Muscle Sarcomere Length Research Articles

Effect of Ultrasound and Calcium Chloride on the Ultrastructure and the Warner-Bratzler Shear Force Value of Beef Shank Muscle Fibers

Meat tenderness is a major factor in consumer satisfaction. Tenderness may be quantified by changes in muscle sarcomere length, roughness (ultrastructure) and shear force. The ultrastructure and the Warner-Bratzler shear force value of beef shank muscle fibers were experimentally measured in controlled applications of ultrasound and calcium chloride. Uncooked beef samples were treated with ultrasound at 40 kHz at different intensities for 6 min, and then immersed in a range of calcium chloride solutions for 24 h. Warner–Bratzler shear analysis and atomic force microscopy (AFM) data collection were performed for each sample. AFM height derived data (the sarcomere length and roughness values) and the Warner–Bratzler shear force values indicate optimum ranges of ultrasound intensity and calcium chloride concentration for meat tenderization. While the effect of low intensity ultrasound alone was not significant, calcium chloride treatment increased the sarcomere length and decreased shear force values. Furthermore, the combination of ultrasound and calcium chloride increased the sarcomere length significantly and simultaneously reduced the Warner–Bratzler shear force tolerance of the sample. These data suggested that low intensity and frequency ultrasound alongside Calcium Chloride immersion can promote the transmembrane transport of calcium ions via the calpain system to improve meat tenderness.

Regulation of Skeletal Muscle Sarcomere Length through Titin Changes in iMSBmal1 Mice

Prior research shows that sarcomere length correlates with isometric force production, but the specific structural elements regulating resting sarcomere length are not well understood. Based on the assumption that sarcomeres operate similarly throughout a muscle fiber, heterogeneity in sarcomere lengths should decrease maximum active tension production. One structural protein that has been proposed to maintain sarcomere length is titin. Our lab developed an inducible skeletal muscle specific Bmal1 knockout mouse, which exhibits muscle weakness and a shift to longer titin isoform. My project takes a histological approach to study phenotypic changes in sarcomere length and A‐band centrality in the iMSBmal1−/− skeletal muscle. I hypothesized that the iMSBmal1−/− mice would exhibit an increased variance in sarcomere length, as well as a change in the A‐band centrality index (i.e. the distance of peak fluorescence of the myosin filament from the center of the sarcomere). After collecting tibilalis anterior (TA) muscle from these mice at 5 weeks after recombination, the muscles were frozen in OCT compound using liquid nitrogen cooled isopentane. These samples (n=3 animals/group) were then longitudinally cryosectioned at 6 microns. The samples were stained using primary antibodies that target α‐actinin and myosin heavy chain followed by fluorescent secondary antibodies. α‐actinin labels the Z‐lines, allowing us to measure sarcomere lengths in the longitudinal sections, while myosin heavy chain allows us to visualize the A‐band. I measured the peak fluorescence of both fluorescent secondary antibodies and measured where the myosin heavy chain peak was in relation to the Z‐line on either side to determine an A‐band centering index. The data was compiled by sample (~450 sarcomeres measured per sample) and then averaged both by sample and then by comparing iMSBmal1+/+ to iMSBmal1−/−. Results of iMSBmal1+/+ to iMSBmal1−/− showed a trend toward a longer sarcomere length from 2.64 μm to 2.82 μm (not significant, p=0.1), but the variance did not change. Since the variance of sarcomere lengths did not change, my hypothesis that altered titin expression correlates to more variable sarcomere length was not supported; however, longer titin could lead to the observed increased sarcomere length and my sample number is currently small (n=3/group). The mean and variance of the A‐band centrality index between these groups were not found to be statistically significant (p = 0.7769 and 0.3672, respectively). The lack of change in the A‐band centrality index does not support my hypothesis that a shift would occur. However, there were some technical issues with the antibody that I am repeating and further studying. Further investigations will include increasing my sample size with a focus on sarcomere length. I will also be using site specific titin antibodies to look at the inclusion of longer titin isoforms through the sarcomeres in muscle. Understanding the relationship among titin isoforms, sarcomere length and structure and muscle force will be important for understanding causes of muscle weakness.Support or Funding InformationThis work was made possible by the University of Florida and the funding of the American Physiological Society Undergraduate Summer Research Fellowship Program. Special thanks to Lance A. Riley and Karyn A. Esser for their support and guidance through this process. Thanks also to David Hammers and Cora Coker for their help with microscopy.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effect of high hydrostatic pressure treatment on physical parameters, ultrastructure and shelf life of pre- and post-rigor mortis palm ruff (Seriolella violacea) under chilled storage

To identify processing conditions that better maintain palm ruff quality attributes, high hydrostatic pressure (HHP) was applied to pre- and post-rigor fillets. Physical parameters as whiteness index (WI), water holding capacity (WHC), texture and ultrastructure and shelf life were evaluated after the application of 450 and 550 MPa (3 and 4 min) and during cold storage. Pre-rigor fillets retained less water and were softer than post-rigor, although the onset of rigor increased palm ruff's WHC and firmness. Application of HHP whitened palm ruff's dark flesh; however, this effect reverted at the end of the storage. Pressurized post-rigor samples retained less water than the control and storage caused a WHC increase in samples pressurized at 550 MPa, independent on the rigor condition. Post-rigor fillets softened at pressures of 450–550 MPa appearing to have a lower threshold than beef or cod (above 600 MPa). Ultrastructural changes revealed a subtle contraction (7.4%) of the myofibrils in the unpressurized post-rigor muscle compared to pre-rigor; after 26 days' storage both samples presented extensive muscle degradation and sarcomere length was reduced in 30%. HHP induced pressure-dependent shortening of the sarcomere and modifications to the structure which after 550 MPa was hardly recognizable. After 26 days' storage, there was only slight degradation of the ultrastructure, showing that beyond the structural modifications caused by HHP, post-mortem deterioration is delayed in HHP-treated fillets. Furthermore, HHP extended palm ruff's shelf life to 14–23 days. Thus, HHP may be considered as a technology that maintains the textural quality and shelf life of fresh and stored fish.

Pelvic muscles’ mechanical response to strains in the absence and presence of pregnancy-induced adaptations in a rat model

Maternal birth trauma to the pelvic floor muscles is thought to be consequent to mechanical demands placed on these muscles during fetal delivery that exceed muscle physiological limits. The above is consistent with studies of striated limb muscles that identify hyperelongation of sarcomeres, the functional muscle units, as the primary cause of mechanical muscle injury and resultant muscle dysfunction. However, pelvic floor muscles' mechanical response to strains have not been examined at a tissue level. Furthermore, we have previously demonstrated that during pregnancy, rat pelvic floor muscles acquire structural and functional adaptations in preparation for delivery, which likely protect against mechanical muscle injury by attenuating the strain effect. We sought to determine the mechanical impact of parturition-related strains on pelvic floor muscles' microstructure, and test the hypothesis that pregnancy-induced adaptations modulate muscle response to strains associated with vaginal delivery. Three-month-old Sprague-Dawley late-pregnant (N= 20) and nonpregnant (N= 22) rats underwent vaginal distention, replicating fetal crowning, with variable distention volumes. Age-matched uninjured pregnant and nonpregnant rats served as respective controls. After sacrifice, pelvic floor muscles, which include coccygeus, iliocaudalis, and pubocaudalis, were fixed in situ and harvested for fiber and sarcomere length measurements. To ascertain the extent of physiological strains during spontaneous vaginal delivery, analogous measurements were obtained in intrapartum rats (N= 4) sacrificed during fetal delivery. Data were compared with repeated measures and 2-way analysis of variance, followed by pairwise comparisons, with significance set at P < .05. Gross anatomic changes were observed in the pelvic floor muscles following vaginal distention, particularly in the entheseal region of pubocaudalis, which appeared translucent. The above appearance resulted from dramatic stretch of the myofibers, as indicated by significantly longer fiber length compared to controls. Stretch ratios, calculated as fiber length after vaginal distention divided by baseline fiber length, increased gradually with increasing distention volume. Paralleling these macroscopic changes, vaginal distention resulted in acute and progressive increase in sarcomere length with rising distention volume. The magnitude of strain effect varied by muscle, with the greatest sarcomere elongation observed in coccygeus, followed by pubocaudalis, and a smaller increase in iliocaudalis, observed only at higher distention volumes. The average fetal rat volume approximated 3 mL. Pelvic floor muscle sarcomere lengths in pregnant animals undergoing vaginal distention with 3 mL were similar to intrapartum sarcomere lengths in all muscles (P > .4), supporting the validity of our experimental approach. Vaginal distention resulted in dramatically longer sarcomere lengths in nonpregnant compared to pregnant animals, especially in coccygeus and pubocaudalis (P < .0001), indicating significant attenuation of sarcomere elongation in the presence of pregnancy-induced adaptations in pelvic floor muscles. Delivery-related strains lead to acute sarcomere elongation, a well-established cause of mechanical injury in skeletal muscles. Sarcomere hyperelongation resultant from mechanical strains is attenuated by pregnancy-induced adaptations acquired by the pelvic floor muscles prior to parturition.

Effects of short and extended fasting periods and cattle breed on glycogenolysis, sarcomere shortening and Warner-Bratzler shear force

The effects of short (three hours) and extended (24 hours) feed withdrawal periods and three cattle breeds on muscle energy metabolism, sarcomere length, and meat quality were investigated. Brahman (Br), Nguni (Ng), and Simmental (Sm) bulls were subjected to ante-mortem feed withdrawal of three hours (Br3, n = 10; Ng3, n = 10; and Sm3, n = 10) or 24 hours (Br24, n = 10; Ng24, n = 10; and Sm24, n = 10). M. longissimus was used as the reference muscle for sampling. pH, and temperature was recorded at 1, 2, 3, 6, 9, and 24 hours post mortem. Samples for energy metabolites were removed at 3, 6, 9 and 24 hours post mortem. Glycogen, glucose-6-phosphate and creatine phosphate concentration were determined at each time interval as glycosyl units after hydrolysis. Samples to determine sarcomere length were removed at one and three days post mortem. Homogenates of the samples were placed under a 31,000 magnification microscope and sarcomere lengths were measured. Warner-Bratzler shear force (WBSF) was measured with an Instron meter at 1, 7 and 14 days post mortem. Glycogen was lower for Br24 and Ng24. There were no differences in glucose-6-phosphate, rate of creatine phosphate decline, average sarcomere length, or WBSF for Br24 and Ng24 compared with Br3 and Ng3. There were no differences for WBSF between Br3, Ng3 and Sm3. Glycogen concentration was higher for Sm24 compared with Sm3; glucose-6-phosphate was lower for Sm24 compared with Sm3; and the rate of creatine phosphate decline was higher for Sm24 compared with Sm3. Average sarcomere length was shorter and WBSF was higher for Sm24 compared with Sm3. The effect of prolonged ante-mortem feed withdrawal on tenderness is breed specific. Warner-Bratzler shear force was affected significantly by an extended feed withdrawal period in Simmental cattle only. Keywords: Acidification, creatine phosphate depletion, fasting, hypometabolism, meat tenderness

Meat and fat quality traits of grazing steers supplemented with corn grain and increasing amounts of flaxseed

To evaluate the effect of corn and flaxseed supplementation at finishing on longissimus thoracis color and shear-force and on subcutaneous fat color, forty-eight angus steers were assigned to four dietary treatments (DIET): no-supplement, supplemented with 0.7% live weight (LW) of cracked corn grain plus 0.0%, 0.125% and 0.25% LW of whole flaxseed in two finishing trials (Trial-1, early spring and Trial-2, late spring). None of the evaluated variables were affected by DIET in either trial, nor in a combined statistical analysis of the pooled data. The lack of DIET effect observed on muscle color and shear-force, are in agreement with the similar longissimus muscle pH@45min, temp@45min, pH@24h, and glycogen content at slaughter observed between DIET. Lack of shear force difference between supplementation treatments is in agreement with their similar muscle sarcomere length, total and soluble collagen content and proportion of intact troponin-T. Subcutaneous fat color was also similar between supplementation treatments. Corn or flaxseed supplementation of steers grazing a high-quality pasture did not improve meat or subcutaneous fat color and meat shear force.

Can postmortem proteolysis explain tenderness differences in various bovine muscles?

This study investigated the relationship between postmortem proteolysis, muscle pH decline, sarcomere length (SL), intramuscular fat (IMF) and Warner-Bratzler shear force (WBSF) in four bovine muscles (biceps femoris (BF), infraspinatus (IS), longissimus lumborum (LL), psoas major (PM). The WBSF was low in BF, IS and PM, while LL had a higher value (P<0.001), but still considered as tender. The PM had fastest pH decline (P<0.001), ultimate pH was lowest in LL and PM and highest for IS (P<0.001), sarcomeres were longest for PM and shortest for BF and LL (P<0.001), while IS and PM had more IMF than BF and LL (P=0.038). Troponin T degradation was similar in all muscles after 2d postmortem, however after 13d LL had more degradation than IS (P=0.003). The MMP-2 activity increased during storage (P=0.001), while IS had less activity than the other muscles (P=0.022). Although the variation in proteolytic activity could not explain the variation in WBSF, the study provides useful knowledge for the meat industry for optimising processing and storage procedures for different beef muscles.

The link between exercise and titin passive stiffness

What is the topic of this review? This review focuses on how in vivo and molecular measurements of cardiac passive stiffness can predict exercise tolerance and how exercise training can reduce cardiac passive stiffness. What advances does it highlight? This review highlights advances in understanding the relationship between molecular (titin-based) and in vivo (left ventricular) passive stiffness, how passive stiffness modifies exercise tolerance, and how exercise training may be therapeutic for cardiac diseases with increased passive stiffness. Exercise can help alleviate the negative effects of cardiovascular disease and cardiovascular co-morbidities associated with sedentary behaviour; this may be especially true in diseases that are associated with increased left ventricular passive stiffness. In this review, we discuss the inverse relationship between exercise tolerance and cardiac passive stiffness. Passive stiffness is the physical property of cardiac muscle to produce a resistive force when stretched, which, in vivo, is measured using the left ventricular end diastolic pressure-volume relationship or is estimated using echocardiography. The giant elastic protein titin is the major contributor to passive stiffness at physiological muscle (sarcomere) lengths. Passive stiffness can be modified by altering titin isoform size or by post-translational modifications. In both human and animal models, increased left ventricular passive stiffness is associated with reduced exercise tolerance due to impaired diastolic filling, suggesting that increased passive stiffness predicts reduced exercise tolerance. At the same time, exercise training itself may induce both short- and long-term changes in titin-based passive stiffness, suggesting that exercise may be a treatment for diseases associated with increased passive stiffness. Direct modification of passive stiffness to improve exercise tolerance is a potential therapeutic approach. Titin passive stiffness itself may be a treatment target based on the recent discovery of RNA binding motif 20, which modifies titin isoform size and passive stiffness. Translating these discoveries that link exercise and left ventricular passive stiffness may provide new methods to enhance exercise tolerance and treat patients with cardiovascular disease.

Effect of a grazing period prior to finishing on a high concentrate diet on meat quality from bulls and steers

Bulls and steers (n=60) were assigned to a pre-finishing grazing period and subsequently finished on concentrates or offered concentrates without grazing until slaughter (19months). Colour and pH of longissimus thoracis were measured (48h post-slaughter), and samples collected for proximate composition, collagen, sarcomere length, muscle fibre and enzymatic profile analysis. Steaks for texture, cook loss and sensory were aged (14days). Castration increased intramuscular fat content, cook loss and myosin isoforms IIa and I proportions, and decreased IIx proportion (P<0.05). Steer meat was positively correlated to overall tenderness, texture and acceptability (P<0.05). The presence of a pre-finishing grazing period decreased intramuscular fat and increased the proportion of IIa compared with animals on concentrates, while no differences were found in sensory. Muscle colour, collagen, sarcomere length and instrumental texture were not modified by diet or castration. In conclusion, beef sensory characteristics were unaffected by diet, whereas castration resulted in a small improvement; however all the treatments produced an acceptable product.

Dietary glycerin does not affect meat quality of Ile de France lambs

The objective of this study was to evaluate the physicochemical and nutritional characteristics of meat from three muscles of Ile de France lambs fed diets containing different levels of glycerin as an alternative to corn. Thirty non-castrated feedlot Ile de France lambs weighing 15±0.2 kg were housed in individual pens and slaughtered at 32±0.2 kg. Treatments consisted of three diets: sugarcane + 0% glycerin; sugarcane + 10% glycerin; and sugarcane + 20% glycerin; and three muscles were evaluated: triceps brachii, semimembranosus, and longissimus lumborum. No interactions were observed among glycerin levels and muscle types for the quality characteristics evaluated. Muscles were similar in terms of physicochemical and nutritional characteristics, and only differed in sarcomere length, diameter of muscle fibers, and cholesterol content. Dietary glycerin did not affect pH, water holding capacity (24 h and 30 days after slaughter), weight loss by thawing and cooking, shear force, diameter of muscle fibers, sarcomere length, lipid oxidation, chemical composition, or meat cholesterol content. Dietary glycerin resulted in a linear decrease of muscle yellowness from 3.99 to 3.29. Dietary glycerin altered intramuscular fat fatty acid profile, increasing the concentration of margaric acid (1.47 to 2.61%). Up to 20% glycerin can be included in the diet of feedlot lambs without any harm to meat quality.

In vivo simultaneous evaluations of sarcomere imaging and muscle fiber tension

Muscle fiber tension and sarcomere length play critical roles in regulating muscle functions and adaptations under pathological conditions. However, methods are lacking to quantify these two variables simultaneously in vivo. A novel force microscope was developed with the unique capabilities of estimating muscle fiber tension and acquiring sarcomere images simultaneously in vivo. The force microscope consisting of a custom microscopic imaging system and a force sensor was used to quantify in vivo sarcomere length, muscle fiber tension and stress of the tibialis cranialis muscle at plantar-flexed and dorsi-flexed positions from 11 rat hind limbs. Results showed that sarcomere images and fiber tension could be measured together in vivo with significantly higher muscle fiber tension and stress and longer sarcomere length at the plantar-flexed position when compared to their counterparts at the dorsi-flexed position. The fiber tension estimated using the force microscope had close agreement with the direct measurements of the fiber tension. The present force microscope with simultaneous characterizations of fiber tension and sarcomere imaging provides us a useful in vivo tool to investigate the roles of muscle tension in regulating sarcomere and muscle fiber functions under physiological and pathological conditions.

Anatomical parameters for musculoskeletal modeling of the hand and wrist

A musculoskeletal model of the hand and wrist can provide valuable biomechanical and neurophysiological insights, relevant for clinicians and ergonomists. Currently, no consistent data-set exists comprising the full anatomy of these upper extremity parts. The aim of this study was to collect a complete anatomical data-set of the hand and wrist, including the intrinsic and extrinsic muscles. One right lower arm, taken from a fresh frozen female specimen, was studied. Geometrical data for muscles and joints were digitized using a 3D optical tracking system. For each muscle, optimal fiber length and physiological cross-sectional area were assessed based on muscle belly mass, fiber length, and sarcomere length. A brief description of model, in which these data were imported as input, is also provided. Anatomical data including muscle morphology and joint axes (48 muscles and 24 joints) and mechanical representations of the hand are presented. After incorporating anatomical data in the presented model, a good consistency was found between outcomes of the model and the previous experimental studies.

Effects of epimuscular myofascial force transmission on sarcomere length of passive muscles in the rat hindlimb

Results from imaging studies and finite element models suggest epimuscular myofascial effects on sarcomere lengths in series within muscle fibers. However, experimental evidence is lacking. We evaluated epimuscular myofascial effects on (1) muscle belly, fiber, and mean sarcomere length and (2) sarcomere length distribution within passive fibers of the rat tibialis anterior (TA) and soleus (SO) muscles. Hindlimbs (n = 24) were positioned in predefined knee (55°, 90°, 125°, 160°) and ankle (either 90° or 125°) angles, and fixed in a formaldehyde solution. Varying knee joint angle causes changes in muscle–tendon unit length of SO and TA’s synergists, but not of SO and TA. Whole fibers were taken from SO and TA and photographed along their length. Mean sarcomere length was assessed for the entire fiber and for the proximal, intermediate, and distal thirds (fiber segments) separately. Mean sarcomere length of the fiber was not affected by knee angle, neither for SO (mean: 2.44 ± 0.03 μm and 2.19 ± 0.05 μm for ankle angles of 90° and 125°, respectively) nor for TA (mean: 2.33 ± 0.05 μm and 2.51 ± 0.07 μm for ankle angle set to 90° and 125°, respectively). Only for TA, a significant interaction between knee angle and fiber segment was found, indicating changes in the distribution of lengths of in-series sarcomeres. Thus, while epimuscular myofascial force transmission did not cause mean sarcomere length changes within passive SO and TA, it did alter the length distribution of sarcomeres within passive TA.

Bipedally trained rats have an increased hindlimb muscle force‐generating capacity compared to quadrupedally trained rats

The shift from bipedalism to quadrupedalism in human evolution led to major morphological changes because of new demands placed on the locomotor system including include larger forces on each limb. The ability for a muscle to generate forces and length change to respond to these demands depends on the muscle's architecture, or the arrangement of fibers within a muscle. Muscle force is determined by the cross-sectional area of its fibers (physiological cross-sectional area or PCSA), while muscle excursion is determined by fiber length. In this study, we asked whether changes in muscle architecture respond to the demands of bipedal locomotion and if this response can occur during the lifetime of an individual. To test this question, we trained rats to stand or walk bipedally on a treadmill for one hour a day for five days a week over 90 days using a harness that loaded the hindlimbs to 90% of body mass. A control group was trained quadrupedally over the same time period. The rats were sacrificed and their hindlimbs were fixed in formalin. Muscles were dissected from one hindlimb each of the bipedally (BI) and quadrupedally trained rats (QU). Measurements of muscle mass, length, fiber length, sarcomere length, and pennation angle were obtained and used to calculate normalized fiber length and PCSA. PCSA was normalized to body mass. We found an increase in the mass-specific PCSA of muscles in the BI rats compared to QU rat muscles. Furthermore, the larger PCSA in BI rats was achieved through a decrease in fiber length as opposed to an increase in muscle mass. These results indicate that muscle architecture responds to the varied demands of bipedal locomotion and these changes are a plastic response that can occur over a short time period.

Relationships between sarcomere length and basic composition ofinfraspinatus and longissimus dorsi muscle

The purpose of this study was to determine the relationship between sarcomere length and basic composition (water, protein, ash, collagen, and fat content including marbling) of beef muscles with fast-twitch glycolytic (longissimus dorsi muscle) and slow-twitch oxidative fibers (infraspinatus muscle) in Limousin bulls. Samples (n = 15) were obtained from federally inspected slaughter facilities, and animals used in the experiment were treated according to standard ethical norms. The shortest sarcomeres were observed for longissimus dorsi from striploin, and the longest for infraspinatus muscle. Additionally, the influence of both cut and animal was observed. In both cases of muscles, sarcomere length was positively correlated with marbling level and negatively correlated with protein content. Moreover, sarcomere length was very strongly correlated with marbling level in the case of longissimus dorsi et lumborum. Differences in sarcomere length were associated with cut and animal, and this correlation may partly explain differences in composition, especially in protein content.

Research on Structural and Ultra-Structural Changes of Grass Carp during Different Crisp Periods

In order to study the influence of muscle structural changes on the special brittleness mechanism of crisp grass carp (Ctenopharyngodon idellus C.et V, CGC) during different crisping process of grass carp (Ctenopharyngodon idellus, GC), samples were periodically acquired and then prepared for light and transmission electron microscopy during the whole process. For GC and CGC, all muscle tissues had typical features, whereas there were much more interstitial tissues and narrower intermyofibrillar spaces in CGC than those in GC. Meanwhile, the muscle fibers diameter, intermyofibrillar space and myofibrils to endomysium detachment lengths of GC were significantly decreased with increasing duration of crisping time. While the muscle fibers density and sarcomere lengths were significantly greater in CGC than in GC. In view of the structural and ultra-structural differences in these samples, it was conceivable to suggest that short muscle fiber diameter, high fiber density, narrow intermyofibrillar spaces and wide lengths of sarcomere were the main causes of increased muscle brittleness.

High resolution muscle measurements provide insights into equinus contractures in patients with cerebral palsy.

Muscle contractures that occur after upper motor neuron lesion are often surgically released or lengthened. However, surgical manipulation of muscle length changes a muscle's sarcomere length (Ls ), which can affect force production. To predict effects of surgery, both macro- (fascicle length (Lf )) and micro- (Ls ) level structural measurements are needed. Therefore, the purpose of this study was to quantify both Ls and Lf in patients with cerebral palsy (CP) as well as typically developing (TD) children. Soleus ultrasound images were obtained from children with CP and TD children. Lf was determined and, with the joint in the same position, CP biopsies were obtained and formalin fixed, and Ls was measured by laser diffraction. Since soleus Ls values were not measurable in TD children, TD Ls values were obtained using three independent methods. While average Lf did not differ between groups (CP=3.6±1.2 cm, TD=3.5±0.9 cm; p>0.6), Ls was dramatically longer in children with CP (4.07±0.45 µm vs. TD=2.17±0.24 µm; p<0.0001). While Lf values were similar between children with CP and TD children, this was due to highly stretched sarcomeres within the soleus muscle. Surgical manipulation of muscle-tendon unit length will thus alter muscle sarcomere length and change force generating capacity of the muscle.

Sarcomere length organization as a design for cooperative function amongst all lumbar spine muscles

The functional design of spine muscles in part dictates their role in moving, loading, and stabilizing the lumbar spine. There have been numerous studies that have examined the isolated properties of these individual muscles. Understanding how these muscles interact and work together, necessary for the prediction of muscle function, spine loading, and stability, is lacking. The objective of this study was to measure sarcomere lengths of lumbar muscles in a neutral cadaveric position and predict the sarcomere operating ranges of these muscles throughout full ranges of spine movements. Sarcomere lengths of seven lumbar muscles in each of seven cadaveric donors were measured using laser diffraction. Using published anatomical coordinate data, superior muscle attachment sites were rotated about each intervertebral joint and the total change in muscle length was used to predict sarcomere length operating ranges. The extensor muscles had short sarcomere lengths in a neutral spine posture and there were no statistically significant differences between extensor muscles. The quadratus lumborum was the only muscle with sarcomere lengths that were optimal for force production in a neutral spine position, and the psoas muscles had the longest lengths in this position. During modeled flexion the extensor, quadratus lumborum, and intertransversarii muscles lengthened so that all muscles operated in the approximate same location on the descending limb of the force-length relationship. The intrinsic properties of lumbar muscles are designed to complement each other. The extensor muscles are all designed to produce maximum force in a mid-flexed posture, and all muscles are designed to operate at similar locations of the force-length relationship at full spine flexion.

The effect of age on rat rotator cuff muscle architecture

Understanding rotator cuff muscle function during disease development and after repair is necessary for preventing degeneration and improving postsurgical outcomes, respectively. The rat is a commonly used rotator cuff animal model; however, unlike humans, rats continue to grow throughout their lifespan, so age-related changes in muscle structure may complicate an understanding of muscle adaptations to injury. Infraspinatus and supraspinatus muscle mass, fiber length, pennation angle, sarcomere length, and physiological cross-sectional area (PCSA) were measured in Sprague-Dawley rats (n=30) with a body mass ranging from 51 to 814g (approximately 3weeks to approximately 18months). Both the supraspinatus and infraspinatus showed a striking conservation of sarcomere length throughout growth. There was linear growth in muscle mass and PCSA, nonlinear growth in muscle length and fiber bundle length, and a linear relationship between humeral head diameter and fiber bundle length, suggesting that muscle fiber length (serial sarcomere number) adjusted according to skeletal dimensions. These muscle growth trajectories allowed sarcomere length to remain nearly constant. During the typical rat rotator cuff experimental period (animal mass, 400-600g), muscle mass will increase by 30%, fiber length will increase by 7%, and PCSA will increase by 27%, but sarcomere lengths are nearly constant. Therefore, these normal growth-induced changes in architecture must be considered when muscle atrophy or fiber shortening is measured after rotator cuff tears in this model.

Myosin phosphorylation and force potentiation in skeletal muscle: evidence from animal models

The contractile performance of mammalian fast twitch skeletal muscle is history dependent. The effect of previous or ongoing contractile activity to potentiate force, i.e. increase isometric twitch force, is a fundamental property of fast skeletal muscle. The precise manifestation of force potentiation is dependent upon a variety of factors with two general types being identified; staircase potentiation referring to the progressive increase in isometric twitch force observed during low frequency stimulation while posttetanic potentiation refers to the step-like increase in isometric twitch force observed following a brief higher frequency (i.e. tetanic) stimulation. Classic studies established that the magnitude and duration of potentiation depends on a number of factors including muscle fiber type, species, temperature, sarcomere length and stimulation paradigm. In addition to isometric twitch force, more recent work has shown that potentiation also influences dynamic (i.e. concentric and/or isotonic) force, work and power at a range of stimulus frequencies in situ or in vitro, an effect that may translate to enhanced physiological function in vivo. Early studies performed on both intact and permeabilized models established that the primary mechanism for this modulation of performance was phosphorylation of myosin, a modification that increased the Ca(2+) sensitivity of contraction. More recent work from a variety of muscle models indicates, however, the presence of a secondary mechanism for potentiation that may involve altered Ca(2+) handling. The primary purpose of this review is to highlight these recent findings relative to the physiological utility of force potentiation in vivo.