Muscle Force Measurements Research Articles

Molecular determinants of skeletal muscle force loss in response to 5days of dry immersion in human.

Astronauts in Earth's orbit experience microgravity, resulting in a decline of skeletal muscle mass and function. On Earth, models simulating microgravity have shown that the extent of the loss in muscle force is greater than the loss in muscle mass. The reasons behind this disproportionate loss of muscle force are still poorly understood. In the present study, we hypothesize that alongside the loss in skeletal muscle mass, modifications in the expression profile of genes encoding critical determinants of resting membrane potential, excitation-contraction coupling and Ca2+ handling contribute to the decline in skeletal muscle force. Healthy male volunteers (n=18) participated in a 5-day dry immersion (DI) study, an Earth-based model of simulated microgravity. Muscle force measurement and MRI analysis of the cross-sectional area of thigh muscles were performed before and after DI. Biopsies of the vastus lateralis skeletal muscle performed before and after DI were used for the determination Ca2+ properties of isolated muscle fibres, molecular and biochemical analyses. The extent of the decline in force, measured as maximal voluntary contraction of knee extensors (-11.1%, P<0.01) was higher than the decline in muscle mass (-2.5%, P<0.01). The decline in muscle mass was molecularly supported by a significant repression of the anabolic IGF-1/Akt/mTOR pathway (-19.9% and -40.9% in 4E-BP1 and RPS6 phosphorylation, respectively), a transcriptional downregulation of the autophagy-lysosome pathway and a downregulation in the mRNA levels of myofibrillar protein slow isoforms. At the single fibre level, biochemical and tension-pCa curve analyses showed that the loss in force was independent of fibre type (-11% and -12.3% in slow and fast fibres, respectively) and Ca2+ activation properties. Finally, we showed a significant remodelling in the expression of critical players of resting membrane potential (aquaporin 4: -24.9%, ATP1A2: +50.4%), excitation-contraction coupling (CHRNA1: +75.1%, CACNA2D1: -23.5%, JPH2: -24.2%, TRDN: -15.6%, S100A1: +27.2%), and Ca2+ handling (ATP2A2: -32.5%, CASQ1: -15%, ORAI1: -36.2%, ATP2B1: -19.1%). These findings provide evidence that a deregulation in the expression profile of critical molecular determinants of resting membrane potential, excitation-contraction coupling, and Ca2+ handling could be involved in the loss of muscle force induced by DI. They also provide the paradigm for the understanding of muscle force loss during prolonged bed rest periods as those encountered in intensive care unit.

A Natural History Study of Hindlimb Physiology and Histopathology in a Heterozygous OPMD Mouse Model

ABSTRACTBackgroundOculopharyngeal muscular dystrophy (OPMD) is an adult‐onset autosomal dominant myopathy. OPMD is caused by a short alanine expansion in the gene encoding for poly(A) binding protein nuclear 1 (PABPN1) forming insoluble nuclear aggregates. OPMD patients are predominantly heterozygous, and the knock‐in Pabpn1+/A17 mouse, which expresses one copy of the expanded Pabpn1 gene under the PABPN1 promoter genetically, mimics OPMD. Insights into the A17/+ mouse model are necessary to evaluate its preclinical value and test therapeutics for OPMD. Here, we performed a natural disease history study for the A17/+ model.MethodsWe combined muscle force measurements of the tibialis anterior with magnetic resonance imaging (MRI) measurements of the calf muscles made in 4‐, 8‐ and 12‐month‐old wild‐type and A17/+ mice. These measures were complemented by muscle histopathology staining and image quantification to detect PABPN1 aggregates and to assess muscle wasting. Statistical significance between genotypes over the three time points was assessed using ANOVA or Student's t test.ResultsPABPN1 nuclear aggregates were found in the 12‐month‐old A17/+ mice at similar quantities of ~2% across hindlimb muscles. We did not observe changes in muscle strength of the tibialis anterior in A17/+ mice. MRI analyses of hindlimb muscles revealed no metabolic difference, no fatty infiltration and limited muscle atrophy between A17/+ and +/+ mice. The plantaris muscle in A17/+ showed 30% atrophy at 12 months of age, which was corroborated by a 30% myofiber shift in the myosin heavy chain −2A to −2B ratio. Histopathologic staining did not reveal muscle wasting in the hindlimb muscles.ConclusionsDespite the presence of PABPN1 insoluble aggregates in hindlimb muscles, muscle involvement in the 12‐month‐old A17/+ mice was limited. Our results query the usefulness of A17/+ hindlimb muscles for preclinical studies.

Integrated Wearable System for Monitoring Skeletal Muscle Force of Lower Extremities.

Continuous monitoring of lower extremity muscles is necessary, as the muscles support many human daily activities, such as maintaining balance, standing, walking, running, and jumping. However, conventional electromyography and physiological cross-sectional area methods inherently encounter obstacles when acquiring precise and real-time data pertaining to human bodies, with a notable lack of consideration for user comfort. Benefitting from the fast development of various fabric-based sensors, this paper addresses these current issues by designing an integrated smart compression stocking system, which includes compression garments, fabric-embedded capacitive pressure sensors, an edge control unit, a user mobile application, and cloud backend. The pipeline architecture design and component selection are discussed in detail to illustrate a comprehensive user-centered STIMES design. Twelve healthy young individuals were recruited for clinical experiments to perform maximum voluntary isometric ankle plantarflexion contractions. All data were simultaneously collected through the integrated smart compression stocking system and a muscle force measurement system (Humac NORM, software version HUMAC2015). The obtained correlation coefficients above 0.92 indicated high linear relationships between the muscle torque and the proposed system readout. Two-way ANOVA analysis further stressed that different ankle angles (p = 0.055) had more important effects on the results than different subjects (p = 0.290). Hence, the integrated smart compression stocking system can be used to monitor the muscle force of the lower extremities in isometric mode.

Consensus for experimental design in electromyography (CEDE) project: Application of EMG to estimate muscle force

Skeletal muscles power movement. Deriving the forces produced by individual muscles has applications across various fields including biomechanics, robotics, and rehabilitation. Since direct in vivo measurement of muscle force in humans is invasive and challenging, its estimation through non-invasive methods such as electromyography (EMG) holds considerable appeal. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, summarizes recommendations on the use of EMG to estimate muscle force. The matrix encompasses the use of bipolar surface EMG, high density surface EMG, and intra-muscular EMG (1) to identify the onset of muscle force during isometric contractions, (2) to identify the offset of muscle force during isometric contractions, (3) to identify force fluctuations during isometric contractions, (4) to estimate force during dynamic contractions, and (5) in combination with musculoskeletal models to estimate force during dynamic contractions. For each application, recommendations on the appropriateness of using EMG to estimate force and justification for each recommendation are provided. The achieved consensus makes clear that there are limited scenarios in which EMG can be used to accurately estimate muscle forces. In most cases, it remains important to consider the activation as well as the muscle state and other biomechanical and physiological factors— such as in the context of a formal mechanical model. This matrix is intended to encourage interdisciplinary discussions regarding the integration of EMG with other experimental techniques and to promote advances in the application of EMG towards developing muscle models and musculoskeletal simulations that can accurately predict muscle forces in healthy and clinical populations.

TRIM Enhances Angiogenesis in Dystrophic Muscle 140 Days Post Treatment

Background: Duchenne Muscular Dystrophy (DMD) causes limb muscle dysfunction and impaired mobility while promoting ischemia in effected muscles. For treatment, we developed a novel biomaterial, Time Release Ion Matrix (TRIM), that promotes angiogenesis and enhances muscle structure and function in dystrophic (D2. mdx) mice at 14- and 70-days post injection (dpi). Whether TRIM remains effective at 140 dpi is unknown. Hypothesis: At 140 dpi in D2. mdx mice TRIM will: 1) enhance isometric force and myofiber cross-sectional area (CSA) of the tibialis anterior (TA) muscle; and 2) stimulate angiogenic growth factors and increase vascularity. Methods: The right and left TA of adult male D2. mdx mice (male, n=7-8, age 7 months) were injected with saline alone (control) or 250 μg of TRIM suspended in saline. At 140 dpi, the left TA muscle was isolated for in situ muscle force measurements and histological analysis; the right TA was frozen for biochemical analysis. Muscle cross-sections were immunostained for laminin (myofiber borders), DAPI (nuclei), and CD31 (endothelial cells). Vascular endothelial growth factor (VEGF) ELISA and immunoblots were performed on muscle homogenates. Results: No difference in peak isometric force was recorded between groups at 140 dpi (means ± SE: TRIM, 65.4 ± 2.7g; Saline, 62.7 ± 1.5g; P= 0.34). TRIM enhanced the relative frequency of myofibers &gt;1000 μm2 by 27% vs. saline (TRIM, 56 ± 2%; Saline, 44 ± 3%; P= 0.006). DAPI staining revealed a 36% increase in central nuclei indicating myofiber regeneration (TRIM, 30.0 ± 1.7%; Saline, 22.0% ± 2.5; P=0.02). Microvessel area (CD31 staining, μm2) was 2.5-fold greater in treated TA’s (TRIM, 131 ± 12 μm2; Saline, 53 ± 6 μm2, P=0.0001). In muscle homogenates, TRIM had no effect on [VEGF] (TRIM, 102 ± 5%; Saline, 100 ± 4%; P= 0.79), while STAT3 activation was greater (TRIM, 129 ± 11%; Saline, 100 ± 7%; P= 0.04). Conclusion: At 140 days following injection of TRIM into the TA muscle of dystrophic mice, peak isometric force was unchanged while myofiber CSA was enhanced. Furthermore, TRIM promotes angiogenic growth factors and muscle vascularity, which should promote the ability to sustain physical activity. University of Missouri Coulter Biomedical Accelerator (SSS), American Physiological Society SURF (AGN), Texas A&amp;M Department of Kinesiology and Sport Management (ABM). 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.

TRIM Enhances Dystrophic Muscle Function 70 Days Post Treatment

Background: Duchenne Muscular Dystrophy (DMD) is associated with limb muscle dysfunction caused by progressive weakness and degeneration of skeletal muscle. Current FDA-approved therapeutic strategies are less effective than originally hoped. Our alternative approach was to develop a novel biomaterial, Time Release Ion Matrix (TRIM), shown to release angiogenic ions and enhance muscle structure and function at 14 days post injection (dpi) in dystrophic (D2. mdx) mice. However, the long-term effects of TRIM are unknown. Hypothesis: At 70 days post injection in D2. mdx mice TRIM will: 1) enhance isometric force and myofiber cross-sectional area (CSA) of the tibialis anterior (TA) muscle; and 2) promote vascular endothelial growth factor (VEGF) availability and muscle vascularity. Methods: The right and left TA of adult male D2. mdx mice (n=6-7, age 5 months) were injected with saline alone (control) or 250 μg of TRIM suspended in saline. At 70 dpi, the left TA was isolated for in situ muscle force measurements and histological analysis; the right TA was frozen for biochemical analysis. Muscle cross-sections were immunostained for laminin (myofiber borders), DAPI (nuclei), and CD31 (endothelial cells). An ELISA for VEGF was performed on homogenates of frozen muscles. Results: TA’s injected with TRIM increased peak isometric force (means ± SE: TRIM 70.9 ± 1.8 g; Saline, 65.9 ± 1.1 g; P= 0.04). TRIM-treated TA’s had a 2.2-fold increase in relative frequency of myofibers &gt;1000 μm2 (TRIM, 31.8 ± 3.7%; Saline, 14.4 ± 1.9%; P= 0.001), and reduced the frequency of myofibers &lt;400 μm2 by 32% (TRIM, 38.6% ± 2.8; Saline, 56.5% ± 3.5; P= 0.001). DAPI staining revealed a 50% increase in central myonuclei indicative of regenerated myofibers (TRIM, 30.2 ± 1.7%; Saline, 20.2 ± 1.3%; P=0.0006). Microvessel area (CD31 staining; μm2) was 2.1-fold greater in treated TA’s (TRIM, 135.6 ± 8.7 μm2; Saline, 63.6 ± 4.5 μm2; P=0.0001). In muscle homogenates, TRIM increased [VEGF] (TRIM, 139 ± 10%; Saline, 100 ± 6%; P= 0.009) Conclusion: Results support the hypothesis that 70 days following injection of TRIM into the TA of dystrophic mice, muscle force and myofiber CSA are enhanced. Moreover, TRIM enhances VEGF with increased vascularity, suggesting that the angiogenic effects of TRIM complement those on myogenesis and contractile function towards restoring muscle structure and function in DMD. University of Missouri Coulter Biomedical Accelerator (SSS), Texas A&amp;M Department of Kinesiology and Sport Management (ABM). 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.

Prepared for landing: A simple activation strategy scales muscle force to landing height

Before landing from a jump or fall, animals preactivate muscles to stiffen their limb joints but it is unclear how muscles tune limb stiffness and how collision forcefulness is anticipated. We measured electromyography and force from the lateral gastrocnemius muscle during landings in turkeys, an animal model that allows for direct measurements of muscle force. Many studies of landings in humans and other animals have found the duration of muscle preactivation to be constant, starting approximately 100 ms before impact, irrespective of fall duration. Therefore, we hypothesized a lack of relationship between fall duration (as dictated by drop height), muscle activity onset-time, and force at toe-down. Contrary to our expectations, both muscle activity and force rose from briefly after fall initiation until toe-down. Preactivation duration was proportional to fall height, while the rate of force rise was consistent across drop heights, resulting in force at landing and leg stiffness being proportional to fall height. Onset of muscle activity lagged 22 ± 7 ms (mean ± S.E.M.) from fall initiation, consistent with a reflex response initiation of the force ramp-up. Together, our results suggest that a constant (clock-like) rate of motor unit recruitment, initiated at fall initiation provides a preactivation that is proportional to drop height. The result is a tuning of pre-landing muscle force, providing a limb stiffening that is proportional to impact intensity, possibly without using information about fall distance.

Upper Extremity Muscle Activation Pattern Prediction Through Synergy Extrapolation and Electromyography-Driven Modeling.

Patients with neuromuscular disease fail to produce necessary muscle force and have trouble maintaining joint moment required to perform activities of daily living. Measuring muscle force values in patients with neuromuscular disease is important but challenging. Electromyography (EMG) can be used to obtain muscle activation values, which can be converted to muscle forces and joint torques. Surface electrodes can measure activations of superficial muscles, but fine-wire electrodes are needed for deep muscles, although it is invasive and require skilled personnel and preparation time. EMG-driven modeling with surface electrodes alone could underestimate the net torque. In this research, authors propose a methodology to predict muscle activations from deeper muscles of the upper extremity. This method finds missing muscle activation one at a time by combining an EMG-driven musculoskeletal model and muscle synergies. This method tracks inverse dynamics joint moments to determine synergy vector weights and predict muscle activation of selected shoulder and elbow muscles of a healthy subject. In addition, muscle-tendon parameter values (optimal fiber length, tendon slack length, and maximum isometric force) have been personalized to the experimental subject. The methodology is tested for a wide range of rehabilitation tasks of the upper extremity across multiple healthy subjects. Results show this methodology can determine single unmeasured muscle activation up to Pearson's correlation coefficient (R) of 0.99 (root mean squared error, RMSE = 0.001) and 0.92 (RMSE = 0.13) for the elbow and shoulder muscles, respectively, for one degree-of-freedom (DoF) tasks. For more complicated five DoF tasks, activation prediction accuracy can reach up to R = 0.71 (RMSE = 0.29).

Neuromuscular junction transmission failure in aging and sarcopenia: The nexus of the neurological and muscular systems.

Sarcopenia, or age-related decline in muscle form and function, exerts high personal, societal, and economic burdens when untreated. Integrity and function of the neuromuscular junction (NMJ), as the nexus between the nervous and muscular systems, is critical for input and dependable neural control of muscle force generation. As such, the NMJ has long been a site of keen interest in the context of skeletal muscle function deficits during aging and in the context of sarcopenia. Historically, changes of NMJ morphology during aging have been investigated extensively but primarily in aged rodent models. Aged rodents have consistently shown features of NMJ endplate fragmentation and denervation. Yet, the presence of NMJ changes in older humans remains controversial, and conflicting findings have been reported. This review article describes the physiological processes involved in NMJ transmission, discusses the evidence that supports NMJ transmission failure as a possible contributor to sarcopenia, and speculates on the potential of targeting these defects for therapeutic development. The technical approaches that are available for assessment of NMJ transmission, whether each approach has been applied in the context of aging and sarcopenia, and the associated findings are summarized. Like morphological studies, age-related NMJ transmission deficits have primarily been studied in rodents. In preclinical studies, isolated synaptic electrophysiology recordings of endplate currentsor potentials have been mostly used, and paradoxically, have shown enhancement, rather than failure, with aging. Yet, in vivo assessment of single muscle fiber action potential generation using single fiber electromyography and nerve-stimulated muscle force measurements show evidence of NMJ failure in aged mice and rats. Together these findings suggest that endplate response enhancement may be a compensatory response to post-synaptic mechanisms of NMJ transmission failure in aged rodents. Possible, but underexplored, mechanisms of this failure are discussed including the simplification of post-synaptic folding and altered voltage-gated sodium channel clustering or function. In humans, there is limited clinical data that has selectively investigated single synaptic function in the context of aging. If sarcopenic older adults turn out to exhibit notable impairments in NMJ transmission (this has yet to be examined but based on available evidence appears to be plausible) then these NMJ transmission defects present a well-defined biological mechanism and offer a well-defined pathway for clinical implementation. Investigation of small molecules that are currently available clinically or being testing clinically in other disorders may provide a rapid route for development of interventions for older adults impacted by sarcopenia.

Effects on Bone and Muscle upon Treadmill Interval Training in Hypogonadal Male Rats.

Testosterone deficiency in males is linked to various pathological conditions, including muscle and bone loss. This study evaluated the potential of different training modalities to counteract these losses in hypogonadal male rats. A total of 54 male Wistar rats underwent either castration (ORX, n = 18) or sham castration (n = 18), with 18 castrated rats engaging in uphill, level, or downhill interval treadmill training. Analyses were conducted at 4, 8, and 12 weeks postsurgery. Muscle force of the soleus muscle, muscle tissue samples, and bone characteristics were analyzed. No significant differences were observed in cortical bone characteristics. Castrated rats experienced decreased trabecular bone mineral density compared to sham-operated rats. However, 12 weeks of training increased trabecular bone mineral density, with no significant differences among groups. Muscle force measurements revealed decreased tetanic force in castrated rats at week 12, while uphill and downhill interval training restored force to sham group levels and led to muscle hypertrophy compared to ORX animals. Linear regression analyses showed a positive correlation between bone biomechanical characteristics and muscle force. The findings suggest that running exercise can prevent bone loss in osteoporosis, with similar bone restoration effects observed across different training modalities.

Timed Release Ion Matrix Enhances Muscle Force in Dystrophic Mice

Background: Duchenne Muscular Dystrophy (DMD) is an x-linked, recessive chromosomal mutation of the gene dystrophin, affecting 1 in 3,500 males. DMD is associated with limb muscle dysfunction from repetitive damage to myofibers beginning at 2 years of age. In healthy muscle, dystrophin is responsible for maintaining the sarcolemma through the dystrophin-glycoprotein complex, while muscle with dystrophin deficiencies seen in DMD patients experience severe sarcolemmal disruption following muscle contraction. Despite major advances in genetic engineering, the functional outcomes of FDA-approved therapeutic strategies have failed to enhance the function of dystrophin-deficient myofibers. We have taken an alternative approach to apply a novel biomaterial, Time Release Ion Matrix (TRIM), that has the potential to release ions favorable to regeneration, restoring muscle structure and function in DMD mice. Hypothesis: We hypothesized that muscle force and myofiber cross-sectional area (CSA) of the tibialis anterior muscle (TA) in an established mouse model of muscle dystrophy [D2.B10 mdx (DBA. mdx)] would be enhanced 14 days post-injection of TRIM. Methods: The left TA of adult male DBA. mdx mice (n=3-9, age 6-7 months) was injected with 250 μg of TRIM suspended in saline or saline alone (vehicle control). At 14 days post injection, the TA muscle was isolated for in situ muscle force measurements using direct electrical stimulation. The TA was then removed and frozen in optimal cutting temperature compound (OCT) for histological analysis. Muscle cross-sections (thickness, 10 μm) were labeled with laminin to identify myofiber borders and myosin heavy chain as a marker of myofibers able to generate force. Images were analyzed using Nikon NIS-Elements software. To simulate body fluid, 300 mg of TRIM was suspended in physiological salt solution (PSS) for 120 hours. Results: The dissolution of TRIM in PSS over 120 hours was 40% (179 mg remaining). Peak isometric force of TAs injected with TRIM (60.0 ± 2.3 g) was 11% greater compared to saline alone (52.4 ± 2.4 g) (means ± SE, n=6-9 per group; P=0.048). There were fewer small diameter myofibers (&lt; 400 μm2) in TAs injected with TRIM (10 ± 7%) compared to saline controls (24 ± 5 %) (n=3/group; P=0.03). Conclusion: 14 days following injection of TRIM into the TA of dystrophic mice, muscle force and myofiber CSA were enhanced by 11 and 14% respectively. We suggest that TRIM may be an effective treatment to restore muscle function in DMD and serve as a powerful tool for discovering therapeutic targets against DMD. University of Missouri Coulter Biomedical Accelerator (SSS, ABM), Texas A&amp;M Department of Kinesiology and Sport Management (JAK, ABM) This is the full abstract presented at the American Physiology Summit 2023 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.

The Implication of Hinge 1 and Hinge 4 in Micro-Dystrophin Gene Therapy for Duchenne Muscular Dystrophy.

Duchenne muscular dystrophy (DMD) is a fatal muscle disease caused by dystrophin deficiency. Dystrophin consists of the amino terminus, central rod domain with 24 spectrin-like repeats and four hinges (H), cysteine-rich domain, and carboxyl terminus. Several highly abbreviated micro-dystrophins (μDys) are currently in clinical trials. They all carry H1 and H4. In this study, we investigated whether these two hinges are essential for μDy function in murine DMD models. Three otherwise identical μDys were engineered to contain H1 and/or H4 and were named H1/H4 (with both H1 and H4), ΔH1 (without H1), and ΔH4 (without H4). These constructs were packaged in adeno-associated virus serotype-9 and delivered to the tibialis anterior muscle of 3-month-old male mdx4cv mice (1E12 vector genome particles/muscle). Three months later, we detected equivalent μDys expression in total muscle lysate. However, only H1/H4 and ΔH1 showed correct sarcolemmal localization. ΔH4 mainly existed as sarcoplasmic aggregates. H1/H4 and ΔH1, but not ΔH4, fully restored the dystrophin-associated protein complex and significantly improved the specific muscle force. Eccentric contraction-induced force decline was best protected by H1/H4, followed by ΔH1, but not by ΔH4. Next, we compared H1/H4 and ΔH1 in 6-week-old male mdx mice by intravenous injection (1E13 vector genome particles/mouse). Four months postinjection, H1/H4 significantly outperformed ΔH1 in extensor digitorum longus muscle force measurements but two constructs yielded comparable electrocardiography improvements. We conclude that H4 is essential for μDys function and H1 facilitates force production. Our findings will help develop next-generation μDys gene therapy.

Optimization Reduces Knee-Joint Forces During Walking and Squatting: Validating the Inverse Dynamics Approach for Full Body Movements on Instrumented Knee Prostheses.

Because of the redundancy of our motor system, movements can be performed in many ways. While multiple motor control strategies can all lead to the desired behavior, they result in different joint and muscle forces. This creates opportunities to explore this redundancy, for example, for pain avoidance or reducing the risk of further injury. To assess the effect of different motor control optimization strategies, a direct measurement of muscle and joint forces is desirable, but problematic for medical and ethical reasons. Computational modeling might provide a solution by calculating approximations of these forces. In this study, we used a full-body computational musculoskeletal model to (a)predict forces measured in knee prostheses during walking and squatting and (b)study the effect of different motor control strategies (i.e.,minimizing joint force vs. muscle activation) on the joint load and prediction error. We found that musculoskeletal models can accurately predict knee joint forces with a root mean squared error of <0.5 body weight (BW) in the superior direction and about 0.1BW in the medial and anterior directions. Generally, minimization of joint forces produced the best predictions. Furthermore, minimizing muscle activation resulted in maximum knee forces of about 4BW for walking and 2.5BW for squatting. Minimizing joint forces resulted in maximum knee forces of 2.25BW and 2.12BW, that is, a reduction of 44% and 15%, respectively. Thus, changing the muscular coordination strategy can strongly affect knee joint forces. Patients with a knee prosthesis may adapt their neuromuscular activation to reduce joint forces during locomotion.

Force Sharing Problem During Gait Using Inverse Optimal Control

Human gait patterns have been intensively studied, both from medical and engineering perspectives, to understand and compensate pathologies. However, the muscle-force sharing problem is still debated as acquiring individual muscle force measurements is challenging, requiring the use of invasive devices. Recent studies, using various objective functions, suggest muscle-force sharing may result from an optimization process. This study proposes using inverse optimal control to identify an objective function. Two popular methods of inverse optimal control, bilevel and inverse Karush-Kuhn-Tucker, were investigated. The identified objective functions were then used to predict muscle forces during gait, and their performances were compared to an exhaustive list of biological cost functions from the literature. The best prediction was achieved by the bilevel inverse optimal control method, with a root-mean-squared error of 176 N (162 N) and a correlation coefficient of 0.76 (0.68) for the stance (swing) phase of the gait cycle. These muscle force predictions were thereafter used to compute joint stiffness, exhibiting an average root-mean-square error of 42 Nm.rad <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{-1}$</tex-math></inline-formula> and a correlation coefficient of 0.90 when compared to the reference. The bilevel method's prevalence in terms of robustness over inverse Karush-Kuhn-Tucker was demonstrated on human data and explained on a toy example.

Genetic Manipulation of CB1 Cannabinoid Receptors Reveals a Role in Maintaining Proper Skeletal Muscle Morphology and Function in Mice.

The endocannabinoid system (ECS) refers to a widespread signaling system and its alteration is implicated in a growing number of human diseases. Cannabinoid receptors (CBRs) are highly expressed in the central nervous system and many peripheral tissues. Evidence suggests that CB1Rs are expressed in human and murine skeletal muscle mainly in the cell membrane, but a subpopulation is present also in the mitochondria. However, very little is known about the latter population. To date, the connection between the function of CB1Rs and the regulation of intracellular Ca2+ signaling has not been investigated yet. Tamoxifen-inducible skeletal muscle-specific conditional CB1 knock-down (skmCB1-KD, hereafter referred to as Cre+/-) mice were used in this study for functional and morphological analysis. After confirming CB1R down-regulation on the mRNA and protein level, we performed in vitro muscle force measurements and found that peak twitch, tetanus, and fatigue were decreased significantly in Cre+/- mice. Resting intracellular calcium concentration, voltage dependence of the calcium transients as well as the activity dependent mitochondrial calcium uptake were essentially unaltered by Cnr1 gene manipulation. Nevertheless, we found striking differences in the ultrastructural architecture of the mitochondrial network of muscle tissue from the Cre+/- mice. Our results suggest a role of CB1Rs in maintaining physiological muscle function and morphology. Targeting ECS could be a potential tool in certain diseases, including muscular dystrophies where increased endocannabinoid levels have already been described.

Electromyography-driven model-based estimation of ankle torque and stiffness during dynamic joint rotations in perturbed and unperturbed conditions

The simultaneous modulation of joint torque and stiffness enables humans to perform large repertoires of movements, while versatilely adapting to external mechanical demands. Multi-muscle force control is key for joint torque and stiffness modulation. However, the inability to directly measure muscle force in the intact moving human prevents understanding how muscle force causally links to joint torque and stiffness. Joint stiffness is predominantly estimated via joint perturbation-based experiments in combination with system identification techniques. However, these techniques provide joint-level stiffness estimations with no causal link to the underlying muscle forces. Moreover, the need for joint perturbations limits the generalizability and applicability to study natural movements. Here, we present an electromyography (EMG)-driven musculoskeletal modeling framework that can be calibrated to match reference joint torque and stiffness profiles simultaneously via a multi-term objective function. EMG-driven models calibrated on <2 s of reference torque and stiffness data could blindly estimate reference profiles across 100 s of data not used for calibration. Model calibrations using an objective function comprising torque and stiffness terms always provided less feasible solutions than an objective function comprising solely a torque term, thereby reducing the space of feasible muscle–tendon parameters. Results also showed the proposed framework’s ability to estimate joint stiffness in unperturbed conditions, while capturing differences against stiffness profiles derived during perturbed conditions. The proposed framework may provide new ways for studying causal relationships between muscle force and joint torque and stiffness during movements in interaction with the environment, with broad implications across biomechanics, rehabilitation and robotics.

Shear wave elastography characterizes passive and active mechanical properties of biceps brachii muscle in vivo

Mechanical characterization of individual muscles in their in vivo environment is not well studied. Shear wave elastography (SWE) as a non-invasive technique was shown to be promising in quantifying the local mechanical properties of skeletal muscles. This study aimed to investigate the mechanics of the biceps brachii muscle (BB) derived from SWE in relation to elbow joint position and contraction intensity during isometric contraction.14 healthy, young subjects participated in the study and five different joint positions (60°–180° elbow angle) were investigated. Shear elastic modulus and surface electromyography (sEMG) of the BB and elbow torque were measured simultaneously, both in passive (i.e., resting) and active states during slow, sub-maximal isometric ramp contractions up to 25%, 50%, and 75% of the maximum voluntary contraction.At passive state, the shear elastic modulus of the BB increased with increasing elbow angle (p < 0.001). Maximum elbow flexion torque was produced at 60° and it decreased with increasing elbow angle (p = 0.001). During sub-maximal contractions, both elbow angle (p < 0.001) and contraction intensity (p < 0.001) had significant effects on the shear elastic modulus but only contraction intensity (p < 0.001) affected sEMG amplitude of the BB. Although torque was decreased at extended elbow positions (150°, 180°), higher active shear elastic modulus of BB muscle was found compared to flexed positions (60°, 90°). Linear regression of the BB sEMG amplitude over elbow torque showed good agreement for all joint positions (R2 between 0.69 and 0.89) while the linear agreement between shear elastic modulus of BB and elbow torque differed between flexed (R2 = 0.70 at 60° and R2 = 0.79 at 90°) and extended positions (with the lowest R2 = 0.57 at 150°).We conclude that using SWE, we can detect length-dependent mechanical changes of BB both in passive and active states. More importantly, SWE can be used to characterize active muscle properties in vivo. The present findings have critical importance for developing muscle stiffness as a measure of individual muscle force to validate muscle models and using SWE in clinical diagnostics.

P-022: THERAPEUTIC EFFECTS OF GBT1118 ON SKELETAL MUSCLE ENERGETICS AND FUNCTION IN SICKLE CELL MICE

Purpose: Sickle cell disease (SCD) is an hemoglobinopathy ultimately resulting red blood cells sickling. SCD patients suffer from anemia, arterial oxyhemoglobin desaturation, hematologic and hemodynamic abnormalities, all of which disrupt oxygen supply and can reduce exercise capacity and quality of life. Strenuous exercise in SCD patients may induce hemolysis, an early lactate accumulation, an intracellular acidosis, all of which are potential triggering factors for sickling and vaso-occlusive crises. GBT1118 is a HbS polymerization inhibitor which can increase hemoglobin affinity for oxygen and the corresponding supply to tissues. On that basis, we hypothesized that GBT1118 could dampen skeletal muscle energetics and function. Materials and methods: Skeletal muscle energetics and function of sixteen Townes mice (2 ± 0.5 month-old) were assessed in response to a standardized Rest-Stimulation(1Hz)-Recovery protocol before and after an 8-week period (8WP). Investigations were conducted using a setup designed to be operational inside a 7T horizontal MRI scanner (PharmaScan-Bruker). The setup allows (i) magnetic resonance imaging, (ii) muscle force measurements and (iii) 31P-magnetic resonance spectroscopy. Assessments conducted before and after the 8WP were identified with number 1 (SCDGBT1) and two (SCDGBT2). Ten mice received a chronic supplementation of GBT1118 (4g/kg in chow) during the 8WP (SCDGBT), whereas six mice had access to not supplemented chow (SCDCON). Results: The specific peak force (sPf) measured at the beginning of stimulation significantly increased over the 8WP (SCDCON2 +68%, p < 0.05). This increased initial sPf was also present in SCDGBT2 as compared to SCDGBT1 (+ 30%, p < 0.05). Over the entire stimulation session, the overall specific [TM1] force production (sTFP) significantly increased over the 8WP (SCDCON2 +32%, p < 0.05, Figure 1). The increased sTFP was also present in SCDGBT2 as compared to SCDGBT1 (+ 25 %, p < 0.05). However, the sTFP-normalized pHi reduction and sTFP-normalized phosphocreatine (PCr) breakdown significantly decreased in SCDGBT2 as compared to SCDGBT1 (p < 0.05, Table 1). Conclusion: In the control condition, the 8-week period was accompanied by a significant increase in specific total force production. Such an increase has been previously reported from 3 to 10 weeks of age (+ 33%). Overall, the increased specific total force production we reported was related to time and cannot be accounted for by GBT supplementation. In SCDGBT, the expected increased force production related to time was associated to a reduced acidosis and PCr breakdown which were not found in the control condition. For a given amount of force, SCDGBT mice muscle would experience a lower net proton accumulation and a reduced net PCr consumption. This reduction of both acidosis and PCr consumption could illustrate a diminished cost of contraction in response to GBT1118 supplementation which could be related to an improved contractile efficiency as previously described in humans. SCD mice treated with GBT1118 exhibit a reduced acidosis for a given force. Given that acidosis is a potential triggering factor of sickling, GBT would protect SCD muscle against the consequences of sickling. One could speculate that SCD patients receiving GBT1118 might be able to perform exercise with a reduced exposure to sickling.Specific total force production in SCDCON and SCDGBT before and after the 8-week period. Each point represents a single measurement and lines are linking consecutive points. p-values are mentioned on the top of the figure.Metabolic indices in SCDCON and SCDGBT before and after the 8-week period. The authors do not declare any conflict of interest

QS63. Severity of Neuropathic Pain, Sensorimotor Impairments, and Sensory Neuron Abnormalities in Rat Model of Radiation Induced Neuropathy is Dose Dependent

Purpose: Radiation therapy (XRT) is one of the mainstays of cancer treatment, with approximately 50% of cancer patients requiring XRT as part of their treatment regimen. Of these patients, almost 31% subsequently suffer from debilitating neuropathic pain and sensory nerve abnormalities. XRT induced peripheral neuropathy results in progressive, intractable, and often irreversible nerve pain which can have a devastating effect on patients’ quality of life and potentially reduce the overall benefit of the therapy itself. We have previously established a novel rodent model of radiation induced peripheral neuropathy. The purpose of the present study was to assess neuropathic pain, sensorimotor impairments, and sensory neuron abnormalities in both clinically relevant and clinically high dose radiated animals. Methods: Eighteen male Lewis rats were randomly assigned to one of three experimental groups: (1) radiation of 35 Gray (Gy) divided into 5 daily doses of 7 Gy/day; (2) radiation of 70 Gray divided into 5 daily doses of 14 Gy/day, and (3) sham-radiated controls. All animals were tested at baseline in both pain and sensorimotor behavioral tests. Pain related measures included mechanical allodynia (von Frey test), cold allodynia (Acetone test), and thermal allodynia (Hargreaves test). Sensorimotor testing consisted of evaluation of overground locomotion using walkings tracks with calculation of the Sciatic Functional Index (SFI). Animals were tested serially over an 8 week period. At study endpoint, we performed nerve conduction analysis, electromyography (EMG), muscle force measurements, and all sciatic nerves were taken for histological/histomorphometrical analysis. In a separate cohort of animals, retrograde labeling was conducted in order to assess the level of sensory neuron death following radiation. Results: All animals that underwent radiation displayed significantly greater hypersensitivity to mechanical stimulation as compared to sham-radiated animals, indicative of chronic neuropathic pain. SFI values indicated sensorimotor impairments in radiated animals, and significantly lower twitch and tetanic muscle forces. Sensory neuron abnormalities and cell death were greater in radiated animals, with the high dose group displaying the most severe decrease. Conclusion: Results from this study demonstrate that neuropathic pain, sensorimotor behavioral impairment, and sensory neuron abnormalities vary as a function of radiation dose administered. Ongoing research is currently assessing novel therapeutic compounds to attenuate debilitating radiation induced sequelae, with the goal of improving the quality of life for cancer patients undergoing XRT.

Therapeutic Exon Skipping Through a CRISPR-Guided Cytidine Deaminase Rescues Dystrophic Cardiomyopathy in Vivo.

Loss of dystrophin protein causes Duchenne muscular dystrophy (DMD), characterized by progressive degeneration of cardiac and skeletal muscles, and mortality in adolescence or young adulthood. Although cardiac failure has risen as the leading cause of mortality in patients with DMD, effective therapeutic interventions remain underdeveloped, in part, because of the lack of a suitable preclinical model. We analyzed a novel murine model of DMD created by introducing a 4-bp deletion into exon 4, one of the exons encoding the actin-binding domain 1 of dystrophin (referred to as DmdE4* mice). Echocardiography, microcomputed tomography, muscle force measurement, and histological analysis were performed to determine cardiac and skeletal muscle defects in these mice. Using this model, we examined the feasibility of using a cytidine base editor to install exon skipping and rescue dystrophic cardiomyopathy in vivo. AAV9-based CRISPR/Cas9-AID (eTAM) together with AAV9-sgRNA was injected into neonatal DmdE4* mice, which were analyzed 2 or 12 months after treatment to evaluate the extent of exon skipping, dystrophin restoration, and phenotypic improvements of cardiac and skeletal muscles. DmdE4* mice recapitulated many aspects of human DMD, including shortened life span (by ≈50%), progressive cardiomyopathy, kyphosis, profound loss of muscle strength, and myocyte degeneration. A single-dose administration of AAV9-eTAM instituted >50% targeted exon skipping in the Dmd transcripts and restored up to 90% dystrophin in the heart. As a result, early ventricular remodeling was prevented and cardiac and skeletal muscle functions were improved, leading to an increased life span of the DmdE4* mice. Despite gradual decline of AAV vector and base editor expression, dystrophin restoration and pathophysiological rescue of muscular dystrophy were long lasted for at least 1 year. Our study demonstrates the feasibility and efficacy to institute exon skipping through an enhanced TAM (eTAM) for therapeutic application(s).