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.