Muscle Fiber Contractile Properties Research Articles

Contractile Properties of MHC I and II Fibers From Highly Trained Arm and Leg Muscles of Cross-Country Skiers.

IntroductionLittle is known about potential differences in contractile properties of muscle fibers of the same type in arms and legs. Accordingly, the present study was designed to compare the force-generating capacity and Ca2+ sensitivity of fibers from arm and leg muscles of highly trained cross-country skiers.MethodSingle muscle fibers of m. vastus lateralis and m. triceps brachii of eight highly trained cross-country skiers were analyzed with respect to maximal Ca2+-activated force, specific force and Ca2+ sensitivity.ResultThe maximal Ca2+-activated force was greater for myosin heavy chain (MHC) II than MHC I fibers in both the arm (+62%, P < 0.001) and leg muscle (+77%, P < 0.001), with no differences between limbs for each MHC isoform. In addition, the specific force of MHC II fibers was higher than that of MHC I fibers in both arms (+41%, P = 0.002) and legs (+95%, P < 0.001). The specific force of MHC II fibers was the same in both limbs, whereas MHC I fibers from the m. triceps brachii were, on average, 39% stronger than fibers of the same type from the m. vastus lateralis (P = 0.003). pCa50 was not different between MHC I and II fibers in neither arms nor legs, but the MHC I fibers of m. triceps brachii demonstrated higher Ca2+ sensitivity than fibers of the same type from m. vastus lateralis (P = 0.007).ConclusionComparison of muscles in limbs equally well trained revealed that MHC I fibers in the arm muscle exhibited a higher specific force-generating capacity and greater Ca2+ sensitivity than the same type of fiber in the leg, with no such difference in the case of MHC II fibers. These distinct differences in the properties of fibers of the same type in equally well-trained muscles open new perspectives in muscle physiology.

Fiber-type phenotype of the jaw-closing muscles in Gorilla gorilla, Pan troglodytes, and Pan paniscus: A test of the Frequent Recruitment Hypothesis

Skeletal muscle fiber types are important determinants of the contractile properties of muscle fibers, such as fatigue resistance and shortening velocity. Yet little is known about how jaw-adductor fiber types correlate with feeding behavior in primates. Compared with chimpanzees and bonobos, gorillas spend a greater percentage of their daily time feeding and shift to herbaceous vegetation when fruits are scarce. We thus used the African apes to test the hypothesis that chewing with unusually high frequency is correlated with the expression in the jaw adductors of a high proportion of type 1 (slow, fatigue-resistant) fibers at the expense of other fiber types (the Frequent Recruitment Hypothesis). We used immunohistochemistry to determine the presence and distribution of the four major myosin heavy chain (MHC) isoforms in the anterior superficial masseter (ASM), superficial anterior temporalis, and deep anterior temporalis of four Gorilla gorilla, two Pan paniscus, and four Pan troglodytes. Serial sections were stained against slow (MHC-1/-α-cardiac) and fast (MHC-2/-M) fibers. Fibers were counted and scored for staining intensity, and fiber cross-sectional areas (CSAs) were measured and used to estimate percentage of CSA of each MHC isoform. Hybrid fibers accounted for nearly 100% of fiber types in the masseter and temporalis of all three species, resulting in three main hybrid phenotypes. As predicted, the gorilla ASM and deep anterior temporalis comprised a greater percentage of CSA of the slower, fatigue-resistant hybrid fiber type, significantly so for the ASM (p = 0.015). Finally, the results suggest that fiber phenotype of the chewing muscles contributes to behavioral flexibility in ways that would go undetected in paleontological studies relying solely on morphology of the bony masticatory apparatus.

Myosin heavy chain expression relationships to power-load and velocity-load curves

Velocity- and power-based training are popular methods of determining training session loads and volumes. One factor that may influence load-velocity and load-power properties of an individual is the myosin heavy chain (MHC) composition of the muscle. The aim of this study was to examine the relationship between MHC composition and both load-velocity and load-power properties of muscle performance. Forty-two men with a variety of training backgrounds took part in this study (mean±SD; age=22.4±3.5 yrs, hgt=1.78±0.07 m, BW=78.7±13.3 kg). After testing leg extension one repetition maximum (1 RM), subjects performed maximal effort leg extensions at loads from 30% to 90% 1 RM. Muscle biopsies from the vastus lateralis were analyzed via SDS-PAGE electrophoresis technique for MHC content (IIx=13.8±12.9%, IIa=49.4±10.3%, I=36.8±11.3%). Leg extension rotational velocity and power were plotted against relative loads for all subjects. Significant correlations (P<0.05) were observed for MHC IIa with all performance variables (i.e. slopes, intercepts, peaks and relative loads). Relationships indicated that greater %MHC IIa was associated with greater velocity intercepts, more negative load-velocity slopes, greater maximal power, and with maximal power occurring at a lower relative intensity (% 1 RM). These data indicate that muscle velocity and power characteristics appear to be partially influenced by MHC content in a manner consistent with single muscle fiber contractile properties.

Ca2+ Sensitivity of Limb Muscle Fibers in Young and Older Adults

Age‐induced declines in skeletal muscle contractile function have been attributed to multiple cellular factors, including a loss of peak force (PO), decreased Ca2+ sensitivity, and reduced shortening velocity (VO). However, changes in these cellular properties with aging remain unresolved, especially in older women, and the effect of submaximal Ca2+ on contractile function is unknown. Thus, we compared contractile properties of muscle fibers from 19 young (24±3 years; 11 women) and 21 older adults (77±7 years; 7 women) under maximal and submaximal Ca2+ and assessed the abundance of three proteins thought to influence Ca2+ sensitivity. Fast fibers showed marked atrophy with aging in both men (young=8,599 μm2; old=5,404 μm2) and women (young=4,909 μm2; old=3,174 μm2) that corresponded with a decreased absolute PO in men (young=1.48 mN; old=0.92 mN) and women (young=0.85 mN; old=0.60 mN). There were no differences in fast fiber size‐specific PO, indicating the age‐related decline in force was explained by differences in fiber size. Except for fiber size and absolute PO, no age or sex differences were observed in Ca2+ sensitivity, rate of force development (ktr), or VO. Submaximal Ca2+ depressed ktr and VO, but the effects were not altered by age in either sex. Contrary to rodent studies, regulatory light chain (RLC) and myosin binding protein‐C abundance and RLC phosphorylation were unaltered by age or sex. These data suggest the age‐induced decline in whole‐muscle contractile function is primarily due to the atrophy of fast fibers and that caution is warranted when extending results from rodent studies to humans.Support or Funding InformationNational Institute on Aging grant (R01AG048262) to Robert Fitts and Sandra Hunter and an American Heart Association postdoctoral fellowship (19POST34380411) to Christopher Sundberg

Baboon (Papio ursinus) single fibre contractile properties are similar to that of trained humans.

This study investigated the contractile properties of single skeletal muscle fibres from the Vastus lateralis (VL) of two male adult chachma baboons (Papio ursinus) and compared it to that from five male human cyclists. Species comparisons are observational and statistical analyses were not performed due to the low sample size. The histological analyses revealed that the baboon muscles contained more type II fibres than their human counterparts. Cross-sectional areas of type I and type II fibres from human VL were similar in size, whereas baboon type I and type II fibres appeared smaller and larger compared to humans, respectively. On average, type II fibres from baboons and type IIAX fibres from humans produced the highest specific force (88 ± 41 and 155 ± 4 kN/m2, respectively), compared to 57 ± 27 and 68 ± 5 kN/m2 for baboon and human type I fibres. Maximum shortening velocity appeared highest in human type IIAX fibres, but fairly similar between human and baboon type I and II fibres. Baboon and human type I (2.2 ± 0.4 vs. 1.5 ± 0.5 kN/m2Fl/s) and type II (6.0 ± 2.8 vs. 7.7 ± 1.0 kN/m2Fl/s) fibres appeared similar in maximum power output. From these observations, it seems that baboon and human muscle fibre contractile properties appear similar to one another, and that fibre type composition itself may play a determining role in muscle strength between these two species.

Hand Grip Strength and Its Relation to Thigh-derived Single Muscle Fiber Contractile Properties

PURPOSE Grip strength has been well documented to represent whole body strength. This study was performed to investigate whether absolute and relative grip strength is associated with thigh strength, and thigh-derived single muscle fiber contractile properties. METHODS Twelve healthy young adults (mean age 27.42 years) received needle biopsy on vastus lateralis muscles. Chemically skinned muscle fibers (n=215) were used to analyze single muscle fiber contractile properties (cross sectional area, maximal force, specific force, maximal shortening velocity) and MHC isoform composition. Pearson correlation was tested to identify the relationship between dominant hand grip strength, thigh strength, and thigh-derived single muscle fiber contractile properties. RESULTS The distribution of MHC I was higher in all the subjects compared to MHC II (type I: 57.2%, IIa: 37.2%, IIa/IIx: 2.3%, IIx: 3.3%). Specific force (8.2%) and maximal shortening velocity (74%) in MHC II were greater than those of MHC I. Absolute and relative grip strength normalized by height had a positive correlation with knee extensor isometric (r=.777, r=.75) and isokinetic (r=.632, r=.603) strength. In addition, the positive correlation of knee extensor isometric and isokinetic strength was observed against both CSA and Po in the thigh-derived single muscle fiber. Finally, Absolute and relative grip strength normalized by height were positively correlated with CSA (r=.279, r=.267) and Po (r=.373, r=.351) in MHC II, but not in MHC I. CONCLUSIONS These data suggest that hand grip strength is highly associated with not only whole thigh muscle strength, but also thigh-derived fast single muscle fiber contractile properties (MHC II). 색인어: ì• ë ¥, 단일 ê·¼ì„¬ìœ , ì „ì‹ ê·¼ë ¥, 지근, 속근 Keywords: Hand grip strength, Whole muscle strength, Single muscle fiber, Slow twitch fiber, Fast twitch fiber

Contractile Properties of Single Muscle Fiber and Their Relations to Whole Muscle Strength in Korean Young Male

PURPOSE This study investigated the muscle fiber type-related contractile properties and examined the relationship between whole limb muscle strengths and single muscle fiber contractile properties in Korean men. METHODS Six Korean men (29.8±1.49 yr) were recruited and participated in the study. Samples were obtained from vastus lateralis muscles. CSA, Po, SF, Vo of single fiber segments were measured using an isometric force transducer and a high-speed motor. Silver staining was performed to identify MHC isoform composition of single muscle fiber segments. Multiple regression was tested to identify the relationship between single muscle fiber contractile properties and whole muscle strength. RESULTS MHC isoforms were distributed in different proportion (Type I: 57.3%, IIa: 34.2%, IIa/IIx: 4.3%, IIx: 4.3%). CSA of type IIx were smaller compared to type I (-50%) and type IIa (-57.5%). Po in type IIa was 12.9% higher compared to type I (p<.05). While SF in type IIa were 7.7% higher than type I, type IIx were higher than type I, IIa, IIa/IIx (31.4%, 25.7%, 30.9%). Vo increased in the order type I<IIa<IIa/IIx<IIx. There was positive correlation between single fiber and properties (Po vs. thigh strength: r2=.248, Po vs. thigh power: r2=.058, Vo vs. thigh power: r2=.095). CONCLUSIONS These results suggest that single muscle fiber contractile properties were exclusively dependent on fiber type isoforms, ruling out a possibility of race-induced difference in fiber type-matched contractile properties. 색인어: 한국남성, ë‹¨ì¼ê·¼ì„¬ìœ , ê·¼ë ¥, ê·¼ì„¬ìœ ìœ í˜•, í•˜ì´ë¸Œë¦¬ë“œì„¬ìœ Keywords: Korean men, Single muscle fiber, Whole muscle strength, MHC isoform, Hybrid fiber

Disuse skeletal muscle atrophy in humans. Proteomic and molecular adaptations

Disuse skeletal muscle atrophy in humans. Proteomic and molecular adaptations

Mitochondrial respiration is fiber‐type specific in skeletal muscles

IntroductionMitochondria are the power houses of the cells and they also play critical roles in the cellular oxidative stress and apoptosis. It is known that mitochondrial density and morphology are fiber‐type specific. Based on differential contractile properties of different types of skeletal muscles, we hypothesized that the mitochondria respiration is fiber‐type specific. The aim of the study is to compare and contrast mitochondria respiration between type I and type II muscles.MethodSoleus (predominantly composed of type I fibers) and plantaris (predominantly composed of type II fibers) muscles were taken from adult rats. Mitochondrial respiration was determined in duplicate by a high‐resolution respirometry OROBOROS Oxygraph‐2k (O2k). The mitochondrial substrate‐uncoupler‐inhibitor titration (SUIT) protocol was used to determine mitochondrial oxygen consumption rate (O2 flux) of permeabilized myofibers. Paired t test was used to compare parameters between soleus and plantaris muscles.ResultsWe found that basal mitochondrial respiration (i.e. O2 flux_ complex I plus complex II) and leak of permeabilized muscle fibers was similar between soleus and plantaris muscles. Similarly, mitochondrial coupling control ratio defined by leak divided by basal mitochondrial respiration (i.e. leak/O2 flux_ complex I plus complex II) is similar between soleus and plantaris muscles. Maximal electron transport system capacity (maximal mitochondrial respiration) of soleus muscles was greater than that of plantaris muscles. In addition, mitochondrial respiratory reserve capacity defined by subtracting basal mitochondrial respiration from maximal mitochondrial respiration was greater in soleus muscles than in plantaris muscles.ConclusionBasal mitochondrial respiration and leak are similar between type I and type II muscles. However, mitochondria in type I muscles have greater respiratory reserve compared to that in type II muscles. This fiber‐type specific mitochondrial respiration is likely associated with the differential fuel metabolism and contractile properties of muscle fibers.

Traumatic Brain Injury Results in Cellular, Structural and Functional Changes Resembling Motor Neuron Disease.

Traumatic brain injury (TBI) has been suggested to increase the risk of amyotrophic lateral sclerosis (ALS). However, this link remains controversial and as such, here we performed experimental moderate TBI in rats and assessed for the presence of ALS-like pathological and functional abnormalities at both 1 and 12 weeks post-injury. Serial in-vivo magnetic resonance imaging (MRI) demonstrated that rats given a TBI had progressive atrophy of the motor cortices and degeneration of the corticospinal tracts compared with sham-injured rats. Immunofluorescence analyses revealed a progressive reduction in neurons, as well as increased phosphorylated transactive response DNA-binding protein 43 (TDP-43) and cytoplasmic TDP-43, in the motor cortex of rats given a TBI. Rats given a TBI also had fewer spinal cord motor neurons, increased expression of muscle atrophy markers, and altered muscle fiber contractile properties compared with sham-injured rats at 12 weeks, but not 1 week, post-injury. All of these changes occurred in the presence of persisting motor deficits. These findings resemble some of the pathological and functional abnormalities common in ALS and support the notion that TBI can result in a progressive neurodegenerative disease process pathologically bearing similarities to a motor neuron disease.

Mechanisms of in vivo muscle fatigue in humans: investigating age‐related fatigue resistance with a computational model

Muscle fatigue can be defined as the transient decrease in maximal force that occurs in response to muscle use. Fatigue develops because of a complex set of changes within the neuromuscular system that are difficult to evaluate simultaneously in humans. The skeletal muscle of older adults fatigues less than that of young adults during static contractions. The potential sources of this difference are multiple and intertwined. To evaluate the individual mechanisms of fatigue, we developed an integrative computational model based on neural, biochemical, morphological and physiological properties of human skeletal muscle. Our results indicate first that the model provides accurate predictions of fatigue and second that the age-related resistance to fatigue is due largely to a lower reliance on glycolytic metabolism during contraction. This model should prove useful for generating hypotheses for future experimental studies into the mechanisms of muscle fatigue. During repeated or sustained muscle activation, force-generating capacity becomes limited in a process referred to as fatigue. Multiple factors, including motor unit activation patterns, muscle fibre contractile properties and bioenergetic function, can impact force-generating capacity and thus the potential to resist fatigue. Given that neuromuscular fatigue depends on interrelated factors, quantifying their independent effects on force-generating capacity is not possible in vivo. Computational models can provide insight into complex systems in which multiple inputs determine discrete outputs. However, few computational models to date have investigated neuromuscular fatigue by incorporating the multiple levels of neuromuscular function known to impact human in vivo function. To address this limitation, we present a computational model that predicts neural activation, biomechanical forces, intracellular metabolic perturbations and, ultimately, fatigue during repeated isometric contractions. This model was compared with metabolic and contractile responses to repeated activation using values reported in the literature. Once validated in this way, the model was modified to reflect age-related changes in neuromuscular function. Comparisons between initial and age-modified simulations indicated that the age-modified model predicted less fatigue during repeated isometric contractions, consistent with reports in the literature. Together, our simulations suggest that reduced glycolytic flux is the greatest contributor to the phenomenon of age-related fatigue resistance. In contrast, oxidative resynthesis of phosphocreatine between intermittent contractions and inherent buffering capacity had minimal impact on predicted fatigue during isometric contractions. The insights gained from these simulations cannot be achieved through traditional in vivo or in vitro experimentation alone.

Single muscle fiber contractile properties in diabetic RAT muscle.

Diabetes is associated with accelerated loss of muscle mass and function. We compared the contractile properties of single muscle fibers in young rat soleus muscle of uncontrolled streptozotocin-induced diabetic animals (n = 10) and nondiabetic controls (n = 10). Single fiber maximal force, shortening velocity, and power were assessed during maximal activation with calcium using the slack test 4 weeks after induction. Myosin heavy chain expression was determined using sodium dodecyl sulfate polyacrylamide gel electrophoresis. Oxidized myosin levels were detected by analyzing protein carbonyls in muscle homogenates. All fibers expressed the type I myosin heavy chain isoform. Diabetic rats had higher blood glucose (537 vs. 175 mg/dl; P < 0.001) and lower body weight (171 vs. 356 g; P < 0.001) than controls. Muscle fibers from diabetic rats showed smaller cross-sectional area (1128 vs. 1812 μm(2) ), lower maximal force (258 vs. 492 μN), and reduced absolute power (182 vs. 388 μN FL/s) (all P < 0.0001). No differences were seen in shortening velocity, specific force or specific power. Myosin carbonylation was higher (P < 0.01) in diabetic rats. After 4 weeks of untreated diabetes, there are significant alterations in muscle at the level of isolated single fibers and myosin protein, although some contractile properties seem to be protected. Muscle Nerve, 2015 Muscle Nerve 53: 958-964, 2016.

Muscle structural assembly and functional consequences.

The relationship between muscle structure and function has been a matter of investigation since the Renaissance period. Extensive use of anatomical dissections and the introduction of the scientific method enabled early scholars to lay the foundations of muscle physiology and biomechanics. Progression of knowledge in these disciplines led to the current understanding that muscle architecture, together with muscle fibre contractile properties, has a major influence on muscle mechanical properties. Recently, advances in laser diffraction, optical microendoscopy and ultrasonography have enabled in vivo investigations into the behaviour of human muscle fascicles and sarcomeres with varying joint angle and muscle contraction intensity. With these technologies it has become possible to identify the length region over which fascicles and sarcomeres develop maximum isometric force in vivo as well as the operating ranges of fascicles and sarcomeres during real-life activities such as walking. Also, greater insights into the remodelling of muscle architecture in response to overloading and unloading, and in ageing, have been obtained by the use of ultrasonography; these have led to the identification of clinical biomarkers of disuse atrophy and sarcopenia. Recent evidence also shows that the pattern of muscle hypertrophy in response to chronic loading is contraction-mode dependent (eccentric versus concentric), as similar gains in muscle mass, but through differing addition of sarcomeres in series and in parallel (as indirectly inferred from changes in fascicle length and pennation angle), have been found. These innovative observations prompted a new set of investigations into the molecular mechanisms regulating this contraction-specific muscle growth.

Human skeletal muscle fibre contractile properties and proteomic profile: adaptations to 3 weeks of unilateral lower limb suspension and active recovery.

It is generally assumed that muscle fibres go through atrophy following disuse with a loss of specific force and an increase in unloaded shortening velocity. However, the underlying mechanisms remain to be clarified. Most studies have focused on events taking place during the development of disuse, whereas the subsequent recovery phase, which is equally important, has received little attention. Our findings support the hypotheses that the specific force of muscle fibres decreased following unilateral lower limb suspension (ULLS) and returned to normal after 3weeks of active recovery as a result of a loss and recovery of myosin and actin content. Furthermore, muscle fibres went through extensive qualitative changes in muscle protein pattern following ULLS, and these were reversed by active recovery. Resistance training was very effective in restoring both muscle mass and qualitative muscle changes, indicating that long-term ULLS did not prevent the positive effect of exercise on human muscle. Following disuse, muscle fibre function goes through adaptations such as a loss of specific force (PO /CSA) and an increase in unloaded shortening velocity, which could be a result of both quantitative changes (i.e. atrophy) and qualitative changes in protein pattern. The underlying mechanisms remain to be clarified. In addition, little is known about the recovery of muscle mass and strength following disuse. In the present study, we report an extensive dataset describing, in detail,the functional and protein content adaptations of skeletal muscle in response to both disuse and re-training. Eight young healthy subjects were subjected to 3weeks of unilateral lower limb suspension (ULLS), a widely used human model of disuse skeletal muscle atrophy. Needle biopsies samples were taken from the vastus lateralis muscle Pre-ULLS, Post-ULLS and after 3weeks of recovery during which heavy resistance training was performed. After disuse, cross-sectional area (CSA), PO /CSA and myosin concentration (MC) decreased in both type 1 and 2A skinned muscle fibres. After recovery, CSA and MC returned to levels comparable to those observed before disuse, whereas Po/CSA and unloaded shortening velocity reached a higher level. Myosin heavy chain isoform composition of muscle samples did not differ among the experimental groups. To study the mechanisms underlying such adaptations, a two-dimensional proteomic analysis was performed. ULLS induced a reduction of myofibrillar, metabolic (glycolytic and oxidative) and anti-oxidant defence system protein content. Resistance training was very effective in counteracting ULLS-induced alterations, indicating that long-term ULLS did not prevent the positive effect of exercise on human muscle.

Single muscle fibre contractile properties differ between body-builders, power athletes and control subjects.

What is the central question of this study? Do the contractile properties of single muscle fibres differ between body-builders, power athletes and control subjects? What is the main finding and its importance? Peak power normalized for muscle fibre volume in power athletes is higher than in control subjects. Compared with control subjects, maximal isometric tension (normalized for muscle fibre cross-sectional area) is lower in body-builders. Although this difference may be caused in part by an apparent negative effect of hypertrophy, these results indicate that the training history of power athletes may increase muscle fibre quality, whereas body-building may be detrimental. We compared muscle fibre contractile properties of biopsies taken from the vastus lateralis of 12 body-builders (BBs; low- to moderate-intensity high-volume resistance training), six power athletes (PAs; high-intensity, low-volume combined with aerobic training) and 14 control subjects (Cs). Maximal isotonic contractions were performed in single muscle fibres, typed with SDS-PAGE. Fibre cross-sectional area was 67 and 88% (P<0.01) larger in BBs than in PAs and Cs, respectively, with no significant difference in fibre cross-sectional area between PAs and Cs. Fibres of BBs and PAs developed a higher maximal isometric tension (32 and 50%, respectively, P<0.01) than those of Cs. The specific tension of BB fibres was 62 and 41% lower than that of PA and C fibres (P<0.05), respectively. Irrespective of fibre type, the peak power (PP) of PA fibres was 58% higher than that of BB fibres (P<0.05), whereas BB fibres, despite considerable hypertrophy, had similar PP to the C fibres. This work suggests that high-intensity, low-volume resistance training with aerobic exercise improves PP, while low- to moderate-intensity high-volume resistance training does not affect PP and results in a reduction in specific tension. We postulate that the decrease in specific tension is caused by differences in myofibrillar density and/or post-translational modifications of contractile proteins.

Inappropriate interpretation of surface EMG signals and muscle fiber characteristics impedes understanding of the control of neuromuscular function

as the final common pathway from the nervous system to muscle, the motor unit transmits an activation signal generated by the nervous system to engage the contractile proteins and produce the muscle forces needed for reflex responses, automatic behaviors, and voluntary actions ([11][1]). The net

Single Muscle Fiber Characteristics of The Oldest-Old

PURPOSE: To examine single muscle fiber function in the oldest-old. METHODS: Vastus lateralis muscle biopsies were obtained from five healthy independently living individuals (3F/2M, 89 ± 1 y) enrolled in the Health, Aging and Body Composition (Health ABC) Study. Isolated myosin heavy chain (MHC) I and IIa muscle fibers were examined for cross-sectional area (CSA) and contractile properties prior to MHC isoform identification via SDS-PAGE. RESULTS: MHC I muscle fibers were 21% larger (P < 0.05) than MHC IIa. A summary of muscle fiber characteristics is provided below (values are means ± SE). CONCLUSION: This investigation was the first to examine single muscle fiber contractile properties of independently living individuals > 85 y. In line with previous aging research, MHC IIa fibers were smaller than MHC I, indicative of MHC IIa-specific atrophy. Normalized power (an indicator of muscle fiber quality incorporating size, strength and speed) of MHC I fibers from these individuals, when compared to previous research from our lab using identical procedures, was 28% higher than healthy 25 y olds and similar to younger octogenarians (∼82 y). MHC IIa normalized power was 63% greater than 25 y olds and 39% greater than younger octogenarians. These data support the idea that power output per unit size (i.e. muscle quality) of surviving muscle fibers improves with advancing age, a phenomenon more pronounced in MHC IIa fibers. Thus, a likely key contributor to the loss of muscle function with age is a quantitative loss of whole muscle specific force rather than a reduction in muscle quality of remaining muscle fibers.Table: No title available.

Aging-associated exacerbation in fatty degeneration and infiltration after rotator cuff tear

Rotator cuff tears are one of the most common musculoskeletal complaints and a substantial source of morbidity in elderly patients. Chronic cuff tears are associated with muscle atrophy and an infiltration of fat to the area, a condition known as "fatty degeneration." To improve the treatment of cuff tears in elderly patients, a greater understanding of the changes in the contractile properties of muscle fibers and the molecular regulation of fatty degeneration is essential. Using a full-thickness, massive supraspinatus and infraspinatus tear model in elderly rats, we measured fiber contractility and determined changes in fiber type distribution that develop 30 days after tear. We also measured the expression of messenger RNA and micro-RNA transcripts involved in muscle atrophy, lipid accumulation, and matrix synthesis. We hypothesized that a decrease in specific force of muscle fibers, an accumulation of type IIb fibers, and an upregulation in atrophic, fibrogenic, and inflammatory gene expression would occur in torn cuff muscles. Thirty days after the tear, we observed a reduction in muscle fiber force and an induction of RNA molecules that regulate atrophy, fibrosis, lipid accumulation, inflammation, and macrophage recruitment. A marked accumulation of advanced glycation end products and a significant accretion of macrophages in areas of fat accumulation were observed. The extent of degenerative changes in old rats was greater than that observed in adults. In addition, we identified that the ectopic fat accumulation that occurs in chronic cuff tears does not occur by activation of canonical intramyocellular lipid storage and synthesis pathways.

Neuromuscular Dysfunction in Diabetes

The purpose of this study was to investigate the effect of diabetes, motor nerve impairment, and training status on neuromuscular function by concurrent assessment of the torque-velocity relationship and muscle fiber conduction velocity (MFCV). Four groups were studied (n = 12 each): sedentary patients with diabetes in the first (lower) and fourth (higher) quartile of motor nerve conduction velocity (D1 and D4, respectively), trained diabetic (TD) patients, and nondiabetic sedentary control (C) subjects. Maximal isometric and isokinetic contractions were assessed over a wide range of angular velocities for the elbow flexors and knee extensors to evaluate the torque-velocity relationship. Simultaneously, MFCV was estimated from surface electromyography of the vastus lateralis and biceps brachii. Isometric strength was similar among groups. The dynamic strength of elbow flexors was reduced in patients with diabetes at the higher contraction speeds. The strength of knee extensors was lower in sedentary patients with diabetes at all velocities considered, with significantly lower values in D1 than that in D4 at 60°, 90°, and 120°·s(-1), whereas it was similar between TD and C subjects, especially at low contraction velocities. At the vastus lateralis, but not the biceps brachii, MFCV was lower in D1 and D4 as compared with TD and C subjects, showing similar values. Muscle weakness in diabetes affects also the upper limb, although to a lower extent than the lower limb, is only partly related to motor nerve impairment, and is dependent on contraction velocity. Exercise training might counteract diabetes-induced alterations in muscle fiber contractile properties and MFCV.

Role of alpha-actinin-3 in contractile properties of human single muscle fibers: a case series study in paraplegics.

A common nonsense polymorphism in the ACTN3 gene results in the absence of α-actinin-3 in XX individuals. The wild type allele has been associated with power athlete status and an increased force output in numeral studies, though the mechanisms by which these effects occur are unclear. Recent findings in the Actn3−/− (KO) mouse suggest a shift towards ‘slow’ metabolic and contractile characteristics of fast muscle fibers lacking α-actinin-3. Skinned single fibers from the quadriceps muscle of three men with spinal cord injury (SCI) were tested regarding peak force, unloaded shortening velocity, force-velocity relationship, passive tension and calcium sensitivity. The SCI condition induces an ‘equal environment condition’ what makes these subjects ideal to study the role of α-actinin-3 on fiber type expression and single muscle fiber contractile properties. Genotyping for ACTN3 revealed that the three subjects were XX, RX and RR carriers, respectively. The XX carrier’s biopsy was the only one that presented type I fibers with a complete lack of type IIx fibers. Properties of hybrid type IIa/IIx fibers were compared between the three subjects. Absence of α-actinin-3 resulted in less stiff type IIa/IIx fibers. The heterozygote (RX) exhibited the highest fiber diameter (0.121±0.005 mm) and CSA (0.012±0.001 mm2) and, as a consequence, the highest peak force (2.11±0.14 mN). Normalized peak force was similar in all three subjects (P = 0.75). Unloaded shortening velocity was highest in R-allele carriers (P<0.001). No difference was found in calcium sensitivity. The preservation of type I fibers and the absence of type IIx fibers in the XX individual indicate a restricted transformation of the muscle fiber composition to type II fibers in response to long-term muscle disuse. Lack of α-actinin-3 may decrease unloaded shortening velocity and increase fiber elasticity.