The workflow to simulate motion with recorded data usually starts with selecting a generic musculoskeletal model and scaling it to represent subject-specific characteristics. Simulating muscle dynamics with muscle-tendon parameters computed from existing scaling methods in literature, however, yields some inconsistencies compared to measurable outcomes. For instance, simulating fiber lengths and muscle excitations during walking with linearly scaled parameters does not resemble established patterns in the literature. This study presents a tool that leverages reported in vivo experimental observations to tune muscle-tendon parameters and evaluates their influence in estimating muscle excitations and metabolic costs during walking. From a scaled generic musculoskeletal model, we tuned optimal fiber length, tendon slack length, and tendon stiffness to match reported fiber lengths from ultrasound imaging and muscle passive force-length relationships to match reported in vivo joint moment-angle relationships. With tuned parameters, muscle contracted more isometrically, and soleus\'s operating range was better estimated than with linearly scaled parameters. Also, with tuned parameters, on/off timing of nearly all muscles\' excitations in the model agreed with reported electromyographic signals, and metabolic rate trajectories varied significantly throughout the gait cycle compared to linearly scaled parameters. Our tool, freely available online, can customize muscle-tendon parameters easily and be adapted to incorporate more experimental data.

A skeletal muscle\'s peak force production and excursion are based on its architectural properties that are, in turn, determined by its mass, muscle fiber length and physiological cross-sectional area (PCSA). In the classic interspecific study of mammalian muscle scaling, it was demonstrated that muscle mass scales positively allometrically with body mass whereas fiber length scales isometrically with body mass, indicating that larger mammals have stronger leg muscles than they would if they were geometrically similar to smaller ones. Although this relationship is highly significant across species, there has never been a detailed intraspecific architectural scaling study. We have thus created a large dataset of 896 muscles across 34 human lower extremities (18 females and 16 males) with a size range including approximately 90% and 70% of the United States population height and mass, respectively, across the range 36-103 years. Our purpose was to quantify the scaling relationships between human muscle architectural properties and body size. We found that human muscles depart greatly from isometric scaling because muscle mass scales with body mass1.3 (larger exponent than isometric scaling of 1.0) and muscle fiber length scales with negative allometry with body mass0.1 (smaller exponent than isometric scaling of 0.33). Based on the known relationship between architecture and function, these results suggest that human muscles place a premium on muscle force production (mass and PCSA) at the expense of muscle excursion (fiber length) with increasing body size, which has implications for understanding human muscle design as well as biomechanical modeling.

Maintaining meat quality is essential to sustainable livestock management. Therefore, identifying alternative feed materials while considering consumer acceptance is crucial. So, the aim of this study was to evaluate the effect of C. glomerata-biomass-supplemented feeds on rabbit muscles\' physical properties, sensory profiles, and evaluators\' emotional responses to them. A total of thirty 52-day-old weaned Californian breed rabbits were randomly allocated to one of three dietary treatments: standard compound diet (SCD), SCD supplemented with 4% C. glomerata (CG4), or SCD supplemented with 8% C. glomerata (CG8). After the 122-day-old rabbits were slaughtered, post-mortem dissection of the rabbit Longissimus dorsi (LD) and hind leg (HL) muscles was conducted. The physical and histomorphometric features, sensory analyses, and emotional responses to the rabbit\'s muscles were determined. Study results revealed CG4 and CG8 treatments significantly increased rabbit muscle moisture, while CG8 increased cooking losses in HL muscles (p < 0.05). Moreover, both CG treatments reduced the darkness and redness of fresh and cooked rabbit muscles compared to SCD (p < 0.05). CG8 treatment compared to SCD resulted in longer LD muscle fibers (p < 0.05). Evaluators discovered that the average scores for each sensory description of rabbit meat are acceptable and that consuming CG8-HL muscles can increase happiness based on emotional responses. Consequently, replacing traditional feed materials in rabbit feed with C. glomerata can lead to not only more sustainable production but also more consumer-acceptable rabbit meat.

Musculoskeletal models rely heavily on the use of an anatomical dataset and clearly defined assumptions to accurately model the subject being studied. Therefore, it is important to understand the limitations of using musculoskeletal models to study individuals. This paper describes a method of measuring in vivo gracilis muscle-tendon unit length and presents a comparison of experimental data versus predictions from four musculoskeletal models in OpenSim. The largest errors occurred when the knee was fully extended. At this position, the absolute average muscle-tendon unit length error was 7% and the absolute average fiber length error was between 15% and 32%. However, the variability of these errors was significant. Manual linear scaling based on an anthropometric database did not capture the variability observed in subjects. The fiber length errors observed are predicted to have a significant impact on muscle force production that may not represent true subject specific force-length relationship of the gracilis.

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.

The purposes f this study were to (i) explore the possibility of splitting the selected forearm muscles into separate compartments in human subjects; (ii) quantify the architectural properties of each neuromuscular compartment; and (iii) discuss the implication of these properties in split tendon transfer procedures. Twenty upper limbs from 10 fresh human cadavers were used in this study. Ten limbs of five cadavers were used for intramuscular nerve study by modified Sihler\'s staining technique, which confirmed the neuromuscular compartments. The other 10 limbs were included for architectural analysis of neuromuscular compartments. The architectural features of the compartments including muscle weight, muscle length, fiber length, pennation angle, and sarcomere length were determined. Physiological cross-sectional area and fiber length/muscle length ratio were calculated. Five of the selected forearm muscles were ideal candidates for splitting, including flexor carpi ulnaris, flexor carpi radials, extensor carpi radialis brevis, extensor carpi ulnaris and pronator teres. The humeral head of pronator teres contained the longest fiber length (6.23 ± 0.31 cm), and the radial compartment of extensor carpi ulnaris contained the shortest (2.90 ± 0.28 cm). The ulnar compartment of flexor carpi ulnaris had the largest physiological cross-sectional area (5.17 ± 0.59 cm(2)), and the ulnar head of pronator teres had the smallest (0.67 ± 0.06 cm(2)). Fiber length/muscle length ratios of the neuromuscular compartments were relatively low (average 0.27 ± 0.09, range 0.18-0.39) except for the ulnar head of pronator teres, which had the highest one (0.72 ± 0.05). Using modified Sihler\'s technique, this research demonstrated that each compartment of these selected forearm muscles has its own neurovascular supply after being split along its central tendon. Data of the architectural properties of each neuromuscular compartment provide insight into the \'design\' of their functional capability. In addition to improving our understanding of muscle anatomy and function, elucidation of forearm neuromuscular compartments architecture may ultimately provide information useful for selection of muscle subdivisions used in tendon transfer.

BACKGROUND: Second-harmonic generation microendoscopy is a minimally invasive technique to image sarcomeres and measure their lengths in humans, but motion artifact and low signal have limited the use of this novel technique.
METHODS: We discovered that an excitation wavelength of 960 nm maximized image signal; this enabled an image acquisition rate of 3 frames/s, which decreased motion artifact. We then used microendoscopy to measure sarcomere lengths in the human extensor carpi radialis brevis with the wrist at 45° extension and 45° flexion in 7 subjects. We also measured the variability in sarcomere lengths within single fibers.
RESULTS: Average sarcomere lengths in 45° extension were 2.93±0.29 μm (±SD) and increased to 3.58±0.19 μm in 45° flexion. Within single fibers the standard deviation of sarcomere lengths in series was 0.20 μm.
CONCLUSIONS: Microendoscopy can be used to measure sarcomere lengths at different body postures. Lengths of sarcomeres in series within a fiber vary substantially.

The aim of this study was to examine anatomical properties of the adductor magnus through a detailed classification, and to hypothesize its function and size to gather enough information about morphology. Ten cadaveric specimens of the adductor magnus were used. The muscle was separated into four portios (AM1-AM4) based on the courses of the corresponding perforating arteries, and its volume, muscle length, muscle fiber length and physiological cross-sectional area were assessed. The architectural characteristics of these four portions of the adductor magnus were then classified with the aid of principal component analysis. The results led us into demarcating the most proximal part of the adductor magnus (AM1) from the remaining parts (AM2, AM3, and AM4). Classification of the adductor magnus in terms of architectural characteristics differed from the more traditional anatomical distinction. The AM2, AM3, and AM4, having longer muscle fiber lengths than the AM1, appear to be designed as displacers for moving the thigh through a large range of motion. The AM1 appears instead to be oriented principally toward stabilizing the hip joint. The large mass of the adductor magnus should thus be regarded as a complex of functionally differentiable muscle portions.