## 4 Discussion and Implication
### Study Limitations and Future Challenges  
A primary limitation of this study is the small sample size, consisting of only two collegiate athletes. As a pilot study, the aim was to initially evaluate the feasibility and accuracy of the RTK GNSS methods. Consequently, the limited sample may not represent the broader population of 400 m sprinters, and the performance levels of the subjects (55.06 s and 73.07 s for the 400 m sprint) could influence the outcomes. Athletes with different skill levels or running styles may exhibit varying biomechanical patterns, potentially affecting the generalizability of the findings. Future research should include a larger and more diverse cohort of athletes encompassing various skill levels, genders, and training backgrounds to validate the methods across different populations and competitive contexts.  
The GNSS antenna was securely attached to the rugby headgear using adhesive tape, and the headgear itself was firmly fastened to the athlete's head to minimize relative motion between the head and the antenna. This set-up provided a stable platform for signal reception; however, slight movements of the headgear relative to the athlete's head may have still occurred at high running speeds, potentially leading to minor measurement errors. While the participants reported no significant discomfort while running, indicating the suitability of this system for training applications which require detailed step-by-step analysis, its practicality for competitive races remains limited. Future refinements, such as improved fixation methods or alternative sensor placements--such as a lightweight back-mounted set-up--could further enhance usability for both training and competition. Additionally, continued miniaturization of GNSS devices may further optimize their application in high-performance sprinting environments.  
Additionally, while all trials in the 3D video analysis sections maintained a 'Fix' solution, indicating optimal GNSS accuracy, Subject A experienced approximately 50% 'Float' solution periods during the 400 m sprint. 'Float' solutions indicate reduced positional accuracy due to unresolved integer ambiguities, often influenced by satellite signal conditions and the measurement environment [37]. These periods of reduced accuracy could impact the reliability of the running parameter estimates. Future studies should explore techniques to minimize 'Float' solution periods, such as enhancing satellite signal reception in environments with minimal obstructions and incorporating advanced correction algorithms like those proposed by [PERSON] [44] to improve the overall accuracy of GNSS measurements.  
Furthermore, integrating RTK GNSS data with additional sensor technologies, such as IMUs, could enhance the precision of IC timing detection. While IMUs are susceptible to drift, they offer superior accuracy in detecting contact and toe-off events compared with a GNSS alone. Sensor fusion methods may allow for more accurate identification of IC and toe-off events, enabling calculations of the stance time, flight time, and other detailed temporal parameters. This integration could capitalize on both technologies' strengths--RTK GNSS for accurate spatial measurements and IMUs for precise temporal event detection--thereby enhancing running parameter estimations' overall reliability and robustness.