A fibre digitisation study of the architecture of the converging components of the Achilles tendon in a neonatal cadaveric sample

Abstract

The gastrocnemius and soleus muscles combine distally as the calcaneal (Achilles) tendon, forming the triceps surae muscle-tendon unit, which is fundamental for ambulation. Despite this essential role, the architectural development of the triceps surae from birth to the onset of weight-bearing, a period of rapid neuromuscular change, remains poorly characterised in developmental anatomy. This study therefore aimed to construct detailed three-dimensional (3D) models of the triceps surae musculoaponeurotic and tendinous architecture in zero-day-old neonatal anatomical donors, to identify innate and adaptive features of this unit. Eight formalin-fixed neonatal gastrocnemius and soleus muscle-tendon units were volumetrically reconstructed via serial coordinate-point sampling and dissection. Coordinate points were captured with digitising hardware (MicroScribe® I, Revware Inc., Raleigh, NC 27675-0786, 2018, USA) and translated into 3D models in specialised software (Maya®, Autodesk, Inc., 2024, San Rafael, CA, USA), with custom plug-ins, enabling quantification of functional architectural parameters (i.e. fibre bundle length, pennation angle, muscle volume, physiological cross-sectional area) and morphological analyses. Nerve branching and connective tissue structures (i.e., aponeuroses and tendons) were comparable to adult records. In contrast, architectural features such as bundle length and physiological cross-sectional area displayed adaptive differences between neonatal and older age groups, suggesting early adaptation to future functional demands. These records represent the first normative architectural dataset of the neonatal triceps surae, demonstrating that architectural specialisation for force production and excursion is present at birth, while features contributing to stabilisation develop with growth and weight-bearing. By quantifying architectural parameters at the fibre bundle level, this study provides the most detailed records of muscle architecture in a previously unrepresented age group. The findings offer a foundation for future studies in functional biomechanics, developmental pathology, and clinical intervention.