Fabella evolution and biomechanical effects in human motion

This study explores the biomechanical role of the fabella in human locomotion and its evolutionary history alongside other posterior knee sesamoids within the primate family. While fabella biomechanical effects related to its functions remain underexplored, fabella excisions are commonly performed to address associated knee issues. Understanding the fabella’s evolutionary history within the primate context and its relationship with other posterior knee sesamoids can provide valuable insights into its role in humans. Sesamoids are periarticular skeletal elements associated with tendons and ligaments, exhibiting variable composition as cartilage, fibrocartilage, or bone. The first study employs phylogenetic comparative methods to explore the evolution of knee sesamoids in primates. The findings reveal that knee sesamoids are highly conserved across primates, with their presence/absence uncorrelated to locomotor mode. Moreover, the medial and lateral fabellae frequently co-occur in most primate clades, challenging previous assumptions that linked the lateral fabella with the cyamella. In contrast, the Hominoidea display a decoupled pattern, with humans uniquely retaining only the lateral fabella. This suggests a distinct developmental pathway potentially linked to bipedalism and endurance running. The second and third studies investigate the biomechanical effects of the fabella in human locomotion. In the second study, a portable handheld ultrasound device was employed to identify fabella presence in a healthy population in London, yielding a prevalence rate of 17.33%. This aligns with European prevalence rates from a 2018 meta-analysis. Unlike most clinical studies, which often feature skewed samples, this in vivo study of a healthy population provides robust validation of European prevalence rates. The final study examines the kinematics, kinetics, and muscle activation patterns of individuals with and without fabellae during walking, running, and two-legged hopping. Using a matched-control design, it identifies reduced gastrocnemius activation during running in individuals with fabellae. This finding supports the hypothesis that the fabella enhances the gastrocnemius's moment arm, reducing the force required to IV produce joint movement (mechanical advantage). The results suggest that the fabella’s mechanical advantage is more significant during running than walking or hopping. These findings have evolutionary implications, leading to two hypotheses: first, that bipedalism and endurance running generated the mechanical stimuli necessary for the development of the lateral fabella in humans; and second, that the lateral fabella, once present, may have been selectively retained due to its biomechanical benefits for endurance running.