Background:The isometric steady-state following active lengthening is associated with greater torque production and lower activation,as measured by electromyographic activity(EMG),in comparison with a purely isometric...Background:The isometric steady-state following active lengthening is associated with greater torque production and lower activation,as measured by electromyographic activity(EMG),in comparison with a purely isometric contraction(ISO)at the same joint angle.This phenomenon is termed residual force enhancement(RFE).While there has been a great deal of research investigating the basic mechanisms of RFE,little work has been performed to understand the everyday relevance of RFE.The purpose of this study was to investigate whether neuromuscular control strategies differ between ISO and RFE by measuring torque steadiness of the human ankle plantar flexors.Methods:Following ISO maximal voluntary contractions in 12 males(25±4 years),an active lengthening contraction was performed at 15°/s over a 30°ankle excursion,ending at the same joint angle as ISO(5°dorsiflexion;RFE).Surface EMG of the tibialis anterior and soleus muscles was recorded during all tasks.Torque steadiness was determined as the standard deviation(SD)and coefficient of variation(CV)of the torque trace in the ISO and RFE condition during activation-matching(20%and 60%integrated EMG)and torque-matching(20%and 60%maximal voluntary contraction)experiments.Two-tailed,paired t tests were used,within subjects,to determine the presence of RFE/activation reduction(AR)and whether there was a difference in torque steadiness between ISO and RFE conditions.Results:During the maximal and submaximal conditions,there was 5%-9%RFE with a 9%-11%AR(p<0.05),respectively,with no difference in antagonist coactivation between RFE and ISO(p>0.05).There were no differences in SD and CV of the torque trace for the 20%and60%activation-matching or the 60%and maximal torque-matching trials in either the RFE or ISO condition(p>0.05).During the 20%torquematching trial,there were～37%higher values for SD and CV in the RFE as compared with the ISO condition(p<0.05).A significant moderate-to-strong negative relationship was identified between the reduction in torque steadiness following active lengthening and the accompanying AR(p<0.05).Conclusion:It appears that while the RFE-associated AR provides some improved neuromuscular economy,this comes at the cost of increased torque fluctuations in the isometric steady-state following active lengthening during submaximal contractions.展开更多
Direct numerical simulations of Mach 6 hypersonic flow over a 34°compression corner subject to steady jet are conducted.Distributions of skin friction coefficient,wall pressure,mean velocity and temperature,bound...Direct numerical simulations of Mach 6 hypersonic flow over a 34°compression corner subject to steady jet are conducted.Distributions of skin friction coefficient,wall pressure,mean velocity and temperature,boundary layer thickness and Stanton number demonstrate that the flow changes dramatically in the shock wave/turbulent boundary layer interaction area.It is found that the steady jet has no effect on suppressing flow separation unexpectedly,but increases its spatial scale instead.Instantaneous flow structures show that the turbulence amplification can be observed after the application of flow control,and abundant Görtler-like vorticities appear,but the strength of the main shock decreases.Analyzing the wall fluctuating pressure signals using weighted power spectral density,we found an interesting thing.That is,although the low-frequency oscillation phenomenon induced by separation shock is suppressed by the steady jet,wall fluctuating pressure beneath the jet shock is oscillating at a frequency lower than 0.1u∞/δref.Results of coherent and intermittency factor reveal that it is related to the backand-forth movement of the jet shock itself.展开更多
Steady forcing can induce the amplitude death in chaotic systems, which generally exists in coupled dynamic systems. Using the Lorenz system as a typical example, this paper investigates the dynamic behaviours of the ...Steady forcing can induce the amplitude death in chaotic systems, which generally exists in coupled dynamic systems. Using the Lorenz system as a typical example, this paper investigates the dynamic behaviours of the chaotic system with steady forcing numerically, and finds that amplitude death can occur as the strength of the steady forcing goes beyond a critical constant.展开更多
基金supported by the Natural Sciences and Engineering Research Council of Canada
文摘Background:The isometric steady-state following active lengthening is associated with greater torque production and lower activation,as measured by electromyographic activity(EMG),in comparison with a purely isometric contraction(ISO)at the same joint angle.This phenomenon is termed residual force enhancement(RFE).While there has been a great deal of research investigating the basic mechanisms of RFE,little work has been performed to understand the everyday relevance of RFE.The purpose of this study was to investigate whether neuromuscular control strategies differ between ISO and RFE by measuring torque steadiness of the human ankle plantar flexors.Methods:Following ISO maximal voluntary contractions in 12 males(25±4 years),an active lengthening contraction was performed at 15°/s over a 30°ankle excursion,ending at the same joint angle as ISO(5°dorsiflexion;RFE).Surface EMG of the tibialis anterior and soleus muscles was recorded during all tasks.Torque steadiness was determined as the standard deviation(SD)and coefficient of variation(CV)of the torque trace in the ISO and RFE condition during activation-matching(20%and 60%integrated EMG)and torque-matching(20%and 60%maximal voluntary contraction)experiments.Two-tailed,paired t tests were used,within subjects,to determine the presence of RFE/activation reduction(AR)and whether there was a difference in torque steadiness between ISO and RFE conditions.Results:During the maximal and submaximal conditions,there was 5%-9%RFE with a 9%-11%AR(p<0.05),respectively,with no difference in antagonist coactivation between RFE and ISO(p>0.05).There were no differences in SD and CV of the torque trace for the 20%and60%activation-matching or the 60%and maximal torque-matching trials in either the RFE or ISO condition(p>0.05).During the 20%torquematching trial,there were～37%higher values for SD and CV in the RFE as compared with the ISO condition(p<0.05).A significant moderate-to-strong negative relationship was identified between the reduction in torque steadiness following active lengthening and the accompanying AR(p<0.05).Conclusion:It appears that while the RFE-associated AR provides some improved neuromuscular economy,this comes at the cost of increased torque fluctuations in the isometric steady-state following active lengthening during submaximal contractions.
基金supported by the National Natural Science Foundation of China(Grant Nos.12202488 and 12002377)the Natural Science Program of National University of Defense Technology(Grant No.ZK22-30)Independent Cultivation Project for Young Talents in College of Aerospace Science and Engineering.
文摘Direct numerical simulations of Mach 6 hypersonic flow over a 34°compression corner subject to steady jet are conducted.Distributions of skin friction coefficient,wall pressure,mean velocity and temperature,boundary layer thickness and Stanton number demonstrate that the flow changes dramatically in the shock wave/turbulent boundary layer interaction area.It is found that the steady jet has no effect on suppressing flow separation unexpectedly,but increases its spatial scale instead.Instantaneous flow structures show that the turbulence amplification can be observed after the application of flow control,and abundant Görtler-like vorticities appear,but the strength of the main shock decreases.Analyzing the wall fluctuating pressure signals using weighted power spectral density,we found an interesting thing.That is,although the low-frequency oscillation phenomenon induced by separation shock is suppressed by the steady jet,wall fluctuating pressure beneath the jet shock is oscillating at a frequency lower than 0.1u∞/δref.Results of coherent and intermittency factor reveal that it is related to the backand-forth movement of the jet shock itself.
基金Project supported by the National Natural Science Foundation of China （Grant No 40325015）, the National Key Program Development for Basic Research （Grant No 2006CB400503）. The authors would like to thank Professors Yang Jun-Zhong and Zhu Fu-Cheng for help, and especially thank the anonymous reviewers for their valuable comments.
文摘Steady forcing can induce the amplitude death in chaotic systems, which generally exists in coupled dynamic systems. Using the Lorenz system as a typical example, this paper investigates the dynamic behaviours of the chaotic system with steady forcing numerically, and finds that amplitude death can occur as the strength of the steady forcing goes beyond a critical constant.
