## 4 Discussion
The excluded dataset was acquired using the vascular preset resulting in poor image contrast, and the display of extra artificial posterior aponeuroses, as explained previously. The average values of the functional parameters remained fairly constant throughout the race, which is expected based on literature studies examining alterations in muscle thickness and pennation angles in endurance runners and resistance training [29, 30, 31]. In these studies, the muscle thickness and pennation angles are measured statically prior and post training. These studies showed no adaptions in muscle thickness and pennation angle after 10 weeks of endurance running or after running a marathon, whereas during resistance training augmentations of both parameters were observed. However, it should be noted that in this study the ultrasound measurements were performed dynamically instead of the static ultrasound measurements performed in the previously mentioned studies. Still, the stability of both functional parameters suggests the preservation of field-of-view throughout the race. The functional parameter fluctuations observed throughout the gait cycles are a consequence of muscle contraction. Moreover, during contraction, out-of-plane motion occurred to some degree in all runners. The muscle contraction-related variations are similar in all endurance runners and remained roughly constant over time. The feasibility of long-term dynamic ultrasound image acquisition and functional evaluation of the vastus lateralis has been demonstrated. However, some limitations were observed in this study. The main limiting factor was the inability to acquire functional and architectural information throughout the complete gait cycle in most subjects, which was mainly caused by probe-skin contact loss. However, probe-skin contact loss could be overcome by the incorporation of a dedicated gel patch during the acquisitions or by further optimization of the probe fixation device. The frequency and the impact of the issues hampering posterior aponeurosis detection (e.g., hypo-echoicity of aponeurosis) are expected to decrease largely once the probe-skin contact loss has been minimized. Additionally, the currently used algorithm could be improved further by taking into account an estimation of the aponeurosis motion during a gait cycle based on the successfully analyzed frames of (other) gait cycles. Alternatively, deep learning models could be used for feature detection [8, 37]. Three-dimensional acquisitions could further improve the dynamic functional evaluation. In 3-D ultrasound, out-of-plane motions can be tracked and stabilized during post-processing. Furthermore, feature detection and functional assessment of architectural and mechanical parameters is expected be more robust. Therefore, local functional analysis will also be improved in 3-D. However, the effect of the spatial and temporal resolution reduction on the performance of the dynamic functional measurements should be analyzed. Moreover, there are no 3-D portable ultrasound systems on the market, which is due to the higher level of complexity in both hard- and software. Additionally, the use of ultrafast imaging would be beneficial for the analyses proposed, but an ultrafast portable solution is also unavailable. The results observed in this study are expected to be applicable to other muscles situated in the leg or arm. Hands-free, continuous ultrasound acquisitions are applicable in the field of sports to analyze and improve muscle activation and motion, as well as gait. Monitoring of the muscles during exercise could also aid in the creation of optimized training schemes, or in the assessment of the athlete's fitness level. Another application of hands-free, continuous ultrasound evaluation of the muscle is during the revalidation process after a muscle injury. The acquisitions could help to determine when it is safe to return to play with minimal chance of injury repetition. In other ultrasound fields, the application of hands-free, continuous acquisitions could also be beneficial.