Stretch Velocity Research Articles

Impact of lengthening velocity on the generation of eccentric force by slow-twitch muscle fibers in long stretches

After an initial increase, isovelocity elongation of a muscle fiber can lead to diminishing (referred to as Give in the literature) and subsequently increasing force. How the stretch velocity affects this behavior in slow-twitch fibers remains largely unexplored. Here, we stretched fully activated individual rat soleus muscle fibers from 0.85 to 1.3 optimal fiber length at stretch velocities of 0.01, 0.1, and 1 maximum shortening velocity, vmax, and compared the results with those of rat EDL fast-twitch fibers obtained in similar experimental conditions. In soleus muscle fibers, Give was 7%, 18%, and 44% of maximum isometric force for 0.01, 0.1, and 1 vmax, respectively. As in EDL fibers, the force increased nearly linearly in the second half of the stretch, although the number of crossbridges decreased, and its slope increased with stretch velocity. Our findings are consistent with the concept of a forceful detachment and subsequent crossbridge reattachment in the stretch’s first phase and a strong viscoelastic titin contribution to fiber force in the second phase of the stretch. Interestingly, we found interaction effects of stretch velocity and fiber type on force parameters in both stretch phases, hinting at fiber type-specific differences in crossbridge and titin contributions to eccentric force. Whether fiber type-specific combined XB and non-XB models can explain these effects or if they hint at some not fully understood properties of muscle contraction remains to be shown. These results may stimulate new optimization perspectives in sports training and provide a better understanding of structure–function relations of muscle proteins.

Mapping of spastic muscle activity after stroke: difference between passive stretch and active contraction

BackgroundInvestigating the spatial distribution of muscle activity would facilitate understanding the underlying mechanism of spasticity. The purpose of this study is to investigate the characteristics of spastic muscles during passive stretch and active contraction by high-density surface electromyography (HD-sEMG).MethodsFourteen spastic hemiparetic subjects and ten healthy subjects were recruited. The biceps brachii (BB) muscle activity of each subject was recorded by HD-sEMG during passive stretch at four stretch velocities (10, 60, 120, 180˚/s) and active contraction at three submaximal contraction levels (20, 50, 80%MVC). The intensity and spatial distribution of the BB activity were compared by the means of two-way analysis of variance, independent sample t-test, and paired sample t-test.ResultsCompared with healthy subjects, spastic hemiparetic subjects showed significantly higher intensity with velocity-dependent heterogeneous activation during passive stretch and more lateral and proximal activation distribution during active contraction. In addition, spastic hemiparetic subjects displayed almost non-overlapping activation areas during passive stretch and active contraction. The activation distribution of passive stretch was more distal when compared with the active contraction.ConclusionsThese alterations of the BB activity could be the consequence of deficits in the descending central control after stroke. The complementary spatial distribution of spastic BB activity reflected their opposite motor units (MUs) recruitment patterns between passive stretch and active contraction. This HD-sEMG study provides new neurophysiological evidence for the spatial relationship of spastic BB activity between passive stretch and active contraction, advancing our knowledge on the mechanism of spasticity.Trial registrationChiCTR2000032245.

Muscle function and structure in a mouse model of dysfunctional central nervous system myelin

INTRODUCTION. Claudin-11(CLDN11)-deficient mice have been reported to have dysfunctional central nervous system myelin, and altered motor control and muscle tone in their hindlimbs. OBJECTIVE. We compared CLDN11-deficient mice with strain-, age-, and sex-matched controls (CLDN11-suffent mice). We hypothesized that CLDN11-deficient mice will have reduced muscle strength and will show a greater resistance to high stretch velocities compared to their CLDN11-suffcient counterparts. METHODS. We studied the ankle dorsiflexor and plantarflexor muscles in a convenience sample of ten CLDN11-deficient and four CLDN11-suffcient mice (3-4 months, males). We recorded the maximum isometric contractile torque of the ankle plantarflexor and dorsiflexor muscles measured in milli-Newton-millimeter (mNmm), and the maximum contractile torque at low (100 °/s) and high (1200 °/s) muscle stretch velocities during eccentric contractions to assess spasticity. We also studied structural changes in muscle cross sections stained with hematoxylin and eosin. RESULTS. Data are reported as mean ± SD. Maximum plantarflexor contractile torque was 7800 ± 1651 mNmm and 9646 ± 2637 mNmm in CLDN11-deficient and -suffcient mice, respectively. Maximum dorsiflexor contractile torque was 1956 ± 347 mNmm and 2975 ± 195 mNmm in CLDN11-deficient and -suffcient mice, respectively. Spasticity assessed as the ratio of maximum eccentric torque measured at 1200 °/s to 100 °/s was 0.94 ± 0.07 and 0.93 ± 0.01 in the plantarflexors of CLDN11-deficient and -suffcient mice, respectively, and 1.41 ± 0.19 and 2.1 ± 0.33 in the dorsiflexors of CLDN11-deficient and -suffcient mice, respectively. One-way ANOVAs indicated that CLDN11-deficient and -suffcient mice were not statistically different (p>0.05) in maximum torque production and spasticity. CLDN11-deficient mice showed no remarkable alterations in muscle structure. CONCLUSION. Claudin 11 deficiency in mice does not affect maximum contractile torque and stretch-velocity-dependent muscle tone when studied by robotic dynamometry under general anesthesia, nor does it prominently affect muscle structure. We thank the WSU UROP funding mechanism for supporting Ms. Iman Manzoor’s involvement on this project. We thank the Wayne State University Doctor of Physical Therapy (DPT) Program for supporting student research and providing research training through coursework and research experiences. We thank Dr. Alexander Gow for sharing mice and mentorship. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A Velocity Stretch Reflex Threshold Based on Muscle-Tendon Unit Peak Acceleration to Detect Possible Occurrences of Spasticity during Gait in Children with Cerebral Palsy.

Spasticity might affect gait in children with cerebral palsy. Quantifying its occurrence during locomotion is challenging. One approach is to determine kinematic stretch reflex thresholds, usually on the velocity, during passive assessment and to search for their exceedance during gait. These thresholds are determined through EMG-Onset detection algorithms, which are variable in performance and sensitive to noisy data, and can therefore lack consistency. This study aimed to evaluate the feasibility of determining the velocity stretch reflex threshold from maximal musculotendon acceleration. Eighteen children with CP were recruited and underwent clinical gait analysis and a full instrumented assessment of their soleus, gastrocnemius lateralis, semitendinosus, and rectus femoris spasticity, with EMG, kinematics, and applied forces being measured simultaneously. Using a subject-scaled musculoskeletal model, the acceleration-based stretch reflex velocity thresholds were determined and compared to those based on EMG-Onset determination. Their consistencies according to physiological criteria, i.e., if the timing of the threshold was between the beginning of the stretch and the spastic catch, were evaluated. Finally, two parameters designed to evaluate the occurrence of spasticity during gait, i.e., the proportion of the gait trial time with a gait velocity above the velocity threshold and the number of times the threshold was exceeded, were compared. The proposed method produces velocity stretch reflex thresholds close to the EMG-based ones. For all muscles, no statistical difference was found between the two parameters designed to evaluate the occurrence of spasticity during gait. Contrarily to the EMG-based methods, the proposed method always provides physiologically consistent values, with median electromechanical delays of between 50 and 130 ms. For all subjects, the semitendinosus velocity during gait usually exceeded its stretch reflex threshold, while it was less frequent for the three other muscles. We conclude that a velocity stretch reflex threshold, based on musculotendon acceleration, is a reliable substitute for EMG-based ones.

Muscle preflex response to perturbations in locomotion: In vitro experiments and simulations with realistic boundary conditions.

Neuromuscular control loops feature substantial communication delays, but mammals run robustly even in the most adverse conditions. In vivo experiments and computer simulation results suggest that muscles' preflex-an immediate mechanical response to a perturbation-could be the critical contributor. Muscle preflexes act within a few milliseconds, an order of magnitude faster than neural reflexes. Their short-lasting action makes mechanical preflexes hard to quantify in vivo. Muscle models, on the other hand, require further improvement of their prediction accuracy during the non-standard conditions of perturbed locomotion. Our study aims to quantify the mechanical work done by muscles during the preflex phase (preflex work) and test their mechanical force modulation. We performed in vitro experiments with biological muscle fibers under physiological boundary conditions, which we determined in computer simulations of perturbed hopping. Our findings show that muscles initially resist impacts with a stereotypical stiffness response-identified as short-range stiffness-regardless of the exact perturbation condition. We then observe a velocity adaptation to the force related to the amount of perturbation similar to a damping response. The main contributor to the preflex work modulation is not the change in force due to a change in fiber stretch velocity (fiber damping characteristics) but the change in magnitude of the stretch due to the leg dynamics in the perturbed conditions. Our results confirm previous findings that muscle stiffness is activity-dependent and show that also damping characteristics are activity-dependent. These results indicate that neural control could tune the preflex properties of muscles in expectation of ground conditions leading to previously inexplicable neuromuscular adaptation speeds.

Validity, Intra-Rater Reliability and Normative Data of the Neuroflexor™ Device to Measure Spasticity of the Ankle Plantar Flexors after Stroke.

Quantification of lower limb spasticity after stroke and the differentiation of neural from passive muscle resistance remain key clinical challenges. The aim of this study was to validate the novel NeuroFlexor foot module, to assess the intrarater reliability of measurements and to identify normative cut-off values. Fifteen patients with chronic stroke with clinical history of spasticity and 18 healthy subjects were examined with the NeuroFlexor foot module at controlled velocities. Elastic, viscous and neural components of passive dorsiflexion resistance were quantified (in Newton, N). The neural component, reflecting stretch reflex mediated resistance, was validated against electromyography activity. A test-retest design with a 2-way random effects model permitted study of intra-rater reliability. Finally, data from 73 healthy subjects were used to establish cutoff values according to mean + 3 standard deviations and receiver operating characteristic curve analysis. The neural component was higher in stroke patients, increased with stretch velocity and correlated with electromyography amplitude. Reliability was high for the neural component (intraclass correlation coefficient model 2.1 (ICC2,1) ≥ 0.903) and good for the elastic component (ICC2,1 ≥ 0.898). Cutoff values were identified, and all patients with neural component above the limit presented pathological electromyography amplitude (area under the curve (AUC) = 1.00, sensitivity = 100%, specificity = 100%). The NeuroFlexor may offer a clinically feasible and non-invasive way to objectively quantify lower limb spasticity.

Revising the stretch reflex threshold method to measure stretch hyperreflexia in cerebral palsy.

Hyper-resistance is an increased resistance to passive muscle stretch, a common feature in neurological disorders. Stretch hyperreflexia, an exaggerated stretch reflex response, is the neural velocity-dependent component of hyper-resistance, and has been quantitatively measured using the stretch reflex threshold (i.e., joint angle at the stretch reflex electromyographic onset). In this study, we introduce a correction in how the stretch reflex threshold is calculated, by accounting for the stretch reflex latency (i.e., time between the stretch reflex onset at the muscle spindles and its appearance in the electromyographic signal). Furthermore, we evaluated how this correction affects the stretch reflex threshold in children and young adults with spastic cerebral palsy. A motor-driven ankle dynamometer induced passive ankle dorsiflexions at four incremental velocities in 13 children with cerebral palsy (mean age: 13.5years, eight males). The stretch reflex threshold for soleus and medial gastrocnemius muscles was calculated as 1) the joint angle corresponding to the stretch reflex electromyographic onset (i.e., original method); and as 2) the joint angle corresponding to the electromyographic onset minus the individual Hoffmann-reflex latency (i.e., latency corrected method). The group linear regression slopes between stretch velocity and stretch reflex threshold differed in both muscles between methods (p < 0.05). While the original stretch reflex threshold was velocity dependent in both muscles (p < 0.05), the latency correction rendered it velocity independent. Thus, the effects of latency correction on the stretch reflex threshold are substantial, especially at higher stretch velocities, and should be considered in future studies.

How velocity impacts eccentric force generation of fully activated skinned skeletal muscle fibers in long stretches

Ecccentric muscle contractions are fundamental to everyday life. They occur markedly in jumping, running, and accidents. Following an initial force rise, stretching of a fully activated muscle can result in a phase of decreasing force ("Give") followed by force redevelopment. However, how the stretch velocity affects "Give" and force redevelopment remains largely unknown. We investigated the force produced by fully activated single-skinned fibers of rat extensor digitorum longus muscles during long stretches. Fibers were pulled from length 0.85 to 1.3 optimal fiber length at a rate of 1%, 10%, and 100% of the estimated maximum shortening velocity. "Give" was absent in slow stretches. Medium and fast stretches yielded a clear "Give." After the initial force peak, forces decreased by 11.2% and 27.8% relative to the initial peak force before rising again. During the last half of the stretch (from 1.07 to 1.3 optimal fiber length, which is within the range of the expected descending limb of the force-length relationship), the linear force slope tripled from slow to medium stretch and increased further by 60% from medium to fast stretch. These results are compatible with forcible cross-bridge detachment and redevelopment of a cross-bridge distribution, and a viscoelastic titin contribution to fiber force. Accounting for these results can improve muscle models and predictions of multibody simulations.NEW & NOTEWORTHY Eccentric muscle contractions are part of our daily lives. We found that force increased monotonically during slow stretches of fully activated muscle fibers, whereas higher stretch velocities resulted in an increasing drop in force after an initial increase and a final steeper rise in force. Cross-bridges cannot explain the observed force traces. This requires a viscoelastic non-cross-bridge contribution. Considering these results can improve muscle models and predictions of multibody simulations.

Fluid flow over a vertical stretching surface within a porous medium filled by a nanofluid containing gyrotactic microorganisms

In this paper, the boundary layer flow of a nanofluid containing gyrotactic microorganisms over a vertical stretching surface embedded in a porous medium. The fluid contains gyrotactic microorganisms. Nonlinear velocity stretches the surface, and the surface is isothermal. The governing boundary layer equations for steady, laminar, and two-dimensional are transformed into ordinary differential equations using a suitable similarity transformation. The fourth/fifth-order Runge–Kutta method solved the system of equations after linearization. The shooting method guesses the missing boundary conditions. Pertinent results are presented graphically and discussed quantitatively with respect to variation in the controlling parameters such as bioconvection Lewis number (Lb), Lewis number (Le), Peclet Lewis number (Pe), buoyancy ratio parameter (Nr), bioconvection Rayleigh number(Rb), Brownian motion parameter (Nb), thermophoresis parameter (Nt), Richardson number (Ri), motile parameter (sigma), porosity parameter (user2{ }lambda), and Prandtl number (Pr) on dimensionless velocity, temperature, nanoparticle concentration, and microorganisms conservation. It is observed that increasing the porosity parameter retards the thermal boundary layer thickness which from application point of view, it is obvious that the surface cooling effect is enhanced and thus nanofluids are appropriate as heat transfer fluids. The comparison with the previous validates the applied numerical scheme.

Residual force enhancement is attenuated for quick stretch conditions

It is generally accepted that the magnitude of residual force enhancement is not affected by stretching velocity. However, we recently found that when the stretching velocity was too large, the magnitude of residual force enhancement was attenuated or vanished in cat soleus muscle in situ. Therefore, the purpose of this study was to extend the knowledge by conducting very quick stretches in skinned rabbit psoas and soleus muscle fibers. Skinned psoas (N = 17) or soleus (N = 13) muscle fibers were activated isometrically at an average sarcomere length of 2.7 μm, and then, actively stretched to an average sarcomere length of 3.0 μm in 0.5 or 500 ms, followed by an isometric contraction at that length. In addition, a purely isometric reference contraction was conducted at an average sarcomere length of 3.0 μm to calculate the magnitude of residual force enhancement for both stretch velocities. The magnitude of residual force enhancement was significantly different between 0.5 ms stretch (101.5 ± 5.6% for psoas, and 101.5 ± 2.3% for soleus) and 500 ms stretch (106.2 ± 5.9% for psoas, and 106.8 ± 5.1% for soleus) while the magnitude of residual force enhancement was not significantly different between muscles. In line with our previous finding, the magnitude of residual force enhancement can vary when the stretch velocity is very large. This finding can contribute to clarify the key mechanism for inducing residual force enhancement and to explain contradicting results obtained in previous studies.

Regulation of the intermittent release of giant unilamellar vesicles under osmotic pressure

Osmotic pressure can break the fluid balance between intracellular and extracellular solutions. In hypo-osmotic solution, water molecules, which transfer into the cell and burst, are driven by the concentration difference of solute across the semi-permeable membrane. The complicated dynamic processes of intermittent bursts have been previously observed. However, the underlying physical mechanism has yet to be thoroughly explored and analyzed. Here, the intermittent release of inclusion in giant unilamellar vesicles was investigated quantitatively, applying the combination of experimental and theoretical methods in the hypo-osmotic medium. Experimentally, we adopted a highly sensitive electron multiplying charge-coupled device to acquire intermittent dynamic images. Notably, the component of the vesicle phospholipids affected the stretch velocity, and the prepared solution of vesicles adjusted the release time. Theoretically, we chose equations and numerical simulations to quantify the dynamic process in phases and explored the influences of physical parameters such as bilayer permeability and solution viscosity on the process. It was concluded that the time taken to achieve the balance of giant unilamellar vesicles was highly dependent on the molecular structure of the lipid. The pore lifetime was strongly related to the internal solution environment of giant unilamellar vesicles. The vesicles prepared in viscous solution were able to visualize long-lived pores. Furthermore, the line tension was measured quantitatively by the release velocity of inclusion, which was of the same order of magnitude as the theoretical simulation. In all, the experimental values well matched the theoretical values. Our investigation clarified the physical regulatory mechanism of intermittent pore formation and inclusion release, which provides an important reference for the development of novel technologies such as gene therapy based on transmembrane transport as well as controlled drug delivery based on liposomes.

The Mechanical Property and Strengthening Mechanism of the Mg-0.5Zn-0.2Mn-0.2Sm-0.3La alloy

Tensile properties at TR with different stretch velocity, and tensile properties at T150°C were investigated in this study, and the strengthening mechanism was discussed preliminarily. Firstly, tensile properties of the Mg-0.5Zn-0.2Mn-0.2Sm-0.3La alloy at TR with the stretch velocity increases from 1 to 6 mm·min−1, and results suggest that the studied alloy exhibits low strength sensitivity as both of Rm and Rp0.2 maintaining at 240 MPa and 190 MPa, which is attributed to the not enough time for the dynamic recrystallization at the higher deformation velocity. Meanwhile, as the stretch velocity increases, although A increases first and then decreases because of the more stress concentration near grain boundaries, A maintains above 20%, suggesting the good plastic sensitivity. Secondly, when stretched at T150°C both of Rm and Rp0.2 of the Mg-0.5Zn-0.2Mn-0.2Sm-0.3La alloy decrease compared to that at TR, which is attributed to the increased dislocation movement at the higher temperature, while A increases at T150°C than at TR because of the decreased critical shear stress at the higher temperature. Even though, because the Mg12La phases impede the dislocation movement and the Sm atoms result in the solution strengthening, the mechanical properties of the Mg-0.5Zn-0.2Mn-0.2Sm-0.3La alloy are still higher than that of the Mg-0.5Zn-0.2Mn alloy at the higher temperature.

Reliability of isokinetic tests of velocity‐ and contraction intensity‐dependent plantar flexor mechanical properties

“Flexibility” tests are traditionally performed voluntarily relaxed by rotating a joint slowly; however, functional activities are performed rapidly with voluntary/reflexive muscle activity. Here, we describe the reliabilities and differences in maximum ankle range of motion (ROMmax) and plantar flexor mechanical properties at several velocities and levels of voluntary force from a new test protocol on a commercially available dynamometer. Fifteen participants had their ankle joint dorsiflexed at 5, 30, and 60° s−1 in two conditions: voluntarily relaxed and while producing 40% and 60% of maximal eccentric torque. Commonly reported variables describing ROMmax and resistance to stretch were subsequently calculated from torque and angle data. Absolute (coefficient of variation (CV%) and typical error) and relative (ICC2,1) reliabilities were determined across two testing days (≥72 h). ROMmax relative reliability was good in voluntarily relaxed tests at 30 and 60° s−1 and moderate at 5° s−1, despite CVs ≤ 10% for all velocities. Tests performed with voluntary muscle activity were only reliable when performed at 5° s−1, and ROMmax reliability was moderate and CV ≤ 8%. For most variables, the rank order of participants differed between the slow‐velocity, relaxed test, and those performed at faster speeds or with voluntary activation, indicating different information. A person's flexibility status during voluntarily relaxed fast or active stretches tended to differ from their status in the traditional voluntarily relaxed, slow‐velocity test. Thus, “flexibility” tests should be completed under conditions of different stretch velocity and levels of muscle force production, and clinicians and researchers should consider the slightly larger between‐day variability from slow‐velocity voluntarily relaxed tests.

Muscle Characteristics in Pediatric Hereditary Spastic Paraplegia vs. Bilateral Spastic Cerebral Palsy: An Exploratory Study.

Hereditary spastic paraplegia (HSP) is a neurological, genetic disorder that predominantly presents with lower limb spasticity and muscle weakness. Pediatric pure HSP types with infancy or childhood symptom onset resemble in clinical presentation to children with bilateral spastic cerebral palsy (SCP). Hence, treatment approaches in these patient groups are analogous. Altered muscle characteristics, including reduced medial gastrocnemius (MG) muscle growth and hyperreflexia have been quantified in children with SCP, using 3D-freehand ultrasound (3DfUS) and instrumented assessments of hyperreflexia, respectively. However, these muscle data have not yet been studied in children with HSP. Therefore, we aimed to explore these MG muscle characteristics in HSP and to test the hypothesis that these data differ from those of children with SCP and typically developing (TD) children. A total of 41 children were retrospectively enrolled including (1) nine children with HSP (ages of 9–17 years with gross motor function levels I and II), (2) 17 age-and severity-matched SCP children, and (3) 15 age-matched typically developing children (TD). Clinically, children with HSP showed significantly increased presence and severity of ankle clonus compared with SCP (p = 0.009). Compared with TD, both HSP and SCP had significantly smaller MG muscle volume normalized to body mass (p ≤ 0.001). Hyperreflexia did not significantly differ between the HSP and SCP group. In addition to the observed pathological muscle activity for both the low-velocity and the change in high-velocity and low-velocity stretches in the two groups, children with HSP tended to present higher muscle activity in response to increased stretch velocity compared with those with SCP. This exploratory study is the first to reveal MG muscle volume deficits in children with HSP. Moreover, high-velocity-dependent hyperreflexia and ankle clonus is observed in children with HSP. Instrumented impairment assessments suggested similar altered MG muscle characteristics in pure HSP type with pediatric onset compared to bilateral SCP. This finding needs to be confirmed in larger sample sizes. Hence, the study results might indicate analogous treatment approaches in these two patient groups.

Applying Stretch to Evoke Hyperreflexia in Spasticity Testing: Velocity vs. Acceleration.

In neurological diseases, muscles often become hyper-resistant to stretch due to hyperreflexia, an exaggerated stretch reflex response that is considered to primarily depend on the muscle's stretch velocity. However, there is still limited understanding of how different biomechanical triggers applied during clinical tests evoke these reflex responses. We examined the effect of imposing a rotation with increasing velocity vs. increasing acceleration on triceps surae muscle repsonse in children with spastic paresis (SP) and compared the responses to those measured in typically developing (TD) children. A motor-operated ankle manipulator was used to apply different bell-shaped movement profiles, with three levels of maximum velocity (70, 110, and 150°/s) and three levels of maximum acceleration (500, 750, and 1,000°/s2). For each profile and both groups, we evaluated the amount of evoked triceps surae muscle activation. In SP, we evaluated two additional characteristics: the intensity of the response (peak EMG burst) and the time from movement initiation to onset of the EMG burst. As expected, the amount of evoked muscle activation was larger in SP compared to TD (all muscles: p < 0.001) and only sensitive to biomechanical triggers in SP. Further investigation of the responses in SP showed that peak EMG bursts increased in profiles with higher peak velocity (lateral gastrocnemius: p = 0.04), which was emphasized by fair correlations with increased velocity at EMG burst onset (all muscles: r > 0.33–0.36, p ≤ 0.008), but showed no significant effect for acceleration. However, the EMG burst was evoked faster with higher peak acceleration (all muscles p < 0.001) whereas it was delayed in profiles with higher peak velocity (medial gastrocnemius and soleus: p < 0.006). We conclude that while exaggerated response intensity (peak EMG burst) seems linked to stretch velocity, higher accelerations seem to evoke faster responses (time to EMG burst onset) in triceps surae muscles in SP. Understanding and controlling for the distinct effects of different biological triggers, including velocity, acceleration but also length and force of the applied movement, will contribute to the development of more precise clinical measurement tools. This is especially important when aiming to understand the role of hyperreflexia during functional movements where the biomechanical inputs are multiple and changing.

Rate-dependent fracture behavior of tough polyelectrolyte complex hydrogels from biopolymers

Tough bio-based hydrogels based on the association of physical bonds are promising materials for use in biomedical applications due to their biocompatibility, biomimicry and biodegradability. Understanding the mechanical mechanisms at work during the fracture in such applications is indispensable. Using tough and self-healing bio-based hydrogels attained by the complexation of the negatively charged sodium hyaluronate (HA) and positively charged chitosan as a model system, an investigation of the tensile and fracture behavior over five decades of stretch velocities in physically and chemically crosslinked HA/chitosan hydrogels was systematically performed. In this study, the tensile analysis shows that the both of physically and chemically crosslinked hydrogels at small deformation are viscoelastic at a low stretch velocity, while they exhibit quasi-elastic character at a relatively high stretch velocity. In comparison to the physically and lightly chemically crosslinked HA/chitosan hydrogels exhibiting plastic-like behavior, the highly chemically crosslinked hydrogels demonstrate strong finite chain extensibility before fracture. The fracture analysis shows that the fracture energy of the highly chemically crosslinked hydrogels scales with the crack velocity as a power law relation Γ∝Vca (a= 0.15), where the exponent a can be well predicted from the viscoelastic spectrum of the bulk sample. These findings indicate that the viscoelastic energy dissipation around the crack tip dominates the fracture energy of the highly chemically crosslinked hydrogels. On the other hand, the fracture energy of the physically and lightly chemically crosslinked hydrogels is noted to increase with the crack velocity as a complex growth function, which can be effectively predicated by a viscoplastic model. The observed finding shows that the fracture energy of the physically and lightly chemically crosslinked hydrogel is dominated by the unzipping of the polyion complex structure and viscous energy dissipation due to the pulled out chains from the polymer network structure.

Diverse and complex muscle spindle afferent firing properties emerge from multiscale muscle mechanics.

Despite decades of research, we lack a mechanistic framework capable of predicting how movement-related signals are transformed into the diversity of muscle spindle afferent firing patterns observed experimentally, particularly in naturalistic behaviors. Here, a biophysical model demonstrates that well-known firing characteristics of mammalian muscle spindle Ia afferents - including movement history dependence, and nonlinear scaling with muscle stretch velocity - emerge from first principles of muscle contractile mechanics. Further, mechanical interactions of the muscle spindle with muscle-tendon dynamics reveal how motor commands to the muscle (alpha drive) versus muscle spindle (gamma drive) can cause highly variable and complex activity during active muscle contraction and muscle stretch that defy simple explanation. Depending on the neuromechanical conditions, the muscle spindle model output appears to 'encode' aspects of muscle force, yank, length, stiffness, velocity, and/or acceleration, providing an extendable, multiscale, biophysical framework for understanding and predicting proprioceptive sensory signals in health and disease.

Increased Muscle Activity Accompanying With Decreased Complexity as Spasticity Appears: High-Density EMG-Based Case Studies on Stroke Patients.

Spasticity is a major contributor to pain, disabilities and many secondary complications after stroke. Investigating the effect of spasticity on neuromuscular function in stroke patients may facilitate the development of its clinical treatment, while the underlying mechanism of spasticity still remains unclear. The aim of this study is to explore the difference in the neuromuscular response to passive stretch between healthy subjects and stroke patients with spasticity. Five healthy subjects and three stroke patients with spastic elbow flexor were recruited to complete the passive stretch at four angular velocities (10°/s, 60°/s, 120°/s, and 180°/s) performed by an isokinetic dynamometer. Meanwhile, the 64-channel electromyography (EMG) signals from biceps brachii muscle were recorded. The root mean square (RMS) and fuzzy entropy (FuzzyEn) of EMG recordings of each channel were calculated, and the relationship between the average value of RMS and FuzzyEn over 64-channel was examined. The two groups showed similar performance from results that RMS increased and FuzzyEn decreased with the increment of stretch velocity, and the RMS was negatively correlated with FuzzyEn. The difference is that stroke patients showed higher RMS and lower FuzzyEn during quick stretch than the healthy group. Furthermore, compared with the healthy group, distinct variations of spatial distribution within the spastic muscle were found in the EMG activity of stroke patients. These results suggested that a large number of motor units were recruited synchronously in the presence of spasticity, and this recruitment pattern was non-uniform in the whole muscle. Using a combination of RMS and FuzzyEn calculated from high-density EMG (HD-EMG) recordings can provide an innovative insight into the physiological mechanism underlying spasticity, and FuzzyEn could potentially be used as a new indicator for spasticity, which would be beneficial to clinical intervention and further research on spasticity.

Sub-maximally Activated Rat Soleus Fibers Exhibit Stretch Activation

Stretch activation occurs when a muscle develops force after an imposed stretch. The effect is large in some types of insect flight muscle and is also thought to contribute to cardiac function. The potential role of stretch activation in mammalian skeletal muscle is less clear. We activated chemically permeabilized rat soleus fibers in solutions with pCa values (=log10[Ca2+]) ranging from 9.0 to 4.5 and imposed ramp and hold length changes of different sizes (0.002 to 0.07 muscle lengths) and different stretch velocities (0.07 to 1.40 muscle lengths per second). All experiments were performed at 22C. When fibers were stretched in pCa 9.0 solution, force increased linearly during the stretch and decayed during the hold portion of the trial. This is consistent with the behavior of a simple visco-elastic system. Fibers immersed in solutions with pCa values between 6.6 and 6.0 exhibited more complex properties. During the stretch, the force response was biphasic due to the muscle's short-range stiffness. Immediately after the stretch, force dropped transiently, and then rose to a peak value of up to 130% of the pre-stretch isometric force. This is stretch activation. The peak stretch activated force increased with the magnitude of the stretch but was not markedly changed by stretch velocity. Fibers immersed in solutions with pCa values less than 6.0 exhibited a short-range stiffness but force declined during the hold portion of the trial and did not exhibit a delayed activation. Our measurements demonstrate that mammalian skeletal muscle can exhibit prominent stretch activation in solutions with sub-maximal calcium concentrations. To investigate the mechanisms responsible for stretch activation, we ran preliminary simulations using the MyoSim model of dynamically coupled myofilaments. These calculations suggest that force-dependent recruitment from the myosin OFF state may contribute to stretch activation.

Musculoskeletal simulation framework for impairment-based exoskeletal assistance post-stroke.

Assistive technology for the lower extremities has shown great promise towards improving gait function in people following neuromuscular injuries. However, our previous work assisting knee flexion torque in post-stroke Stiff-Knee gait found that augmenting strength can induce secondary complications such as spasticity due to stretching of the rectus femoris. In this work we explore whether we could have obtained improved knee flexion but avoided a spastic response by simulating combinations of hip and knee flexion torques using musculoskeletal modeling and simulation. We explore previously collected data on a case-by-case basis to determine individual-specific quadriceps reflex thresholds based on estimated rectus femoris muscle fiber stretch velocities. We then implemented a forward simulation framework to identify the subject-specific hip-knee assistance prescription to improve knee range of motion without initiating a spastic response. The obtained subject-specific assistive prescription informs the development of new gait assistance strategies for post-stroke gait and could be extended to other neuromuscular gait impairments.