# Relationship Between Front Crawl Trunk Incline and Lower Limbs' Biomechanics in Non-Expert Swimmers
## 4 Discussion  
The aim of this study was to analyze the relationship between the lower limbs' kinematics or kinetics and trunk inclination in front crawl performed by non-expert swimmers. Main findings indicate that a greater TI in non-expert swimmers can result from a weaker knee motion, which negatively affects velocity and efficiency. The mean kicking force is a kinetic measure that seems to determine the degree of TI in non-expert swimmers. As such, our hypothesis was partially confirmed, as both the kinematics and kinetics of the lower limbs were related to the angle of the trunk's inclination to a similar extent. Figure 3: Correlation matrix between TI and kinetic variables. *, \(p\leq 0.05\); **, \(p\leq 0.01\). TI, trunk incline; Fmax, maximum kicking force; Fmean, mean kicking force; Dp max, maximum passive drag; Dp min, minimum passive drag; Dp mean, mean passive drag. Figure 2: Correlation matrix between TI and kinematic variables. *, \(p\leq 0.05\); **, \(p\leq 0.01\). TI, trunk incline; v, mean velocity; SR, stroke rate; SL, stroke length; SI, stroke index; KneeMax, maximum knee angle; KneeMin, minimum knee angle; KneeROM, knee range of motion; DurKick, kicking cycle duration; DurDesc, descendent phase duration; DurAsc, ascendant phase duration. The swimmers participating in this study exhibited trunk inclination angles of approximately 15-16\({}^{\circ}\) during front crawl swimming. Those values came from a 25 m maximal-effort test, which likely influences how their body segments behaved. It is likely, under lower-intensity efforts (e.g., a submaximal training pace) or longer-distance swimming, where fatigue comes into play, these TI values could become higher. Still, this finding highlights the fundamental difference in body position between non-expert and elite swimmers, who typically demonstrate inclination values closer to 10\({}^{\circ}\)[3]. While direct comparisons within a similar non-expert sample are unavailable due to a lack of studies on this specific metric, previous research on elite swimmers suggests that their better position in the water allows them to swim more smoothly than non-expert swimmers [18]. A greater TI in non-expert swimmers may contribute to increased frontal drag, decelerating the body, which leads to higher energy demands and reduced swimming efficiency. This steeper angle suggests that these swimmers may struggle to maintain a streamlined posture, potentially due to a lack of body awareness, insufficient technical control, or suboptimal buoyancy distribution. Although this has been considered by the swimming science community, no attempts have been made until now to a deeper understanding of how TI is affected by the lower limbs' behavior in these swimmers. A negative correlation was observed between the TI and knee range of motion (ROM) during the kicking action. Specifically, swimmers with a greater TI exhibited a reduced KneeROM and a reduced KneeMin during kicking. This suggests that a more inclined body position may be, at least in part, a consequence of a weaker or less effective knee action, ultimately impairing velocity. An increased KneeROM could be a counterproductive strategy, limiting the ability to generate sufficient propulsion, as happens in other cyclic actions like running [19]. An effective kick relies on a well-coordinated movement of the hip, knee, and ankle, where the propulsive force is part of a seamless cycle reflecting the synergy of the lower extremities' muscles [20]. When knee flexion is restricted, the foot may not achieve an optimal position for generating thrust. When knee flexion is exaggerated, the legs may increase hydrodynamic resistance during the ascendant phase, reducing the overall effectiveness of the kick. So, some kind of a comfortable interval of kicking should be found for each swimmer where he/she will benefit by remaining more horizontal. This relationship between TI and the lower limbs' action also highlights a potential cycle of inefficiency, as seen in the negative association between TI and SI.