Furthermore, the echo intensity of the muscle area might expose neurological disorders or is an indication of muscle acidification [22]. Structures adhering to the muscle (e.g., tendons and its fibers, cartilage, bursa, bones) can also be visualized with high quality [23, 24]. Dynamic imaging of the skeletal muscles is strenuous and often impossible to perform manually, since the transducer has to be held in a fixed position. Manual acquisitions might be feasible during low intensity exercise, though the musculoskeletal function parameters are likely to vary between different exercise intensity levels. In static interval measurements, the imaging plane is difficult to reproduce. Out-of-plane motion as a consequence of an unstable field-of-view renders measurements, such as Doppler velocity imaging and strain, impossible. A constant field-of-view is crucial for accurate estimation of muscle tissue parameters [25]. Fixation of the transducer enables dynamic imaging, and has been shown to overcome part of the limitations associated with manual acquisitions. Probe fixation has been shown to yield improved or similar field-of-view stability and repeatability of ultrasound acquisitions [18, 26, 27]. Additionally, probe fixation has shown to decline shoulder abduction and forearm muscle activation of sonographers without influencing the acquisition time in echocardiography [28]. A demonstration of short-term probe fixation during a few minutes of cycling exercise was already presented [18]. However, the feasibility of dynamic musculoskeletal ultrasound acquisitions during long-term (running) exercise still has to be demonstrated. In this study, the feasibility of obtaining dynamic ultrasound acquisitions and evaluating the skeletal muscle function is investigated in ten endurance runners. To the best of our knowledge, no hands-free, outdoor, long distance, ultrasound acquisitions have been obtained previously. In endurance running, the risk of probe displacement due to external factors (e.g., other participants, attributes, the ground) is minimal. Additionally, in endurance running, the exercise performed is fairly invariable, implying deviations in functional measurements are most likely caused by probe dislocation or detachment [29, 30]. The objectives of this study were to quantitatively analyze the image quality and demonstrate the possibility of functional parameter computation from ultrasound images obtained hands-free during endurance exercise. The image quality was assessed by estimation of maintained probe-skin contact, and field-of-view stability analysis using the complex-wavelet structural similarity index method (CW-SSIM). Potential probe-skin contact loss was expected to occur due to sweating, or slight probe fixation device shifts. The field-of-view was hypothesized to stay constant, based on a previous study executed during a couple of minutes of cycling exercise [18]. Furthermore, the functional measurements were expected to remain constant over time and distance ran, since this study was performed during endurance exercise. The pennation angle and muscle thickness have shown to remain constant when measured prior and post endurance running [29; 30; 31].