Down-regulation of MyoD gene expression in rat diaphragm muscle with heart failure

Diaphragm myopathy has been described in patients with heart failure (HF), with alterations in myosin heavy chains (MHC) expression. The pathways that regulate MHC expression during HF have not been described, and myogenic regulatory factors (MRFs) may be involved. The purpose of this investigation was to determine MRF mRNA expression levels in the diaphragm. Diaphragm muscle from both HF and control Wistar rats was studied when overt HF had developed, 22 days after monocrotaline administration. MyoD, myogenin and MRF4 gene expression were determined by RT-PCR and MHC isoforms by polyacrylamide gel electrophoresis. Heart failure animals presented decreased MHC IIa/IIx protein isoform and MyoD gene expression, without altering MHC I, IIb, myogenin and MRF4. Our results show that in HF, MyoD is selectively down-regulated, which might be associated with alterations in MHC IIa/IIx content. These changes are likely to contribute to the diaphragm myopathy caused by HF.     

Heart failure alters matrix metalloproteinase gene expression and activity in rat skeletal muscle

Heart failure is associated with a skeletal muscle myopathy with cellular and extracellular alterations. The hypothesis of this investigation is that extracellular changes may be associated with enhanced mRNA expression and activity of matrix metalloproteinases (MMP). We examined MMP mRNA expression and MMP activity in Soleus (SOL), extensor digitorum longus (EDL), and diaphragm (DIA) muscles of young Wistar rat with monocrotaline-induced heart failure. Rats injected with saline served as age-matched controls. MMP2 and MMP9 mRNA contents were determined by RT-PCR and MMP activity by electrophoresis in gelatin-containing polyacrylamide gels in the presence of SDS under non-reducing conditions. Heart failure increased MMP9 mRNA expression and activity in SOL, EDL and DIA and MMP2 mRNA expression in DIA. These results suggest that MMP changes may contribute to the skeletal muscle myopathy during heart failure.     

Myosin heavy chain expression and atrophy in rat skeletal muscle during transition from cardiac hypertrophy to heart failure

The purpose of this investigation was to determine whether changes in myosin heavy chain (MHC) expression and atrophy in rat skeletal muscle are observed during transition from cardiac hypertrophy to chronic heart failure (CHF) induced by aortic stenosis (AS). AS and control animals were studied 12 and 18 weeks after surgery and when overt CHF had developed in AS animals, 28 weeks after the surgery. The following parameters were studied in the soleus muscle: muscle atrophy index (soleus weight/body weight), muscle fibre diameter and frequency and MHC expression. AS animals presented decreases in both MHC1 and type I fibres and increases in both MHC2a and type IIa fibres during late cardiac hypertrophy and CHF. Type IIa fibre atrophy occurred during CHF. In conclusion, our data demonstrate that skeletal muscle phenotype changes occur in both late cardiac hypertrophy and heart failure; this suggests that attention should be given to the fact that skeletal muscle phenotype changes occur prior to overt heart failure symptoms.     

Apoptosis and changes in contractile protein pattern in the skeletal muscle in heart failure

Chronic heart failure is characterized as a clinical disorder by exercise intolerance. There are two factors that are independently responsible for the reduced exercise capacity: (a) a shift from myosin heavy chain 1 (MHC1) to MHC2a and MHC2b and (b) muscle atrophy. We have demonstrated, both in experimental models of heart failure and in man, that the more severe the heart failure, the greater the magnitude of skeletal muscle apoptosis. In the monocrotaline treated rat, that develops a severe right‐sided heart failure, the increased number of apoptotic nuclei was paralleled by increasing levels of circulating TNFα. In agreement with some recent observations showing that sphingolipids can mediate programmed cell death, we found that in animals with heart failure and high number of apoptotic nuclei, circulating levels of sphingosine were significantly increased. In a study conducted in patients with heart failure we found a correlation between exercise capacity limitation and skeletal myocytes apoptosis. There was also a correlation between degree of muscle atrophy and magnitude of apoptosis. The shift in MHCs, although with a different mechanism, is also responsible for the reduced exercise capacity in these patients. In fact there is a strong correlation between indices of severity of CHF and MHC composition. Muscle fatigue, appears earlier in patients that have a greater skeletal muscle expression of ‘fast’ MHCs. We have also demonstrated that MHCs shift and apoptosis can be prevented by using angiotensin II converting enzyme inhibitors and angiotensin II receptor blockers.     

Co-existence of myosin heavy chain I and IIa isoforms in human skeletal muscle fibres with endurance training

The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis was analysed by one-dimensional electrophoresis and immunoblotting in three groups of young men with distinct difference in physical activity patterns. No major co-existence of MHC isoforms was found in the group with some daily physical activity. In the very sedentary group, however, 19±5% (P<0.05) of the fibres exhibited coexistence of MHC type IIa and IIb. Further, in the endurance trained group co-existence of MHC type I and IIa was manifested in 36±4% (P<0.05) of the fibres. Disuse and extreme usage of muscle both give rise to an elevation in co-expression of MHC isoforms in single muscle fibres but of markedly different combination of isoforms.     

运动在骨骼肌肌纤维类型转变中的作用

骨骼肌纤维类型及其表达的专一蛋白同功型的多样性,是骨骼肌功能和适应性的结构和分子基础.肌球蛋白重链同功型被认为是决定肌纤维快、慢类型的主要因素,已成为区分肌纤维类型和研究肌适应性的分子标志.运动可以导致肌球蛋白重链不同亚型之间的转变.本文就肌球蛋白重链与骨骼肌纤维类型的关系,以及不同运动模式对骨骼肌纤维肌球蛋白重链同功型转变的影响作一综述.     

Postnatal development of skeletal muscle in pigs with intrauterine growth restriction: morphofunctional phenotype and molecular mechanisms

Intrauterine growth restriction (IUGR) is a serious condition which impairs the achievement of the fetus' full growth potential and occurs in a natural and severe manner in pigs as a result of placental insufficiency. Reduced skeletal muscle mass in the fetus with IUGR persists into adulthood and may contribute to increased metabolic disease risk. To investigate skeletal muscle postnatal development, histomorphometrical patterns of the semitendinosus muscle, myosin heavy chain (MyHC; embryonic I, IIA, IIB and IIX isoforms) fiber composition and the relative expression of genes related to myogenesis, adipogenesis and growth during three specific periods: postnatal myogenesis (newborn to 100 days old), hypertrophy (100–150 days old), and postnatal development (newborn to 150 days old) were evaluated in female pigs with IUGR and normal birth weight (NW) female littermates. NW females presented higher body weights compared to their IUGR counterparts at all ages evaluated (P < 0.05). Moreover, growth restriction in utero affected the semitendinosus muscle weight, muscle fiber diameter, and muscle cross-sectional area, which were smaller in IUGR pigs at birth (P < 0.05). Notwithstanding the effects on muscle morphology, IUGR also affected muscle fiber composition, as the percentage of MyHC-I myofibers was higher at birth (P < 0.05), and, in 150-day-old gilts, a lower percentage of MyHC-IIX isoform (P < 0.05) and the presence of embryonic MyHC isoform were also observed. Regarding the pattern of gene expression in both the postnatal myogenesis and postnatal development periods, IUGR led to the downregulation of myogenic factors, which delayed skeletal muscle myogenesis (PAX7, MYOD, MYOG, MYF5 and DES). Altogether, growth restriction in utero affects muscle fiber number and size at birth and muscle fiber composition through the downregulation of myogenic factors, which determines the individual´s postnatal growth rate. This fact, associated with delayed myofiber development in growth-restricted animals, may affect meat quality characteristics in animal production. Hence, knowledge of the morphofunctional phenotype of the skeletal muscle throughout postnatal development in individuals with IUGR, and the mechanism that governs it, may provide a better understanding of the mechanisms that limit postnatal muscle growth, and help the establishment of potential strategies to improve muscle development and prevent the onset of later-life metabolic diseases.     

Effects of anabolic/androgenic steroids on regenerating skeletal muscles in the rat

We have examined the effect of male sexual hormones on the regeneration of skeletal muscles. Degeneration/regeneration of the left soleus and extensor digitorum longus muscles (EDL) of Wistar male rats was induced by an injection of snake venom (2 μg, Notechis scutatus scutatus). During the muscle regeneration (25 days), rats were treated with either oil (CON), nandrolone (NAN), NAN combined with exercise (NAN + EXE) or were castrated (CAS). Muscle growth and myosin heavy chain (MyHC) isoform content of regenerating muscles were studied. Castration altered the concentrations of MyHC in venom-treated EDL (P < 0.01) and soleus (P < 0.05). NAN increased the mass (P < 0.01) of regenerating soleus and decreased the relative amount of fast MyHC protein (% of total, P < 0.05). The effect of NAN + EXE on the fast MyHC proteins of venom-treated soleus was opposite (P < 0.05). NAN and NAN + EXE were without effect on the regenerating EDL (P > 0.05). In conclusion, it is possible that male sexual hormones play a role in the growth (synthesis of contractile proteins) of regenerating muscles in rat. In addition, contrary to NAN + EXE, NAN could be beneficial to soleus regeneration.     

Differential titin isoform expression in human skeletal muscle

Mammalian skeletal muscle expresses at least two isoforms of the cytoskeletal protein titin (connectin; MW ≈ 3000 kDa). These isoforms are associated with different passive force curves, and thus may affect physical performance. To study the distribution of titin and its possible influence on performance in humans, muscle biopsies were obtained from 15 males (X ± SE; age = 25.4 ± 2.9 years, height = 177.7 ± 1.8 cm, weight = 76.5 ± 2.2 kg). Two biopsies were obtained on separate occasions from both the right and left vastus lateralis, and one biopsy each from the lateral head of the right gastrocnemius and the right soleus, with all biopsies handled identically. Fibre type analyses were performed via mATPase histochemistry. Expression of titin and myosin heavy chain isoforms were determined by SDS-PAGE. Titin bands in the resulting gels were highly repeatable and were verified by migration patterns, as well as Western blot analysis. Two groups of subjects were identified: group 1 (n=10) expressed only one titin isoform (titin-1) in all biopsies, and group 2 (n=5) expressed two titin isoforms (titin-1 and titin-2) in all biopsies. No significant differences (P> 0.05) between groups were observed for percentage fibre types, percentage fibre type areas, fibre type cross-sectional areas, and percentage myosin heavy chain expression when comparing individual muscles, sampling times or bilateral comparisons. This is the first report of differential titin isoform expression in healthy, mature human skeletal muscle, but it is not clear why this occurs or what influence this may have on performance.     

Rat skeletal muscle metabolism in experimental heart failure: effects of physical training

Skeletal muscle metabolic abnormalities exist in chronic heart failure. The influence of physical training on muscle metabolism after myocardial infarction was studied in a rat model. ³¹P magnetic resonance spectroscopy and enzyme assays were performed in Wistar rats 12 weeks after coronary artery ligation. Infarcted rats were allocated randomly to either 6 weeks of training or non-training. Spectra were collected from the calf muscles during sciatic nerve stimulation at 2 Hz. Fibre typing and enzymatic assays were performed on the muscles of the contralateral non stimulated leg. Post-mortem rats were also divided into severe and moderate heart failure according to the lung weight per body weight. At 200 g twitch tension, phosphocreatine and pH were found to be significantly lower in the non-trained severe heart failure group compared with the other groups. Phosphocreatine recovery half-time was significantly longer in the non-trained group with severe heart failure and correlated with the citrate synthase activity in the muscle. The training did not induce a change in the enzyme activities in the infarcted animals with moderate heart failure but did correct the lower citrate synthase activity in the non-trained severe heart failure animals. This normalization of muscle metabolism was achieved by training without any change in calf muscle mass, making atrophy unlikely to be the sole cause of the metabolic changes in heart failure. Training in rats with severe heart failure can reverse the abnormalities of skeletal muscle metabolism, implicating decreased physical activity in the aetiology of these changes.     

Regional differences in fiber characteristics in the rat temporalis muscle

The behavioral differences in muscle use are related to the fiber type composition of the muscles among other variables. The aim of this study was to examine the degree of heterogeneity in the fiber type composition in the rat temporalis muscle. The temporalis muscle was taken from 10‐week‐old Wistar strain male rats (n = 5). Fiber types were classified by immunohistochemical staining according to their myosin heavy chain content. The anterior temporalis revealed an obvious regional difference of the fiber type distribution, whereas the posterior temporalis was homogeneous. The deep anterior temporalis showed a predominant proportion of type IIA fibers and was the only muscle portion displaying slow type fibers (< 10%). The other two muscle portions, the superficial anterior and posterior temporalis, did not differ significantly from each other and contained mainly type IIB fibers. Moreover, the deep anterior temporalis was the only muscle portion showing slow type fibers (< 10%). In the deep portion, type IIX fibers revealed the largest cross‐sectional area (1943.1 ± 613.7 µm²), which was significantly (P < 0.01) larger than those of type IIA and I + IIA fibers. The cross‐sectional area of type IIB fibers was the largest in the remaining two muscle portions and was significantly (P < 0.01) larger than that of type IIX fibers. In conclusion, temporalis muscle in rats showed an obvious heterogeneity of fiber type composition and fiber cross‐sectional area, which suggests multiple functions of this muscle.     

Fibre types in skeletal muscle: a personal account

Muscle performance is in part dictated by muscle fibre composition and a precise understanding of the genetic and acquired factors that determine the fibre type profile is important in sport science, but is also relevant to neuromuscular diseases and to metabolic diseases, such as type 2 diabetes. The dissection of the signalling pathways that determine or modulate the muscle fibre phenotype has thus potential clinical significance. In this brief review, I examine the evolution of the notion of muscle fibre types, discuss some aspects related to species differences, point at problems in the interpretation of transgenic and knockout models and show how in vivo transfection can be used to identify regulatory factors involved in fibre type diversification, focusing on the calcineurin-nuclear factor of activated T cells (NFAT) pathway.     

Human skeletal muscle myosin function at physiological and non-physiological temperatures

Aim: The aim of the study was to assess the function of human skeletal muscle myosin across a wide range of temperatures, including physiological. Methods: We used a single fibre in vitro motility assay. The in vitro motility speed of actin filaments propelled by myosin extracted from fibres expressing type I myosin heavy chain (MyHC; n = 9), IIa MyHC (n = 6), IIax MyHC (n = 4) and I/IIa MyHC (n = 1) was measured at 15, 20, 25, 30 and 35 °C. Results: The motility speed between groups of fibres expressing different MyHC differed significantly (P ? 0.001). The increase in motility speed with an increase in temperature was statistically significant (P ? 0.001) between all temperatures. The relative difference in motility speed between the slow type I and the fast IIax MyHC fibres decreased with increasing temperature, i.e. a 7.5‐fold difference at 15 °C was reduced to twofold at 35 °C. Furthermore, the twofold difference in motility speed between type IIa and IIax MyHC at 15 °C disappeared completely at 35 °C. The activation energy, E_A, and temperature coefficient, Q₁₀, over the 15–35 °C temperature range was higher for type I MyHC, 54.47 ± 4.37 kJ mol^?1 and 2.09 ± 0.12, respectively, than for type IIa MyHC, 45.41 ± 3.12 kJ mol^?1 (P < 0.001) and 1.85 ± 0.08 (P < 0.001), or IIax MyHC, 34.71 ± 1.75 kJ mol^?1 (P ? 0.001) and 1.60 ± 0.04 (P ? 0.001). Conclusion: The present results suggest a significantly reduced difference in shortening velocity between different human muscle fibre types at physiological temperature than previously reported at lower temperatures (12 or 15 °C) with implications for human in vivo muscle function.     

Correlation of dystrophin–glycoprotein complex and focal adhesion complex with myosin heavy chain isoforms in rat skeletal muscle

Aim: The dystrophin–glycoprotein complex (DGC) and focal adhesion complex (FAC) are transmembrane structures in muscle fibres that link the intracellular cytoskeleton to the extracellular matrix. DGC and FAC proteins are abundant in slow‐type muscles, indicating the structural reinforcement which play a pivotal role in continuous force output to maintain posture for long periods. The aim of the present study was to examine the expression of these structures across fast‐type muscles containing different myosin heavy chain (MHC) isoform patterns which reflect the fatigue‐resistant characteristics of skeletal muscle. Methods: We measured the expression of dystrophin and β1 integrin (representative proteins of DGC and FAC respectively) in plantaris, extensor digitorum longus, tibialis anterior, red and white portions of gastrocnemius, superficial portion of vastus lateralis and diaphragm, in comparison with soleus (SOL) and cardiac muscle from rats. Results: The expression of dystrophin and β1 integrin correlated positively with the percentage of type I, IIa and IIx MHC isoforms and negatively with that of type IIb MHC isoform in fast‐type skeletal muscles, and their expression was abundant in SOL and cardiac muscle. Conclusion: Our results support the idea that DGC and FAC are among the factors that explain the fatigue‐resistant property not only of slow‐type but also of fast‐type skeletal muscles.     

Functional characteristics of the rat jaw muscles: daily muscle activity and fiber type composition

Skeletal muscles have a heterogeneous fiber type composition, which reflects their functional demand. The daily muscle use and the percentage of slow‐type fibers have been shown to be positively correlated in skeletal muscles of larger animals but for smaller animals there is no information. The examination of this relationship in adult rats was the purpose of this study. We hypothesized a positive relationship between the percentage of fatigue‐resistant fibers in each muscle and its total duration of use per day. Fourteen Wistar strain male rats (410–450 g) were used. A radio‐telemetric device was implanted to record muscle activity continuously from the superficial masseter, deep masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time) exceeding specified levels of the peak activity (2, 5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of the fibers by means of immunohistochemical staining. At lower activity levels (exceeding 2 and 5% of the peak activity), the duty time of the anterior belly of digastric muscle was significantly (P < 0.01) longer than those of the other muscles. The anterior belly of digastric muscle also contained the highest percentage of slow‐type fibers (type I fiber and hybrid fiber co‐expressing myosin heavy chain I + IIA) (ca. 11%; P < 0.05). By regression analysis for all four muscles, an inter‐muscular comparison showed a positive relationship between the duty time (exceeding 50% of the peak activity) and the percentage of type IIX fibers (P < 0.05), which demonstrate intermediate physiological properties relative to type IIA and IIB fibers. For the jaw muscles of adult male rats, the variations of fiber type composition and muscle use suggest that the muscle containing the largest amounts of slow‐type fibers (the anterior belly of digastric muscle) is mainly involved in low‐amplitude activities and that the amount of type IIX fibers is positively related to the generation of large muscle forces, validating our hypothesis.     

The formation of skeletal muscle: from somite to limb

During embryogenesis, skeletal muscle forms in the vertebrate limb from progenitor cells originating in the somites. These cells delaminate from the hypaxial edge of the dorsal part of the somite, the dermomyotome, and migrate into the limb bud, where they proliferate, express myogenic determination factors and subsequently differentiate into skeletal muscle. A number of regulatory factors involved in these different steps have been identified. These include Pax3 with its target c-met, Lbx1 and Mox2 as well as the myogenic determination factors Myf5 and MyoD and factors required for differentiation such as Myogenin, Mrf4 and Mef2 isoforms. Mutants for genes such as Lbx1 and Mox2, expressed uniformly in limb muscle progenitors, reveal unexpected differences between fore and hind limb muscles, also indicated by the differential expression of Tbx genes. As development proceeds, a secondary wave of myogenesis takes place, and, postnatally, satellite cells become located under the basal lamina of adult muscle fibres. Satellite cells are thought to be the progenitor cells for adult muscle regeneration, during which similar genes to those which regulate myogenesis in the embryo also play a role. In particular, Pax3 as well as its orthologue Pax7 are important. The origin of secondary/fetal myoblasts and of adult satellite cells is unclear, as is the relation of the latter to so-called SP or stem cell populations, or indeed to potential mesangioblast progenitors, present in blood vessels. The oligoclonal origin of postnatal muscles points to a small number of founder cells, whether or not these have additional origins to the progenitor cells of the somite which form the first skeletal muscles, as discussed here for the embryonic limb.     

Heart failure: a model of cardiac and skeletal muscle energetic failure

Chronic heart failure (CHF), the new epidemic in cardiology, is characterized by energetic failure of both cardiac and skeletal muscles. The failing heart wastes energy due to anatomical changes that include cavity enlargement, altered geometry, tachycardia, mitral insufficiency and abnormal loading, while skeletal muscle undergoes atrophy. Cardiac and skeletal muscles also have altered high-energy phosphate production and handling in CHF. Nevertheless, there are differences in the phenotype of myocardial and skeletal muscle myopathy in CHF: cardiomyocytes have a lower mitochondrial oxidative capacity, abnormal substrate utilisation and intracellular signalling but a maintained oxidative profile; in skeletal muscle, by contrast, mitochondrial failure is less clear, and there is altered microvascular reactivity, fibre type shifts and abnormalities in the enzymatic systems involved in energy distribution. Underlying these phenotypic abnormalities are changes in gene regulation in both cardiac and skeletal muscle cells. Here, we review the latest advances in cardiac and skeletal muscle energetic research and argue that energetic failure could be taken as a unifying mechanism leading to contractile failure, ultimately resulting in skeletal muscle energetic failure, exertional fatigue and death.     

Effects of ageing and endurance exercise training on alpha-actinin isoforms in rat plantaris muscle

Aim: We recently reported that α‐actinin adaptation occurs at the isoform level. This study was undertaken to clarify the effects of: (1) ageing‐induced shift of myosin heavy chain (MyHC) composition and (2) endurance exercise training on α‐actinin isoforms in rat plantaris muscle. Methods: Adult (18 mo) and old (28 mo) male Fischer 344 rats were assigned to either sedentary control or endurance exercise training groups. Animals in the training groups ran on a treadmill for 8 week with training intensity adjusted to be equal for adult and old groups. After the training was completed, the plantaris muscles were taken for analyses of α‐actinin‐2, α‐actinin‐3, and MyHC composition and metabolic enzyme activities. Results: The proportion of type IIb MyHC was lower, and that of type I MyHC was higher in old animals than in adult animals. α‐actinin‐3 was significantly lower in old animals than in adult animals. No significant difference was found in α‐actinin‐2 and citrate synthase (CS) activity between adult and old animals. Citrate synthase activity was higher in trained animals than in sedentary animals. Endurance training produced a fast‐to‐slow shift within type II MyHC isoforms in both adult and old animals. α‐actinin‐2 was significantly higher in trained animals than in sedentary animals. No significant difference was found in α‐actinin‐3 between trained and sedentary animals. Conclusion: These results support the α‐actinin adaptation at the isoform level and show that the α‐actinin‐3 expression depends on the amount of type II MyHC, whereas α‐actinin‐2 expression is associated with improvement of muscular aerobic capacity.     

Segmental fibre type composition of the rat iliopsoas muscle

The iliopsoas of the rat is composed of two muscles – the psoas major muscle and the iliacus muscle. The psoas major muscle arises from all the lumbar vertebrae and the iliacus muscle from the fifth and sixth lumbar vertebrae and ilium. Their common insertion point is the lesser trochanter of the femur, and their common action is the lateral rotation of the femur and flexion of the hip joint. Unlike humans, the rat is a quadruped and only occasionally rises up on its hind legs. Therefore, it is expected that the fibre type composition of the rat iliopsoas muscle will be different than that of humans. The iliopsoas muscle of the rat is generally considered to be a fast muscle. However, previous studies of the fibre type composition of the rat psoas muscle showed different results. Moreover, very little is known about the composition of the rat iliacus muscle. The aim of our study was to examine the fibre type composition of the rat iliopsoas muscle in order to better understand the complex function of the listed muscle. The psoas major muscle was examined segmentally at four different levels of its origin. Type I, IIA, IIB and IIX muscle fibres were typed using monoclonal antibodies for myosin heavy chain identification. The percentage of muscle fibre types and muscle fibre cross‐sectional areas were calculated. In our study we showed that in the rat iliopsoas muscle both the iliacus and the psoas major muscles had a predominance of fast muscle fibre types, with the highest percentage of the fastest IIB muscle fibres. Also, the IIB muscle fibres showed the largest cross‐sectional area (CSA) in both muscles. As well, the psoas major muscle showed segmental differences of fibre type composition. Our results showed changes in percentages, as well as the CSAs of muscle fibre types in cranio‐caudal direction. The most significant changes were visible in type IIB muscle fibres, where there was a decrease of percentages and the CSAs from the cranial towards the caudal part of the muscle. From our results it is evident that the rat iliopsoas muscle has a heterogeneous composition and is composed of all four muscle fibre types. Primarily, it is a fast, dynamic muscle with a predominance of fast type IIB muscle fibres with the largest CSAs. The composition of the rat psoas major muscles changes in a cranio‐caudal direction, thus pointing to a more postural role of the caudal part of the muscle.