606 September & October 2025. Volume 14. Num 4 Background and Aims Given the negave impact of chronic ankle instability (CAI) on the quality of life and daily motor funcon, this study aims to compare the acve tensile force of the peroneus longus muscle during the gait iniaon between athletes with CAI and healthy controls using musculoskeletal modeling in OpenSim soſtware, , version 3.2. Methods In this cross-seconal study, 19 professional male and female athletes with CAI (aged 18–25 years) with a history of at least two ankle sprains in the past 3-12 months, and 19 healthy athletes (aged 18–25 years) parcipated. Parcipants stood on a force plate and began walking upon hearing an auditory cue. Using the gait2392 model in OpenSim, the force generated by the peroneus longus muscle during gait iniaon was calculated. Addionally, spaotemporal parameters (stride length, stride speed, stride me) and the ancipatory postural adjustment (APA) phase duraon were extracted based on center of pressure (COP) data and surface marker posions. Results The results showed that the peroneus longus muscle force (P=0.006) and the APA phase duraon (P=0.002) were significantly lower in the CAI group compared to the healthy group. Conversely, stride length (P=0.001), stride speed (P=0.001), and stride me (P=0.04) were significantly higher in the CAI group than in the control group. Conclusion The athletes with CAI have a lower acve tensile force of the peroneus longus muscle and a shorter APA duraon during gait iniaon compared to healthy peers. This suggests a potenal associaon between impaired propriocepve funcon, weakness of the peroneus longus muscle, and deficits in ancipatory postural control. Moreover, the observed increases in spaotemporal gait parameters may serve as compensatory strategies to migate these funconal impairments. Keywords Peroneus longus muscle, Chronic ankle instability (CAI), Gait iniaon, OpenSim A B S T R A C T Received: 15 Apr 2025 Accepted: 14 May 2025 Available Online: 23 Sep 2025 Citaon Bijari Kh, Sadeghi H, Yousefi M. [Evaluaon of Peroneus Longus Muscle Force in Individuals With Chronic Ankle Instability During Gait Iniaon (Persian)]. Scienfic Journal of Rehabilitaon Medicine. 2025; 14(4):606-617. hps://dx.doi.org/10.32598/ SJRM.14.4.3354 https://dx.doi.org/10.32598/SJRM.14.4.3354 Use your device to scan and read the article online Research Paper Evaluaon of Peroneus Longus Muscle Force in Individuals With Chronic Ankle Instability During Gait Iniaon 1. Department of Sports Biomechanics and Injuries, Faculty of Physical Educaon and Sports Sciences, Kharazmi University, Tehran, Iran. 2. Department of Sports Biomechanics and Rehabilitaon, Kinesiology Research Center, Kharazmi University, Tehran, Iran. 3. Department of Sports Biomechanics and Injuries, Faculty of Physical Educaon and Sports Sciences, Birjand University, Birjand, Iran. Khorshid Bijari 1 , *Hydar Sadeghi 1,2 , Mohammad Yousefi 3 * Corresponding Author: Heydar Sadeghi, Professor. Address: Department of Sports Biomechanics and Injuries, Faculty of Physical Educaon and Sports Sciences, Kharazmi University, Tehran, Iran. Tel: +98 (912) 2453175 E-Mail: h.sadeghi@khu.ac.ir, sadeghih@yahoo.com Copyright © 2025 The Author(s); This is an open access article distributed under the terms of the Creative Commons Attribution License (CC-By-NC: https://creativecommons.org/licenses/by-nc/4.0/legalcode.en), which permits use, distribution, and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.