Relationship between Connective Tissue Morphology and Lower-Limb Stiffness in Endurance Runners. A Prospective Study Rubio Peirotén, Alberto García Pinillos, Felipe Jaén Carrillo, Diego Cartón Llorente, Antonio Abat, Ferrán Roche Seruendo, Luis Enrique Foot behavior Tendon Stretch-shortening cycle Running Background: The lower limb behaves like a spring compressing and decompressing during running, where lower-limb stiffness is one of the most influential factors. This prospective observational study is aimed at examining the relationship between the connective tissue morphology and lower-limb stiffness and investigating whether the barefoot/shod condition influences on such relationship. Methods: 14 male amateur runners (10-km time trial <500 ) were included. Data were recorded over one session, where participants ran 2 trials (i.e., barefoot and shod conditions) of 3 minutes at 12 km/h, where running spatiotemporal parameters and vertical (Kvert) and leg stiffness (Kleg) were obtained. Prior to testing trials, thickness and cross-sectional area (CSA) were recorded for Achilles (AT) and patellar tendons (PT) and plantar fascia (PF) with ultrasound. Results: Under barefoot condition, a positive correlation was found between Kleg and AT-thickness and CSA and PF-thickness; and between Kvert and AT-thickness and PF thickness. Under shod condition, a positive correlation was found between Kleg and PT-CSA and PT-thickness, and between Kvert and PT-CSA and PT-thickness. Conclusions: The results reveal a specificity of the relationship between the lower-limb stiffness and the morphology of the connective tissue. Greater tendon shows higher lower-limb stiffness when that tendon is specially demanded by the function. 2021-09-29T08:30:14Z 2021-09-29T08:30:14Z 2021 journal article Rubio-Peirotén, A.; García-Pinillos, F.; Jaén-Carrillo, D.; Cartón-Llorente, A.; Abat, F.; Roche-Seruendo, L.E. Relationship between Connective Tissue Morphology and Lower-Limb Stiffness in Endurance Runners. A Prospective Study. Int. J. Environ. Res. Public Health 2021, 18, 8453. https:// doi.org/10.3390/ijerph18168453 http://hdl.handle.net/10481/70519 10.3390/ijerph18168453 eng http://creativecommons.org/licenses/by/3.0/es/ open access Atribución 3.0 España MDPI