Background:Residual force enhancement(rFE),defined as increased isometric force following active lengthening compared to a fixed-end isometric contraction at the same muscle length and level of activation,is present a...Background:Residual force enhancement(rFE),defined as increased isometric force following active lengthening compared to a fixed-end isometric contraction at the same muscle length and level of activation,is present across all scales of muscle.While rFE is always present at the cellular level,often rFE"non-re sponders"are observed during joint-level voluntary contractions.Methods:We compared rFE between the joint level and single fiber level(vastus lateralis biopsies)in 16 young males.In vivo voluntary kneeextensor rFE was measured by comparing steady-state isometric torque between a stretch-hold(maximal activation at 150°,stretch to 70°,hold)and a fixed-end isometric contraction,with ultrasonographic recording of vastus lateralis fascicle length(FL).Fixed-end contractions were performed at 67.5°,70.0°,72.5°,and 75.0°;the joint angle that most closely matched FL of the stretch-hold contraction's isometric steady-state was used to calculate rFE.The starting and ending FLs of the stretch-hold contraction were expressed as%optimal FL,determined via torqueangle relationship.Resu lts:In single fiber experiments,the starting and ending fiber lengths were matched relative to optimal length determined from in vivo testing,yielding an average sarcomere excursion of~2.2-3.4μm.There was a greater magnitude of rFE at the single fiber(~20%)than joint level(~5%)(p=0.004),with"non-re sponders"only observed at the joint level.Conclusion:By comparing rFE across scales within the same participants,we show the development of the rFE non-responder phenomenon is upstream of rFE's cellular mechanisms,with rFE only lost rather than gained when scaling from single fibers to the joint level.展开更多
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
Natural adult aging is associated with many functional impairments of the human neuromuscular system.One of the more observable alterations is the loss of contractile muscle mass,termed sarcopenia.The loss of muscle m...Natural adult aging is associated with many functional impairments of the human neuromuscular system.One of the more observable alterations is the loss of contractile muscle mass,termed sarcopenia.The loss of muscle mass occurs primarily due to a progressive loss of viable motor units,and accompanying atrophy of remaining muscle fibers.Not only does the loss of muscle mass contribute to impaired function in old age,but alterations in fiber type and myosin heavy chain isoform expression also contribute to weaker,slower,and less powerful contracting muscles.This review will focus on motor unit loss associated with natural adult aging,age-related fatigability,and the age-related differences in strength across contractile muscle actions.展开更多
Background:Females are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities.This sex difference in fatigability becomes more variable during higher int...Background:Females are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities.This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions.While less fatiguing than isometric or concentric contractions,eccentric contractions induce greater and longer lasting impairments in force production.However,it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions.Methods:We investigated the effects of eccentric exercise-induced muscle weakness on time to task failure(TTF)during a sustained submaximal isometric contraction in young(18-30 years)healthy males(n=9)and females(n=10).Participants performed a sustained isometric contraction of the dorsiflexors at 35°plantar flexion by matching a 30%maximal voluntary contraction(MVC)torque target until task failure(i.e.,falling below 5%of their target torque for>2 s).The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions.Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles,respectively.Results:Males were~41%stronger than females.Following eccentric exercise both males and females experienced an~20%decline in maximal voliuntary contraction torque.TTF was-34%longer in females than males prior to eccentic exercise-induced muscle weakness.However,following eccentric exercise-induced muscle weakness,this sex-related difference was abolished,with both groups having an"45%shorter TTF.Notably,there was~100%greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males.Conclusion:This increase in antagonist activation disadvantaged females by decreasing their TTF,resulting in a blunting of their typical fatigability advantage over males.展开更多
Background:Residual torque enhancement(rTE)is the increase in torque observed during the isometric steady state following active muscle lengthening when compared with a fixed-end isometric contraction at the same musc...Background:Residual torque enhancement(rTE)is the increase in torque observed during the isometric steady state following active muscle lengthening when compared with a fixed-end isometric contraction at the same muscle length and level of neuromuscular activation.In the rTE state,owing to an elevated contribution of passive force to total force production,less active force is required,and there is a subsequent reduction in activation.In vivo studies of rTE reporting an activation reduction are often performed using a dynamometer,where participants contract against a rigid restraint,resisting a torque motor.rTE has yet to be investigated during a position task,which involves the displacement of an inertial load with positional control.Methods:A total of 12 participants(6 males,6 females;age=22.8±1.1 years,height=174.7±8.6 cm,mass=82.1±37.7 kg;mean±SD)completed torque-and position-matching tasks at 60%maximum voluntary contraction for a fixed-end isometric contraction and an isometric contraction following active lengthening of the ankle dorsiflexors.Results:There were no significant differences in activation between torque-and position-matching tasks(p=0.743),with^27%activation reduction following active lengthening for both task types(p<0.001).Conclusion:These results indicate that rTE is a feature of voluntary,position-controlled contractions.These findings support and extend previous findings of isometric torque-control conditions to position-controlled contractions that represent different tasks of daily living.展开更多
基金supported by the Natural Sciences and Engineering Research Council of Canada(NSERC,Grant No.RGPIN-2024-03782).
文摘Background:Residual force enhancement(rFE),defined as increased isometric force following active lengthening compared to a fixed-end isometric contraction at the same muscle length and level of activation,is present across all scales of muscle.While rFE is always present at the cellular level,often rFE"non-re sponders"are observed during joint-level voluntary contractions.Methods:We compared rFE between the joint level and single fiber level(vastus lateralis biopsies)in 16 young males.In vivo voluntary kneeextensor rFE was measured by comparing steady-state isometric torque between a stretch-hold(maximal activation at 150°,stretch to 70°,hold)and a fixed-end isometric contraction,with ultrasonographic recording of vastus lateralis fascicle length(FL).Fixed-end contractions were performed at 67.5°,70.0°,72.5°,and 75.0°;the joint angle that most closely matched FL of the stretch-hold contraction's isometric steady-state was used to calculate rFE.The starting and ending FLs of the stretch-hold contraction were expressed as%optimal FL,determined via torqueangle relationship.Resu lts:In single fiber experiments,the starting and ending fiber lengths were matched relative to optimal length determined from in vivo testing,yielding an average sarcomere excursion of~2.2-3.4μm.There was a greater magnitude of rFE at the single fiber(~20%)than joint level(~5%)(p=0.004),with"non-re sponders"only observed at the joint level.Conclusion:By comparing rFE across scales within the same participants,we show the development of the rFE non-responder phenomenon is upstream of rFE's cellular mechanisms,with rFE only lost rather than gained when scaling from single fibers to the joint level.
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 an operating grant to Dr.Charles Rice from the Natural Sciences and Engineering Research Council of Canada (NSERC)supported by the Canadian Institutes of Health Research (CIHR)+1 种基金the Michael Smith Foundation for Health Research (MSFHR)supported by Alberta Innovates Health Solutions (AIHS)
文摘Natural adult aging is associated with many functional impairments of the human neuromuscular system.One of the more observable alterations is the loss of contractile muscle mass,termed sarcopenia.The loss of muscle mass occurs primarily due to a progressive loss of viable motor units,and accompanying atrophy of remaining muscle fibers.Not only does the loss of muscle mass contribute to impaired function in old age,but alterations in fiber type and myosin heavy chain isoform expression also contribute to weaker,slower,and less powerful contracting muscles.This review will focus on motor unit loss associated with natural adult aging,age-related fatigability,and the age-related differences in strength across contractile muscle actions.
基金supported by the Natural Sciences and Engineering Research Council of Canada(NSERC)。
文摘Background:Females are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities.This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions.While less fatiguing than isometric or concentric contractions,eccentric contractions induce greater and longer lasting impairments in force production.However,it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions.Methods:We investigated the effects of eccentric exercise-induced muscle weakness on time to task failure(TTF)during a sustained submaximal isometric contraction in young(18-30 years)healthy males(n=9)and females(n=10).Participants performed a sustained isometric contraction of the dorsiflexors at 35°plantar flexion by matching a 30%maximal voluntary contraction(MVC)torque target until task failure(i.e.,falling below 5%of their target torque for>2 s).The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions.Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles,respectively.Results:Males were~41%stronger than females.Following eccentric exercise both males and females experienced an~20%decline in maximal voliuntary contraction torque.TTF was-34%longer in females than males prior to eccentic exercise-induced muscle weakness.However,following eccentric exercise-induced muscle weakness,this sex-related difference was abolished,with both groups having an"45%shorter TTF.Notably,there was~100%greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males.Conclusion:This increase in antagonist activation disadvantaged females by decreasing their TTF,resulting in a blunting of their typical fatigability advantage over males.
基金supported by the Natural Sciences and Engineering Research Council of Canada(NSERC),Grant number:03829Infrastructure was provided by the University of Guelph start-up funding.
文摘Background:Residual torque enhancement(rTE)is the increase in torque observed during the isometric steady state following active muscle lengthening when compared with a fixed-end isometric contraction at the same muscle length and level of neuromuscular activation.In the rTE state,owing to an elevated contribution of passive force to total force production,less active force is required,and there is a subsequent reduction in activation.In vivo studies of rTE reporting an activation reduction are often performed using a dynamometer,where participants contract against a rigid restraint,resisting a torque motor.rTE has yet to be investigated during a position task,which involves the displacement of an inertial load with positional control.Methods:A total of 12 participants(6 males,6 females;age=22.8±1.1 years,height=174.7±8.6 cm,mass=82.1±37.7 kg;mean±SD)completed torque-and position-matching tasks at 60%maximum voluntary contraction for a fixed-end isometric contraction and an isometric contraction following active lengthening of the ankle dorsiflexors.Results:There were no significant differences in activation between torque-and position-matching tasks(p=0.743),with^27%activation reduction following active lengthening for both task types(p<0.001).Conclusion:These results indicate that rTE is a feature of voluntary,position-controlled contractions.These findings support and extend previous findings of isometric torque-control conditions to position-controlled contractions that represent different tasks of daily living.
