Maximum Unloaded Shortening Velocity Research Articles

One size does not fit all: diversity of length-force properties of obliquely striated muscles.

Obliquely striated muscles occur in 17+ phyla, likely evolving repeatedly, yet the implications of oblique striation for muscle function are unknown. Contrary to the belief that oblique striation allows high force output over extraordinary length ranges (i.e. superelongation), recent work suggests diversity in operating length ranges and length-force relationships. We hypothesize oblique striation evolved to increase length-force relationship flexibility. We predict that superelongation is not a general characteristic of obliquely striated muscles and instead that length-force relationships vary with operating length range. To test these predictions, we measured length-force relationships of five obliquely striated muscles from inshore longfin squid, Doryteuthis pealeii: tentacle, funnel retractor and head retractor longitudinal fibers, and arm and fin transverse fibers. Consistent with superelongation, the tentacle length-force relationship had a long descending limb, whereas all others exhibited limited descending limbs. The ascending limb at 0.6P0 was significantly broader (P<0.001) for the tentacle length-force relationship (0.43±0.04L0; where L0 is the preparation length that produced peak isometric stress, P0) than for the arm (0.29±0.03L0), head retractor (0.24±0.06L0), fin (0.20±0.04L0) and funnel retractor (0.27±0.03L0). The fin's narrow ascending limb differed significantly from those of the arm (P=0.004) and funnel retractor (P=0.012). We further characterized the tentacle preparation's maximum isometric stress (315±78 kPa), maximum unloaded shortening velocity (2.97±0.55L0s-1) and ultrastructural traits (compared with the arm), which may explain its broader length-force relationship. Comparison of obliquely striated muscles across taxa revealed length-force relationship diversity, with only two species exhibiting superelongation.

Posttetanic potentiation improves neuromuscular efficiency of mouse muscle in vitro.

Neuromuscular efficiency is defined as the ratio of work output to stimulation rate. The purpose of these experiments was to test the hypothesis that neuromuscular efficiency would be increased in proportion to posttetanic potentiation, that is, the stimulation-induced increase in work output displayed by rodent fast-twitch muscle. To this end, extensor digitorum longus muscles from wild-type and skeletal myosin light chain kinase knockout (skMLCK-/- ) mice were surgically isolated and suspended in vitro (25°C). Concentric force development during shortening at 70% of maximal unloaded shortening velocity was tested at stimulus frequencies between 10 and 80 Hz both before and after a potentiating tetanus. A strong genotype-dependent difference in the potentiation of concentric work output was observed; concentric work output of wild-type muscles was increased by 51%-88% while that of skMLCK-/- muscles was increased by only 20%-34% across the frequencies tested. As a result, comparison of work - frequency plots revealed that the frequency required for peak and 50% peak unpotentiated work of wild-type muscles was decreased from ~80 to 52 Hz and from ~48 to 21 Hz, respectively. By contrast, the frequency required for peak and 50% peak unpotentiated work of skMLCK-/- muscles was decreased from ~80 to 68 Hz and from ~51 to 41 Hz, respectively. Thus, wild-type muscles with the ability to phosphorylate myosin displayed larger increases in neuromuscular efficiency than skMLCK-/- muscles (25-30 vs 10-15 Hz, respectively). This suggests that the presence of myosin phosphorylation may ameliorate or mitigate fatigue mechanisms associated with high-frequency stimulation rates.

Passive force and viscoelastic properties of single fibers in human aging muscles.

Changes in stiffness or extensibility of the muscle or muscle-tendon unit with aging could lead to impaired function and an increased vulnerability to injury. We aimed to investigate the passive force and viscoelastic properties of single muscle fibers in older adults. Seven older adults (mean age 79.0 ± 3.8years) and 10 young control (mean age 25.6 ± 4.5years) were recruited. Biopsy specimens were obtained percutaneously from m. vastus lateralis and skinned single fibers were used for the experiments. Slack tests were performed to determine maximal force and maximal unloaded shortening velocity. Passive force was measured in pCa 9.0 solution using a stepwise stretch technique with increment of sarcomere length from 2.4 to 4.2µm. Myosin heavy chain (MHC) isoform was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Specific force was calculated as maximal force divided by cross-sectional area. Passive force, peak passive force, time to half stress relaxation (T1/2) and force decay index (a force time integral under a stress relaxation curve) were measured. No difference between the groups were found in specific force and shortening velocity. Passive force and peak passive force were greater in both MHC I and IIa fibers of older adults (p < 0.001, p = 0.012, respectively, at 4.2mm SL). Force decay index was higher in older adults. (p = 0.001 at 4.2µm SL). There were no significant differences in passive force and viscoelastic properties between fiber types. We demonstrated greater passive force and viscoelastic properties at the level of single fibers in older adults.

The Effect of the Stretch-Shortening Cycle in the Force-Velocity Relationship and Its Association With Physical Function in Older Adults With COPD.

This study aimed to evaluate the effect of the stretch-shortening cycle (SSC) on different portions of the force–velocity (F–V) relationship in older adults with and without chronic obstructive pulmonary disease (COPD), and to assess its association with physical function. The participants were 26 older adults with COPD (79 ± 7 years old; FEV1 = 53 ± 36% of predicted) and 10 physically active non-COPD (77 ± 4 years old) older adults. The F–V relationship was evaluated in the leg press exercise during a purely concentric muscle action and compared with that following an eccentric muscle action at 10% intervals of maximal unloaded shortening velocity (V0). Vastus lateralis (VL) muscle thickness, pennation angle (PA), and fascicle length (FL) were assessed by ultrasound. Habitual gait speed was measured over a 4-m distance. COPD subjects exhibited lower physical function and concentric maximal muscle power (Pmax) values compared with the non-COPD group (both p < 0.05). The SSC increased force and power values among COPD participants at 0–100 and 1–100% of V0, respectively, while the same was observed among non-COPD participants only at 40–90 and 30–90% of V0, respectively (all p < 0.05). The SSC induced greater improvements in force, but not power, among COPD compared with non-COPD subjects between 50 and 70% of V0 (all p < 0.05). Thus, between-group differences in muscle power were not statistically significant after the inclusion of the SSC (p > 0.05). The SSC-induced potentiation at 50–100% of V0 was negatively associated with physical function (r = -0.40–0.50), while that observed at 80–100% of V0 was negatively associated with VL muscle thickness and PA (r = -0.43–0.52) (all p < 0.05). In conclusion, older adults with COPD showed a higher SSC-induced potentiation compared with non-COPD subjects, which eliminated between-group differences in muscle power when performing SSC muscle actions. The SSC-induced potentiation was associated with lower physical function, VL muscle thickness, and VL PA values. The SSC-induced potentiation may help as a compensatory mechanism in those older subjects with a decreased ability to produce force/power during purely concentric muscle actions.

Effects of Chronic Myocardial Infarction on Cardiac Muscle Performance and Structure In-Vivo and In-Vitro

After chronic myocardial infarction (CMI) the myocardium undergoes remodeling which is described initially as ‘hypertrophic’ followed by ‘de-compensation’. We have studied this phenomenon at the sarcomeric level to understand how muscle mechanics, post-translational modifications, tissue morphology and sarcomeric structures are affected in the remaining well-perfused tissue.We use a rat model of left anterior descending (LAD) ligation to create phenotypic chronic myocardial infarction (CMI). A specialized temperature jump (T-jump) approach assess changes to myocardial muscle function during physiological shortening in saturating (32μM) and physiological (1μM) calcium concentrations. During ‘compensation’ we observe higher power (45%) compared to control. During ‘de-compensation’ we observe a 50% reduction in peak power and 40% reduction in maximal unloaded shortening velocity (Vmax). We exploit second harmonic generation (SHG) by two-photon excitation microscopy (TPE) to assess collagen abundance and infiltration. Global type I collagen abundance and infiltration increases 2-fold post-AMI but does not change during disease progression. Phos-Tag SDS-PAGE is used to assess Troponin I (TnI), Myosin Binding Protein C (MyBP-C) and Regulatory Light Chain (RLC) phosphorylation. Electron microscopy is used to assess sarcomeric organization and mitochondrial health and abundance. Sarcomeric disorder becomes evident with disease progression as well as a global reduction in functional mitochondria. This study demonstrates that sarcomeric protein modification is an important modifier of cardiac muscle function during the evolution of disease, which mirrors the route of progression from ‘compensated’ to ‘decompensated’ phenotypes that is part of the wider picture of heart failure that encompasses changes to tissue structure and organelle morphology.

In Vitro Smooth Muscle Hypercontractility Induced by Cd4+ T Cells is Transient

Smooth muscle (SM) hypercontractility is a characteristic of several pathologies, including asthma. Inflammatory cells and/or the cytokines they release may alter contractile protein expression, thereby also altering the SM mechanics. We characterized the maximal unloaded shortening velocity (Vmax), stress (force/cross sectional area), and contractile protein expression of Brown Norway rat airway SM co-cultured with activated CD4+ T cells for 24h and 48h. After 24h incubation, Vmax (mean±SEM) was significantly increased (0.15±0.01 l/s vs. 0.29±0.02 l/s; control vs. T cells, p = 0.0012). However, this alteration in Vmax was transient and disappeared after 48h of co-incubation (0.21±0.01 vs. 0.25±0.01 l/s, p = 0.099). No significant differences in stress were observed at 24h or 48h. Western blot analysis showed a significant increase in the levels of MLCK (0.20±0.03 vs. 0.41± 0.09; control vs. T cells; p<0.05, p=0.018) and the (+)insert SM myosin isoform (0.55±0.04 vs. 0.81±0.05; p = 0.0023) after 24h incubation with the CD4+ T cells. These alterations were also transient; all values returned to baseline after 48h exposure to the CD4+ T cells. Thus, the increase in Vmax was correlated with the increase in MLCK and (+)insert isoform expression. Conversely, there were no differences in levels of total SM myosin heavy chain or α-smooth muscle actin, suggesting there was no increase in muscle mass. These results show that indeed inflammatory cells and/or their mediators can alter SM contractile protein expression and SM function, but that these effects may be transient. This transient behavior may be important to consider when studying SM function in disease.Supported by: CIHR, NIH-RO1HL103405.

Detection of an aging-related increase in advanced glycation end products in fast- and slow-twitch skeletal muscles in the rat

Glycation, a non-enzymatic addition of reducing sugars to ε-amino groups of proteins, is a post-translational modification that results in the formation of irreversible advanced glycation end products (AGEs). Ageing related decline in myofibrillar protein function is effected by a number of structural and functional modifications including glycation. Functional properties of skeletal muscles, such as maximum velocity of unloaded shortening, are known to be profoundly affected by ageing at the motor unit, cellular and tissue levels. However, the contribution of protein modifications to a decline in muscle function is not well understood. In this study we measured AGEs of intracellular and sarcolemmal proteins, using an anti-AGE antibody in soleus (SOL) and extensor digiotorum longus (EDL) muscles of male and female rats of five different age groups. Using a fluorescent secondary antibody to visualize AGEs in the confocal microscope, we found that myosin is glycated in both fiber types in all age groups; an ageing related increase in AGEs was observed in both intracellular and sarcolemmal regions in all age groups, with the exception of sarcolemma of SOL (unchanged) and EDL (reduced) in female rats; the greatest concentration of AGEs was found intracellularly in the SOL of the oldest age group (27-30) of females. While an ageing related decline in motor properties can be partially attributed to the observed increase in myofibrillar protein glycation, our results also indicate that intracellular and the less well studied sarcolemmal protein modification likely contribute to an aging-related decline in muscle function. Further studies are required to establish a link between the observed ageing related increase in glycation and muscle function at the motor unit, cellular and tissue levels.

Enhancement of Force Generated by Individual Myosin Heads in Skinned Rabbit Psoas Muscle Fibers at Low Ionic Strength

Although evidence has been presented that, at low ionic strength, myosin heads in relaxed skeletal muscle fibers form linkages with actin filaments, the effect of low ionic strength on contraction characteristics of Ca2+-activated muscle fibers has not yet been studied in detail. To give information about the mechanism of muscle contraction, we have examined the effect of low ionic strength on the mechanical properties and the contraction characteristics of skinned rabbit psoas muscle fibers in both relaxed and maximally Ca2+-activated states. By progressively decreasing KCl concentration from 125 mM to 0 mM (corresponding to a decrease in ionic strength μ from 170 mM to 50 mM), relaxed fibers showed changes in mechanical response to sinusoidal length changes and ramp stretches, which are consistent with the idea of actin-myosin linkage formation at low ionic strength. In maximally Ca2+-activated fibers, on the other hand, the maximum isometric force increased about twofold by reducing KCl concentration from 125 to 0 mM. Unexpectedly, determination of the force-velocity curves indicated that, the maximum unloaded shortening velocity Vmax, remained unchanged at low ionic strength. This finding indicates that the actin-myosin linkages, which has been detected in relaxed fibers at low ionic strength, are broken quickly on Ca2+ activation, so that the linkages in relaxed fibers no longer provide any internal resistance against fiber shortening. The force-velocity curves, obtained at various levels of steady Ca2+-activated isometric force, were found to be identical if they are normalized with respect to the maximum isometric force. The MgATPase activity of muscle fibers during isometric force generation was found not to change appreciably at low ionic strength despite the two-fold increase in Ca2+-activated isometric force. These results can be explained in terms of enhancement of force generated by individual myosin heads, but not by any changes in kinetic properties of cyclic actin-myosin interaction.

Evidence that Actin-Myosy Cycling in Muscle may not Pass through Rigor Configuration

In 1989, Sutoh, Tokunaga and Wakabayashi (J Mol Biol 206:357) prepared a monoclonal antibody (IgG) directed to junctional peptide between 50- and 20-kDa segments of myosin heavy chain, and showed that the antibody attaches around the tip of myosin heads facing actin filaments in muscle. using the same antibody, we successfuly position-mark the tip of myosin heads in synthetic myosin filaments kept in hydrated state in the gas environmental chamber, and succeeded in recording ATP-induced myosin head movement electron microscopically (Sugi et al. PNAS 105: 17396, 2008). We have examined the effect of this antibody on both ATP-dependent in vitro actin-myosin sliding and Ca-activated contraction of permealized rabbit psoas muscle fibers. Unexpectedly, the antibody (concentration, up to 2mg/ml) had no appreciable effect on the velocity profile of actin-myosin sliding and the contraction characteristics of muscle fibers, including magnitude of the maximum Ca-activated isometric force, the maximum unloaded shortening velocity, ans MgATPase activity during contraction. Since the junctional peptide is located between the two main actin- binding sites of myosin heads during formation of rigor linkages, attachment of massive antibodies to myosin heads completely covers their actin- binding sites. The possibility that the antibody does not diffuse into muscle fibers is excluded by the fact that other kinds of antibody diffuse into muscle fibers to inhibit Ca-activated force generation (e.g., Sugi et al. PNAS 89: 6134, 1992). Our work, therefore, suggests strongly that attachment-detachment cycle between actin and myosin in muscle may not pass through rigor configuration. It seems possible that so-called strong actin-myosin binding might be due to some electrostatic mechanism.

Myosin Regulatory Light Chain (RLC) Phosphorylation Change as a Modulator of Cardiac Muscle Contraction in Disease

Rationale: Alterations to the cardiac regulatory light chain (RLC) phosphorylation state correlate with many types of cardiac disease including familial hypertrophic cardiomyopathies (FHC's) and heart failure (HF) post myocardial infarction (MI). However, little is known regarding the role of RLC phosphorylation in regulating cardiac contraction mechanics during shortening. Objective: Determine the mechanical effect of altered cRLC phosphorylation on the force-velocity characteristics of cardiac muscle. Methods and Results: SDS-PAGE electrophoresis determined the effects of heart failure on cRLC phosphorylation on human heart failure (HF) samples and a rat chronic-MI model. We report an increase in RLC phosphorylation in HF progression. Recombinant cRLC was cloned and expressed. Recombinant RLC was differentially phosphorylated in-vitro using cardiac myosin light chain kinase (cMLCK) and ZIP kinase. Recombinant RLC species were exchanged into permeabilised cardiac trabeculae to perform mechanical measurements during shortening (slack test and force-velocity experimentation protocols). We observed a decrease in peak power, velocity at peak power (VPP) and peak isometric force of contraction when reducing RLC phosphorylation. Conversely, enriching RLC phosphorylation increased peak power, VPP and peak maximal unloaded shortening velocity (VMAX) in comparison to control. Conclusions: Altering cRLC phosphorylation has a significant impact on the mechanical function of cardiac muscle and is dramatically altered during HF progression. These results highlight the importance of understanding thick filament regulation and specifically the ability of cRLC phosphorylation to regulate cardiac muscle function.

Contact between Cd4+ T Cells and Smooth Muscle Tissue Enhances Contractile Function

Smooth muscle (SM) hyper-contractility is central to the pathophysiology of several diseases, including asthma. Inflammatory cells and/or their products may alter the expression of contractile proteins, thereby enhancing SM mechanics. The goal of the current study was to characterize the maximum unloaded shortening velocity (Vmax), stress (force/cross sectional area), and contractile protein expression of Brown Norway (BN) rat airway SM in the presence of activated CD4+ T cells. T cells from OVA-sensitized BN rats were co-cultured with tracheal rings for 24h. The SM strips were then mounted in a mechanical testing apparatus, equilibrated and supramaximally stimulated with methacholine. A significant increase in Vmax (mean±S.E.) (0.15±0.01 l/s for control, 0.29±0.02 l/s for sensitized, p=0.0012) was observed whereas no significant changes in stress (84.5±13.9 kPa for control, 107.8±4.8 kPa for sensitized) were seen. Western blot analysis showed that levels of the (+)insert myosin heavy chain isoform and myosin light chain kinase (MLCK) were significantly increased. To determine whether contact between T cells and SM cells is required for this enhanced contractility, CD4+T cells were placed in the upper chamber of a transwell, separated by a membrane from the muscle strips placed below. The membrane had 0.4 μm pores that allowed the cytokines released from the CD4+T cells to reach the SM but prevented direct contact between cells. We found a significant increase in Vmax when the SM were in contact with the T cells (0.32±0.02 l/s) compared to the co-culture without contact (0.25±0.01 l/s) or compared to the SM alone (0.24±0.01 l/s; p=0.01). Thus, contact between SM and CD4+T cells is necessary for modulation of SM contractile properties and this may be mediated, at least in part, by increases in (+)insert myosin heavy chain and MLCK expression. Supported by: CIHR, NIH-RO1HL103405.

Origin of Motion in the Human Ureter: Mechanics, Energetics and Kinetics of the Myosin Molecular Motors

Ureteral peristalsis is the result of coordinated mechanical motor performance of longitudinal and circular smooth muscle layer of the ureter wall. The main aim of this study was to characterize in smooth muscle of proximal segments of human ureter, the mechanical properties at level of muscle tissue and at level of myosin molecular motors. Ureteral samples were collected from 15 patients, who underwent nephrectomy for renal cancer. Smooth muscle strips longitudinally and circularly oriented from proximal segments of human ureter were used for the in vitro experiments. Mechanical indices including the maximum unloaded shortening velocity (Vmax), and the maximum isometric tension (P0) normalized per cross-sectional area, were determined in vitro determined in electrically evoked contractions of longitudinal and circular smooth muscle strips. Myosin cross-bridge (CB) number per mm2 (Ψ) the elementary force per single CB (Ψ) and kinetic parameters were calculated in muscle strips, using Huxley's equations adapted to nonsarcomeric muscles. Longitudinal smooth muscle strips exhibited a significantly (p<0.05) faster Vmax (63%) and a higher P0 (40%), if compared to circular strips. Moreover, longitudinal muscle strips showed a significantly higher unitary force (Ψ) per CB. However, no significant differences were observed in CB number, the attachment (f1) and the detachment (g2) rate constants between longitudinal and circular muscle strips. The main result obtained in the present work documents that the mechanical, energetic and unitary forces per CB of longitudinal layer of proximal ureter are better compared to the circular one; these preliminary findings suggested, unlike intestinal smooth muscle, a major role of longitudinal smooth muscle layer in the ureter peristalsis.

Altered Cross-Bridge Kinetics and Increased Force Production in Presence of a Myosin Heavy Chain IIb Mutant

Hereditary myosinopathies have emerged as a new group of diseases and are characterized by skeletal muscle weakness. These diseases are notably caused by mutations in genes encoding myosin heavy chain (MyHC) isoforms. A new mouse model with a L342Q MyHC IIb mutation has been developed to mimic such myosinopathies. Homozygous mice experience severe skeletal muscle paralysis and, because of ethical reasons, are killed thirteen days after birth. Heterozygous animals have a normal lifespan and no apparent weakness. The underlying mechanisms for such absence of deteriorated phenotype in heterozygous rodents remain unknown. In the current study, we aimed at investigating this particular point by evaluating whether and how the presence of 50% of L342Q MyHC IIb mutants affects the contractile function. We compared isolated membrane-permeabilized muscle fibers from heterozygote (L342Q+/-) and wild-type young adult mice. Results showed, during full activation, at pCa 4.5, an increase in force production in L342Q+/- when compared with wild-type animals (p<0.05) whilst stiffness was unaffected. Additionally, we observed a preserved rate of force redevelopment (ktr) together with an increase in maximum unloaded shortening velocity (V0) (p<0.05). All these findings suggest an enhanced strong myosin binding to actin as well as a faster myosin cross-bridge detachment, in presence of the MyHC IIb mutants. Results also highlighted, during rigor activation, an unchanged force-generating capacity and decreased stiffness in L342Q+/- when compared with wild-type rodents (p<0.05). This indicates that myosin cross-bridges carrying the mutant may not function properly but, at the same time, may perturb the wild-type myosin heads leading to an accelerated cross-bridge detachment and increased force generation.

Changes in the force-velocity relationship of fatigued muscle: implications for power production and possible causes.

Slowing of the contractile properties of skeletal muscle is one of the characteristic features of fatigue. First studied as a slowing of relaxation from an isometric contraction, it has become apparent that this slowing is indicative of functional changes in muscle responsible for a major loss of power with all its functional repercussions. There are three factors contributing to the loss of power in mammalian muscle at physiological temperatures, a decrease in isometric force, which mainly indicates a reduction in the number of active cross bridges, a slowing of the maximum velocity of unloaded shortening and an increased curvature of the force-velocity relationship. This latter change is a major cause of loss of power but is poorly understood. It is probably associated with an increase in the proportion of cross bridges in the low force state but there are no clear candidates for the metabolic changes that are responsible for this shift in cross bridge states. The possibility is discussed that the reduction in activating calcium that occurs with metabolically depleted muscle, alters the distribution of cross bridge states, affecting both shortening velocity and curvature.

Enhancement of shortening velocity, power, and acto‐myosin crossbridge (CB) kinetics following long‐term treatment with propionyl‐L‐carnitine, coenzyme Q10, and omega‐3 fatty acids in BIO TO‐2 cardiomyopathic Syrian hamsters papillary muscle

Impaired functions of myocardial muscle cells in human and animals, is a primary defect associated with idiopathic dilated cardiomyopathy (DCM). The pathophysiological mechanisms implicated in the DCM are yet to be clarified and an effective therapy is still not available. The BIO TO-2 cardiomyopathic Syrian Hamsters (CMSHs) represent an animal model of idiopathic DCM. The aim of this study was to investigate the effect of long-term treatment (2 months) with propionyl-L-carnitine (PLC), coenzyme Q(10), omega-3 fatty acids and a combination of these three agents (formulation HS12607) on mechanical properties and acto-myosin crossbridges (CBs) kinetics of left ventricular (LV) papillary muscle from control and treated 10 month old BIO TO-2 CMSHs. Isometric and isotonic contractile properties of isolated papillary muscle from control and treated CMSHs were investigated, and acto-myosin CB number, force and kinetics were calculated using Huxley's equations. Mechanical parameter values were higher in treated than in control hamsters, particularly when substances were administered together in a coformulation (HS12607). Compared to control, HS12607-treated papillary muscles showed a significant increase of maximum peak isometric tension (P(o)) (30.06 +/- 4.91 vs. 19.74 +/- 5.00 mN/mm(2)), maximum extent of muscle shortening (0.13 +/- 0.03 vs. 0.07 +/- 0.02 L/L(max)), maximum unloaded shortening velocity (1.18 +/- 0.24 vs. 0.53 +/- 0.13 L/L(max) s(-1)) and maximum peak of power output (5.52 +/- 1.61 vs. 1.58 +/- 0.83). The curvature of the hyperbolic force-velocity relationships did not differ between control and treated hamsters. When compared to controls, acto-myosin CB number increased in treated hamsters [(6.67 +/- 1.91) 10(10)/mm(2) vs. (3.55 +/- 2.08) 10(10)/mm(2)], whereas the unitary force of single CB was similar in control and treated animals. The peak value of the rate constant for CB attachment (f(1)) and detachment (g(2)) was higher in treated animals when compared to control. (93.87 +/- 25.82 vs.47.28 +/- 10.88 s(-1) and 214.40 +/- 44.64 vs. 95.56 +/- 23.49 s(-1), respectively). In conclusion, the present study illustrates that supplementation with PLC, CoQ(10) and omega-3 fatty acids improved motor parameters, energetic, and CB kinetics of BIO TO-2 CMSH papillary muscle indicating that these naturally occurring substances may be a valid adjuvant to conventional therapy in DCM.

Variation in the determinants of power of chemically skinned human muscle fibres

We have explored the extent to which the maximal velocity of unloaded shortening (V(max)), the force generated per unit cross-sectional area (P(0)) and the curvature of the force-velocity relationship (a/P(0) in the Hill equation) contribute to differences in peak power of chemically skinned single fibres from the quadriceps muscle of healthy young male subjects. The analysis was restricted to type I and IIA fibres that contained a single type of myosin heavy chain on electrophoretic separation. Force-velocity relationships were determined from isotonic contractions of maximally activated fibres at 15 degrees C. Mean (+/- s.d.) peak powers were 1.99 +/- 0.72 watts per litre (W L(-1)) for type I fibres and 6.92 +/- 2.41 W L(-1), for type IIA fibres. The most notable feature, however, was the very large, sevenfold, range of power outputs within a single fibre type. This wide range was a consequence of variations in each of the three components determining power: P(0), V(max) and a/P(0). Within a single fibre type, P(0) varied threefold, and V(max) and a/P(0) two- to threefold. There were no obvious relationships between P(0) and V(max) or between P(0) and a/P(0). However, there was a suggestion of an inverse relationship between a/P(0) and V(max), the effect being to reduce, somewhat, the impact of differences in V(max) on peak power. In searching for the causes of variation in peak power of fibres of the same type, it appears likely that there are two factors, one that affects P(0) and another that leads to variation in both V(max) and a/P(0).

Causes of fatigue in slow-twitch rat skeletal muscle during dynamic activity

Skeletal muscle fatigue is most often studied in vitro at room temperature and is classically defined as a decline in maximum force production or power output, exclusively linked to repeated isometric contractions. However, most muscles shorten during normal use, and we propose that both the functional correlate of fatigue, as well as the fatigue mechanism, will be different during dynamic contractions compared with static contractions. Under isoflurane anesthesia, fatigue was induced in rat soleus muscles in situ by isotonic shortening contractions at 37 degrees C. Muscles were stimulated repeatedly for 1 s at 30 Hz every 2 s for a total of 15 min. The muscles were allowed to shorten isotonically against a load corresponding to one-third of maximal isometric force. Maximal unloaded shortening velocity (V(0)), maximum force production (F(max)), and isometric relaxation rate (-dF/dt) was reduced after 100 s but returned to almost initial values at the end of the stimulation protocol. Likewise, ATP and creatine phosphate (CrP) were reduced after 100 s, but the level of CrP was partially restored to initial values after 15 min. The rate of isometric force development, the velocity of shortening, and isotonic shortening were also reduced at 100 s, but in striking contrast, did not recover during the remainder of the stimulation protocol. The regulatory myosin light chain (MLC2s) was dephosphorylated after 100 s and did not recover. Although metabolic changes may account for the changes of F(max), -dF/dt, and V(0), dephosphorylation of MLC2s may be involved in the fatigue seen as sustained slower contraction velocities and decreased muscle shortening.

Single muscle fiber contractile properties in adults with muscular dystrophy treated with MYO‐029

Myostatin inhibitors are being investigated as treatments for myopathies. We assessed single muscle fiber contractile properties before and after 6 months of study drug in 6 patients with facioscapulohumeral, Becker, and limb-girdle muscular dystrophy. Five of the patients received MYO-029, a myostatin inhibitor, and 1 received placebo. The chemically skinned single muscle fiber preparation was used to measure single fiber force, specific force, maximum unloaded shortening velocity, power, and specific power in type I and IIa fibers from each subject. In 4 of 5 patients who received MYO-029, improvement was seen in single muscle fiber contractile properties; thus, there may be a beneficial effect of myostatin inhibition on muscle physiology at the cellular level. No improvement was seen in the patient who received placebo. This finding may be clinically relevant in spite of the fact that quantitative muscle strength measurements in our patients did not improve. Further studies of myostatin inhibition as a treatment for muscular dystrophy are warranted, and single muscle fiber contractile studies are a useful assay for muscle function at the cellular level.

Muscle fiber size and function in elderly humans: a longitudinal study

Cross-sectional studies are likely to underestimate age-related changes in skeletal muscle strength and mass. The purpose of this longitudinal study was to assess whole muscle and single muscle fiber alterations in the same cohort of 12 older (mean age: start of study 71.1+/-5.4 yr and end of study 80+/-5.3 yr) volunteers (5 men) evaluated 8.9 yr apart. No significant changes were noted at follow-up in body weight, body mass index, and physical activity. Muscle strength, evaluated using isokinetic dynamometry, and whole muscle specific force of the knee extensors were significantly lower at follow-up. This was accompanied by a significant reduction (5.7%) in cross-sectional area of the total anterior muscle compartment of the thigh as evaluated by computed tomography. Muscle histochemistry showed no significant changes in fiber type distribution or fiber area. Experiments with chemically skinned single muscle fibers (n=411) demonstrated no change in type I fiber size but an increase in IIA fiber diameter. A trend toward an increase in maximal force in both fiber types was observed. Maximum unloaded shortening velocity did not change. In conclusion, single muscle fiber contractile function may be preserved in older humans in the presence of significant alterations at the whole muscle level. This suggests that surviving fibers compensate to partially correct muscle size deficits in an attempt to maintain optimal force-generating capacity.

What Do Single-Fiber Studies Tell Us about Exercise Training?

Many exercise programs designed by trainers have the specific aim to enhance strength, endurance, or power of certain muscle groups to improve athletic performance. Muscle training is also an important component of rehabilitation to limit atrophy associated with immobilization, aging, or disease. To optimize exercise interventions, a good understanding of the response to various training programs and the identification of the mechanisms that influence muscle function are required. Changes in the performance of muscle contraction are associated, among other things, with muscle fiber-type expression and functional properties of the contractile apparatus. Chemically skinned, single fibers is the only model that allows study of the mechanical properties of human muscle fibers as a function of the expression of myosin heavy-chain isoforms. Recently, this model has been used to explore the functional adaptations after different types of solicitations. The current results of the literature indicate that cross-sectional area and force of a specific fiber type are enhanced after resistance training, whereas maximal unloaded shortening velocity is increased by endurance training. Plyometric training improves both fiber force and shortening velocity. Fiber peak power is increased after resistance or plyometric training, and on a long-term basis it is decreased after endurance training. Studies on elderly individuals indicate that this population may react differently with respect to some single-fiber adaptations after training. Generally, training regimens tend to decrease the proportion of type IIx fibers. The available data in the literature make it possible to formulate recommendations applicable to sport science and rehabilitation.