## 4 Discussion  
In this study, CoAIs were calculated and evaluated in terms of reliability to quantify how knee flexor muscles (antagonists) and extensor muscles (agonists) co-contract during the YBT-LQ. Specifically, the activation of flexor muscles (ST and BF) was compared with that of extensor muscles (VM and VL) to assess the contribution of the investigated muscles during the given task. Four distinct CoAIs were calculated for each muscle pair, accounting for different directions, limbs, and trials. The results revealed that the reliability of these sEMG-based CoAIs varied depending on the muscle pairs and movement directions. Notably, CoAIs involving the ST, precisely ST/VL and ST/VM, demonstrated good to excellent reliability, while those involving the BF exhibited lower reliability. These findings highlight the need to consider specific muscle pairs, test directions, and rater consistency when using CoAIs in clinical and research settings. For instance, the high reliability observed in the ST and VL muscle pair could suggest that these muscles effectively coactivate to stabilize the knee during anterior, posteromedial, and posterolateral movements. In contrast, the lower reliability in the BF and VM pair may indicate inconsistent activation, potentially reflecting variability in knee stabilization strategies or individual biomechanical differences. These insights can potentially emphasize the critical role of specific muscle pairs in maintaining dynamic knee stability. The reliability analysis of the CoAIs during the YBT-LQ varied across directions and muscle pairs. CoAI2 (ST/VL) had the highest intra-rater reliability in the anterior direction, reflecting strong consistency, while CoAI3 (BF/VM) showed the lowest reliability in the posterolateral direction. This suggests that the investigated muscle pairs co-work differently for different YBT-LQ directions. The anterior direction likely places greater demand on the knee extensors, explaining the higher reliability of CoAI2 (ST/VL). In contrast, the posterolateral direction requires more complex coordination between the knee flexors and extensors, contributing to the lower reliability of CoAI3 (BF/VM). This highlights the need for direction-specific considerations when assessing knee muscle coactivation using CoAIs. No significant differences were observed between limbs in the four CoAIs, suggesting symmetrical muscle coactivation patterns in healthy recreational athletes. While this indicates balanced neuromuscular control, relying solely on CoAIs may not fully capture the complexity of lower limb symmetry, as muscle coordination involves more than coactivation indices. In clinical populations, such as those recovering from knee injuries, asymmetry is often a marker of impaired neuromuscular function. Exploring how coactivation patterns change in injured individuals could enhance this study's clinical relevance, especially for injury prevention and rehabilitation. Future research could investigate the potential of these indices to monitor asymmetry and guide targeted rehabilitation strategies. An interesting issue was the diversity of the developed indices in terms of their comparison between different directions. For example, CoAI2 showed significantly higher values in the anterior direction compared to the posteromedial and posterolateral directions, which could potentially indicate the dominant role of the ST and VL muscles in stabilizing the knee during anterior motion. In contrast, CoAI4 (BF/VL) demonstrated no significant differences across directions, indicating a stable coactivation pattern. The reason for the variability observed in CoAI1 (ST/VM) and CoAI3 (BF/VM) should be further explored. The developed set of four sEMG-based indices to quantify knee muscle coactivation during the YBT-LQ could potentially be a valuable tool for assessing neuromuscular control and stability.