Regular physical activity, including modalities such as running, has been demonstrated to confer significant health benefits across multiple physiological systems, including the musculoskeletal, cardiovascular, nervous, endocrine, and gastrointestinal systems, thereby enhancing overall quality of life (Fiuza-Luces et al., 2018, Pedisic et al., 2020). Nevertheless, epidemiological data indicate that over 40 % of runners experience running-related musculoskeletal injuries at some point during their participation, with incidence rates notably higher among individuals engaged in high-volume or endurance running activities, such as marathon training and competition (Kakouris et al., 2021). Among marathon runners, the most frequently reported overuse injuries include patellofemoral pain syndrome (10–17 %), iliotibial band syndrome (8–14 %), Achilles tendinopathy (11 %), medial tibial stress syndrome (10 %), lower back pain (9–10 %), plantar fasciitis (8 %), and osteoarthritis (5 %), while acute ankle sprain represents the most common acute injury, comprising approximately 10–15 % of all acute injury cases in this population (Hoffman and Krishnan, 2014, Krabak and Chen, 2020, Small and Relph, 2018, Taunton et al., 2002). Prolonged running imposes substantial physiological demands and is frequently associated with the onset of fatigue, conditions under which it becomes increasingly challenging for runners to maintain their habitual gait patterns (Chen et al., 2022). One notable manifestation of altered gait mechanics under fatigue is increased gait asymmetry. Although a certain degree of gait asymmetry is inherent to repetitive locomotor activities, excessive asymmetry has been identified as a significant risk factor for running-related injuries (Bredeweg et al., 2013). This is particularly pertinent to rearfoot biomechanics, where pronation during the loading response phase facilitates shock absorption via midfoot unlocking and eccentric loading of soft tissues, followed by supination, which stabilizes the foot into a rigid lever to optimize propulsion efficiency (Heiderscheit and Simoneau, 2025).. Given that the majority of running-related injuries are localized to the lower extremities, particularly the ankle–foot complex (Kakouris et al., 2021, Lopes et al., 2012), it is essential to investigate alterations in rearfoot movement symmetry to elucidate the impact of prolonged running on gait mechanics and the underlying pathophysiological mechanisms.

Gait asymmetry influences a range of biomechanical parameters, including fatigue status, injury risk, and energy expenditure. Due to the inherent bilateral function of human locomotion, a certain degree of asymmetry in both kinetic and kinematic variables is naturally present during running, particularly at initial contact and propulsion phases of the stance (Polk et al., 2017). While moderate asymmetry is generally considered benign (Malisoux et al., 2024), prolonged running can exacerbate asymmetrical movement patterns and potentially alter the injury risk profile. Muscular fatigue induced by sustained running has been shown to modify lower limb kinematics across multiple planes of motion, including alterations in ankle dorsiflexion (Mizrahi et al., 2001) and eversion (Koblbauer et al., 2014), tibial internal rotation (Dierks et al., 2010), knee internal rotation and flexion (Derrick et al., 2002), as well as hip flexion (Williams et al., 1991) and adduction (Willwacher et al., 2020). These fatigue-induced changes are often asymmetrically distributed between limbs and can affect all major joints of the lower extremities (Gao et al., 2022a), resulting in uneven load distribution and increased strain on soft tissues. Furthermore, gait asymmetry imposes disproportionate workloads on lower limb actuators and leads to compensatory overactivation of muscles to maintain inter-limb coordination, thereby creating a feedback loop that exacerbates energy expenditure and accelerates the onset of fatigue (Aslani et al., 2016). Collectively, these factors elevate the risk of both acute and chronic musculoskeletal injuries (Fukano et al., 2018). Ultimately, gait asymmetry disrupts the preferred symmetric gait pattern, diminishes mechanical efficiency, and increases net metabolic power requirements (Beck et al., 2018, Melo et al., 2020).

Although substantial progress has been made in the investigation of gait symmetry, several methodological limitations in the existing literature constrain the generalizability and applicability of these findings to the broader running population. Firstly, the majority of studies have been conducted in controlled indoor treadmill environments due to the stationary nature of traditional biomechanical instrumentation. However, evidence indicates that runners exhibit distinct gait patterns (Van Hooren et al., 2020) and asymmetry profiles during outdoor overground running compared to treadmill conditions (Lempke et al., 2025, Robadey et al., 2018). Secondly, most research protocols employ relatively short running distances (typically less than 10 km or 10 min in duration), which may be insufficient to elicit fatigue-induced modifications in running gait (Ament and Verkerke, 2009). Thirdly, the predominant focus of prior studies has been on gait symmetry within the sagittal plane, despite the fact that movement patterns across multiple planes are critical indicators of injury risk and are more susceptible to asymmetric alterations during running (Florenciano Restoy et al., 2021). Lastly, existing studies often report discrete variables—such as peak or mean values within a stance cycle—to characterize gait symmetry, whereas actual asymmetrical changes may occur continuously across multiple phases of the running stance.

Recent advancements in wearable sensor technology now make it possible to continuously track the three-dimensional (3D) movements of multiple body segments using inertial measurement units (IMUs) during long-distance running in outdoor environments (Lempke et al., 2025, Schütte et al., 2018). We further applied statistical parametric mapping to pinpoint both the presence and precise timing of increased gait asymmetry during the stance phase. To the best of our knowledge, this study is among the first to comprehensively examine changes in gait symmetry across multiple motion planes during an outdoor marathon. Based on existing evidence, we hypothesized that gait asymmetry, reflected by changes in segment acceleration or angular velocity, would manifest most prominently during initial contact and propulsion phases of the stance phase.