Purpose:The aim of this study was to extend current half-sarcomere models by involving a recently found force-mediated activation of the thick filament and analyze the effect of this mechanosensing regulation on the l...Purpose:The aim of this study was to extend current half-sarcomere models by involving a recently found force-mediated activation of the thick filament and analyze the effect of this mechanosensing regulation on the length stability of half-sarcomeres arranged in series.Methods:We included a super-relaxed state of myosin motors and its force-dependent activation in a conventional cross-bridge model.We simulated active stretches of a sarcomere consisting of 2 non-uniform half-sarcomeres on the descending limb of the force-length relationship.Results:The mechanosensing model predicts that,in a passive sarcomere on the descending limb of the force-length relationship,the longer half-sarcomere has a higher fraction of myosin motors in the on-state than the shorter half-sarcomere.The difference in the number of myosin motors in the on-state ensures that upon calcium-mediated thin filament activation,the force-dependent thick filament activation keeps differences in active force within 20%during an active stretch.In the classical cross-bridge model,the corresponding difference exceeds 80%,leading to great length instabilities.Conclusion:Our simulations suggest that,in contrast to the classical cross-bridge model,the mechanosensing regulation is able to stabilize a system of non-uniform half-sarcomeres arranged in series on the descending limb of the force-length relationship.展开更多
It is well acknowledged that muscles that are elongated while activated(i.e.,eccentric muscle action)are stronger and require less energy(per unit of force)than muscles that are shortening(i.e.,concentric contraction)...It is well acknowledged that muscles that are elongated while activated(i.e.,eccentric muscle action)are stronger and require less energy(per unit of force)than muscles that are shortening(i.e.,concentric contraction)or that remain at a constant length(i.e.,isometric contraction).Although the cross-bridge theory of muscle contraction provides a good explanation for the increase in force in active muscle lengthening,it does not explain the residual increase in force following active lengthening(residual force enhancement),or except with additional assumptions,the reduced metabolic requirement of muscle during and following active stretch.Aside from the cross-bridge theory,2 other primary explanations for the mechanical properties of actively stretched muscles have emerged:(1)the so-called sarcomere length nonuniformity theory and(2)the engagement of a passive structural element theory.In this article,these theories are discussed,and it is shown that the last of these—the engagement of a passive structural element in eccentric muscle action—offers a simple and complete explanation for many hitherto unexplained observations in actively lengthening muscle.Although by no means fully proven,the theory has great appeal for its simplicity and beauty,and even if over time it is shown to be wrong,it nevertheless forms a useful framework for direct hypothesis testing.展开更多
Background:The steady-state increase in muscle force generating potential following a lengthening contraction is called residual force enhancement(RFE).In this study,we aimed to test for differences in torque,electrom...Background:The steady-state increase in muscle force generating potential following a lengthening contraction is called residual force enhancement(RFE).In this study,we aimed to test for differences in torque,electromyographic activity(EMG),and the associated neuromuscular efficiency(NME)between isometric voluntary contractions of elbow flexors preceded and not preceded by a lengthening contraction.The dependence of such differences on(i)stretch amplitude,(ii)the region of the force-length(FxL)relationship where contraction occurs,and(iii)the individual's ability to produce(negative)work during the stretch was investigated.Methods:Sixteen healthy adults participated in the study.Elbow flexor torque,angle,and biceps brachii EMG for purely isometric contractions(reference contractions)and for isometric contractions preceded by active stretches of 20°and 40°were measured at the ascending,plateau,and descending regions of subject-specific FxL curves.All contractions were performed in an isokinetic dynamometer.Two-factor(stretch×FxL region)repeated measures analysis of variance ANOVAs was used to analyze the effect of active stretch on EMG,torque,and NME across conditions.The relationships between mechanical work during stretch-calculated as the torque-angular displacement integral-and the changes in EMG,torque,and NME were analyzed using Pearson correlation.Results:In general,torque,EMG,and NME following active stretches differed from the values observed for the purely isometric reference contractions.While although the detailed effects of active stretch on torque and EMG differed between regions of the FxL relationship,NME increased by about 19%for all muscle lengths.Up to 30%of the interindividual variability in torque generating potential change in response to active stretching was accounted for by differences in(negative)work capacity between subjects.Conclusion:Our results suggest that(i)RFE contributes to"flatten"the elbow flexor torque-angle relationship,favoring torque production at lengths where the purely isometric torques are reduced substantially,and(ii)RFE contributes to a reduction in energy cost of torque production during isometric contractions for the entire operating range.展开更多
Objective:We reviewed and appraised the existing evidence of in vivo manifestations of residual force enhancement in human skeletal muscles and assessed,through a meta-analysis,the effect of an immediate history of ec...Objective:We reviewed and appraised the existing evidence of in vivo manifestations of residual force enhancement in human skeletal muscles and assessed,through a meta-analysis,the effect of an immediate history of eccentric contraction on the subsequent torque capacity of voluntary and electrically evoked muscle contractions.Methods:Our search was conducted from database inception to May 2020.Descriptive information was extracted from,and quality was assessed for,45 studies.Meta-analyses and metaregressions were used to analyze residual torque enhancement and its dependence on the angular amplitude of the preceding eccentric contraction.Results:Procedures varied across studies with regards to muscle group tested,angular stretch amplitude,randomization of contractions,time window analyzed,and verbal command.Torque capacity in isometric(constant muscle tendon unit length and joint angle)contractions preceded by an eccentric contraction was typically greater compared to purely isometric contractions,and this effect was greater for electrically evoked muscle contractions than voluntary contractions.Residual torque enhancement differed across muscle groups for the voluntary contractions,with a significant enhancement in torque observed for the adductor pollicis,ankle dorsiflexors,ankle plantar flexors,and knee extensors,but not for the elbow and knee flexors.Meta-regressions revealed that the angular amplitude of the eccentric contraction(normalized to the respective joints full range of motion)was not associated with the residual torque enhancement observed.Conclusion:There is evidence of residual torque enhancement for most,but not all,muscle groups,and residual torque enhancement is greater for electrically evoked than for voluntary contractions.Contrary to our hypothesis,and contrary to generally accepted findings on isolated muscle preparations,residual torque enhancement in voluntary and electrically evoked contractions does not seem to depend on the angular amplitude of the preceding eccentric contraction.展开更多
文摘Purpose:The aim of this study was to extend current half-sarcomere models by involving a recently found force-mediated activation of the thick filament and analyze the effect of this mechanosensing regulation on the length stability of half-sarcomeres arranged in series.Methods:We included a super-relaxed state of myosin motors and its force-dependent activation in a conventional cross-bridge model.We simulated active stretches of a sarcomere consisting of 2 non-uniform half-sarcomeres on the descending limb of the force-length relationship.Results:The mechanosensing model predicts that,in a passive sarcomere on the descending limb of the force-length relationship,the longer half-sarcomere has a higher fraction of myosin motors in the on-state than the shorter half-sarcomere.The difference in the number of myosin motors in the on-state ensures that upon calcium-mediated thin filament activation,the force-dependent thick filament activation keeps differences in active force within 20%during an active stretch.In the classical cross-bridge model,the corresponding difference exceeds 80%,leading to great length instabilities.Conclusion:Our simulations suggest that,in contrast to the classical cross-bridge model,the mechanosensing regulation is able to stabilize a system of non-uniform half-sarcomeres arranged in series on the descending limb of the force-length relationship.
文摘It is well acknowledged that muscles that are elongated while activated(i.e.,eccentric muscle action)are stronger and require less energy(per unit of force)than muscles that are shortening(i.e.,concentric contraction)or that remain at a constant length(i.e.,isometric contraction).Although the cross-bridge theory of muscle contraction provides a good explanation for the increase in force in active muscle lengthening,it does not explain the residual increase in force following active lengthening(residual force enhancement),or except with additional assumptions,the reduced metabolic requirement of muscle during and following active stretch.Aside from the cross-bridge theory,2 other primary explanations for the mechanical properties of actively stretched muscles have emerged:(1)the so-called sarcomere length nonuniformity theory and(2)the engagement of a passive structural element theory.In this article,these theories are discussed,and it is shown that the last of these—the engagement of a passive structural element in eccentric muscle action—offers a simple and complete explanation for many hitherto unexplained observations in actively lengthening muscle.Although by no means fully proven,the theory has great appeal for its simplicity and beauty,and even if over time it is shown to be wrong,it nevertheless forms a useful framework for direct hypothesis testing.
文摘Background:The steady-state increase in muscle force generating potential following a lengthening contraction is called residual force enhancement(RFE).In this study,we aimed to test for differences in torque,electromyographic activity(EMG),and the associated neuromuscular efficiency(NME)between isometric voluntary contractions of elbow flexors preceded and not preceded by a lengthening contraction.The dependence of such differences on(i)stretch amplitude,(ii)the region of the force-length(FxL)relationship where contraction occurs,and(iii)the individual's ability to produce(negative)work during the stretch was investigated.Methods:Sixteen healthy adults participated in the study.Elbow flexor torque,angle,and biceps brachii EMG for purely isometric contractions(reference contractions)and for isometric contractions preceded by active stretches of 20°and 40°were measured at the ascending,plateau,and descending regions of subject-specific FxL curves.All contractions were performed in an isokinetic dynamometer.Two-factor(stretch×FxL region)repeated measures analysis of variance ANOVAs was used to analyze the effect of active stretch on EMG,torque,and NME across conditions.The relationships between mechanical work during stretch-calculated as the torque-angular displacement integral-and the changes in EMG,torque,and NME were analyzed using Pearson correlation.Results:In general,torque,EMG,and NME following active stretches differed from the values observed for the purely isometric reference contractions.While although the detailed effects of active stretch on torque and EMG differed between regions of the FxL relationship,NME increased by about 19%for all muscle lengths.Up to 30%of the interindividual variability in torque generating potential change in response to active stretching was accounted for by differences in(negative)work capacity between subjects.Conclusion:Our results suggest that(i)RFE contributes to"flatten"the elbow flexor torque-angle relationship,favoring torque production at lengths where the purely isometric torques are reduced substantially,and(ii)RFE contributes to a reduction in energy cost of torque production during isometric contractions for the entire operating range.
文摘Objective:We reviewed and appraised the existing evidence of in vivo manifestations of residual force enhancement in human skeletal muscles and assessed,through a meta-analysis,the effect of an immediate history of eccentric contraction on the subsequent torque capacity of voluntary and electrically evoked muscle contractions.Methods:Our search was conducted from database inception to May 2020.Descriptive information was extracted from,and quality was assessed for,45 studies.Meta-analyses and metaregressions were used to analyze residual torque enhancement and its dependence on the angular amplitude of the preceding eccentric contraction.Results:Procedures varied across studies with regards to muscle group tested,angular stretch amplitude,randomization of contractions,time window analyzed,and verbal command.Torque capacity in isometric(constant muscle tendon unit length and joint angle)contractions preceded by an eccentric contraction was typically greater compared to purely isometric contractions,and this effect was greater for electrically evoked muscle contractions than voluntary contractions.Residual torque enhancement differed across muscle groups for the voluntary contractions,with a significant enhancement in torque observed for the adductor pollicis,ankle dorsiflexors,ankle plantar flexors,and knee extensors,but not for the elbow and knee flexors.Meta-regressions revealed that the angular amplitude of the eccentric contraction(normalized to the respective joints full range of motion)was not associated with the residual torque enhancement observed.Conclusion:There is evidence of residual torque enhancement for most,but not all,muscle groups,and residual torque enhancement is greater for electrically evoked than for voluntary contractions.Contrary to our hypothesis,and contrary to generally accepted findings on isolated muscle preparations,residual torque enhancement in voluntary and electrically evoked contractions does not seem to depend on the angular amplitude of the preceding eccentric contraction.
