revealed that significant distortions might occur during vibration tests using shakers and these distortions significantly affect the determination of the structure response.

Twitch Interpolation in Human Muscles: Mechanisms and Implications for Measurement of Voluntary Activation

1999 ◽  
Vol 82 (5) ◽  
pp. 2271-2283 ◽  
Author(s):  
R. D. Herbert ◽  
S. C. Gandevia
Keyword(s):  
Time Course ◽  
Full Range ◽  
Electrical Stimulus ◽  
Voluntary Contraction ◽  
Voluntary Activation ◽  
Time To Peak ◽  
Motoneuron Pool ◽  
Twitch Interpolation ◽  
Muscle Nerve ◽  
Force Drop

An electrical stimulus delivered to a muscle nerve during a maximal voluntary contraction usually produces a twitchlike increment in force. The amplitude of this “interpolated twitch” is widely used to measure voluntary “activation” of muscles. In the present study, a computer model of the human adductor pollicis motoneuron pool was used to investigate factors that affect the interpolated twitch. Antidromic occlusion of naturally occurring orthodromic potentials was modeled, but reflex effects of the stimulus were not. In simulations, antidromic collisions occurred with probabilities of between ∼16% (in early recruited motoneurons) and nearly 100% (in late recruited motoneurons). The model closely predicted experimental data on the amplitude and time course of the rising phase of interpolated twitches over the full range of voluntary forces, except that the amplitude of interpolated twitches was slightly overestimated at intermediate contraction intensities. Small interpolated twitches (4.7% of the resting twitch) were evident in simulated maximal voluntary contractions, but were nearly completely occluded when mean peak firing rate was increased to ∼60 Hz. Simulated interpolated twitches did not show the marked force drop that follows the peak of the twitch, and when antidromic collisions were excluded from the model interpolated twitch amplitude was slightly increased and time-to-peak force was prolonged. These findings suggest that both antidromic and reflex effects reduce the amplitude of the interpolated twitch and contribute to the force drop that follows the twitch. The amplitude of the interpolated twitch was related to “excitation” of the motoneuron pool in a nonlinear way, so that at near-maximal contraction intensities (>90% maximal voluntary force) increases in excitation produced only small changes in interpolated twitch amplitude. Thus twitch interpolation may not provide a sensitive measure of motoneuronal excitation at near-maximal forces. Increases in the amplitude of interpolated twitches such as have been observed in fatigue and various pathologies may reflect large reductions in excitation of the motoneuron pool.

scholarly journals Akt activation prevents the force drop induced by eccentric contractions in dystrophin-deficient skeletal muscle

2008 ◽  
Vol 17 (23) ◽  
pp. 3686-3696 ◽  
Author(s):  
Bert Blaauw ◽  
Cristina Mammucari ◽  
Luana Toniolo ◽  
Lisa Agatea ◽  
Reimar Abraham ◽  
...  
Keyword(s):  
Skeletal Muscle ◽  
Eccentric Contractions ◽  
Akt Activation ◽  
Force Drop

scholarly journals Transient kinetics measured with force steps discriminate between double-stranded DNA elongation and melting and define the reaction energetics

2013 ◽  
Vol 42 (5) ◽  
pp. 3436-3449 ◽  
Author(s):  
Lorenzo Bongini ◽  
Luca Melli ◽  
Vincenzo Lombardi ◽  
Pasquale Bianco
Keyword(s):  
Entropy Change ◽  
Structural Factors ◽  
Transient Kinetics ◽  
Double Stranded Dna ◽  
Phage Dna ◽  
Thermal Melting ◽  
Kinetics Of ◽  
Entropic Contribution ◽  
Force Drop

Abstract Under a tension of ∼65 pN, double-stranded DNA undergoes an overstretching transition from its basic (B-form) conformation to a 1.7 times longer conformation whose nature is only recently starting to be understood. Here we provide a structural and thermodynamic characterization of the transition by recording the length transient following force steps imposed on the λ-phage DNA with different melting degrees and temperatures (10–25°C). The shortening transient following a 20–35 pN force drop from the overstretching force shows a sequence of fast shortenings of double-stranded extended (S-form) segments and pauses owing to reannealing of melted segments. The lengthening transients following a 2–35 pN stretch to the overstretching force show the kinetics of a two-state reaction and indicate that the whole 70% extension is a B-S transition that precedes and is independent of melting. The temperature dependence of the lengthening transient shows that the entropic contribution to the B-S transition is one-third of the entropy change of thermal melting, reinforcing the evidence for a double-stranded S-form that maintains a significant fraction of the interstrand bonds. The cooperativity of the unitary elongation (22 bp) is independent of temperature, suggesting that structural factors, such as the nucleic acid sequence, control the transition.

scholarly journals Dystrophin restoration therapy improves both the reduced excitability and the force drop induced by lengthening contractions in dystrophic mdx skeletal muscle

2016 ◽  
Vol 6 (1) ◽  
Author(s):  
Pauline Roy ◽  
Fredérique Rau ◽  
Julien Ochala ◽  
Julien Messéant ◽  
Bodvael Fraysse ◽  
...  
Keyword(s):  
Skeletal Muscle ◽  
Lengthening Contractions ◽  
Force Drop

Acoustic Emission and Force Drop in Grain Crushing of Carbonate Sands

2019 ◽  
Vol 145 (9) ◽  
pp. 04019057 ◽  
Author(s):  
Yang Xiao ◽  
Lei Wang ◽  
Xiang Jiang ◽  
T. Matthew Evans ◽  
Armin W. Stuedlein ◽  
...  
Keyword(s):  
Acoustic Emission ◽  
Grain Crushing ◽  
Force Drop

scholarly journals Determination of thermodynamics and kinetics of RNA reactions by force

2006 ◽  
Vol 39 (4) ◽  
pp. 325-360 ◽  
Author(s):  
Ignacio Tinoco ◽  
Pan T. X. Li ◽  
Carlos Bustamante
Keyword(s):  
Free Energy ◽  
Single Molecule ◽  
Rate Constants ◽  
Magnetic Tweezers ◽  
Constant Force ◽  
Thermodynamics And Kinetics ◽  
Wide Range ◽  
Kinetic Mechanisms ◽  
Kinetics Of ◽  
Force Drop

1. Introduction 3262. Instrumentation 3282.1 Instruments to study mechanical properties of RNA 3282.1.1 AFM 3282.1.2 Magnetic tweezers 3282.1.3 Optical tweezers 3302.2 Optical trap instrumentation 3302.3 Calibrations 3322.3.1 Calibration of trap stiffness 3322.3.2 Calibration of force 3332.3.3 Calibration of distance 3342.4 Types of experiments 3342.4.1 Force-ramp 3342.4.2 Force-clamp or constant-force experiments 3352.4.3 Extension-clamp or constant extension experiments 3352.4.4 Force-jump, Force-drop 3362.4.5 Passive mode 3363. Thermodynamics 3363.1 Reversibility 3363.2 Gibbs free energy 3373.2.1 Stretching free energy 3383.2.1.1 Rigid molecules 3383.2.1.2 Compliant or flexible molecules 3393.2.2 Free energy of a reversible unfolding transition 3393.2.3 Free energy of unfolding at zero force 3403.2.4 Free energy of an irreversible unfolding transition 3403.2.4.1 Jarzynski's method 3413.2.4.2 Crooks fluctuation theorem 3434. Kinetics 3454.1 Measuring rate constants 3454.1.1 Hopping 3454.1.2 Force-jump, Force-drop 3474.1.3 Force-ramp 3484.1.4 Instrumental effects 3504.2 Kinetic mechanisms 3514.2.1 Free-energy landscapes 3514.2.2 Kinetics of unfolding 3535. Relating force-measured data to other measurements 3545.1 Thermodynamics 3545.2 Kinetics 3576. Acknowledgements 3577. References 358Single-molecule methods have made it possible to apply force to an individual RNA molecule. Two beads are attached to the RNA; one is on a micropipette, the other is in a laser trap. The force on the RNA and the distance between the beads are measured. Force can change the equilibrium and the rate of any reaction in which the product has a different extension from the reactant. This review describes use of laser tweezers to measure thermodynamics and kinetics of unfolding/refolding RNA. For a reversible reaction the work directly provides the free energy; for irreversible reactions the free energy is obtained from the distribution of work values. The rate constants for the folding and unfolding reactions can be measured by several methods. The effect of pulling rate on the distribution of force-unfolding values leads to rate constants for unfolding. Hopping of the RNA between folded and unfolded states at constant force provides both unfolding and folding rates. Force-jumps and force-drops, similar to the temperature jump method, provide direct measurement of reaction rates over a wide range of forces. The advantages of applying force and using single-molecule methods are discussed. These methods, for example, allow reactions to be studied in non-denaturing solvents at physiological temperatures; they also simplify analysis of kinetic mechanisms because only one intermediate at a time is present. Unfolding of RNA in biological cells by helicases, or ribosomes, has similarities to unfolding by force.

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