## 1 Introduction  
Low back pain (LBP) is one of the main alterations of the musculoskeletal system and one of the complaints with the highest incidence in rehabilitation centers [1]. The prevalence is so high that it is thought that approximately 70-80% of the population will suffer from this disease at some point in their lives [2], and in 40% of cases, the initial discomfort may become chronic [3]. In athletes, LBP, a musculoskeletal disorder, is associated with limitations in joint range, muscle spasms, a loss of muscle strength, and postural problems [4]. Competitive sports can lead to the development of LBP. The results of some studies have shown that low back pain affects more athletes than non-athletes and that low back pain increases with increasing frequency of training sessions [5]. In the case of rhythmic gymnastics as a discipline of sports, [PERSON] et al. (2021) confirmed that it is an exhausting sport that requires considerable physical and mental effort to continuously adapt to the harmony between biomechanical and aesthetic efforts [6]. Gymnasts require coordination, joint mobility, postural adaptation, strength, speed, rhythm, agility, and dynamism. In rhythmic gymnastics, a refined quality of motor control, excellent capacity for expression, and elegance of technical gestures are very important [7; 8]. Therefore, from an early age, high-performance training is required for the correct execution of sports gestures to achieve appropriate skills. The average amount of training in this sport discipline, according to [PERSON] et al. (2021) [6], is 25-30 h per week. This is because of the high technical demands of this discipline [6; 9]. Because of what they involve, rhythmic and artistic gymnastics are sports that put athletes at risk of musculoskeletal disorders such as LBP, postural disorders, and other important injuries mainly caused by the excessive use and repetition of the exercises, the same gestures, several times for each type of training [6; 10; 11; 12; 13; 14]. LBP plays an important role in individual demographic and physical characteristics, effort, high training pace, reduced joint mobility, accelerated growth of the gymnast, excessive muscle flexibility, lack of muscle strength, years of experience, and practice of the sport [15; 16]. The spinal alignment and functional symmetry of the trunk--particularly in the frontal plane--are critical for rhythmic gymnasts, where even minor asymmetries can compromise performance and increase the risk of chronic LBP. Interventions aiming to improve frontal spinal curvatures, trunk lateral mobility, and postural control are thus of special interest [17]. Two prominent postural approaches include the [PERSON] method and isostretching, both of which aim to restore postural balance, correct musculoskeletal dysfunctions, and reduce pain. The Mezieres method is based on global postural re-education, focusing on muscular chain elongation, breathing control, and vertebral realignment through tailored static postures [18]. In contrast, isostretching combines isometric contractions with stretching exercises to enhance core stability, spinal alignment, and functional mobility, often using symmetrical movement sequences and controlled respiration [19]. Despite the potential benefits of both interventions, most existing studies using these methods have focused on sagittal posture, flexibility, or performance-related outcomes such as vertical jump and trunk range of motion [17; 18; 19], leaving a gap in the understanding of how these techniques affect spinal symmetry and function in the frontal plane. This represents a crucial limitation, particularly for sports like rhythmic gymnastics where lateral trunk control and segmental vertebral alignment are fundamental. This study aims to evaluate the effects of a 12-week intervention using the Mezieres method compared with an equivalent isostretching protocol on frontal-plane spinal curvatures and pain perception in elite female rhythmic gymnasts with LBP.