Response to fatigue protocol on running ground reaction forces in individuals with anterior cruciate ligament reconstruction and pronated feet: a cross-sectional observational study

This study examined how running-induced fatigue protocol affects ground reaction forces (GRFs) in individuals with anterior cruciate ligament reconstruction (ACLR) and pronated feet (ACLR-PF) at 6-, 12-, and 18-month post-surgery, with the aim of identifying potentially high-risk loading patterns rather than predicting long-term joint outcomes. Forty-five males with ACLR-PF were divided into three groups based on post-surgery time (6, 12, and 18-months) and compared to 15 healthy controls. This study used a cross-sectional observational design. Pronated foot posture was confirmed using the Foot Posture Index (FPI ≥ 6) and navicular drop (> 10 mm). A standardized running-induced fatigue protocol was performed on a treadmill, and fatigue was operationally defined as the completion of this protocol. GRFs were recorded using a force plate during running at 3.2 m/s immediately before (pre-fatigue) and immediately after (post-fatigue) the fatigue protocol, and the pre–post changes in GRFs were used as the objective fatigue-related parameters. Significant group-by-fatigue interactions were observed for several GRF parameters. The 6-month ACLR-PF group showed a more pronounced high-risk loading response to fatigue, with notable increases in impact vertical peak force (p = 0.016; η²=0.167, large effect), medial force (p = 0.042; η²=0.135, moderate effect), and loading rate (p = 0.002; η²=0.232, large effect), along with shorter time-to-peak forces. Conversely, the 12- and 18-month ACLR-PF groups displayed a different adaptive response under fatigue, with decreases in impact vertical peak force and loading rate. Nonetheless, all ACLR-PF groups continued to exhibit biomechanical deviations from the healthy control group regardless of time since surgery. The biomechanical response to fatigue in ACLR-PF individuals varies across recovery stages. Early after surgery (6-months), fatigue appears to exacerbate potentially high-risk loading patterns, whereas in later stages (12 and 18-months) force-reducing compensatory strategies may emerge. Because of the cross-sectional design, these findings should not be interpreted as evidence of causality or as predictors of future osteoarthritis. Despite partial adaptations, movement patterns were not fully normalized by 18 months. Rehabilitation programs may benefit from incorporating fatigue-resistant training tailored to recovery stage to help reduce excessive joint loading that may contribute to long-term joint problems. This study used a cross-sectional observational design and conformed to the ethical guidelines of the latest version of the Declaration of Helsinki, and the procedures were approved by the Ethics Committee of the University of Mohaghegh Ardabili, Iran (IR.UMA.REC.1404.019).