## 1 Introduction  
Swimming is a highly competitive sport; only a small time difference can affect the outcome of a race. Start performance is a crucial aspect of a swimmer's performance, especially in elite competitions [1]. It has been reported that the start time can contribute nearly 30% of the swimming time in sprinting events [2]. Furthermore, it has been observed that this percentage contribution tends to increase as the race distance becomes shorter [3]. The swimming start is defined as the distance to the 15 m mark in the race [4]. In a study conducted by [PERSON] et al., the start phase was further segmented into six distinct phases: the block phase, flight phase, entry phase, glide phase, leg kicking phase, and swimming phase [5]. Currently, the majority of sports biomechanics research in the field of swimming relies on traditional statistical and theoretical methods. Most of the research on swim starts has aimed to identify the most optimal technique for maximizing performance [6; 7; 8]. Previous studies have primarily focused on analyzing factors such as foot placement, body position, dive angle, and force production [9; 10; 11]. These investigations typically employ straightforward and easily interpretable data, including time, distance, speed, block height,and other related metrics. While straightforward data analysis of start performance can be helpful for coaches and swimmers to quickly identify areas for improvement and make necessary adjustments [12; 13; 14], it often fails to fully capture the intricate interplay between different body segments and the coordination patterns that underlie efficient movement execution [9; 15]. It only focuses on the external observable aspects of the technique, without considering the intricate inter-limbs or segment adjustments [16; 17; 18]. Such inter-segmental coordination involves the relative behavior of one body segment about another [19]. For example, during a ball throw, various body segments, such as the shoulder, arm, and trunk, collaborate in a coordinated manner. The arm extends, the shoulder rotates, and the trunk shifts weight to generate power and accuracy in the throw. Continuous relative phase (CRP) is a measure used in motor control research to analyze the coordination between two or more segments during a movement pattern [20]. It is derived from the phase plots of the segments, which show the relationship between their movement over time [21]. Since [PERSON] reported the dynamics of finger oscillations, many other types of behavior have been described by using CRP [20]. Primarily utilized in the study of cyclic movements, such as walking or running, CRP analysis enables the assessment of coordination patterns among joints or limbs [22; 23]. However, the application of CRP in investigating discrete movements has received relatively limited attention [24; 25]. Discrete movements often involve nonlinear dynamics, where the relationship between segments is not linear or predictable. CRP analysis could capture and analyze these nonlinear dynamics, offering a deeper understanding of the complex interplay between different body segments. [PERSON] et al. used it to characterize the inter-segment coordination (i.e., shank-thigh and thigh-ascrum pairs) during the take-off in ski jumping [26], and the authors reveal the correlation between lower inter-segmental coordination and jump length during take-off extension. There is a need for a detailed description regarding inter-segmental coordination during the swimming start. Calculating the CRP between segments and trunk during the swimming start can help identify any coordination or timing issues between different body segments that may affect their ability to generate power and propulsion during the swimming start. Previous studies on swimming start are commonly based on able-bodied swimmers, while there is very little relevant research for disabled swimmers. The disabled swimmers may differ depending on the nature of their impairment in start performance.