The Achilles tendon (AT) consists of fibers originating from the soleus muscle (SOL), which lies deep, and the medial (GM) and lateral (GL) heads of the gastrocnemius muscle, which lie superficial. As the fibers descend toward the insertion of the AT, the individual subtendons twist around each other. The aim of this study was to investigate the twisted structure of the AT and its individual subtendons. Specimens of the AT, with preserved calcaneal bone and a fragment of the triceps surae muscle, were obtained from 53 fresh-frozen, male cadavers (n=106 lower limbs). The angle of torsion of each of the AT's subtendons was measured using a specially designed and 3D-printed tool. The mean distance between the most distal fibers of the triceps surae muscle and the superior border of the calcaneal bone was 60.77±14.15 mm. The largest component of the AT at the level of its insertion into the calcaneal bone is the subtendon from the GL (44.43%), followed by the subtendon from SOL (27.89%), and the subtendon from GM (27.68%). The fibers originating from the GM rotate on average 28.17±15.15°, while the fibers originating from the GL and SOL twist 135.98±33.58° and 128.58±29.63°, respectively. The torsion of superficial fibers (GM) comprising the AT is significantly lower than that of deeper fibers (GL and SOL). The cross-sectional area of the AT is smaller at the level of the musculo-tendinous junction than at the level of its insertion. This study illustrates the three types of the AT with differently twisting subtendons, as well as a generalized model of the AT. Types of AT torsion may potentially alter the biomechanical properties of the tendon, thus possibly influencing the pathophysiologic mechanisms leading to the development of various tendinopathies.

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A study in accelerated humidity–temperature ageing and it is numerical modelling for short fibre reinforced polymer composites (SFRPC) based on poly(butylene terephthalate) (PBT) is reported. Authors described experimental results of humidity–temperature ageing of PBT reinforced with glass fibres and proposed a novel computation method of strength and durability analysis for SFRPC parts. Experimental results showed different ageing behaviours, which depend on fibre alignment, e.g. a decrease of Young’s modulus in longitudinal fibre alignment in tension after ageing, an increase of Young’s modulus in transverse direction in tension after ageing, and the increase of the shear modulus and decrease of shear strength after ageing in both directions. Proposed modelling procedure takes the fibre orientation from mould filling analysis as an independent material orientation, using a developed ageing dependent material model, based on tensile, compressive, and shear properties for longitudinal and transverse fibre alignments, and calculates failure criteria as a function of the ageing time and fibre alignment. An innovative approach is to create a fibre alignment dependent material ageing model which takes into account changes of material properties depending on the direction of the reinforcement. This methodology was extended to arbitrary models and validated on real parts made of SFRPC.