Packing of muscles in the rabbit shank influences three-dimensional architecture of M. soleus.

Isolated and packed muscles (e.g. in the calf) exhibit different three-dimensional muscle shapes. In packed muscles, cross-sections are more angular compared to the more elliptical ones in isolated muscles. As far as we know, it has not been examined yet, whether the shape of the muscle in its packed condition influences its internal arrangement of muscle fascicles and accordingly the contraction behavior in comparison to the isolated condition. To evaluate the impact of muscle packing, we examined the three-dimensional muscle architecture of isolated and packed rabbit M. soleus for different ankle angles (65°, 75°, 85°, 90°, and 95°) using manual digitization (MicroScribe® MLX). In general, significantly increased values of pennation angle and fascicle curvature were found in packed compared to isolated M. soleus (except for fascicle curvature at 90° ankle angle). On average, fascicle length of isolated muscles exceeded fascicle lengths of packed muscles by 2.6%. Reduction of pennation angle in the packed condition had only marginal influence on force generation (about 1% of maximum isometric force) in longitudinal direction (along the line of action) although an increase of transversal force component (perpendicular to the line of action) of about 26% is expected. Results of this study provide initial evidence that muscle packing limits maximum muscle performance observed in isolated M. soleus. Besides an enhanced understanding of the impact of muscle packing on architectural parameters, the outcomes of this study are essential for realistic three-dimensional muscle modeling and model validation.

Novel microstructural findings in M. plantaris and their impact during active and passive loading at the macro level.

There are several studies dealing with experimental and structural analyses of skeletal muscles that are aimed at gaining a better understanding of three-dimensional muscle deformation and force generation. A variety of these contributions have performed structural or mechanical analyses, but very few have combined these approaches at different levels. To fill this gap, the present study aims to bring together three-dimensional micro-structural and mechanical findings in rabbit M. plantaris to study load transfer mechanisms inside the muscle during passive loading and active muscle contraction. During these two deformation states, the three-dimensional surface of the aponeurosis-tendon complex was recorded using optical measurement systems. In this way, the strain distribution on the muscle can be calculated to interpret the load transfer mechanisms inside the muscle. The results show that the three-dimensional strain distribution during muscle activation is completely different from the distribution during passive loading. Under both loading conditions, the strain distribution is irregular. To interpret these findings, the gross try and the fascicle architecture of the M. plantaris were determined. In doing so, a highly complex microstructure featuring tube- and sail-like structure was identified. Moreover, a compartmentalisation of the muscle into two compartments was detected. The smaller, bipennated muscle compartment was embedded into the larger, unipennated compartment. To the authors' knowledge, this type of inner structure has never been previously documented in single-headed muscles.

Three-Dimensional Muscle Architecture and Comprehensive Dynamic Properties of Rabbit Gastrocnemius, Plantaris and Soleus: Input for Simulation Studies.

The vastly increasing number of neuro-muscular simulation studies (with increasing numbers of muscles used per simulation) is in sharp contrast to a narrow database of necessary muscle parameters. Simulation results depend heavily on rough parameter estimates often obtained by scaling of one muscle parameter set. However, in vivo muscles differ in their individual properties and architecture. Here we provide a comprehensive dataset of dynamic (n = 6 per muscle) and geometric (three-dimensional architecture, n = 3 per muscle) muscle properties of the rabbit calf muscles gastrocnemius, plantaris, and soleus. For completeness we provide the dynamic muscle properties for further important shank muscles (flexor digitorum longus, extensor digitorum longus, and tibialis anterior; n = 1 per muscle). Maximum shortening velocity (normalized to optimal fiber length) of the gastrocnemius is about twice that of soleus, while plantaris showed an intermediate value. The force-velocity relation is similar for gastrocnemius and plantaris but is much more bent for the soleus. Although the muscles vary greatly in their three-dimensional architecture their mean pennation angle and normalized force-length relationships are almost similar. Forces of the muscles were enhanced in the isometric phase following stretching and were depressed following shortening compared to the corresponding isometric forces. While the enhancement was independent of the ramp velocity, the depression was inversely related to the ramp velocity. The lowest effect strength for soleus supports the idea that these effects adapt to muscle function. The careful acquisition of typical dynamical parameters (e.g. force-length and force-velocity relations, force elongation relations of passive components), enhancement and depression effects, and 3D muscle architecture of calf muscles provides valuable comprehensive datasets for e.g. simulations with neuro-muscular models, development of more realistic muscle models, or simulation of muscle packages.