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Bayberry is known to be a rich source of natural flavonoids and has been reported to have various health-promoting abilities. However, its function on regulating skeletal muscle fiber type remains unclear. This study examined whether bayberry extract affects skeletal muscle fiber type by promoting fast myofiber formation, as well as the potential molecular mechanism. After eight weeks, the "Biqi" bayberry extract (BBE) groups mice displayed markedly enhanced grip strength and improved metabolic rate compared to the control group mice. BBE also significantly increased myofibers size, LDH activity, MyHC-IIb (fast-twitch mRNA) expression, and the percentage of fast-twitch myofibers, while decreasing SDH activity, MyHC-I (slow-twitch mRNA) expression, and slow-twitch myofibers percentage in the skeletal muscle of the mice. The effect of BBE on regulating skeletal muscle fiber type remodeling is likely attributed to its activation of the Akt-FoxO1 pathway. Our findings indicated that BBE can effectively regulate the expression and proportion of fast-twitch fibers, making it a potential therapy for improving glucose homeostasis of skeletal muscle. [ABSTRACT FROM AUTHOR]

Objective To compare the effects of chronic intermittent hypoxia exposure on different fiber types of muscle and investigate its possible mechanism. Methods Sixteen SD rats were randomly divided into normoxia control group (C group, placed in a normoxic chamber) and chronic intermittent hypoxia group (H group, placed in a hypoxic chamber with an oxygen concentration of 12.4%, 8 h per day, for 4 weeks). Their body mass was weighed during the intervention, and the grasping force and lean body mass were tested after the intervention. The wet masses of soleus muscle (SOL) and musculi tibialis anterior (TA), muscle fiber cross-sectional area (FCSA) were measured. The protein levels of puromycin (Puro) and ubiquitin (Ub) were detected with Western blotting. RNA sequencing was carried out for TA tissues to screen the differential genes between the 2 groups. And the biological process and pathways enriched by these differential genes were analyzed. Results ① During the intervention, the body mass of the C and H groups continued to increase, but it of the H group was always lower than that of the C group at each time point. The intervention also resulted in the decreases in the lean body mass and its percentage, grasping force and relative grasping force in the H group than the C group, but there were no statistical differences. ② The wet mass of TA was significantly lower in the H group than the C group (P < 0.05). In H group, both SOL and TA muscle fibers were damaged, and the FCSA of SOL was significantly lower than C group (P < 0.05). ③Puro and Ub in SOL tissues were obviously higher in the H group than the C group. The ratio of Puro/Ub was decreased significantly in SOL tissues (P < 0.05), but increased significantly in TA tissues (P < 0.01). ④ There were 665 differential genes found in SOL tissues and 228 differential genes in TA tissues between the 2 groups. Among them, 58 differential genes were up-regulated and 35 were down-regulated in both muscles. Those up-regulated differential genes in SOL tissues were mainly enriched in inflammatory response and protein synthesis and involved in TNF and Jak/STAT signaling pathways. The up-regulated differential genes in TA tissues were mainly enriched in antioxidant and glucose metabolism and glucocorticoid and involved in PPAR signaling pathway and glutathione metabolism. The down-regulated genes were concentrated in oxidative stress and transformation between fast- and slow-twitch muscles. Conclusion Chronic intermittent hypoxia could promote SOL atrophy by enhancing inflammatory response, and TA may resist hypoxia-induced muscle atrophy by improving antioxidant capacity.

Bayberry is known to be a rich source of natural flavonoids and has been reported to have various health-promoting abilities. However, its function on regulating skeletal muscle fiber type remains unclear. This study examined whether bayberry extract affects skeletal muscle fiber type by promoting fast myofiber formation, as well as the potential molecular mechanism. After eight weeks, the “Biqi” bayberry extract (BBE) groups mice displayed markedly enhanced grip strength and improved metabolic rate compared to the control group mice. BBE also significantly increased myofibers size, LDH activity, MyHC-IIb (fast-twitch mRNA) expression, and the percentage of fast-twitch myofibers, while decreasing SDH activity, MyHC-I (slow-twitch mRNA) expression, and slow-twitch myofibers percentage in the skeletal muscle of the mice. The effect of BBE on regulating skeletal muscle fiber type remodeling is likely attributed to its activation of the Akt-FoxO1 pathway. Our findings indicated that BBE can effectively regulate the expression and proportion of fast-twitch fibers, making it a potential therapy for improving glucose homeostasis of skeletal muscle.

Human muscle fibers are generally classified by myosin heavy chain (MHC) isoforms characterized by slow to fast contractile speeds. Type I, or slow-twitch fibers, are seen in high abundance in elite endurance athletes, such as long-distance runners and cyclists. Alternatively, fast-twitch IIa and IIx fibers are abundant in elite power athletes, such as weightlifters and sprinters. While cross-sectional comparisons have shown marked differences between athletes, longitudinal data have not clearly converged on patterns in fiber type shifts over time, particularly between slow and fast fibers. However, not all fiber type identification techniques are created equal and, thus, may limit interpretation. Hybrid fibers, which express more than one MHC type (I/IIa, IIa/IIx, I/IIa/IIx), may make up a significant proportion of fibers. The measurement of the distribution of fibers would necessitate the ability to identify hybrid fibers, which is best done through single fiber analysis. Current evidence using the most appropriate techniques suggests a clear ability of fibers to shift between hybrid and pure fibers as well as between slow and fast fiber types. The context and extent to which this occurs, along with the limitations of current evidence, are discussed herein.

Dietary nitrate supplementation increases circulating nitrite concentration, and the subsequent reduction of nitrite to nitric oxide is promoted in hypoxic environments. Given that PO2 is lower in Type II compared with Type I muscle, this article examines the hypothesis that the ergogenicity of nitrate supplementation is linked to specific effects on vascular, metabolic, and contractile function in Type II muscle. [ABSTRACT FROM AUTHOR]

Objectives: This study evaluates the effect of post-activation potentiation (PAP) after 5x5s maximal voluntary isometric contractions (activation stimulus, AS) on tensiomyography (TMG) and torque twitch contractile parameters of vastus lateralis (VL) and medialis (VM), respectively. Further, we validated the decomposition of TMG response to separate responses of three fiber types., Methods: 15 healthy individuals participated in this study (40% women; age 19±2.3 years). A decomposition of VL TMG response was done after optimal fitting of three exponential curves., Results: We found main effects in contraction time (Tc) for muscle, method and time. Furthermore, we found interactions between muscle*method, method*time and muscle*method*time. Compared to PRE AS, we found shorter TMG Tc in VL and VM during the first two minutes after AS. Torque Tc remained unchanged in VL, while it increased in VM within 30 seconds after AS. A decomposition of VL TMG response confirmed PAP effects being present only in decomposed type IIb muscle fibers., Conclusion: The TMG is a sensitive method to detect PAP effects with a sensor mounted directly above the muscle belly. After the decomposition of the TMG signal to three separate muscle fiber phenotypes, we provided a non-invasive insight in the contribution of each muscle fiber phenotype to the PAP of the whole muscle., Competing Interests: The authors have no conflict of interest.

The alpha-actinin-3 (ACTN3) gene encodes a Z-disc structural protein which is found only in fast glycolytic muscle fibers. A common nonsense polymorphism in codon 577 of the ACTN3 gene (R577X) results in alpha-actinin-3 deficiency in XX homozygotes. Previous reports have shown a lower proportion of the ACTN3 XX genotype in power-oriented athletes compared to the general population. In the present study we tested whether XX genotype was under-represented in Russian power-oriented athletes. The study involved 486 Russian power-oriented athletes of regional or national competitive standard. ACTN3 genotype and allele frequencies were compared to 1,197 controls. The frequencies of the ACTN3 XX genotype (6.4 vs. 14.2%; P < 0.0001) and X allele (33.3 vs. 38.7%; P = 0.004) were significantly lower in power-oriented athletes compared to controls. Furthermore, the lowest (3.4%) frequency of the ACTN3 XX genotype was found in a group of highly elite athletes, supporting the hypothesis that the presence of alpha-actinin-3 has a beneficial effect on the function of skeletal muscle in generating forceful contractions at high velocity. In conclusion, ACTN3 R577X polymorphism was associated with power athlete status in Russians. [ABSTRACT FROM AUTHOR]

It has been observed consistently and is well accepted that the steady-state isometric force after active muscle stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions. However, this so-called force enhancement has not been studied for submaximal voluntary efforts; therefore, it is not known whether this property affects everyday movements. The purpose of this study was to determine whether there was force enhancement during submaximal voluntary contractions. Human adductor pollicis muscles (n = 17) were studied using a custom-built dynamometer, and both force and activation were measured while muscle activation and force were controlled at a level of 30% of maximal voluntary contraction. The steady-state isometric force and activation after active stretch were compared with the corresponding values obtained during isometric reference contractions. There was consistent and reliable force enhancement in 8 of the 17 subjects, whereas there was no force enhancement in the remaining subjects. Subjects with force enhancement had greater postactivation potentiation and a smaller resistance to fatigue in the adductor pollicis. We conclude from these results that force enhancement exists during submaximal voluntary contractions in a subset of the populations and suggest that it may affect everyday voluntary movements in this subset. On the basis of follow-up testing, it appears that force enhancement during voluntary contractions is linked to potentiation and fatigue resistance and therefore possibly to the fiber-type distribution in the adductor pollicis muscle. [ABSTRACT FROM AUTHOR]

Human muscle fibers are generally classified by myosin heavy chain (MHC) isoforms characterized by slow to fast contractile speeds. Type I, or slow-twitch fibers, are seen in high abundance in elite endurance athletes, such as long-distance runners and cyclists. Alternatively, fast-twitch IIa and IIx fibers are abundant in elite power athletes, such as weightlifters and sprinters. While cross-sectional comparisons have shown marked differences between athletes, longitudinal data have not clearly converged on patterns in fiber type shifts over time, particularly between slow and fast fibers. However, not all fiber type identification techniques are created equal and, thus, may limit interpretation. Hybrid fibers, which express more than one MHC type (I/IIa, IIa/IIx, I/IIa/IIx), may make up a significant proportion of fibers. The measurement of the distribution of fibers would necessitate the ability to identify hybrid fibers, which is best done through single fiber analysis. Current evidence using the most appropriate techniques suggests a clear ability of fibers to shift between hybrid and pure fibers as well as between slow and fast fiber types. The context and extent to which this occurs, along with the limitations of current evidence, are discussed herein. [ABSTRACT FROM AUTHOR]