## 4 Discussion
Thus, the heightened BF activation observed in these configurations supports efficient musculoskeletal function without increasing joint laxity or mobility risks. This synergy may enhance knee centration and reduce musculoskeletal strain while maintaining effective power transmission during cycling. Lastly, we showed results of the right and left erector spinae (RES and LES) muscles, particularly active during the early phase of the pedalling cycle (10-40%). Both muscles demonstrated a significant peak in this phase; however, peak amplitudes varied between positions and between the right and left sides, indicating that positioning adjustments impact the erector spinae asymmetrically. Position P5, the custom-fitted position, elicited the highest activation of the RES muscle compared to other positions, especially during the peak phase of the cycle. This suggests that P5 may engage the RES more effectively, potentially contributing to improved spinal stability and posture on the right side during cycling. In contrast, the LES muscle demonstrated significantly higher activation in position P1, particularly in the early and late phases, than P5. This elevated LES engagement in P1 suggests that certain configurations, such as the aft saddle with low handlebars, place greater demands on the left side for stabilisation and dynamic balance, possibly due to altered trunk posture and pelvic positioning. In the comparison of P2 vs. P5, an asymmetrical activation pattern was also noted, with the LES muscle showing significantly higher activation in P2, especially during the early and late phases. In contrast, the RES muscle exhibiteda more balanced activation between positions. This asymmetry in LES activation in P2 suggests that the aft saddle with high handlebars may place additional demands on the left side, likely for posture stabilisation. At the same time, the RES remains relatively unaffected by this configuration. Similarly, P3 vs. P5 comparisons showed slightly higher peaks in both RES and LES activity in P5. However, differences were not statistically significant, suggesting that P5 and P3 configurations maintain comparable levels of spinal stability without imposing asymmetrical demands. Position P4 vs. P5 comparisons indicated slightly higher, albeit statistically insignificant, RES and LES activity in P5, hinting that the customised position may promote slightly increased spinal muscle engagement without significantly impacting symmetry. Overall, the findings emphasise that although P5 may generally induce more excellent RES activity, other positions, particularly P1 and P2, tend to increase LES activation. Asymmetrical muscle activity between RES and LES may have considerable implications for cycling performance and bike fitting practices. The erector spinae muscles are essential for maintaining a stable and upright posture, efficient power transfer, and optimal pedalling technique. Asymmetry between RES and LES can result in uneven force distribution and lead to inefficient mechanics, potentially diminishing power output and pedalling efficiency. Moreover, prolonged asymmetrical loading may contribute to musculoskeletal imbalances, increasing the risk of overuse injuries, discomfort, and strain on the spine. Identifying and addressing asymmetry in erector spinae activation is critical from a bike-fitting perspective. Practical bike fitting aims to balance the engagement of both RES and LES muscles by optimising saddle height and position, handlebar reach and height, and cleat alignment. By promoting symmetrical posture and spinal stability, bike fitting can mitigate risks associated with asymmetrical muscle activity [38], enhance rider comfort, and reduce injury likelihood [42; 43]. Additionally, by achieving balanced muscle activity, cyclists may experience improved proprioception and neuromuscular coordination, leading to better control, stability, and endurance--particularly valuable during prolonged rides or high-intensity efforts [44].