Muscle Fiber Length Research Articles

Volumetric muscle loss disrupts length-dependent architectural and functional characteristics of skeletal muscle

ABSTRACT Purpose/Aim: Skeletal muscle architecture is a primary determinant of function. Volumetric muscle loss (VML) injury is destructive; however, the impact on muscle architecture is uncharacterized. Methods: Architectural and functional effects of VML were assessed in rat tibialis anterior (TA) muscle model 4 weeks post-injury. Results: VML caused a 31% and 33% reduction in muscle weight (p < 0.001) and fiber length (p = 0.002), respectively, culminating a 34% reduction of fiber to muscle length ratio (FL:ML; p < 0.001). Fiber pennation angle (+14%; p = 0.150) and physiological cross-sectional area (PCSA; −12%; p = 0.220) were unchanged. VML injury reduced peak isometric force (Po) by 36% (p < 0.001), specific force (sPo = Po/PCSA) by 41% (vs. Po, p > 0.999), and force per gram muscle weight (Po/mw) by 18% (vs. Po, p < 0.001). VML injury increased the length at which Po was produced (Lo) by 8% (p = 0.009), and reduced functional excursion by 35% (p = 0.035). Conclusion: The architectural changes after VML injury preserved PCSA, and therefore preserved “potential” maximal force-producing capacity. At most, only half the Po deficit was due directly to the cumulative effect of horizontal and longitudinal tissue loss. Highlighting the impact of longitudinal muscle loss, VML injury reduced fiber length, and FL:ML and grossly disrupted length-dependent functional properties. These findings raise the importance of augmenting length-dependent muscle properties to optimize functional recovery after VML injury.

Sex Differences In The Morphology Of The Vastus Lateralis After An Anterior Cruciate Ligament Tear

Females are three times more susceptible to tear their anterior cruciate ligament (ACL) and have significantly weaker quadriceps after injury. Despite the high frequency of injury, little is known about whether the morphology of the quadriceps alters in a sex specific manner. Muscle imaging of volume, fibrosis, pennation angle, and fiber tract length could provide valuable insights into these differences. PURPOSE: To investigate sex differences in morphology of the vastus lateralis (VL) after ACL injury. METHODS: 24 ACL deficient patients (11M, 13F, 21.8 ± 5.0 y, 25.0 ± 3.7 kg/m2, days since injury 24.6 ± 16.2) underwent magnetic resonance imaging including multi echo T1ρ, DTI, and 2D turbo spin echo. Data was post-processed in MATLAB, where a mono exponential decay curve was fitted to analyze the T1ρ signal. Muscle fiber bundles were tracked by taking the first eigenvector of diffusion tensor starting at the aponeurosis until the fiber exited the side or top of the selected slices. From these tracks pennation angle and fiber length were calculated. Muscle volume was calculated by manually outlining the border of the VL. Independent t-tests compared differences between males and females. RESULTS: T1ρ times (M: 0.031 ± 0.003 s; F: 0.028 ± 0.002 s; p = 0.06) and pennation angle (M: 16.6 ± 2.6°; F: 14.9 ± 1.8°) of the involved limb was not significantly different between sexes; however, fiber length was significantly longer in females compared to males (F: 55.1 ± 6.2 mm; M: 48.3 ± 7.0 mm; p = 0.04). Volume of the vastus lateralis was not significantly different between sexes (M: 355.1 ± 117.5 cm3; F: 335.9 ± 97.1 cm3; p = 0.6). CONCLUSIONS: We show that ACL injury results in a sex specific difference in muscle fiber length. Fiber length is an important determinant in physiological cross-sectional area (PCSA). The longer fiber lengths seen in the females may lead to decreased PCSA which could have a negative influence on quadriceps muscle strength. Potentially, this longer fiber length prior to surgery may be one factor that drives differences in recovery of muscle strength after surgery. Future work is needed to examine how these morphological aspects change over time following ACL reconstruction.

A comparison of pain responses, hemodynamic reactivity and fibre type composition between Bergström and microbiopsy skeletal muscle biopsies.

This study tested the hypotheses that 1) skeletal muscle biopsies performed with the Bergström needle evoke larger perceptions of pain and greater hemodynamic reactivity compared to biopsies performed with the microbiopsy needle, and 2) both needles yield samples with similar fibre type compositions when samples are collected at similar skeletal muscle depths. Fourteen healthy (age: 21.6 ± 3.2 years; VO2peak: 41.5 ± 5.8 mL/kg/min) males (n = 7) and females (n = 7) provided two resting skeletal muscle biopsies, one with each needle type, following a randomized crossover design. Participants completed the short-form McGill Pain Questionnaire and the Brief Pain Inventory before, during, and after the skeletal muscle biopsies. Hemodynamic reactivity was assessed by measuring heart rate (HR) and mean arterial pressure (MAP) at rest and during the biopsy procedures. Immunofluorescence analysis was used to assess fibre type composition in vastus lateralis samples. Compared to the microbiopsy needle, the Bergström needle elicited a larger perception of pain but similar hemodynamic reactivity during the biopsy. Both needles yielded skeletal muscle samples with similar fibre type composition and resulted in similar perceptions of pain and pain-related interference during the post-biopsy recovery period. Collectively, these findings suggest that studies should consider using the microbiopsy needle rather than the Bergström needle unless large amounts of muscle tissue or certain muscle fibre lengths are required. However, future work should determine whether our findings are generalizable to biopsies performed with different procedures and/or types of Bergström/microbiopsy needles.

Predicting Perturbed Human Arm Movements in a Neuro-Musculoskeletal Model to Investigate the Muscular Force Response.

Human movement is generated by a dynamic interplay between the nervous system, the biomechanical structures, and the environment. To investigate this interaction, we propose a neuro-musculoskeletal model of human goal-directed arm movements. Using this model, we simulated static perturbations of the inertia and damping properties of the arm, as well as dynamic torque perturbations for one-degree-of freedom movements around the elbow joint. The controller consists of a feed-forward motor command and feedback based on muscle fiber length and contraction velocity representing short-latency (25 ms) or long-latency (50 ms) stretch reflexes as the first neuronal responses elicited by an external perturbation. To determine the open-loop control signal, we parameterized the control signal resulting in a piecewise constant stimulation over time for each muscle. Interestingly, such an intermittent open-loop signal results in a smooth movement that is close to experimental observations. So, our model can generate the unperturbed point-to-point movement solely by the feed-forward command. The feedback only contributed to the stimulation in perturbed movements. We found that the relative contribution of this feedback is small compared to the feed-forward control and that the characteristics of the musculoskeletal system create an immediate and beneficial reaction to the investigated perturbations. The novelty of these findings is (1) the reproduction of static as well as dynamic perturbation experiments in one neuro-musculoskeletal model with only one set of basic parameters. This allows to investigate the model's neuro-muscular response to the perturbations that—at least to some degree—represent stereotypical interactions with the environment; (2) the demonstration that in feed-forward driven movements the muscle characteristics generate a mechanical response with zero-time delay which helps to compensate for the perturbations; (3) that this model provides enough biomechanical detail to allow for the prediction of internal forces, including joint loads and muscle-bone contact forces which are relevant in ergonomics and for the development of assistive devices but cannot be observed in experiments.

Muscular architecture of the popliteus muscle and the basic science implications

BackgroundThe function of the popliteus muscle is largely treated as a static stabilizer and has a lack of basic muscular architectural data to enable study of its dynamic function. A large volume of literature supports its static function and the essential need for reconstruction in the posterolateral knee when injured to restore knee stability. Hypothesis/purposeWe hypothesize that the popliteus muscle is more significant as a dynamic presence in the knee. MethodsA collection of popliteus architectural data was collected from 28 cadaver specimens (mean (SD) 76 years (11)). Physiological cross-sectional area of the popliteus and semimembranosus muscles were calculated from muscle volume and fiber length to power future muscle force prediction models. Posterior knee muscle trajectories were measured with respect to the longitudinal axis of the tibia. A 2-tailed T test was performed. ResultsSignificant differences between males and females were found for both the popliteus (p = 1.1E−05) and semimembranosus (p = 2.0E–05) muscle volumes. Significant differences between males and females were also found in PCSA for the popliteus (p = 0.005) and semimembranosus (p = 4.1E–05) muscles. There were no significant differences in fiber length, overall muscle length (with tendon removed), age, and orientation. ConclusionFurther consideration should be given to include the popliteus muscle as a dynamic entity in the knee given its mechanical properties, trajectory, and prior biomechanical evidence showing when and how it is activated. The present study provides data that may shape future directions of research and treatment with regard to posterolateral corner injuries and ligamentous balancing of the knee.

Drosophila Activin signaling promotes muscle growth through InR/TORC1-dependent and -independent processes.

The Myostatin/Activin branch of the TGF-β superfamily acts as a negative regulator of vertebrate skeletal muscle size, in part, through downregulation of insulin/insulin-like growth factor 1 (IGF-1) signaling. Surprisingly, recent studies in Drosophila indicate that motoneuron-derived Activin signaling acts as a positive regulator of muscle size. Here we demonstrate that Drosophila Activin signaling promotes the growth of muscle cells along all three axes: width, thickness and length. Activin signaling positively regulates the insulin receptor (InR)/TORC1 pathway and the level of Myosin heavy chain (Mhc), an essential sarcomeric protein, via increased Pdk1 and Akt1 expression. Enhancing InR/TORC1 signaling in the muscle of Activin pathway mutants restores Mhc levels close to those of the wild type, but only increases muscle width. In contrast, hyperactivation of the Activin pathway in muscles increases overall larval body and muscle fiber length, even when Mhc levels are lowered by suppression of TORC1. Together, these results indicate that the Drosophila Activin pathway regulates larval muscle geometry and body size via promoting InR/TORC1-dependent Mhc production and the differential assembly of sarcomeric components into either pre-existing or new sarcomeric units depending on the balance of InR/TORC1 and Activin signals.

Effect of vasti morphology on peak sprint cycling power of a human musculoskeletal simulation model

Fascicle length of m. vastus lateralis in cyclists has been shown to correlate positively with peak sprint cycling power normalized for lean body mass. We investigated whether vasti morphology affects sprint cycling power via force-length and force-velocity relationships. We simulated isokinetic sprint cycling at pedaling rates ranging from 40 to 150 rpm with a forward dynamic model of the human musculoskeletal system actuated by eight leg muscles. Input of the model was muscle stimulation over time, which was optimized to maximize the average power output over a pedal cycle. This was done for a reference model and for models in which the vasti had equal volume but different morphology. It was found that models with longer muscle fibers but a reduced physiological cross-sectional area of the vasti produced a higher sprint cycling power. This was partly explained by better alignment of the peak power-pedaling rate curve of the vasti with the corresponding curves of the other leg muscles. The highest sprint cycling power was achieved in a model in which the increase in muscle fiber length of the vasti was accompanied by a concomitant shift in optimum knee angle. It was concluded that muscle mechanics can partly explain the positive correlations between fascicle length of m. vastus lateralis and normalized peak sprint cycling power. It should be investigated whether muscle fiber length of the vasti and optimum knee angle are suitable training targets for athletes who want to concurrently improve their sprint and endurance cycling performance.NEW & NOTEWORTHY We simulated isokinetic sprint cycling at pedaling rates ranging from 40 to 150 rpm with a forward dynamic model of the human musculoskeletal system actuated by eight leg muscles. We selectively modified vasti morphology: we lengthened the muscle fibers and reduced the physiological cross-sectional area. The modified model was able to produce a higher sprint cycling power.

Architecture of the Short External Rotator Muscles of the Hip

BackgroundMuscle architecture, or the arrangement of sarcomeres and fibers within muscles, defines functional capacity. There are limited data that provide an understanding of hip short external rotator muscle architecture. The purpose of this study was thus to characterize the architecture of these small hip muscles.MethodsEight muscles from 10 independent human cadaver hips were used in this study (n = 80 muscles). Architectural measurements were made on pectineus, piriformis, gemelli, obturators, quadratus femoris, and gluteus minimus. Muscle mass, fiber length, sarcomere length, and pennation angle were used to calculate the normalized muscle fiber length, which defines excursion, and physiological cross-sectional area (PCSA), which defines force-producing capacity.ResultsGluteus minimus had the largest PCSA (8.29 cm2) followed by obturator externus (4.54 cm2), whereas superior gemellus had the smallest PCSA (0.68 cm2). Fiber lengths clustered into long (pectineus - 10.38 cm and gluteus minimus - 10.30 cm), moderate (obturator internus - 8.77 cm and externus - 8.04 cm), or short (inferior gemellus - 5.64 and superior gemellus - 4.85). There were no significant differences among muscles in pennation angle which were all nearly zero. When the gemelli and obturators were considered as a single functional unit, their collective PCSA (10.00 cm2) exceeded that of gluteus minimus as a substantial force-producing group.ConclusionsThe key findings are that these muscles have relatively small individual PCSAs, short fiber lengths, and low pennation angles. The large collective PCSA and short fiber lengths of the gemelli and obturators suggest that they primarily play a stabilizing role rather than a joint rotating role.

Effect of electric stimulation on histological traits and color of carcasses in old duck and chicken

The effect of electric stimulation on some quantitative character of old duck and chicken carcasses were the main aim of the present study. The device for measuring meat tenderness was designed at department of Food Sciences, College of Agriculture, and University of Basrah. A total of 36 old duck and same number of spent layer chickens aged78.21 week’s birds were slaughtered with knife manually. They de-feathered, and all internal organs were removed. Carcasses were divided into three groups, the control group (no electric stimulation), the second treatment group (electric conductivity 3.67 V/cm) with low voltage [110 volt]) and 1% saline solution, the third treatment group (7.33 V/cm) with 220 V voltage and 1% saline solution. Traits were measured at 25 min, 6 hrs and 24 hrs after electric stimulation. The carcasses were stored for 30 and 60 days. The following histological parameters were determined; sarcomere length, muscle fiber length, muscle fibres breaking index and color. The results indicated that length of sarcomere and muscle fibre breaking index were significantly(P≤0.05) affected by electric stimulation, because there was an increase in sarcomere length and muscle fiber breaking index of third treatment in chicken with an increasing interval of time. There was a significant (P≤0.05) decrease in muscle fiber diameter in electric stimulation treatment three (duck and chicken) with the time progress. Duck differed significantly when compared to chicken. Statistical analysis showed a significant effect of electric stimulation on color lightness (L). Duck appeared to be darker in color than chicken. Treatment group three revealed blue coloration at 25min, 60 days in both duck and chicken. Whereas other treatment groups showed yellowness (b) color. Similarly, stimulation affected color redness (a) where all groups showed greenish except 30 and 60 day intervals, their color was reddish and value of (a) of duck was less than of chicken. Electric stimulation (especially 220 V voltage) improve meat tenderness of both chicken and duck.

In vivo human lower limb muscle architecture dataset obtained using diffusion tensor imaging.

‘Gold standard’ reference sets of human muscle architecture are based on elderly cadaveric specimens, which are unlikely to be representative of a large proportion of the human population. This is important for musculoskeletal modeling, where the muscle force-generating properties of generic models are defined by these data but may not be valid when applied to models of young, healthy individuals. Obtaining individualized muscle architecture data in vivo is difficult, however diffusion tensor magnetic resonance imaging (DTI) has recently emerged as a valid method of achieving this. DTI was used here to provide an architecture data set of 20 lower limb muscles from 10 healthy adults, including muscle fiber lengths, which are important inputs for Hill-type muscle models commonly used in musculoskeletal modeling. Maximum isometric force and muscle fiber lengths were found not to scale with subject anthropometry, suggesting that these factors may be difficult to predict using scaling or optimization algorithms. These data also highlight the high level of anatomical variation that exists between individuals in terms of lower limb muscle architecture, which supports the need of incorporating subject-specific force-generating properties into musculoskeletal models to optimize their accuracy for clinical evaluation.

3D analysis of capillary network in skeletal muscle of obese insulin-resistant mice.

In obesity, the skeletal muscle capillary network regresses and the insulin-mediated capillary recruitment is impaired. However, it has been shown that in the early stage of advanced obesity, an increased functional vascular response can partially compensate for other mechanisms of insulin resistance. The present study aimed to investigate the changes in the capillary network around individual muscle fibres during the early stage of obesity and insulin resistance in mice using 3D analysis. Capillaries and muscle fibres of the gluteus maximus muscles of seven high-fat-diet-induced obese and insulin-resistant mice and seven age-matched lean healthy mice were immunofluorescently labelled in thick transverse muscle sections. Stacks of images were acquired using confocal microscope. Capillary network characteristics were estimated by methods of quantitative image analysis. Muscle fibre typing was performed by histochemical analysis of myosin heavy chain isoforms on thin serial sections of skeletal muscle. Capillary length per muscle fibre length and capillary length per muscle fibre surface were increased by 27% and 23%, respectively, around small muscle fibres in obese mice, while there were no significant comparative differences around large fibres of obese and lean mice. Furthermore, the capillarization was larger around small compared to large fibres and there was a shift toward fast type myosin heavy chain isoforms, with no significant changes in muscle fibre diameters, tortuosity and anisotropy in obese mice. Overall, the results show that obese insulin-resistant mice have selective increase in capillarization around small predominantly intermediate muscle fibres, which is most likely related to the impaired glucose metabolism characteristic of type 2 diabetes.

Assessment of the energy-related cost function over a range of walking speeds.

Cost funtions are needed for calculation of muscle forces in musculoskeletal models. The behavior of the energy-related cost function, proposed by Praagman et al. (J Biomech 39(4):758-765, 2006. https://doi.org/10.1016/j.jbiomech.2004.11.034 ) (CFP), can be used as an optimization criteria in musculoskeletal models for studying gait. In particular, in this work, its performance is compared against two empirical phenomenological models at different walking speed conditions. Also, the sensitivity of the CFP function to model parameters, such as muscle mass, maximal isometric muscle force, optimal muscle fiber length and maximum muscle velocity of the contractile element, was analyzed. The obtained results showed that CFP presents different behavior (in terms of the normalized root-mean-squared deviation (NRMSD) and the coefficient of multiple correlation (CMC)) for different muscles. Also, it provided estimates with median of NRMSD between 0.176 and 0.299 and median of CMC between 0.703 and 0.865 both metrics for slow, free and fast walking speed, which could be considered as acceptable results. Furthermore, the results indicated that CFP is insensitive to changes in muscle mass and relatively sensitive to maximal isometric muscle force. However, CFP presented a noisy behavior on estimations of muscle energy rate for some muscle as compared to phenomenological models. Finally, estimations by CFP during gait are within the values obtained by the empirical phenomenological models.

Determining Subject-Specific Lower-Limb Muscle Architecture Data for Musculoskeletal Models Using Diffusion Tensor Imaging.

Accurate individualized muscle architecture data are crucial for generating subject-specific musculoskeletal models to investigate movement and dynamic muscle function. Diffusion tensor imaging (DTI) magnetic resonance (MR) imaging has emerged as a promising method of gathering muscle architecture data in vivo; however, its accuracy in estimating parameters such as muscle fiber lengths for creating subject-specific musculoskeletal models has not been tested. Here, we provide a validation of the method of using anatomical magnetic resonance imaging (MRI) and DTI to gather muscle architecture data in vivo by directly comparing those data obtained from MR scans of three human cadaveric lower limbs to those from dissections. DTI was used to measure fiber lengths and pennation angles, while the anatomical images were used to estimate muscle mass, which were used to calculate physiological cross-sectional area (PCSA). The same data were then obtained through dissections, where it was found that on average muscle masses and fiber lengths matched well between the two methods (4% and 1% differences, respectively), while PCSA values had slightly larger differences (6%). Overall, these results suggest that DTI is a promising technique to gather in vivo muscle architecture data, but further refinement and complementary imaging techniques may be needed to realize these goals.

Hydrogen Sulfide Improves Aberrant Gastric Smooth Muscle Function in Duchenne Muscular Dystrophy Mice

Deficiency of dystrophin, a cytoskeletal protein localized in the inner face of the plasma membrane in skeletal, cardiac and smooth muscle, results in Duchenne Muscular Dystrophy (DMD). Through its interactions with extracellular matrix and plasma membrane proteins, dystrophin plays a role in contraction and signal transduction. In DMD, gastrointestinal disorders such as gastric dilation and intestinal pseudo‐obstruction resulting from altered motility have been reported. The role of dystrophin in the regulation of contractile protein expression and smooth muscle function, per se, is not known. Studies have suggested that inflammation contributes to the pathophysiology of DMD. Exogenous H2S had been shown to exert beneficial cardiovascular and gastrointestinal functions, probably via exerting anti‐inflammatory actions.AimTo test the hypothesis that a lack of dystrophin causes a decrease in contractile protein expression and smooth muscle function and that treatment with H2S restores the effects of dystrophin deficiency.MethodsThe role of dystrophin was examined using mice deficient in dystrophin alone (mdx) and mice deficient in dystrophin plus telomerase RNA (mdx/mTR), which exhibit increased disease severity. The effect of an orally‐active, slow releasing H2S agent (SG1002) was tested in mdx/mTR mice (40 mg/kg body weight in chow/every 3 days starting from 3 weeks to 9 months). Contraction in response to acetylcholine (ACh) was measured in gastric muscle strips isolated from mdx, mdx/mTR and SG1002‐treated mdx/mTR mice. Age‐matched control mice were used for each group. Contraction was also measured in muscle cells isolated from the stomach of control and mdx mice by scanning micrometry and expressed as the percent decrease in muscle cell length. Expression of contractile proteins such as smoothelin, caldesmon, calponin and tropomyosin was measured by qRT‐PCR and western blot.ResultsAcetylcholine‐induced contraction was reduced in muscle strips from mdx/mTR mice (19 ± 5 mN/100 mg tissue) compared to age‐matched 9‐month old control mice (34 ± 7 mN/100 mg tissue). Treatment of mdx/mTR mice with SG1002 restored contraction to above normal levels (56 ± 8 mN/100 mg tissue). Contraction was also reduced in mdx mice (22 ± 8 mN/100 mg tissue) compared to age‐matched 3‐month old control mice (40 ± 5 mN/100 mg tissue). Acetylcholine‐induced contraction was also decreased in gastric muscle cells from mdx mice (25 ± 3% decrease in cell length) compared to control mice (41 ± 5% decrease in cell length). Expression of contractile proteins was decreased in gastric smooth muscle from mdx and mdx/mTR mice compared to age‐matched controls. Treatment of mdx/mTR mice with SG1002 restored expression of contractile proteins to levels similar to control.ConclusionThe results support our hypothesis that dystrophin deficiency affects contractile protein expression leading to decreased muscle contraction. Furthermore, treatment with H2S restores gastric smooth muscle function and contractile protein expression suggesting therapeutic potential of H2S in the treatment of motility disorders in DMD.Support or Funding InformationSupported by DK15564, DK28300, DK34153, and HL133167.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Mechanical similarity across ontogeny of digging muscles in an Australian marsupial (Isoodon fusciventer).

Many mammals dig, either during foraging to access subsurface food resources, or in creating burrows for shelter. Digging requires large forces produced by muscles and transmitted to the soil via the skeletal system; thus fossorial mammals tend to have characteristic modifications of the musculoskeletal system that reflect their digging ability. Bandicoots (Marsupialia: Peramelidae) scratch-dig mainly to source food, searching for subterranean food items including invertebrates, seeds, and fungi. They have musculoskeletal features for digging, including shortened, robust forelimb bones, large muscles, and enlarged muscle attachment areas. Here, we compared changes in the ontogenetic development of muscles associated with digging in the Quenda (Isoodon fusciventer). We measured muscle mass (m m ), pennation angle, and fiber length (FL) to calculate physiological cross-sectional area (PCSA; a proxy of maximum isometric force) as well as estimate the maximum isometric force (Fmax) for 34 individuals ranging in body size from 124 to 2,390 g. Males grow larger than females in this bandicoot species, however, we found negligible sex differences in mass-specific m m , PCSA or FL for our sample. Majority of the forelimb muscles PCSA showed a positive allometric relationship with total body mass, while m m and FL in the majority of forelimb muscles showed isometry. Mechanical similarity was tested, and two thirds of forelimb muscles maximum isometric forces (Fmax) scaled with isometry; therefore the forelimb is primarily mechanical similar throughout ontogeny. PCSA showed a significant difference between scaling slopes between main movers in the power stroke, and main movers of the recovery stroke of scratch-digging. This suggests that some forelimb muscles grow with positive allometry, specially these associated with the power stroke of digging. Intraspecific variation in PCSA is rarely considered in the literature, and thus this is an important study quantifying changes in muscle architectural properties with growth in a mammalian model of scratch-digging.

Plantarflexor fiber and tendon slack length are strong determinates of simulated single-leg heel raise height

Achilles tendon ruptures have been linked with detrimental changes in muscle-tendon structure, which may help explain long-term functional deficits. However, the causal effects of muscle-tendon structure on joint function have not been tested in a controlled setting. Therefore, the purpose of this study was to test the implications of muscle-tendon unit parameters on simulated single-leg heel raise height. We hypothesized that muscle fiber length and resting ankle angle – a clinical surrogate measure of tendon slack length – would predict single-leg heel raise height more strongly than other parameters. To test this hypothesis, we developed a two-part simulation paradigm that recreated clinically relevant muscle-tendon scenarios and then tested these parameters on single-leg heel raise height. We found that longer muscle fibers had the greatest positive effect on single-leg heel raise height. However, tendon slack length, determined by simulating resting ankle angles in a secondary analysis, revealed a stronger negative correlation with heel raise height. Our findings support previous clinical observations that both muscle fascicle length and resting tendon length are important muscle-tendon parameters for patient function. In addition to minimizing tendon elongation following rupture, treatment plans should focus on preserving plantarflexor muscle structure to mitigate functional loses following Achilles tendon ruptures.

Efeito do treinamento pliométrico e isométrico na força explosiva de atletas de handebol

Plyometrics uses the reactive capacity of the neuromuscular system to increase the power of movement from an eccentric to a concentric contraction. Isometry refers to a static mode of training, ie there is no change in muscle fiber length. Handball is a collective sport that involves a large amount and variety of movement associated with ball manipulation and interaction with other athletes. In terms of movement, handball can be considered a complete sport because it uses a rich combination of fundamental and "natural" motor skills of the human motor repertoire. The aim of this research is to analyze the influence of a 4 week plyometric and isometric training on the explosive strength of young handball athletes. The sample consisted of 10 women, with an average age of 16.1 ± 0.56 years, who had been practicing handball for at least three months. The evaluation protocol used was: horizontal jump (HS) to measure lower limb explosive strength. The girls were divided into two statistically equal groups, the plyometric group (GP) and the isometric group (GI). Data were analyzed using paired t-test and Effect Size, as well as the Proesp manual. Initially the tests presented the following results: GP showed 1.32 ± 0.13m in HS and GI 1.41 ± 0.10m. After four weeks of training, the tests were redone and the GP showed 1.37 ± 0.08m in HS and the GI 1.43 ± 0.09m. Statistically the changes were not significant, the magnitude of the effect and the lower explosive strength results were superficial. It is concluded that, although statistically insignificant, both methods present little evolution in the explosive force of lower limbs, and it is necessary to further analyze the methods in these gains and also to analyze the concurrent proposal between both methods.

Do the relationships between hindlimb anatomy and sprint speed variation differ between sexes in Anolis lizards?

The ability of an animal to run fast has important consequences on its survival capacity and overall fitness. Previous studies have documented how variation in the morphology of the limbs is related to variation in locomotor performance. Although these studies have suggested direct relations between sprint speed and hindlimb morphology, few quantitative data exist. Consequently, it remains unclear whether selection acts in limb segment lengths, overall muscle mass or muscle architecture (e.g. muscle fiber length and cross-sectional area). Here, we investigate whether muscle architecture (mass, fiber length and physiological cross-sectional area), hindlimb segment dimensions, or both, explain variation in sprint speed across 14 species of Anolis lizards. Moreover, we test whether similar relationships exist between morphology and performance for both sexes, which may not be the case given the known differences in locomotor behavior and habitat use. Our results show that the main driver of sprint speed is the variation in femur length for both males and females. Our results further show sexual dimorphism in the traits studied and, moreover, show differences in the traits that predict maximal sprint speed in males and females. For example, snout vent length and overall muscle mass are also good predictors of sprint speed in males, whereas no relationships between muscle mass and sprint speed was observed in females. Only a few significant relationships were found between muscle architecture (fiber length, cross-sectional area) and sprint speed in male anoles, suggesting that overall muscles size, rather than muscle architecture, appears to be under selection.

The Interaction of Compliance and Activation on the Force-Length Operating Range and Force Generating Capacity of Skeletal Muscle: A Computational Study using a Guinea Fowl Musculoskeletal Model.

SynopsisA muscle’s performance is influenced by where it operates on its force–length (F–L) curve. Here we explore how activation and tendon compliance interact to influence muscle operating lengths and force-generating capacity. To study this, we built a musculoskeletal model of the lower limb of the guinea fowl and simulated the F–L operating range during fixed-end fixed-posture contractions for 39 actuators under thousands of combinations of activation and posture using three different muscle models: Muscles with non-compliant tendons, muscles with compliant tendons but no activation-dependent shift in optimal fiber length (L0), and muscles with both compliant tendons and activation-dependent shifts in L0. We found that activation-dependent effects altered muscle fiber lengths up to 40% and increased or decreased force capacity by up to 50% during fixed-end contractions. Typically, activation-compliance effects reduce muscle force and are dominated by the effects of tendon compliance at high activations. At low activation, however, activation-dependent shifts in L0 are equally important and can result in relative force changes for low compliance muscles of up to 60%. There are regions of the F–L curve in which muscles are most sensitive to compliance and there are troughs of influence where these factors have little effect. These regions are hard to predict, though, because the magnitude and location of these areas of high and low sensitivity shift with compliance level. In this study we provide a map for when these effects will meaningfully influence force capacity and an example of their contributions to force production during a static task, namely standing.

Limb Kinematics, Kinetics and Muscle Dynamics During the Sit-to-Stand Transition in Greyhounds.

Standing up from a prone position is a critical daily activity for animals: failing to do so effectively may cause an injurious fall or increase predation susceptibility. This sit-to-stand behaviour (StS) is biomechanically interesting because it necessitates transitioning through near-maximal joint motion ranges from a crouched (i.e., poor mechanical advantage) to a more upright posture. Such large joint excursions should require large length changes of muscle-tendon units. Here we integrate experimental and musculoskeletal simulation methods to quantify the joint motions, limb forces, and muscle fibre forces, activations and length changes during StS in an extreme athlete—the greyhound—which has large hindlimb muscles bearing short-fibred distal muscles and long tendons. Study results indicate that hindlimb anti-gravity muscle fibres operate near their ~50% limits of length change during StS; mostly by starting at highly lengthened positions. StS also requires high muscle activations (>50%), in part due to non-sagittal motions. Finally, StS movements require passive non-muscular support in the distal hindlimb where short-fibred muscles are incapable of sustaining StS themselves. Non-locomotor behaviours like StS likely impose important trade-offs between muscle fibre force capacity and length changes, as well as active and passive mechanisms of support, that have been neglected in locomotor biomechanics studies.