Browsing by Author "Siebert, Tobias"

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Open Access
3D ultrasound-based determination of skeletal muscle fascicle orientations
(2024) Sahrmann, Annika S.; Vosse, Lukas; Siebert, Tobias; Handsfield, Geoffrey G.; Röhrle, Oliver
Architectural parameters of skeletal muscle such as pennation angle provide valuable information on muscle function, since they can be related to the muscle force generating capacity, fiber packing, and contraction velocity. In this paper, we introduce a 3D ultrasound-based workflow for determining 3D fascicle orientations of skeletal muscles. We used a custom-designed automated motor driven 3D ultrasound scanning system for obtaining 3D ultrasound images. From these, we applied a custom-developed multiscale-vessel enhancement filter-based fascicle detection algorithm and determined muscle volume and pennation angle. We conducted trials on a phantom and on the human tibialis anterior (TA) muscle of 10 healthy subjects in plantarflexion (157 ± 7 ∘), neutral position (109 ± 7 ∘, corresponding to neutral standing), and one resting position in between (145 ± 6 ∘). The results of the phantom trials showed a high accuracy with a mean absolute error of 0.92 ± 0.59 ∘. TA pennation angles were significantly different between all positions for the deep muscle compartment; for the superficial compartment, angles are significantly increased for neutral position compared to plantarflexion and resting position. Pennation angles were also significantly different between superficial and deep compartment. The results of constant muscle volumes across the 3 ankle joint angles indicate the suitability of the method for capturing 3D muscle geometry. Absolute pennation angles in our study were slightly lower than recent literature. Decreased pennation angles during plantarflexion are consistent with previous studies. The presented method demonstrates the possibility of determining 3D fascicle orientations of the TA muscle in vivo.
Open Access
Active exoskeleton reduces erector spinae muscle activity during lifting
(2023) Walter, Tobias; Stutzig, Norman; Siebert, Tobias
Musculoskeletal disorders (MSD) are a widespread problem, often regarding the lumbar region. Exoskeletons designed to support the lower back could be used in physically demanding professions with the intention of reducing the strain on the musculoskeletal system, e.g., by lowering task-related muscle activation. The present study aims to investigate the effect of an active exoskeleton on back muscle activity when lifting weights. Within the framework of the study, 14 subjects were asked to lift a 15 kg box with and without an active exoskeleton which allows the adjustment of different levels of support, while the activity of their M. erector spinae (MES) was measured using surface electromyography. Additionally, the subjects were asked about their overall rating of perceived exertion (RPE) during lifting under various conditions. Using the exoskeleton with the maximum level of support, the muscle activity was significantly lower than without exoskeleton. A significant correlation was found between the exoskeleton’s support level and the reduction of MES activity. The higher the support level, the lower the observed muscle activity. Furthermore, when lifting with the maximum level of support, RPE was found to be significantly lower than without exoskeleton too. A reduction in the MES activity indicates actual support for the movement task and might indicate lower compression forces in the lumbar region. It is concluded that the active exoskeleton supports people noticeably when lifting heavy weights. Exoskeletons seem to be a powerful tool for reducing load during physically demanding jobs and thus, their use might be helpful in lowering the risk of MSD.
Open Access
Architectural model for muscle growth during maturation
(2021) Papenkort, Stefan; Böl, Markus; Siebert, Tobias
Muscle architecture, which includes parameters like fascicle length, pennation angle, and physiological cross-sectional area, strongly influences skeletal muscles' mechanical properties. During maturation, the muscle architecture has to adapt to a growing organism. This study aimed to develop an architectural model capable of predicting the complete 3D fascicle architecture for primarily unipennate muscles of an arbitrary age, based on fascicle data for an initial age. For model development, we collected novel data on 3D muscle architecture of the rabbit (Oryctolagus cuniculus) M. plantaris of eight animals ranging in age from 29 to 106 days. Experimental results show that plantaris muscle belly length increases by 73%, whereas mean fascicle length and mean pennation angle increases by 39 and 14%, respectively. Those changes were incorporated into the model. In addition to the data collected for M. plantaris the predictions of the model were compared to existing literature data of rabbit M. soleus and M. gastrocnemius medialis. With an error of -1.0 ± 8.6% for relative differences in aponeurosis length, aponeurosis width, muscle height, and muscle mass, the model delivered good results matching interindividual differences. For future studies, the model could be utilized to generate realistic architectural data sets for simulation studies.
Open Access
Cervical muscle reflexes during lateral accelerations
(2023) Millard, Matthew; Hunger, Susanne; Broß, Lisa; Fehr, Jörg; Holzapfel, Christian; Stutzig, Norman; Siebert, Tobias
Autonomous vehicles will allow a variety of seating orientations that may change the risk of neck injury during an accident. Having a rotated head at the time of a rear-end collision in a conventional vehicle is associated with a higher risk of acute and chronic whiplash. The change in posture affects both the movement of the head and the response of the muscles. We are studying the reflexes of the muscles of the neck so that we can validate the responses of digital human body models that are used in crash simulations. The neck movements and muscle activity of 21 participants (11 female) were recorded at the Stuttgart FKFS mechanical driving simulator. During the maneuver we recorded the acceleration of the seat and electromyographic (EMG) signals from the sternocleidomastoid (STR) muscles using a Biopac MP 160 system (USA). As intuition would suggest, the reflexes of the muscles of the neck are sensitive to posture and the direction of the acceleration.
Open Access
A comparison of lower body gait kinematics and kinetics between Theia3D markerless and marker-based models in healthy subjects and clinical patients
(2024) D’Souza, Sonia; Siebert, Tobias; Fohanno, Vincent
Three-dimensional (3D) marker-based motion capture is the current gold standard to assess and monitor pathological gait in a clinical setting. However, 3D markerless motion capture based on pose estimation is advancing into the field of gait analysis. This study aims at evaluating the lower-body 3D gait kinematics and kinetics from synchronously recorded Theia3D markerless and CAST marker-based systems. Twelve healthy individuals and 34 clinical patients aged 8-61 years walked at self-selected speed over a 13 m long walkway. Similarity between models was statistically analysed using inter-trial variability, root mean square error, Pearson’s correlation coefficient and Statistical Parametric Mapping. Inter-trial variability was on average higher for clinical patients in both models. Overall, the markerless system demonstrated similar gait patterns although hip and knee rotations were non-comparable. Pelvic anterior tilt was significantly underestimated. Significant differences especially in peak values at specific phases of the gait cycle were observed across all planes for all joints (more so for clinical patients than healthy subjects) as well as in the sagittal powers of the hip, knee and ankle. Theia3D markerless system offers great potential in gait analysis. This study brings awareness to potential clinical users and researchers where they can have confidence, as well as areas where caution should be exercised.
Open Access
Consecutive SSCs increase the SSC effect in skinned rat muscle fibres
(2025) Elst, Tobias; Weidner, Sven; Tomalka, André; Hahn, Daniel; Paternoster, Florian Kurt; Seiberl, Wolfgang; Siebert, Tobias
Muscle function is essential for generating force and movement, with stretch-shortening cycles (SSCs) playing a fundamental role in the economy of everyday locomotion. Compared with pure shortening contractions, the SSC effect is characterised by increased force, work, and power output during the SSC shortening phase. Few studies have investigated whether SSC effects transfer across consecutive SSCs. Therefore, we investigated SSC effects over three consecutive SSCs in skinned rat muscle fibres by analysing the isometric force immediately before stretch onset (Fonset), the peak force at the end of stretching (Fpeak), and the minimum force at the end of shortening (Fmin), along with mechanical (WorkSSC) and shortening work (WorkSHO) at different activation levels (20%, 60%, and 100%). Each SSC was followed by an isometric hold phase, allowing force to return to a steady state. Results indicated an increase in both Fpeak (20.3%) and WorkSSC (60.9%) from SSC1 to SSC3 across all activation levels tested. At 20% and 60% activation, Fonset, Fmin, and WorkSHO increased (range: 4.5-28.5%) from SSC1 to SSC3. However, at 100% activation, Fonset and WorkSHO remained unchanged, while Fmin decreased (- 8.5%) from SSC1 to SSC3. These results suggest that the increase in SSC effects at submaximal activation may be primarily due to increased cross-bridge forces. The absence of increases in Fonset, Fmin, and WorkSHO at 100% activation suggests that increases in Fpeak and WorkSSC may not be attributed to increased cross-bridge force but could instead be caused by additional effects, possibly involving modulation of non-cross-bridge structures, likely titin, and their stiffness.
Open Access
Considerations on the human Achilles tendon moment arm for in vivo triceps surae muscle-tendon unit force estimates
(2020) Holzer, Denis; Paternoster, Florian Kurt; Hahn, Daniel; Siebert, Tobias; Seiberl, Wolfgang
Moment arm-angle functions (MA-a-functions) are commonly used to estimate in vivo muscle forces in humans. However, different MA-a-functions might not only influence the magnitude of the estimated muscle forces but also change the shape of the muscle’s estimated force-angle relationship (F-a-r). Therefore, we investigated the influence of different literature based Achilles tendon MA-a-functions on the triceps surae muscle-tendon unit F-a-r. The individual in vivo triceps torque-angle relationship was determined in 14 participants performing maximum voluntary fixed-end plantarflexion contractions from 18.3° ± 3.2° plantarflexion to 24.2° ± 5.1° dorsiflexion on a dynamometer. The resulting F-a-r were calculated using 15 literature-based in vivo Achilles tendon MA-a-functions. MA-a-functions affected the F-a-r shape and magnitude of estimated peak active triceps muscle-tendon unit force. Depending on the MA-a-function used, the triceps was solely operating on the ascending limb (n = 2), on the ascending limb and plateau region (n = 12), or on the ascending limb, plateau region and descending limb of the F-a-r (n = 1). According to our findings, the estimated triceps muscle–tendon unit forces and the shape of the F-a-r are highly dependent on the MA-a-function used. As these functions are affected by many variables, we recommend using individual Achilles tendon MA-a-functions, ideally accounting for contraction intensity-related changes in moment arm magnitude.
Open Access
Cross-bridge mechanics estimated from skeletal muscles’ work-loop responses to impacts in legged locomotion
(2021) Christensen, Kasper B.; Günther, Michael; Schmitt, Syn; Siebert, Tobias
Legged locomotion has evolved as the most common form of terrestrial locomotion. When the leg makes contact with a solid surface, muscles absorb some of the shock-wave accelerations (impacts) that propagate through the body. We built a custom-made frame to which we fixated a rat (Rattus norvegicus, Wistar) muscle (m. gastrocnemius medialis and lateralis: GAS) for emulating an impact. We found that the fibre material of the muscle dissipates between 3.5 and 23μJ ranging from fresh, fully active to passive muscle material, respectively. Accordingly, the corresponding dissipated energy in a half-sarcomere ranges between 10.4 and 68zJ, respectively. At maximum activity, a single cross-bridge would, thus, dissipate 0.6% of the mechanical work available per ATP split per impact, and up to 16% energy in common, submaximal, activities. We also found the cross-bridge stiffness as low as 2.2pNnm-1, which can be explained by the Coulomb-actuating cross-bridge part dominating the sarcomere stiffness. Results of the study provide a deeper understanding of contractile dynamics during early ground contact in bouncy gait.
Open Access
Decoupling of muscle‐tendon unit and fascicle velocity contributes to the in vivo stretch‐shortening cycle effect in the male human triceps surae muscle
(2024) Holzer, Denis; Hahn, Daniel; Schwirtz, Ansgar; Siebert, Tobias; Seiberl, Wolfgang
During the shortening of stretch‐shortening cycles (SSCs), muscle force output is enhanced compared with pure shortening (SHO), referred to as the SSC‐effect. In general, muscle‐tendon unit (MTU), muscle belly, muscle fascicle, and tendon length changes can be decoupled during contraction, which affects force generation and elastic recoil. We researched whether MTU decoupling contributes to the SSC‐effect. Participants performed electrically stimulated submaximal fixed‐end, SSC, and SHO plantar‐flexions on a dynamometer at two velocities (40, 120°/s) and two ranges of motion (15, 25°). Fascicle and tendon length changes of the gastrocnemius medialis, and ankle joint kinematics were assessed by ultrasound and motion capture, respectively. During SSC shortening, ankle joint torque and work, MTU force and work, and fascicle force were increased by 12%-22% compared with SHO, confirming a SSC‐effect. Further, fascicle length change and velocity during SSCs were significantly reduced compared with SHO condition, and SSC fascicle work was decreased by ~35%. Our results indicate that MTU decoupling leads to a reduction in fascicle shortening amplitude and velocity, thereby increasing the muscle's force capacity while reducing its work output during SSC shortening. MTU decoupling therefore contributes to the SSC‐effect and underlines the limited transferability of joint work measurements to estimated muscle work.
Open Access
Determination of muscle shape deformations of the tibialis anterior during dynamic contractions using 3D ultrasound
(2024) Sahrmann, Annika S.; Vosse, Lukas; Siebert, Tobias; Handsfield, Geoffrey G.; Röhrle, Oliver
Purpose: In this paper, we introduce a novel method for determining 3D deformations of the human tibialis anterior (TA) muscle during dynamic movements using 3D ultrasound. Materials and Methods: An existing automated 3D ultrasound system is used for data acquisition, which consists of three moveable axes, along which the probe can move. While the subjects perform continuous plantar- and dorsiflexion movements in two different controlled velocities, the ultrasound probe sweeps cyclically from the ankle to the knee along the anterior shin. The ankle joint angle can be determined using reflective motion capture markers. Since we considered the movement direction of the foot, i.e., active or passive TA, four conditions occur: slow active, slow passive, fast active, fast passive. By employing an algorithm which defines ankle joint angle intervals, i.e., intervals of range of motion (ROM), 3D images of the volumes during movement can be reconstructed. Results: We found constant muscle volumes between different muscle lengths, i.e., ROM intervals. The results show an increase in mean cross-sectional area (CSA) for TA muscle shortening. Furthermore, a shift in maximum CSA towards the proximal side of the muscle could be observed for muscle shortening. We found significantly different maximum CSA values between the fast active and all other conditions, which might be caused by higher muscle activation due to the faster velocity. Conclusion: In summary, we present a method for determining muscle volume deformation during dynamic contraction using ultrasound, which will enable future empirical studies and 3D computational models of skeletal muscles.
Open Access
Editorial - the stretch-shortening cycle of active muscle and muscle-tendon complex : what, why and how it increases muscle performance?
(2021) Seiberl, Wolfgang; Hahn, Daniel; Power, Geoffrey A.; Fletcher, Jared R.; Siebert, Tobias
Open Access
The effect of multidirectional loading on contractions of the M. medial gastrocnemius
(2021) Ryan, David S.; Stutzig, Norman; Helmer, Andreas; Siebert, Tobias; Wakeling, James M.
Research has shown that compression of muscle can lead to a change in muscle force. Most studies show compression to lead to a reduction in muscle force, although recent research has shown that increases are also possible. Based on methodological differences in the loading design between studies, it seems that muscle length and the direction of transverse loading influence the effect of muscle compression on force production. Thus, in our current study we implement these two factors to influence the effects of muscle loading. In contrast to long resting length of the medial gastrocnemius (MG) in most studies, we use a shorter MG resting length by having participant seated with their knees at a 90° angle. Where previous studies have used unidirectional loads to compress the MG, in this study we applied a multidirectional load using a sling setup. Multidirectional loading using a sling setup has been shown to cause muscle force reductions in previous research. As a result of our choices in experimental design we observed changes in the effects of muscle loading compared to previous research. In the present study we observed no changes in muscle force due to muscle loading. Muscle thickness and pennation angle showed minor but significant increases during contraction. However, no significant changes occurred between unloaded and loaded trials. Fascicle thickness and length showed different patterns of change compared to previous research. We show that muscle loading does not result in force reduction in all situations and is possibly linked to differences in muscle architecture and muscle length.
Open Access
Fast and powerful: biomechanics and bite forces of the mandibles in the American cockroach Periplaneta americana
(2015) Weihmann, Tom; Reinhardt, Lars; Weißing, Kevin; Siebert, Tobias; Wipfler, Benjamin
Knowing the functionality and capabilities of masticatory apparatuses is essential for the ecological classification of jawed organisms. Nevertheless insects, especially with their outstanding high species number providing an overwhelming morphological diversity, are notoriously underexplored with respect to maximum bite forces and their dependency on the mandible opening angles. Aiming for a general understanding of insect biting, we examined the generalist feeding cockroach Periplaneta americana, characterized by its primitive chewing mouth parts. We measured active isometric bite forces and passive forces caused by joint resistance over the entire mandibular range with a custom-built 2D force transducer. The opening angle of the mandibles was quantified by using a video system. With respect to the effective mechanical advantage of the mandibles and the cross-section areas, we calculated the forces exerted by the mandible closer muscles and the corresponding muscle stress values. Comparisons with the scarce data available revealed close similarities of the cockroaches’ mandible closer stress values (58 N/cm2) to that of smaller specialist carnivorous ground beetles, but strikingly higher values than in larger stag beetles. In contrast to available datasets our results imply the activity of faster and slower muscle fibres, with the latter becoming active only when the animals chew on tough material which requires repetitive, hard biting. Under such circumstances the coactivity of fast and slow fibres provides a force boost which is not available during short-term activities, since long latencies prevent a specific effective employment of the slow fibres in this case.
Open Access
Impact of body weight and age on plantar pressure in typically developing children : normative data and methodological considerations
(2025) Behrendt, Anika; Siebert, Tobias; D’Souza, Sonia
Purpose: Pedobarography is frequently employed for the identification and characterisation of foot pathologies in paediatrics. However, the lack of standardised normalisation methods presents a challenge for cross-age comparisons. This cross-sectional study provides normative plantar pressure data for typically developing children aged 4-17 years and compares normalisations and explanatory powers of parameters measuring peak and total load.
Methods: Dynamic foot pressure of 101 typically developing children aged 4-17 years was measured at self-selected speed using the mid-gait protocol. They were divided into five age groups: 4-6, 7-8, 9-11, 12-14 and 15–17 years old. Force and pressure variables measuring peak and total load were normalised by body weight or scaled by maximum value and the foot region where the peak pressure occurred was identified.
Results: The absolute values demonstrated an increase in load with advancing age. In contrast, when normalised to body weight, peak pressure and pressure time integrals decreased. The scaled peak pressure showed a load shift to the forefoot. The results indicate that the normalised parameters exhibit superior qualitative significance, suggesting a more dynamic gait pattern and improved morphology of the foot in relation to body weight with increasing age.
Conclusions: This study shows that standardisation of the measurement protocol is imperative because results in typically developing children can vary depending up parameter selection and normalisation technique.
Level of evidence: 3
Open Access
Impact of contraction intensity and ankle joint angle on calf muscle fascicle length and pennation angle during isometric and dynamic contractions
(2024) Coenning, Corinna; Rieg, Volker; Siebert, Tobias; Wank, Veit
During muscle contraction, not only are the fascicles shortening but also the pennation angle changes, which leads to a faster contraction of the muscle than of its fascicles. This phenomenon is called muscle gearing, and it has a direct influence on the force output of the muscle. There are few studies showing pennation angle changes during isometric and concentric contractions for different contraction intensities and muscle lengths. Therefore, the aim was to determine these influences over a wide range of contraction intensities and ankle joint angles for human triceps surae. Additionally, the influence of contraction intensity and ankle joint angle on muscle gearing was evaluated. Ten sport students performed concentric and isometric contractions with intensities between 0 and 90% of the maximum voluntary contraction and ankle joint angles from 50° to 120°. During these contractions, the m. gastrocnemius medialis and lateralis and the m. soleus were recorded via ultrasound imaging. A nonlinear relationship between fascicle length and pennation angle was discovered, which can be described with a quadratic fit for each of the muscles during isometric contraction. A nearly identical relationship was detected during dynamic contraction. The muscle gearing increased almost linearly with contraction intensity and ankle joint angle.
Open Access
Impact of lengthening velocity on the generation of eccentric force by slow-twitch muscle fibers in long stretches
(2024) Weidner, Sven; Tomalka, André; Rode, Christian; Siebert, Tobias
After an initial increase, isovelocity elongation of a muscle fiber can lead to diminishing (referred to as Give in the literature) and subsequently increasing force. How the stretch velocity affects this behavior in slow-twitch fibers remains largely unexplored. Here, we stretched fully activated individual rat soleus muscle fibers from 0.85 to 1.3 optimal fiber length at stretch velocities of 0.01, 0.1, and 1 maximum shortening velocity, vmax, and compared the results with those of rat EDL fast-twitch fibers obtained in similar experimental conditions. In soleus muscle fibers, Give was 7%, 18%, and 44% of maximum isometric force for 0.01, 0.1, and 1 vmax, respectively. As in EDL fibers, the force increased nearly linearly in the second half of the stretch, although the number of crossbridges decreased, and its slope increased with stretch velocity. Our findings are consistent with the concept of a forceful detachment and subsequent crossbridge reattachment in the stretch’s first phase and a strong viscoelastic titin contribution to fiber force in the second phase of the stretch. Interestingly, we found interaction effects of stretch velocity and fiber type on force parameters in both stretch phases, hinting at fiber type-specific differences in crossbridge and titin contributions to eccentric force. Whether fiber type-specific combined XB and non-XB models can explain these effects or if they hint at some not fully understood properties of muscle contraction remains to be shown. These results may stimulate new optimization perspectives in sports training and provide a better understanding of structure-function relations of muscle proteins.
Open Access
Impact of multidirectional transverse calf muscle loading on calf muscle force in young adults
(2018) Siebert, Tobias; Eb, Manuel; Ryan, David S.; Wakeling, James M.; Stutzig, Norman
It has been demonstrated that unidirectional transversal muscle loading induced by a plunger influences muscle shape and reduces muscle force. The interaction between muscle and transversal forces may depend on specific neuromuscular properties that change during a lifetime. Compression garments, applying forces from all directions in the transverse plane, are widely used in sports for example to improve performance. Differences in the loading direction (unidirectional vs. multidirectional) may have an impact on force generating capacity of muscle and, thus, on muscle performance. The aim of this study was to examine the effect of multidirectional transversal loads, using a sling looped around the calf, on the isometric force during plantarflexions. Young male adults (25.7 ± 1.5 years, n = 15) were placed in a prone position in a calf press apparatus. The posterior tibial nerve was stimulated to obtain the maximal double-twitch force of the calf muscles with (59.4 N and 108.4 N) and without multidirectional transverse load. Compared to the unloaded condition, the rate of force development was reduced by 5.0±8.1 % (p=0.048) and 6.9±10.7 % (p=0.008) for the 59.4 N and the 108.4 N load, respectively. No significant reduction (3.2±4.8 %, p=0.141) in maximum muscle force (Fm) was found for the lower load (59.4 N), but application of the higher load (108.4 N) resulted in a significant reduction of Fm by 4.8±7.0 % (p=0.008). Mean pressures induced in this study (14.3 mm Hg and 26.3 mm Hg corresponding to the 59.4 N and 108.4 N loads, respectively) are within the pressure range reported for compression garments. Taking the results of the present study into account, a reduction in maximum muscle force would be expected for compression garments with pressures ≥ 26.3 mm Hg. However, it should be noted that the loading condition (sling vs. compression garment) differs and that compression garments may influence other mechanisms contributing to force generation. For example, wearing compression garments may enhance sport performance by enhanced proprioception and reduced muscle oscillation. Thus, superposition of several effects should be considered when analyzing the impact of compression garments on more complex sport performance.
Open Access
Improving the accuracy of musculotendon models for the simulation of active lengthening
(2023) Millard, Matthew; Kempter, Fabian; Stutzig, Norman; Siebert, Tobias; Fehr, Jörg
Vehicle accidents can cause neck injuries which are costly for individuals and society. Safety systems could be designed to reduce the risk of neck injury if it were possible to accurately simulate the tissue-level injuries that later lead to chronic pain. During a crash, reflexes cause the muscles of the neck to be actively lengthened. Although the muscles of the neck are often only mildly injured, the forces developed by the neck’s musculature affect the tissues that are more severely injured. In this work, we compare the forces developed by MAT_156, LS-DYNA’s Hill-type model, and the newly proposed VEXAT muscle model during active lengthening. The results show that Hill-type muscle models underestimate forces developed during active lengthening, while the VEXAT model can more faithfully reproduce experimental measurements.
Open Access
Influence of layer separation on the determination of stomach smooth muscle properties
(2021) Borsdorf, Mischa; Böl, Markus; Siebert, Tobias
Uniaxial tensile experiments are a standard method to determine the contractile properties of smooth muscles. Smooth muscle strips from organs of the urogenital and gastrointestinal tract contain multiple muscle layers with different muscle fiber orientations, which are frequently not separated for the experiments. During strip activation, these muscle fibers contract in deviant orientations from the force-measuring axis, affecting the biomechanical characteristics of the tissue strips. This study aimed to investigate the influence of muscle layer separation on the determination of smooth muscle properties. Smooth muscle strips, consisting of longitudinal and circumferential muscle layers (whole-muscle strips [WMS]), and smooth muscle strips, consisting of only the circumferential muscle layer (separated layer strips [SLS]), have been prepared from the fundus of the porcine stomach. Strips were mounted with muscle fibers of the circumferential layer inline with the force-measuring axis of the uniaxial testing setup. The force–length (FLR) and force–velocity relationships (FVR) were determined through a series of isometric and isotonic contractions, respectively. Muscle layer separation revealed no changes in the FLR. However, the SLS exhibited a higher maximal shortening velocity and a lower curvature factor than WMS. During WMS activation, the transversally oriented muscle fibers of the longitudinal layer shortened, resulting in a narrowing of this layer. Expecting volume constancy of muscle tissue, this narrowing leads to a lengthening of the longitudinal layer, which counteracted the shortening of the circumferential layer during isotonic contractions. Consequently, the shortening velocities of the WMS were decreased significantly. This effect was stronger at high shortening velocities.
Open Access
A muscle model for injury simulation
(2023) Millard, Matthew; Kempter, Fabian; Fehr, Jörg; Stutzig, Norman; Siebert, Tobias
Car accidents frequently cause neck injuries that are painful, expensive, and difficult to simulate. The movements that lead to neck injury include phases in which the neck muscles are actively lengthened. Actively lengthened muscle can develop large forces that greatly exceed the maximum isometric force. Although Hill-type models are often used to simulate human movement, this model has no mechanism to develop large tensions during active lengthening. When used to simulate neck injury, a Hill model will underestimate the risk of injury to the muscles but may overestimate the risk of injury to the structures that the muscles protect. We have developed a musculotendon model that includes the viscoelasticity of attached crossbridges and has an active titin element. In this work we evaluate the proposed model to a Hill model by simulating the experiments of Leonard et al. [1] that feature extreme active lengthening.