The Experts below are selected from a list of 183 Experts worldwide ranked by ideXlab platform
Gerald E. Loeb - One of the best experts on this subject based on the ideXlab platform.
-
Measured and modeled properties of mammalian skeletal muscle: IV. Dynamics of activation and deactivation
Journal of Muscle Research & Cell Motility, 2000Co-Authors: Ian E. Brown, Gerald E. LoebAbstract:The interactive effects of length and stimulus frequency on rise and fall times and on sag were investigated in fast-twitch feline Caudofemoralis at normal body temperature. The length and stimulus frequency ranges studied were 0.8–1.2 L _0 and 15–60 pps. Isometric rise times were shortest under two sets of conditions: short lengths + low stimulus frequencies and long lengths + high stimulus frequencies. In contrast the isometric fall time relationship showed a single minimum at short lengths + low stimulus frequencies. Velocity was shown to have an additional effect on fall time, but only at higher stimulus frequencies (40–60 pps): fall times were shorter during movement in either direction as compared to isometric. The effects of sag were greatest at shorter lengths and lower stimulus frequencies during isometric stimulus trains. Potential mechanisms underlying this last effect were investigated by comparing isometric twitches elicited prior to and immediately following a sag-inducing stimulus train. Post-sag twitches produced less force, reached peak force earlier and initially decayed more quickly compared to pre-sag twitches. However, the final rate of force decay and the initial rate of force rise (during the first 15 ms) were unaffected by sag. We construct a logical argument based on these findings to hypothesize that the predominant mechanism underlying sag is an increase in the rate of sarcoplasmic calcium ion removal. All of the above findings were used to construct a model of activation dynamics for fast-twitch muscle, which was then extrapolated to slow-twitch muscle. When coupled with a previous model of kinematic dynamics, the complete model produced accurate predictions of the forces actually recorded during experiments in which we applied concurrent dynamic changes in length, velocity and stimulus frequency.
-
Measured and modeled properties of mammalian skeletal muscle: III. the effects of stimulus frequency on stretch-induced force enhancement and shortening-induced force depression
Journal of Muscle Research & Cell Motility, 2000Co-Authors: Ian E. Brown, Gerald E. LoebAbstract:Stretch-induced force enhancement and shortening-induced force depression were examined in fast-twitch feline Caudofemoralis muscle at 37^∘C. These phenomena were induced by applying ramp length changes during the first 100–200 ms of an otherwise isometric contraction. The effects of various stimulus frequencies ranging from 30 to 120 pps were investigated over lengths ranging from 0.85 to 1.15 L _0. Distributed asynchronous stimulation of bundles of ventral roots was employed to produce smooth contractions at sub-tetanic stimulus frequencies in whole muscle. Of the two components of force enhancement identified by Noble (1992) we observed only the transient component that decays with time; we did not observe residual force enhancement. The force depression that we observed was symmetrical in almost all respects to the transient force enhancement, and was unlike the shortening-induced de-activation and residual force depression identified by Edman (Edman, 1975; Edman et al. , 1993). Both transient force enhancement and depression were independent of work, load and activation. Reversals in the direction of ramp length changes following either an initial stretch or initial shortening were shown to cancel the effects of both transient force enhancement and transient force depression. The distances over which these cancellations could be achieved were different for the lengthening and shortening effects. This asymmetry can be reconciled with the predictions of Huxley's original cross-bridge mechanism by incorporating the recent suggestion that myosin heads can interact with multiple actin binding sites during a single ‘working’ stroke. We conclude that the types of force enhancement/depression that are most likely to be encountered under physiological conditions are the transient effects observed here, but that even these will have relatively little effect on force production during most natural behaviors.
-
Measured and modeled properties of mammalian skeletal muscle. II. The effectsof stimulus frequency on force-length and force-velocity relationships
Journal of Muscle Research & Cell Motility, 1999Co-Authors: Ian E. Brown, Ernest J. Cheng, Gerald E. LoebAbstract:Interactions between physiological stimulus frequencies, fascicle lengths and velocities were analyzed in feline Caudofemoralis (CF), a hindlimb skeletal muscle composed exclusively of fast-twitch fibers. Split ventral roots were stimulated asynchronously to produce smooth contractions at sub-tetanic stimulus frequencies. As described previously, the peak of the sub-tetanic force-length relationship was found to shift to longer lengths with decreases in stimulus frequency, indicating a length dependence for activation that is independent of filament overlap. The sub-tetanic force-velocity (FV) relationship was affected strongly both by stimulus frequency and by length; decreases in either decreased the slope of the FV relationship around isometric. The shapes of the force transients following stretch or shortening revealed that these effects were not due to a change in the instantaneous FV relationship; the relative shape of the force transients following stretch or shortening was independent of stimulus frequency and hardly affected by length. The effects of stimulus frequency and length on the sub-tetanic FV relationship instead appear to be caused by a time delay in the length-dependent changes of activation. In contrast to feline soleus muscle, which is composed exclusively of slow-twitch fibers, CF did not yield at sub-tetanic stimulus frequencies for the range of stretch velocities tested (up to 2 L_0/s). The data presented here were used to build a model of muscle that accounted well for all of the effects described. We extended our model to account for slow-twitch muscle by comparing our fast-twitch model with previously published data and then changing the necessary parameters to fit the data. Our slow-twitch model accounts well for all previous findings including that of yielding.
-
Measured and modeled properties of mammalian skeletal muscle. I. The effects of post-activation potentiation on the time course and velocity dependencies of force production
Journal of Muscle Research & Cell Motility, 1999Co-Authors: Ian E. Brown, Gerald E. LoebAbstract:Activation of mammalian fast-twitch skeletal muscle induces a persistent effect known as post-activation potentiation (PAP), classically defined as an increase in force production at sub-maximal levels of activation. The underlying mechanism is thought to be phosphorylation of the myosin regulatory light chain (MRLC), which leads to an increase in the rate constant for cross-bridge attachment (Sweeney et al., 1993). If true, this suggests the hypothesis that other contractile properties should be affected during PAP. Using a feline fast-twitch whole-muscle preparation (Caudofemoralis) at 37^∘C, we observed that PAP greatly increased tetanic forces during active lengthening, decreased isometric tetanic rise times and delayed isometric tetanic force relaxation. The first two of these effects were length dependent with a greater effect occurring at shorter lengths. These findings confirmed that PAP has other functionally important effects beyond a simple increase in sub-maximal isometric forces. Furthermore, length was found to have an effect independent of PAP on the shortening half of the FV relationship (less force was produced at longer lengths) and on the rate of force relaxation during the later stages of isometric tetanic force decay (slower relaxation at longer lengths). All of these findings can be explained with a simplified, two-state model of cross-bridge dynamics that accounts for the interaction of both interfilament spacing and MRLC phosphorylation on the apparent rate constants for cross-bridge attachment and detachment. These findings are largely consistent with data collected previously from reduced preparations such as skinned fibers at cold, unphysiological temperatures (e.g. 5^∘C). One finding that could not be explained by our model was that twitch fall times in the dispotentiated state were parabolically correlated with length, whereas in the potentiated state the relationship was linear. The time course of decay of this effect did not follow the time course of force dispotentiation, suggesting that there are other activation-dependent processes occurring in parallel with MRLC phosphorylation.
-
Post-Activation Potentiation—A Clue for Simplifying Models of Muscle Dynamics'
Integrative and Comparative Biology, 1998Co-Authors: Ian E. Brown, Gerald E. LoebAbstract:SYNOPSIS. Post-activation potentiation is a phenomena that occurs only in fasttwitch muscle fibers. Its main effect is to enhance muscle force at sub-maximal activation levels for a short duration of time following previous muscle activation. We characterized this phenomenon in feline Caudofemoralis (CF) muscle (composed of 100% fast-twitch muscle fibers) to understand its importance during physiological patterns of activation. During such patterns ( e.g. , 43 pps, 8 pulse trains delivered at 1 sec intervals) CF potentiated rapidly and apparently maximally. When CF was allowed to relax, potentiation decayed slowly with a time constant 20–40 x slower than the rise-time. The level of potentiation reached during the potentiating paradigm was stable in response to a wide range of stimuli, including various stimulation rates (15–120 pps) and various inter-train intervals (up to 10 sec). The shape of the twitch force-length curve for potentiated CF was similar to that of the tetanic force-length curve in either the potentiated or unpotentiated state. In contrast, the shape of the twitch force-length curve for unpotentiated CF was shifted markedly to the right accompanied by a narrowing of the curve's peak. We conclude from our observations that fast-twitch muscle fibers operate and should be modeled in a state of full potentiation, and that modeling the potentiated state may actuaUy be simpler than modeling the unpotentiated state.
Ian E. Brown - One of the best experts on this subject based on the ideXlab platform.
-
Measured and modeled properties of mammalian skeletal muscle: III. the effects of stimulus frequency on stretch-induced force enhancement and shortening-induced force depression
Journal of Muscle Research & Cell Motility, 2000Co-Authors: Ian E. Brown, Gerald E. LoebAbstract:Stretch-induced force enhancement and shortening-induced force depression were examined in fast-twitch feline Caudofemoralis muscle at 37^∘C. These phenomena were induced by applying ramp length changes during the first 100–200 ms of an otherwise isometric contraction. The effects of various stimulus frequencies ranging from 30 to 120 pps were investigated over lengths ranging from 0.85 to 1.15 L _0. Distributed asynchronous stimulation of bundles of ventral roots was employed to produce smooth contractions at sub-tetanic stimulus frequencies in whole muscle. Of the two components of force enhancement identified by Noble (1992) we observed only the transient component that decays with time; we did not observe residual force enhancement. The force depression that we observed was symmetrical in almost all respects to the transient force enhancement, and was unlike the shortening-induced de-activation and residual force depression identified by Edman (Edman, 1975; Edman et al. , 1993). Both transient force enhancement and depression were independent of work, load and activation. Reversals in the direction of ramp length changes following either an initial stretch or initial shortening were shown to cancel the effects of both transient force enhancement and transient force depression. The distances over which these cancellations could be achieved were different for the lengthening and shortening effects. This asymmetry can be reconciled with the predictions of Huxley's original cross-bridge mechanism by incorporating the recent suggestion that myosin heads can interact with multiple actin binding sites during a single ‘working’ stroke. We conclude that the types of force enhancement/depression that are most likely to be encountered under physiological conditions are the transient effects observed here, but that even these will have relatively little effect on force production during most natural behaviors.
-
Measured and modeled properties of mammalian skeletal muscle: IV. Dynamics of activation and deactivation
Journal of Muscle Research & Cell Motility, 2000Co-Authors: Ian E. Brown, Gerald E. LoebAbstract:The interactive effects of length and stimulus frequency on rise and fall times and on sag were investigated in fast-twitch feline Caudofemoralis at normal body temperature. The length and stimulus frequency ranges studied were 0.8–1.2 L _0 and 15–60 pps. Isometric rise times were shortest under two sets of conditions: short lengths + low stimulus frequencies and long lengths + high stimulus frequencies. In contrast the isometric fall time relationship showed a single minimum at short lengths + low stimulus frequencies. Velocity was shown to have an additional effect on fall time, but only at higher stimulus frequencies (40–60 pps): fall times were shorter during movement in either direction as compared to isometric. The effects of sag were greatest at shorter lengths and lower stimulus frequencies during isometric stimulus trains. Potential mechanisms underlying this last effect were investigated by comparing isometric twitches elicited prior to and immediately following a sag-inducing stimulus train. Post-sag twitches produced less force, reached peak force earlier and initially decayed more quickly compared to pre-sag twitches. However, the final rate of force decay and the initial rate of force rise (during the first 15 ms) were unaffected by sag. We construct a logical argument based on these findings to hypothesize that the predominant mechanism underlying sag is an increase in the rate of sarcoplasmic calcium ion removal. All of the above findings were used to construct a model of activation dynamics for fast-twitch muscle, which was then extrapolated to slow-twitch muscle. When coupled with a previous model of kinematic dynamics, the complete model produced accurate predictions of the forces actually recorded during experiments in which we applied concurrent dynamic changes in length, velocity and stimulus frequency.
-
Measured and modeled properties of mammalian skeletal muscle. II. The effectsof stimulus frequency on force-length and force-velocity relationships
Journal of Muscle Research & Cell Motility, 1999Co-Authors: Ian E. Brown, Ernest J. Cheng, Gerald E. LoebAbstract:Interactions between physiological stimulus frequencies, fascicle lengths and velocities were analyzed in feline Caudofemoralis (CF), a hindlimb skeletal muscle composed exclusively of fast-twitch fibers. Split ventral roots were stimulated asynchronously to produce smooth contractions at sub-tetanic stimulus frequencies. As described previously, the peak of the sub-tetanic force-length relationship was found to shift to longer lengths with decreases in stimulus frequency, indicating a length dependence for activation that is independent of filament overlap. The sub-tetanic force-velocity (FV) relationship was affected strongly both by stimulus frequency and by length; decreases in either decreased the slope of the FV relationship around isometric. The shapes of the force transients following stretch or shortening revealed that these effects were not due to a change in the instantaneous FV relationship; the relative shape of the force transients following stretch or shortening was independent of stimulus frequency and hardly affected by length. The effects of stimulus frequency and length on the sub-tetanic FV relationship instead appear to be caused by a time delay in the length-dependent changes of activation. In contrast to feline soleus muscle, which is composed exclusively of slow-twitch fibers, CF did not yield at sub-tetanic stimulus frequencies for the range of stretch velocities tested (up to 2 L_0/s). The data presented here were used to build a model of muscle that accounted well for all of the effects described. We extended our model to account for slow-twitch muscle by comparing our fast-twitch model with previously published data and then changing the necessary parameters to fit the data. Our slow-twitch model accounts well for all previous findings including that of yielding.
-
Measured and modeled properties of mammalian skeletal muscle. I. The effects of post-activation potentiation on the time course and velocity dependencies of force production
Journal of Muscle Research & Cell Motility, 1999Co-Authors: Ian E. Brown, Gerald E. LoebAbstract:Activation of mammalian fast-twitch skeletal muscle induces a persistent effect known as post-activation potentiation (PAP), classically defined as an increase in force production at sub-maximal levels of activation. The underlying mechanism is thought to be phosphorylation of the myosin regulatory light chain (MRLC), which leads to an increase in the rate constant for cross-bridge attachment (Sweeney et al., 1993). If true, this suggests the hypothesis that other contractile properties should be affected during PAP. Using a feline fast-twitch whole-muscle preparation (Caudofemoralis) at 37^∘C, we observed that PAP greatly increased tetanic forces during active lengthening, decreased isometric tetanic rise times and delayed isometric tetanic force relaxation. The first two of these effects were length dependent with a greater effect occurring at shorter lengths. These findings confirmed that PAP has other functionally important effects beyond a simple increase in sub-maximal isometric forces. Furthermore, length was found to have an effect independent of PAP on the shortening half of the FV relationship (less force was produced at longer lengths) and on the rate of force relaxation during the later stages of isometric tetanic force decay (slower relaxation at longer lengths). All of these findings can be explained with a simplified, two-state model of cross-bridge dynamics that accounts for the interaction of both interfilament spacing and MRLC phosphorylation on the apparent rate constants for cross-bridge attachment and detachment. These findings are largely consistent with data collected previously from reduced preparations such as skinned fibers at cold, unphysiological temperatures (e.g. 5^∘C). One finding that could not be explained by our model was that twitch fall times in the dispotentiated state were parabolically correlated with length, whereas in the potentiated state the relationship was linear. The time course of decay of this effect did not follow the time course of force dispotentiation, suggesting that there are other activation-dependent processes occurring in parallel with MRLC phosphorylation.
-
Post-Activation Potentiation—A Clue for Simplifying Models of Muscle Dynamics'
Integrative and Comparative Biology, 1998Co-Authors: Ian E. Brown, Gerald E. LoebAbstract:SYNOPSIS. Post-activation potentiation is a phenomena that occurs only in fasttwitch muscle fibers. Its main effect is to enhance muscle force at sub-maximal activation levels for a short duration of time following previous muscle activation. We characterized this phenomenon in feline Caudofemoralis (CF) muscle (composed of 100% fast-twitch muscle fibers) to understand its importance during physiological patterns of activation. During such patterns ( e.g. , 43 pps, 8 pulse trains delivered at 1 sec intervals) CF potentiated rapidly and apparently maximally. When CF was allowed to relax, potentiation decayed slowly with a time constant 20–40 x slower than the rise-time. The level of potentiation reached during the potentiating paradigm was stable in response to a wide range of stimuli, including various stimulation rates (15–120 pps) and various inter-train intervals (up to 10 sec). The shape of the twitch force-length curve for potentiated CF was similar to that of the tetanic force-length curve in either the potentiated or unpotentiated state. In contrast, the shape of the twitch force-length curve for unpotentiated CF was shifted markedly to the right accompanied by a narrowing of the curve's peak. We conclude from our observations that fast-twitch muscle fibers operate and should be modeled in a state of full potentiation, and that modeling the potentiated state may actuaUy be simpler than modeling the unpotentiated state.
Philip J Currie - One of the best experts on this subject based on the ideXlab platform.
-
The Anatomical and Functional Evolution of the Femoral Fourth Trochanter in Ornithischian Dinosaurs.
Anatomical Record-advances in Integrative Anatomy and Evolutionary Biology, 2019Co-Authors: Walter S. Persons, Philip J CurrieAbstract:: The femoral fourth trochanter is the attachment site of the Caudofemoralis musculature, which is the primary hindlimb retractor in most non-avian dinosaurs. Early ornithischian dinosaurs are uniquely characterized by a fourth trochanter with a prominent pendant process. Throughout the evolution of ornithischians, the fourth trochanter repeatedly converged on two major morphological changes: (1) the distal migration of the trochanter down the femoral shaft and (2) the loss of the pendant process. Both changes, as well as the original evolution of the pendant form, relate to a single major functional shift emphasizing caudofemoral leverage. Direct evidence of muscle scarring across the surface of the pendant process affirms that it served to extend the attachment of the primary Caudofemoralis brevis tendon distally. A proximally located fourth trochanter is the basal condition in dinosaurs and other archosaurs, and the development of a pendant process lengthened the functional lever arm with regard to the insertion of the Caudofemoralis. This adaptation afforded improved mechanical advantage, perhaps beneficial in the context of the newly assumed herbivorous diets of basal ornithischians. As some derived ornithischians increased in body size, a high-leverage system with a more distal Caudofemoralis attachment evolved. In some groups, the fourth trochanter as a whole descended down the femur, eventually reaching a point where the pendant process was unnecessary. Sauropodomorphs, the other great lineage of dinosaur herbivores, converged on the same high-leverage distal fourth trochanter arrangement, but without first transitioning through a prominent pendant form. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc.
-
Lower rotational inertia and larger leg muscles indicate more rapid turns in tyrannosaurids than in other large theropods
PeerJ, 2019Co-Authors: Eric Snively, Anthony P Russell, Heinrich Mallison, Haley D. O'brien, Donald M. Henderson, Lara A. Surring, Michael E. Burns, Thomas R. Holtz, Lawrence M. Witmer, Philip J CurrieAbstract:Synopsis: Tyrannosaurid dinosaurs had large preserved leg muscle attachments and low rotational inertia relative to their body mass, indicating that they could turn more quickly than other large theropods. Methods: To compare turning capability in theropods, we regressed agility estimates against body mass, incorporating superellipse-based modeled mass, centers of mass, and rotational inertia (mass moment of inertia). Muscle force relative to body mass is a direct correlate of agility in humans, and torque gives potential angular acceleration. Agility scores therefore include rotational inertia values divided by proxies for (1) muscle force (ilium area and estimates of m. Caudofemoralis longus cross-section), and (2) musculoskeletal torque. Phylogenetic ANCOVA (phylANCOVA) allow assessment of differences in agility between tyrannosaurids and non-tyrannosaurid theropods (accounting for both ontogeny and phylogeny). We applied conditional error probabilities a(p) to stringently test the null hypothesis of equal agility. Results: Tyrannosaurids consistently have agility index magnitudes twice those of allosauroids and some other theropods of equivalent mass, turning the body with both legs planted or pivoting over a stance leg. PhylANCOVA demonstrates definitively greater agilities in tyrannosaurids, and phylogeny explains nearly all covariance. Mass property results are consistent with those of other studies based on skeletal mounts, and between different figure-based methods (our main mathematical slicing procedures, lofted 3D computer models, and simplified graphical double integration). Implications: The capacity for relatively rapid turns in tyrannosaurids is ecologically intriguing in light of their monopolization of large (>400 kg), toothed dinosaurian predator niches in their habitats.
-
Lower rotational inertia and larger leg muscles indicate more rapid turns in tyrannosaurids than in other large theropods
2018Co-Authors: Eric Snively, Anthony P Russell, Heinrich Mallison, Donald M. Henderson, Lara A. Surring, Michael E. Burns, Lawrence M. Witmer, Haley O'brien, Thomas R Holtz, Jr., Philip J CurrieAbstract:Synopsis: Tyrannosaurid dinosaurs had larger than predicted preserved leg muscle attachments and low rotational inertia relative to their body mass, indicating that they could turn more quickly than other large theropods. Methods: To compare turning capability in theropods, we regressed agility estimates against body mass, incorporating superellipse-based modeled mass, centers of mass, and rotational inertia (mass moment of inertia). Muscle force relative to body mass is a direct correlate of agility in humans, and torque gives potential angular acceleration. Agility scores therefore include rotational inertia values divided by proxies for (1) muscle force (ilium area and estimates of m. Caudofemoralis longus cross-section), and (2) musculoskeletal torque. Phylogenetic ANCOVA (phylANCOVA) allow assessment of differences in agility between tyrannosaurids and non-tyrannosaurid theropods (accounting for both ontogeny and phylogeny). We applied conditional error probabilities a(p) to stringently test the null hypothesis of equal agility. Results: Tyrannosaurids consistently have agility index magnitudes twice those of allosauroids and some other theropods of equivalent mass, turning the body with both legs planted or pivoting over a stance leg. PhylANCOVA demonstrates definitively greater agilities in tyrannosaurids, and phylogeny explains nearly all covariance. Mass property results are consistent with those of other studies based on skeletal mounts, and between different figure-based methods (our main mathematical slicing procedures, lofted 3D computer models, and simplified graphical double integration). Implications: The capacity for relatively rapid turns in tyrannosaurids is ecologically intriguing in light of their monopolization of large (>400 kg), toothed dinosaurian predator niches in their habitats.
-
The functional origin of dinosaur bipedalism: Cumulative evidence from bipedally inclined reptiles and disinclined mammals.
Journal of Theoretical Biology, 2017Co-Authors: W. Scott Persons, Philip J CurrieAbstract:Bipedalism is a trait basal to, and widespread among, dinosaurs. It has been previously argued that bipedalism arose in the ancestors of dinosaurs for the function of freeing the forelimbs to serve as predatory weapons. However, this argument does not explain why bipedalism was retained among numerous herbivorous groups of dinosaurs. We argue that bipedalism arose in the dinosaur line for the purpose of enhanced cursoriality. Modern facultatively bipedal lizards offer an analog for the first stages in the evolution of dinosaurian bipedalism. Many extant lizards assume a bipedal stance while attempting to flee predators at maximum speed. Bipedalism, when combined with a Caudofemoralis musculature, has cursorial advantages because the Caudofemoralis provides a greater source of propulsion to the hindlimbs than is generally available to the forelimbs. That cursorial advantage explains the relative abundance of cursorial facultative bipeds and obligate bipeds among fossil diapsids and the relative scarcity of either among mammals. Having lost their Caudofemoralis in the Permian, perhaps in the context of adapting to a fossorial lifestyle, the mammalian line has been disinclined towards bipedalism, but, having never lost the Caudofemoralis of their ancestors, cursorial avemetatarsalians (bird-line archosaurs) were naturally inclined towards bipedalism.
-
Oviraptorosaur tail forms and functions
Acta Palaeontologica Polonica, 2013Co-Authors: W. Scott Persons, Philip J Currie, Mark A. NorellAbstract:Oviraptorosaur caudal osteology is unique among theropods and is characterized by posteriorly persistent and exceptionally wide transverse processes, anteroposteriorly short centra, and a high degree of flexibility across the pre-pygostyle vertebral series. Three-dimensional digital muscle reconstructions reveal that, while oviraptorosaur tails were reduced in length relative to the tails of other theropods, they were muscularly robust. Despite overall caudal length reduction, the relative size of the M. Caudofemoralis in most oviraptorosaurs was comparable with those of other non-avian theropods. The discovery of a second Nomingia specimen with a pygostyle confirms that the fused terminal vertebrae of the type specimen were not an abnormality. New evidence shows that pygostyles were also present in the oviraptorosaurs Citipati and Conchoraptor. Based on the observed osteological morphology and inferred muscle morphology, along with the recognition that many members of the group probably sported broad tai...
Ernest J. Cheng - One of the best experts on this subject based on the ideXlab platform.
-
Measured and modeled properties of mammalian skeletal muscle. II. The effectsof stimulus frequency on force-length and force-velocity relationships
Journal of Muscle Research & Cell Motility, 1999Co-Authors: Ian E. Brown, Ernest J. Cheng, Gerald E. LoebAbstract:Interactions between physiological stimulus frequencies, fascicle lengths and velocities were analyzed in feline Caudofemoralis (CF), a hindlimb skeletal muscle composed exclusively of fast-twitch fibers. Split ventral roots were stimulated asynchronously to produce smooth contractions at sub-tetanic stimulus frequencies. As described previously, the peak of the sub-tetanic force-length relationship was found to shift to longer lengths with decreases in stimulus frequency, indicating a length dependence for activation that is independent of filament overlap. The sub-tetanic force-velocity (FV) relationship was affected strongly both by stimulus frequency and by length; decreases in either decreased the slope of the FV relationship around isometric. The shapes of the force transients following stretch or shortening revealed that these effects were not due to a change in the instantaneous FV relationship; the relative shape of the force transients following stretch or shortening was independent of stimulus frequency and hardly affected by length. The effects of stimulus frequency and length on the sub-tetanic FV relationship instead appear to be caused by a time delay in the length-dependent changes of activation. In contrast to feline soleus muscle, which is composed exclusively of slow-twitch fibers, CF did not yield at sub-tetanic stimulus frequencies for the range of stretch velocities tested (up to 2 L_0/s). The data presented here were used to build a model of muscle that accounted well for all of the effects described. We extended our model to account for slow-twitch muscle by comparing our fast-twitch model with previously published data and then changing the necessary parameters to fit the data. Our slow-twitch model accounts well for all previous findings including that of yielding.
G E Loeb - One of the best experts on this subject based on the ideXlab platform.
-
Measured and modeled properties of mammalian skeletal muscle. I. The effects of post-activation potentiation on the time course and velocity dependencies of force production.
Journal of muscle research and cell motility, 1999Co-Authors: I E Brown, G E LoebAbstract:Activation of mammalian fast-twitch skeletal muscle induces a persistent effect known as post-activation potentiation (PAP), classically defined as an increase in force production at sub-maximal levels of activation. The underlying mechanism is thought to be phosphorylation of the myosin regulatory light chain (MRLC), which leads to an increase in the rate constant for cross-bridge attachment (Sweeney et al., 1993). If true, this suggests the hypothesis that other contractile properties should be affected during PAP. Using a feline fast-twitch whole-muscle preparation (Caudofemoralis) at 37 degrees C, we observed that PAP greatly increased tetanic forces during active lengthening decreased isometric tetanic rise times and delayed isometric tetanic force relaxation. The first two of these effects were length dependent with a greater effect occurring at shorter lengths. These findings confirmed that PAP has other functionally important effects beyond a simple increase in sub-maximal isometric forces. Furthermore, length was found to have an effect independent of PAP on the shortening half of the FV relationship (less force was produced at longer lengths) and on the rate of force relaxation during the later stages of isometric tetanic force decay (slower relaxation at longer lengths). All of these findings can be explained with a simplified, two-state model of cross-bridge dynamics that accounts for the interaction of both interfilament spacing and MRLC phosphorylation on the apparent rate constants for cross-bridge attachment and detachment. These findings are largely consistent with data collected previously from reduced preparations such as skinned fibers at cold, unphysiological temperatures (e.g. 5 degrees C). One finding that could not be explained by our model was that twitch fall times in the dispotentiated state were parabolically correlated with length, whereas in the potentiated state the relationship was linear. The time course of decay of this effect did not follow the time course of force dispotentiation, suggesting that there are other activation-dependent processes occurring in parallel with MRLC phosphorylation.
-
Functionally complex muscles of the cat hindlimb
Experimental Brain Research, 1991Co-Authors: C. A. Pratt, C. M. Chanaud, G E LoebAbstract:Similarities between the muscle synergies associated with the flexion reflex and locomotion in reduced preparations have suggested that spinal circuits subserving these two motor tasks might share common interneurons. To test this hypothesis in functionally complex muscles, we studied the interaction between low-threshold cutaneous afferents and the locomotor central pattern generator (CPG) during treadmill locomotion in awake, intact cats. Electrical stimuli were delivered via implanted nerve cuff electrodes at all phases of locomotion, and EMGs were recorded from fourteen intramuscular subregions in eight bifunctional thigh muscles (adductor femoris, biceps femoris, Caudofemoralis, gracilis, semimembranosus, semitendinosus, tensor fasciae latae, and tenuissimus). In addition, the EMG patterns recorded during locomotion were compared with those recorded during two other centrally driven rhythmical behaviors, scratching and paw shaking, to determine whether the functional relationships among these intramuscular subregions were fixed or task dependent. Four of the five broad, bifunctional muscles studied (biceps femoris, gracilis, semimembranosus, and tensor fasciae latae) had functional subunits that could be differentially activated in one or more of the three movements studied; adductor femoris was consistently uniformly activated despite its distributed skeletal attachments. The pattern of recruitment of the intramuscular functional subunits was movement-specific. The locomotor CPG and cutaneous reflex pathways both similarly subdivided some bifunctional muscles, but not others, into intramuscular subregions. The results of the present study confirm that some combinations of muscle subregions and cutaneous nerves constitute simple reciprocal categories of flexors and extensors, as described originally by Sherrington (1910). “Typical” low threshold excitatory or inhibitory reflex responses were produced in muscles or muscle subregions that were recruited as “net” flexors of extensors, respectively. However, muscles with complex activation patterns during walking often had very individualized, complex reflex responses during locomotion that did not conform to the background locomotion synergies. All of the reflex responses observed were mediated by low threshold cutaneous afferents. These data indicate that there are multiple, low threshold, excitatory and inhibitory cutaneous reflex pathways that have highly specialized connections with flexor and extensor muscles and even their intramuscular subregions. It is also clear that the premotoneuronal circuits mediating these cutaneous reflex effects are not necessarily synonymous with those of the locomotor CPG. These two systems do interact powerfully, however, suggesting some convergence. The nature of the convergence between the CPG and the many independent subsets of spinal interneurons mediating cutaneous reflexes is specialized and muscle subregion-specific.