## 4 Discussion
The 44 knees examined in this study provide a pilot dataset to suggest this cut-off level; however, further studies are needed on a large number of subjects to set representative reference values for the valgus shift of the knee under load. An important novel finding of the present study that was not described earlier in the literature is that the dynamic valgus shift is very much dependent on the depth of the single-leg squat, and so the widely-used lowest point does not provide comparable metrics unless all subjects perform the squat to the same relative depth--which is hardly the case, especially in a postoperative or osteoarthritic population. Now that we have established this new variable, i.e., lateral knee shift at fixed squat depth measured in the percentage of lower extremity length (e.g., 15%), it is justifiable to recruit a large number of healthy and osteoarthritic subjects to establish exact cut-off values in a future study. It must be noted that there are no gold standards with any method to define dynamic valgus instability of the knee. Even static valgus cut-off values are debated in the literature, let alone the yet to be established dynamic ones. Future studies should define reference values for different populations, such as healthy young adults who possess no valgus or varus deformities as judged by orthopedic surgeons. Based on this reference group, knee lateral movement range and symmetry indices can be established and considered 'normal' values. The current study established that such evaluations can be performed with adequate accuracy using a portable system, opening the possibility for a wide-scale evaluation of dynamic knee valgus. It is well-known that the single-leg squat is not only widespread in sports medicine and sports science to assess injury risk [16; 40], but also commonly applied by clinicians, physiotherapists and orthopedics [38; 41; 42]. Excess knee valgus is the first observable step in a chain of events that may ultimately lead to symptomatic knee osteoarthritis. Dynamic knee valgus is an imbalance that results in excess shear force in the knee joint, which overloads the joint faces; in some patients, it is already associated with mild symptoms such as patellofemoral pain under load. When this imbalance lasts for years or is overloaded with demanding physical activity or sport, a destructive process develops. Furthermore, most non-contact ACL injuries in sports happen during landing with excessive knee valgus [12]. Several studies reported that the ACL-injured and -torn subjects have a greater risk for osteoarthritis [13; 33]. [PERSON] et al. showed that the long-term sequelae of ACL injury includes knee osteoarthritis in up to 90% of patients [14]. Moreover, [PERSON] et al. found osteochondral lesions in more than 80% of patients who underwent acute anterior cruciate ligament ruptures [43]. These considerations together highlight the importance of early and accurate measurement of dynamic knee valgus when it is still corrigible. Based on the scientific literature, there is a need for a relatively low-cost, reliable and user-friendly motion analysis system that is capable of injury risk screening in the orthopedic practice. The Dynaknee software, along with Kinect Azure, incorporates a low-cost, user-friendly and quick instrument, without reliance of on-body markers, that is easily applicable in GP offices, orthopedic physiotherapy practices or even in gyms. The Kinect Azure camera is the newest Microsoft Kinect product, and it can accurately monitor major joint positions during exercise. Our results have shown that the relative evaluation of medial-lateral knee movement seems to be an appropriate method to determine the range of the dynamic knee valgus. Several studies proposed the use of Kinect in other medical fields. It could be a good choice in walking and running analysis during rehabilitation [21; 44] or in hip disorder treatment [19]. Kinect provides data about lower limb functions, such as mobility and safety.