Maximal Force Generation Research Articles

Investigation of Bisphenol A administration on skeletal muscle force generation in mice

Bisphenol A (BPA) is a chemical used in the manufacturing of numerous products including plastics and food‐contact materials and is classified as an endocrine disruptor because it can mimic actions of estrogen in the body. The purpose of this study was to examine BPA effects on muscle force generation since estrogen may influence muscle contractile properties. Eight‐week old male CD‐1 mice were housed in polypropylene cages and provided a soybean oil‐free diet (AIN‐93G) (control; C) for two weeks. Thereafter, animals were randomly assigned to C (n=5), C+BPA (BPA; 50 mg BPA/kg diet; n=4), or C+ethinyl estradiol (EE; 0.1 ppb; n=5) diets for one month. Soleus muscles were harvested for measurement of twitch and tetanic force generation (N/cm2) in vitro. Peak twitch tension of EE (3.5±0.1) was greater (p<0.05) than BPA (2.6±0.1) and C (2.8±0.2). Peak tetanic tension of EE (24.8±1.2) was greater (p<0.05) than BPA (19.2±0.7) but was similar (p>;0.05) to C (22.31±0.3). Soleus force generation between BPA and C was similar (p>;0.05). Post‐experiment animal body mass and soleus muscle mass did not differ between groups (p>;0.05). Preliminary findings revealed augmented force generation of the soleus following administration of EE, but not BPA. Therefore, BPA administration does not appear to influence twitch or maximal force generation of mouse soleus muscle.Supported by the Hill Collaboration on Environmental Medicine (KCD)

Defining feasible bounds on muscle activation in a redundant biomechanical task: practical implications of redundancy

Measured muscle activation patterns often vary significantly from musculoskeletal model predictions that use optimization to resolve redundancy. Although experimental muscle activity exhibits both inter- and intra-subject variability we lack adequate tools to quantify the biomechanical latitude that the nervous system has when selecting muscle activation patterns. Here, we identified feasible ranges of individual muscle activity during force production in a musculoskeletal model to quantify the degree to which biomechanical redundancy allows for variability in muscle activation patterns. In a detailed cat hindlimb model matched to the posture of three cats, we identified the lower and upper bounds on muscle activity in each of 31 muscles during static endpoint force production across different force directions and magnitudes. Feasible ranges of muscle activation were relatively unconstrained across force magnitudes such that only a few (0–13%) muscles were found to be truly “necessary” (e.g. exhibited non-zero lower bounds) at physiological force ranges. Most of the muscles were “optional”, having zero lower bounds, and frequently had “maximal” upper bounds as well. Moreover, “optional” muscles were never selected by optimization methods that either minimized muscle stress, or that scaled the pattern required for maximum force generation. Therefore, biomechanical constraints were generally insufficient to restrict or specify muscle activation levels for producing a force in a given direction, and many muscle patterns exist that could deviate substantially from one another but still achieve the task. Our approach could be extended to identify the feasible limits of variability in muscle activation patterns in dynamic tasks such as walking.

Familial hypertrophic cardiomyopathy: Functional effects of myosin mutation R723G in cardiomyocytes

Familial Hypertrophic Cardiomyopathy (FHC) is frequently caused by mutations in the β-cardiac myosin heavy chain (β-MyHC). To identify changes in sarcomeric function triggered by such mutations, distinguishing mutation effects from other functional alterations of the myocardium is essential. We previously identified a direct effect of mutation R723G (MyHC723) on myosin function in slow Musculus soleus fibers. Here we investigate contractile features of left ventricular cardiomyocytes of FHC-patients with the same MyHC723-mutation and compare these to the soleus data.In mechanically isolated, triton-permeabilized MyHC723-cardiomyocytes, maximum force was significantly lower but calcium-sensitivity was unchanged compared to donor. Conversely, MyHC723-soleus fibers showed significantly higher maximum force and reduced calcium-sensitivity compared to controls. Protein phosphorylation, a potential myocardium specific modifying mechanism, might account for differences compared to soleus fibers. Analysis revealed reduced phosphorylation of troponin I and T, myosin-binding-protein C, and myosin-light-chain 2 in MyHC723-myocardium compared to donor. Saturation of protein-kinaseA phospho-sites led to comparable, i.e., reduced MyHC723-calcium-sensitivity in cardiomyocytes as in M. soleus fibers, while maximum force remained reduced. Myofibrillar disarray and lower density of myofibrils, however, largely account for reduced maximum force in MyHC723-cardiomyocytes.The changes seen when phosphorylation of sarcomeric proteins in myocardium of affected patients is matched to control tissue suggest that the R723G mutation causes reduced Ca++-sensitivity in both cardiomyocytes and M. soleus fibers. In MyHC723-myocardium, however, hypophosphorylation can compensate for the reduced calcium-sensitivity, while maximum force generation, lowered by myofibrillar deficiency and disarray, remains impaired, and may only be compensated by hypertrophy.

Effects of Hcm-Linked Mutations in Myosin Essential Light Chain and the Role of Serine-195 Pseudo-Phosphorylation

In this study we investigated whether phosphorylation of myosin ELC at Serine 195, identified by proteomic analysis as a novel phosphorylation site, and shown to affect contractility in zebrafish cardiomyocytes, can play a role in cardiac muscle contraction. We also investigated whether S195D pseudo phosphorylation of ELC could rescue an ELC induced hypertrophic cardiomyopathy (HCM) phenotype. To test this, we exchanged human recombinant WT-ELC, WT-S195D and ELC-A57G, E143K and M173V mutants in skinned porcine papillary muscle fibers and porcine cardiac myosin, both expressing the β-myosin heavy chain (MHC). In myosin exchange experiments, we observed a stronger binding of WT-S195D to MHC (lower Kd), but the degree of ELC exchange vs. WT was significantly lower (∼48% vs. ∼69%). However, no changes in %-reconstitution were observed among all recombinant ELC mutants when compared to WT. In the ELC-reconstituted fibers, WT-S195D was seen to exchange with a significantly lower efficiency compared with WT (∼33% vs 47%), but it also caused a small increase in maximal force (by ∼3 kN/m2). An increase in maximal tension upon pseudo-phosphorylation of ELC was in accord with the MLCK-induced or pseudo (with S15D mutation)-phosphorylation observed for the myosin RLC. However, a WT-S195D-mediated increase in force was not accompanied by any increase in myofilament calcium sensitivity, as observed in RLC studies. Maximal force generation was not changed in any of ELC-reconstituted fibers when compared to WT-ELC. Two ELC mutants (A57G and E143K) slightly (but significantly) increased Ca2+-sensitivity of force while M173V produced no change. Further studies are needed to reveal whether these subtle changes seen in A57G and E143K-exchanged porcine cardiac preparations could be “rescued” by respective S195D pseudo-phosphorylation mimics. Supported by AHA-12PRE12030412 (WH) and NIH-HL108343 (DSC).

Mechanical Properties and Biological Interaction of Aortic Clamps: Are These All Minimally Invasive?

Although specifically designed aortic clamps are mainstay of minimally invasive cardiac surgery, so far, no comparative reports about their mechanical properties and interaction with the aortic wall have been reported. In this study, the generated force in the clamps' jaws and the biological response of the aorta after clamping are evaluated. The jaw force of five commercially available clamps [Geister, Cygnet, Cardiovision (CV) 195.10, CV 195.40, and CV 195.83] was assessed by clamping a 2.2-mm compression load cell with a dedicated computer universal serial bus interface at the proximal, the middle, and the distal site from the fulcrum. Biological response of the aortic wall was assessed in five minipigs (weight, 38-40 kg) that underwent thoracic aorta clamping and leakage point test. Immunohistochemistry and morphometric analysis were carried out for each aortic segment tested. Force generation pattern is peculiar of each clamp, being higher in the proximal and the middle portion and lower in the distal part. One clamp (Cygnet) exhibited homogeneous maximal force generation at all three sites. All clamps exhibited peculiar crushing artifacts. A variable degree of endothelial layer disruption occurred in all clamping tests; three clamps (CV 195.10, Cygnet, and Geister) had the lower amount of intact endothelium. The clamping force was not associated with the degree of endothelial disruption (P value was not significant). The choice of a clamp that is not only minimally invasive in design but also least traumatic will help avoid complications of aortic manipulation.

The impact of isometric handgrip testing on left ventricular twist mechanics

Left ventricular (LV) rotation occurs due to contraction of obliquely oriented myocardial fibres. Left ventricular twist (LVT) results from rotation of the apex and base in opposite directions. Although LVT is altered in various cardiac diseases, physiological factors that affect LVT remain incompletely understood. Isometric handgrip testing (IHGT), a well-established laboratory-based technique to increase LV afterload, was performed for 3 min at 40% maximum force generation in healthy human subjects (n = 18, mean age 29.7 ± 2.7 years). Speckle-tracking echocardiography was used to measure LV volumes, LV apical and basal rotation, peak systolic LVT and peak early diastolic untwisting rate (UTR) at rest and at peak IHGT. IHGT led to significant increase in systemic blood pressure (systolic, 120.6 ± 9.7 vs. 155.6 ± 14.5 mmHg, P < 0.001; diastolic, 67.5 ± 6.4 vs. 94.1 ± 21.1 mmHg, P < 0.001) and LV end-systolic volume (44.2 ± 7.8 vs. 50.5 ± 10.8 ml, P = 0.005), as well as a significant increase in heart rate (62.8 ± 11.7 vs. 84.7 ± 13.8 beats min−1; P < 0.001). IHGT produced a significant acute reduction in LV stroke volume (63.9 ± 12.0 vs. 49.4 ± 7.8 ml, P < 0.001). In this setting, there was a significant decrease in peak systolic apical rotation (11.9 ± 3.0 vs. 8.6 ± 2.2 deg, P < 0.001) and a resultant 25% decrease in peak systolic LVT (16.6 ± 2.8 vs. 12.5 ± 2.8 deg, P < 0.001). The magnitude of peak early diastolic UTR did not change (−114.5 ± 26.4 vs. −110.6 ± 39.8 deg s−1, P = 0.71). Peak systolic apical rotation and LVT decrease during IHGT in healthy humans. This impairment of LV twist mechanics may in part underlie the LV dysfunction that can occur in the clinical context of acute increase in afterload.

A Mutation in TNNC1-encoded Cardiac Troponin C, TNNC1-A31S, Predisposes to Hypertrophic Cardiomyopathy and Ventricular Fibrillation

Defined as clinically unexplained hypertrophy of the left ventricle, hypertrophic cardiomyopathy (HCM) is traditionally understood as a disease of the cardiac sarcomere. Mutations in TNNC1-encoded cardiac troponin C (cTnC) are a relatively rare cause of HCM. Here, we report clinical and functional characterization of a novel TNNC1 mutation, A31S, identified in a pediatric HCM proband with multiple episodes of ventricular fibrillation and aborted sudden cardiac death. Diagnosed at age 5, the proband is family history-negative for HCM or sudden cardiac death, suggesting a de novo mutation. TnC-extracted cardiac skinned fibers were reconstituted with the cTnC-A31S mutant, which increased Ca(2+) sensitivity with no effect on the maximal contractile force generation. Reconstituted actomyosin ATPase assays with 50% cTnC-A31S:50% cTnC-WT demonstrated Ca(2+) sensitivity that was intermediate between 100% cTnC-A31S and 100% cTnC-WT, whereas the mutant increased the activation of the actomyosin ATPase without affecting the inhibitory qualities of the ATPase. The secondary structure of the cTnC mutant was evaluated by circular dichroism, which did not indicate global changes in structure. Fluorescence studies demonstrated increased Ca(2+) affinity in isolated cTnC, the troponin complex, thin filament, and to a lesser degree, thin filament with myosin subfragment 1. These results suggest that this mutation has a direct effect on the Ca(2+) sensitivity of the myofilament, which may alter Ca(2+) handling and contribute to the arrhythmogenesis observed in the proband. In summary, we report a novel mutation in the TNNC1 gene that is associated with HCM pathogenesis and may predispose to the pathogenesis of a fatal arrhythmogenic subtype of HCM.

G.P.108 Sarcomeric dysfunction contributes to muscle weakness in facioscapulohumeral muscular dystrophy

Abstract The expression of sarcomeric proteins is impaired in facioscapulohumeral muscular dystrophy (FSHD), a common hereditary myopathy characterized by muscle weakness. In addition, overexpression of DUX4, the leading FSHD candidate gene, has been shown to activate pathways involved in sarcomeric protein degradation. Here we investigated whether sarcomeric dysfunction contributes to muscle weakness, using demembranated single muscle fibers of FSHD patients and control subjects. The force generating capacity of sarcomeres is significantly impaired in FSHD. Sarcomeric weakness was restricted to type II muscle fibers, in which maximum force generation was only 70% of normal strength. In contrast to active force measurements, a 5- to 12-fold increase in passive force was seen in type I and type II fibers respectively, indicating stiffening of titin molecules. Corroborating these findings, we observed a decrease in myofilament lattice spacing and increase in calcium sensitivity, both physiological consequences of titin stiffening. Based on these findings, we propose that sarcomeric dysfunction plays a critical role in muscle weakness in FSHD.

Contractile and Electrophysiologic Characterization of Optimized Self-Organizing Engineered Heart Tissue

Engineered heart tissue (EHT) is being developed for clinical implantation in heart failure or congenital heart disease and therefore requires a comprehensive functional characterization and scale-up of EHT. Here we explored the effects of scale-up of self-organizing EHT and present detailed electrophysiologic and contractile functional characterization. Fibers from EHT were generated from self-organizing neonatal rat cardiac cells (0.5×10(6) to 3×10(6)/fiber) on fibrin. We characterized contractile patterns and measured contractile function using a force transducer, and assessed force-length relationship, maximal force generation, and rate of force generation. Action potential and conduction velocity of EHT were measured with optical mapping, and transcript levels of myosin heavy chain beta were measured by reverse transcriptase-polymerase chain reaction. Increasing the cell number per construct resulted in an increase in fiber volume. The force-length relationship was negatively impacted by increasing cell number. Maximal force generation and rate of force generation were also abrogated with increasing cell number. This decrease was not likely attributable to a selective expansion of noncontractile cells as myosin heavy chain beta levels were stable. Irregular contractile behavior was more prevalent in constructs with more cells. Engineered heart tissue (1×10(6)/construct) had an action potential duration of 140.2 milliseconds and a conduction velocity of 23.2 cm/s. Engineered heart tissue displays physiologically relevant features shared with native myocardium. Engineered heart tissue scale-up by increasing cell number abrogates contractile function, possibly as a result of suboptimal cardiomyocyte performance in the absence of vasculature. Finally, conduction velocity approaches that of native myocardium without any electrical or mechanical conditioning, suggesting that the self-organizing method may be superior to other rigid scaffold-based EHT.

Further Development of a Tissue Engineered Muscle Repair ConstructIn Vitrofor Enhanced Functional Recovery Following ImplantationIn Vivoin a Murine Model of Volumetric Muscle Loss Injury

Volumetric muscle loss (VML) can result from trauma and surgery in civilian and military populations, resulting in irrecoverable functional and cosmetic deficits that cannot be effectively treated with current therapies. Previous work evaluated a bioreactor-based tissue engineering approach in which muscle derived cells (MDCs) were seeded onto bladder acellular matrices (BAM) and mechanically preconditioned. This first generation tissue engineered muscle repair (TEMR) construct exhibited a largely differentiated cellular morphology consisting primarily of myotubes, and moreover, significantly improved functional recovery within 2 months of implantation in a murine latissimus dorsi (LD) muscle with a surgically created VML injury. The present report extends these initial observations to further document the importance of the cellular phenotype and composition of the TEMR construct in vitro to the functional recovery observed following implantation in vivo. To this end, three distinct TEMR constructs were created by seeding MDCs onto BAM as follows: (1) a short-term cellular proliferation of MDCs to generate primarily myoblasts without bioreactor preconditioning (TEMR-1SP), (2) a prolonged cellular differentiation and maturation period that included bioreactor preconditioning (TEMR-1SPD; identical to the first generation TEMR construct), and (3) similar treatment as TEMR-1SPD but with a second application of MDCs during bioreactor preconditioning (TEMR-2SPD); simulating aspects of "exercise" in vitro. Assessment of maximal tetanic force generation on retrieved LD muscles in vitro revealed that TEMR-1SP and TEMR-1SPD constructs promoted either an accelerated (i.e., 1 month) or a prolonged (i.e., 2 month postinjury) functional recovery, respectively, of similar magnitude. Meanwhile, TEMR-2SPD constructs promoted both an accelerated and prolonged functional recovery, resulting in twice the magnitude of functional recovery of either TEMR-1SP or TEMR-1SPD constructs. Histological and molecular analyses indicated that TEMR constructs mediated functional recovery via regeneration of functional muscle fibers either at the interface of the construct and the native tissue or within the BAM scaffolding independent of the native tissue. Taken together these findings are encouraging for the further development and clinical application of TEMR constructs as a VML injury treatment.

In vivo studies of motor nerve re‐growth following skeletal muscle damage by lengthening contractions

Very little is known about the pattern of changes in motor neurons that occur following skeletal muscle injury and during muscle regeneration during ageing in rodents or man. Our hypothesis is that physiologically relevant episodes of damage to muscle lead to a transient loss of innervation of the damaged muscle fibres with subsequent re‐growth of the terminal axons, interaction with the regenerating muscle fibres and formation of new neuromuscular junctions. Studies were undertaken in yellow fluorescent protein (YFP)‐H mice that only express YFP in neuronal cells. These mice provide a novel approach to assess the extent of motor neuron degeneration and regeneration. One extensor digitorum longus (EDL) muscle of adult YFP‐H mice was subjected to a protocol of 450 lengthening contractions involving a stretch to 120% of fibre length. The EDL muscle from the contralateral leg was used as control. This protocol produces an equivalent level of damage to 60–70% of the muscle fibres in muscles from adult and old mice. Maximal force generation by the EDL muscle in vivo was measured to examine the extent of functional denervation at 3, 14 and 28 days following damage. Mice were then killed, the EDL muscle was removed and the extent of loss and regrowth of peripheral motor neurons was examined by visualisation of labelled axons by fluorescence microscopy in the EDL. Supported by Research into Ageing, UK.

Overexpression of HSP10 in transgenic mice protects skeletal muscle against endotoxic shock‐induced loss of force generation

Impairment of skeletal muscle function and wasting are consistent and severe features of sepsis and are associated with poor outcome. The rate of loss of muscle in septic patients with multiple organ failure is between 2 and 4% per day. We have demonstrated that overexpression of the mitochondrial chaperone HSP10 prevents age related loss of maximum force generation and cross sectional area and we hypothesised that HSP10 would be beneficial in preventing the loss of muscle function during sepsis.WT and HSP10 overexpressor mice were treated with LPS at 2.5mg/Kg and maximum force generation of EDL muscles was measured after 24hrs. Overexpression of HSP10 resulted in significant protection against LPS induced loss of maximum force generation compared with WT mice and additionally protected against the systemic symptoms of endotoxic shock observed in WT mice. HSP10 overexpression significantly reduced serum protein levels and muscle mRNA content of TNF‐α and IL‐6 compared with WT mice. Muscle fibres were isolated from FDB muscles and treated with LPS at 1μg/ml for 5hr. A significant reduction in the TNF‐α release was observed from fibres from HSP10 overexpressor mice compared with WT.These data suggest that anti‐inflammatory effects of HSP10 protect against the loss of maximum force and the severity of systemic illness during endotoxic shock and therefore provides a potential therapeutic target in sepsis. Funded by MRC.

Effects of early-stage aging on locomotor dynamics and hindlimb muscle force production in the rat

Attenuation of locomotor function is common in many species of animals as they age. Dysfunctions may emerge from a constellation of age-related impairments, including increased joint stiffness, reduced ability to repair muscle tissue, and decreasing fine motor control capabilities. Any or all of these factors may contribute to gait abnormalities and substantially limit an animal's speed and mobility. In this study we examined the effects of aging on whole-animal locomotor performance and hindlimb muscle mechanics in young adult rats aged 6-8 months and 'early aged' 24-month-old rats (Rattus norvegicus, Fischer 344 × Brown Norway crosses). Analyses of gaits and kinematics demonstrated that aged rats moved significantly more slowly, sustained longer hindlimb support durations, moved with a greater proportion of asymmetrical gaits, were more plantigrade, and moved with a more kyphotic spinal posture than the young rats. Additionally, the external mechanical energy profiles of the aged animals were variable across trials, whereas the younger rats moved predominantly with bouncing mechanics. In situ analyses of the ankle extensor/plantar flexor muscle group (soleus, plantaris, and medial and lateral gastrocnemii) revealed reduced maximum force generation with aging, despite minimal changes in muscle mass. The weakened muscles were implicated in the degradation of hindfoot posture, as well as variability in center-of-mass mechanics. These results demonstrate that the early stages of aging have consequences for whole-body performance, even before age-related loss of muscle mass begins.

The influence of PKA treatment on the Ca2+ activation of force generation by trout cardiac muscle

β-Adrenergic stimulation of the mammalian heart increases heart rate, the strength of contraction as well as the kinetics of force generation and relaxation. These effects are due to the phosphorylation of select membrane and thin filament proteins by cAMP-activated protein kinase (PKA). At the level of the sarcomere, it is typically the phosphorylation of cardiac myosin binding protein C (cMyBP-C) and cardiac troponin I (cTnI) that is responsible for the change in the kinetics of contraction and relaxation. Trout cTnI (ScTnI) lacks two critical PKA targets within the N-terminus of the protein that, when phosphorylated in mammalian cTnI, cause a reduction in myofilament Ca(2+) affinity. To determine what role the contractile element plays in the response of the trout heart to β-adrenergic stimulation, we characterized the influence of PKA treatment on the Ca(2+) activation of skinned preparations dissected from ventricular trabeculae. In these experiments, isometric force generation and the rate of force development were measured over a range of Ca(2+) concentrations. The results demonstrate that PKA treatment does not influence the Ca(2+) sensitivity of force generation but it decreases maximum force generation by 25% and the rate of force re-development at maximal activation by 46%. Analysis of the trabeculae preparations for phosphoproteins revealed that PKA treatment phosphorylated myosin light chain 2 but not cTnI or cMyBP-C. These results indicate that the function of the trout cardiac contractile element is altered by PKA phosphorylation but in a manner different from that in mammalian heart.

Uncovering Effects of the Familial Hypertrophic Cardiomyopathy (fHCM) Related ß-Myosin Mutation Arg723gly in Human Cardiomyocytes

Mutations in sarcomeric proteins have been identified as the major cause of fHCM. The primary functional effects of these mutations at the sarcomere level of cardiomyocytes, however, are still largely unknown. To address this point we studied cardiomyocytes of the left ventricle (LV) from explanted hearts of two patients with ß-myosin (ß-MHC) mutation R723G (Enjuto et al., JMCC 2000). Our measurements revealed reduced maximum force generation of myocytes but unchanged calcium-sensitivity. In contrast, previous studies on slow skeletal muscle fibers expressing ß-MHC with the same mutation showed reduced calcium-sensitivity and increased maximum force. To address this discrepancy we first determined the expression of mutated and wildtype ß-MHC-mRNA in LV tissue. 62±2% of total ß-MHC-mRNA was found to contain the mutation, just as seen in M. soleus. Secondly, similar to previous findings for failing human heart, gel electrophoresis of the fHCM cardiac tissue showed reduced phosphorylation of troponins I (TnI) and T, myosin binding protein C (MyBP-C), and myosin light chain 2 compared to donor tissue. Adjustment of phosphorylation of TnI and MyBP-C in donor and HCM myocytes by treatment with protein kinase A (PKA), however, uncovered an originally masked reduction in calcium-sensitivity while maximum force was not affected by PKA treatment. Electron microscopy showed reduced myofibrillar density in cardiac tissue samples of the patients which may account for the reduced force. In conclusion, our study reveals that secondary, adaptational processes, triggered by the fHCM-related mutation, can obscure primary effects.

Levosimendan improves calcium sensitivity of diaphragm muscle fibres from a rat model of heart failure

Diaphragm muscle weakness occurs in patients with heart failure (HF) and is associated with exercise intolerance and increased mortality. Reduced sensitivity of diaphragm fibres to calcium contributes to diaphragm weakness in HF. Here we have investigated the ability of the calcium sensitizer levosimendan to restore the reduced calcium sensitivity of diaphragm fibres from rats with HF. Coronary artery ligation in rats was used as an animal model for HF. Sham-operated rats served as controls. Fifteen weeks after induction of HF or sham operations animals were killed and muscle fibres were isolated from the diaphragm. Diaphragm fibres were skinned and activated with solutions containing incremental calcium concentrations and 10 µM levosimendan or vehicle (0.02% DMSO). Developed force was measured at each calcium concentration, and force-calcium concentration relationships were plotted. Calcium sensitivity of force generation was reduced in diaphragm muscle fibres from HF rats, compared with fibres from control rats (P < 0.01). Maximal force generation was ∼25% lower in HF diaphragm fibres than in control fibres (P < 0.05). Levosimendan significantly increased calcium sensitivity of force generation in diaphragm fibres from HF and control rats, without affecting maximal force generation. Levosimendan enhanced the force generating capacity of diaphragm fibres from HF rats by increasing the sensitivity of force generation to calcium concentration. These results provide strong support for testing the effect of calcium sensitizers on diaphragm muscle weakness in patients with HF.

Diaphragm Muscle Fiber Weakness in Pulmonary Hypertension

Recently it was suggested that patients with pulmonary hypertension (PH) suffer from inspiratory muscle dysfunction. However, the nature of inspiratory muscle weakness in PH remains unclear. To assess whether alterations in contractile performance and in morphology of the diaphragm underlie inspiratory muscle weakness in PH. PH was induced in Wistar rats by a single injection of monocrotaline (60 mg/kg). Diaphragm (PH n = 8; controls n = 7) and extensor digitorum longus (PH n = 5; controls n = 7) muscles were excised for determination of in vitro contractile properties and cross-sectional area (CSA) of the muscle fibers. In addition, important determinants of protein synthesis and degradation were determined. Finally, muscle fiber CSA was determined in diaphragm and quadriceps of patients with PH, and the contractile performance of single fibers of the diaphragm. In rats with PH, twitch and maximal tetanic force generation of diaphragm strips were significantly lower, and the force-frequency relation was shifted to the right (i.e., impaired relative force generation) compared with control subjects. Diaphragm fiber CSA was significantly smaller in rats with PH compared with controls, and was associated with increased expression of E3-ligases MAFbx and MuRF-1. No significant differences in contractility and morphology of extensor digitorum longus muscle fibers were found between rats with PH and controls. In line with the rat data, studies on patients with PH revealed significantly reduced CSA and impaired contractility of diaphragm muscle fibers compared with control subjects, with no changes in quadriceps muscle. PH induces selective diaphragm muscle fiber weakness and atrophy.

Investigating the role of the shoulder musculature during maximum unilateral isometric exertions

The extent to which specific muscles may limit maximum isometric force production is largely unknown. This study investigated shoulder muscle activity in six muscles and maximum force generation at the hand in three directions, while in eight different working positions. Ten right hand dominant, university-aged female participants completed twenty-four maximal isometric force hand exertions against a handle positioned by a robot arm within a 3-dimensional simulated workspace. A multivariate, full factorial ANOVA indicated a reach distance main effect where 11% greater force production occurred at the lesser reach distance. A target handle elevation and force direction interaction effect on maximum force production also existed. These findings add to normative static strength data for hand locations typical of an operating work envelope. Although hand force was significantly influenced by position in each direction (p &lt; 0.05), muscle EMG was not influenced in any of the six muscles measured. No muscle achieved 100% MVE in any of the tested conditions, with the highest total muscle activity recorded at 86% MVE for the Pectoralis Major – sternal origin. Collectively, these data better demonstrate that the location and direction of work presentation influences force outputs more than specific muscle demands (of those measured), and should be considered in evaluating workstations.

The masticatory system under varying functional load. Part 1: structural adaptation of rabbit jaw muscles to reduced masticatory load

Skeletal muscle fibres can change their myosin heavy-chain (MyHC) isoform and cross-sectional area, which determine their contraction velocity and maximum force generation, respectively, to adapt to varying functional loads. In general, reduced muscle activity induces transition towards faster fibres and a decrease in fibre cross-sectional area. In order to investigate the effect of a reduction in masticatory load on three functionally different jaw muscles, the MyHC composition and the corresponding cross-sectional area of fibres were determined in the superficial masseter, superficial temporalis, and digastric muscles of male juvenile New Zealand White rabbits that had been raised on a soft diet (n=8) from 8 to 20 weeks of age and in those of normal diet controls (n=8). Differences between groups were tested for statistical significance using a Mann-Whitney rank sum test. The proportion and cross-sectional area of fibres co-expressing MyHC-I and MyHC-cardiac alpha were significantly smaller in the masseter muscles of the animals that had been fed soft food than in those of the controls. In contrast, the proportions and cross-sectional areas of the various fibre types in the temporalis and digastric muscles did not differ significantly between the groups. The results suggest that reducing the masticatory load during development affects the contraction velocity and maximum force generation of the jaw-closing muscles that are primarily responsible for force generation during chewing. These muscles adapt structurally to the reduced functional load with changes in the MyHC composition and cross-sectional area mainly within their slow fibre compartment.

Does central fatigue exist under low-frequency stimulation of a low fatigue-resistant muscle?

The aim of the present study was to determine whether central fatigue occurs when fatigue is electrically induced in the abductor pollicis brevis muscle. Three series of 17 trains (30 Hz, 450 μs, 4 s on/6 s off, at the maximal tolerated intensity) were used to fatigue the muscle. Neuromuscular tests consisting of electrically evoked and voluntary contractions were performed before and after every 17-train series. Both the force induced by the stimulation trains and maximal voluntary force generation capacity significantly decreased throughout the protocol (-27 and -20%, respectively, at the end of the protocol, P < 0.001). These decreases were accompanied by failure in muscle excitability (P < 0.01), as assessed by the muscle compound action potential (M-wave or Mmax), leading to significant impairment in the muscle contractile properties (P < 0.05), as assessed by the muscle mechanical response (Pt). Central fatigue indices (level of activation, RMS/Mmax and H reflex) were not significantly changed at any point in the protocol. This gives evidence of preserved motor command reaching the motor neurons and preserved spinal excitability. The results indicate that this low-frequency stimulation protocol entails purely peripheral fatigue development when applied to a low fatigue-resistant muscle.