## 4 Discussion  
The introduction of portable ultrasound systems and ultrasound transducer fixation devices has made dynamic evaluation of the skeletal muscles possible. The feasibility of short-term dynamic ultrasound acquisitions has already been demonstrated previously in cycling exercise in ultrasonography of skeletal muscles and in echocardiography [18, 27]. In this study, the feasibility of long-term dynamic functional measurements has been investigated in ten endurance runners. Endurance runners were selected as participants for this research, since their labor is relatively invariable and there is no danger of fixated transducer motion as a consequence of external forces (e.g., contact with other athletes, attributes, or the ground). In addition, the forces that are generated when the feet come into contact with the ground might also have an effect on the feasibility of long-term dynamic acquisitions during endurance running. When using an ultrasound transducer designed for smart mobile usage, usually only the preset, gain, and power can be modified. In this study, a linear array Philips Lumify L12-4 was used. For this transducer, five presets were available (i.e., lung, musculoskeletal, soft tissue, superficial, and vascular), each optimized for their specific application. The musculoskeletal preset was most likely optimized for static acquisitions, since it has a high persistence (i.e., temporal compounding). A high persistence in static musculoskeletal acquisitions will lead to an augmented contrast and a reduction in noise. However, in dynamic measurements, high persistence results in the materialization of artificial posterior aponeuroses. Therefore, the musculoskeletal preset is deficient for dynamic measurements. In the vascular preset, the persistence appeared to be higher and the contrast lower in comparison to the musculoskeletal preset. Consequently, the aponeuroses detection algorithm failed to track the posterior aponeurosis during the gait cycle. Hence, this dataset was excluded from functional evaluation. The lung preset was most suitable for these dynamic measurements, since the contrast was sufficient and no temporal compounding seemed to be applied. The image quality was evaluated by quantification of probe-skin contact preservation and the field-of-view stability. In four volunteers, the probe-skin contact was preserved during the entire measurement. However, in five volunteers, temporary probe-skin contact loss occurred for some or all acquisitions. Probe-skin contact loss could be a result of gel loss caused by sweating, a slight probe displacement, muscle deformation during contraction, or by probe dislocation induced by gravitational pull or inertia of the probe motion countered by the ground impact forces. Within an acquisition, probe-skin contact loss occurred during part of the gait, which was approximately identical for each gait cycle captured within the acquisition. Although functional analysis of the complete gait cycle was impossible in these acquisitions, functional parameter alterations within these gait sections over time still contain valuable information. The field-of-view stability was estimated using the CW-SSIM. The initial CW-SSIM index drop was highest, which could be caused by a dislocation of the transducer, or by a general temporal decorrelation of the ultrasound images provoked by noise and/or slight out-of-imaging-plane motion. A dislocation of the transducer would result in a relatively big alteration in the measured functional parameters. However, the difference in muscle thickness and pennation angle between the first and second kilometer is comparable to the average variation of these functional parameters. Accordingly, the initial drop in CW-SSIM indices is expected to be caused by temporal decorrelation. The functional parameters in this study consisted of the muscle thickness and pennation angles of the fascia with respect to the posterior aponeurosis. In eight out of nine runners, a sufficient portion of gait cycle could be analyzed.