Paul Medina-González; Karen Moreno; Valentina Bernal-Fernández; Marcelo Gómez-Jaramillo; Rodrigo Muñoz-Cofré & Pedro Aburto-Valdebenito
The passive stay apparatus (PSA) in horses is an example of an efficient biomechanical structure that enables upright posture with minimal muscle activity for extended periods of time. This function is achieved through the synergy of ligaments, tendons, muscles and bone elements. The intermediate tubercle (IT) of the humerus that optimizes the biceps brachii muscle in the forelimb and the medial trochlear ridge (MTR) that blocks the sliding of the patella, are structures established as phylogenetic signals of Equidae that have not been evaluated during the life cycle of horses. We explore the ontogeny of morphological variables of IT and MTR of Chilean horses. We selected a humerus and femur osteotechnics from Chilean horses for ontogenetic stage of newborn, juvenile, sub-adult, adult and senescent. We photographed proximal views of the humerus and distal views of the femur, in which lengths were measured in 2 dimensions using the Tracker 4.11.0 program. In the humerus, the length of the IT, the total length from the center of rotation (TLCR) and its percentage ratio [(TI/TLCR)×100] were measured. In the femur, the length of the MTR, lateral (LTR) and its percentage ratio [(MTR/LTR)×100] were measured. IT/TLCR ratio was for newborn = 0 %; juvenile = 45 %; adult = 54 %; sub-adult = 59 % and senescent = 57 %. MTR/LTR is in newborn = 1.6 %; juvenile = 24 %; adult = 26 % and senescent = 31%. The relative increase in IT and MTR is observed during ontogeny, and this developmental pattern parallels trends observed in the phylogeny of Equidae.
KEY WORDS: Developmental Biology; Equidae; Thoracic limb; Pelvic limb; Biomechanical Phenomena; Paleontology.
MEDINA-GONZÁLEZ, P.; MORENO, K.; BERNAL-FERNÁNDEZ, V.; GÓMEZ-JARAMILLO, M.; MUÑOZ-COFRÉ, R. & ABURTO-VALDEBENITO, P. Ontogeny of morphological adaptations in the passive stay apparatus of Chilean horses: Implications for key phylogenetic structures. Int. J. Morphol., 43(4):1313-1320, 2025.
