**4. Discussion**

The main findings of the study were, firstly, that knee joint stability (kinematics) was affected by the waist-width of the ski, as well as by the level of fatigue. Secondly, hypotheses H1a and H1b were only partly confirmed as only knee abduction increased with the ski waist-width and with the level of fatigue but not the knee rotation. Concerning the comparison of the functional stability in the simulated skiing position using different ski waist-widths, it was demonstrated that the fatigue caused a significant deterioration in knee stability with wide skis compared to narrow ones. Thirdly, fatigue resulted in an increase in CoP movement compared to prefatigue values, confirming hypothesis H2a. The fatigue effect on balance deterioration was significantly more influential with narrow skis compared to wide ones. Thus, hypothesis H2b was not confirmed. With most CoP parameters, it was shown that the effect of fatigue on balance was in accordance with previous studies [21,22,30].

Previous on-snow [19] and laboratory [20] studies demonstrated that knee rotation was the primary adaptation mechanism to avoid an increase in knee-joint torque when using wide skis. The knee abduction was independent of the ski waist-width [20]. In the present investigation, where the muscular fatigue effect was studied, knee abduction increased in the fatigue state with both ski widths, while rotation remained unchanged or there was even a trend of diminishing external rotation. One possible explanation is that, in a state of fatigue, abduction took on the role of minimizing torque in the knee joint instead of external rotation in combination with flexion, as found in a previous study. However, the knee-joint abduction that presently occurred imposes an additional strain on the medial collateral ligament [31]. The stiffness of this ligament is increased by lower-limb muscle activation [32], which is considered as an additional mediolateral knee stabilizer. This additional active stabilization mechanism could be hampered in the state of muscle fatigue. Thus, the knee abducted/valgus position becomes more pronounced and nearer to the ligamentous limitation of the end range of the knee valgus position, which might represent the risk of acute medial collateral injury in the case of additional sudden external valgus thrust [31], which may occur during skiing.

It is known from other biomechanical studies that knee-joint malalignment predisposes the knee joint to degenerative changes [33] via the local overload of joint surfaces. In our study, it was shown that, in the state of fatigue, and even more so in connection with wide skis, the knee is forced to the

pronounced valgus position in the simulated ski turn. It can be assumed that, in such cases, the lateral knee compartment might be notably more loaded or, in the worst case, even overloaded. Nevertheless, knowing that ground reaction forces in recreational skiing are as high as two body weights [34] and in competitive skiing as high as 4.2 body weights [4,5], in combination with vibrations [35–37], this may increase the risk of chronic joint conditions. This especially applies to competitive and advanced recreational skiers/ski instructors because of their high number of ski runs/turns per season.

With most CoP movement parameters, the fatigue e ffect was most significantly expressed immediately after the fatigue procedure, in accordance with a previous study conducted on an isokinetic dynamometer [12]. Some of the parameters (VAP, MFAP, MFML with narrow ski, and MFAP with wide ski) did not return to baseline even at the time of the last measurement (4 min after fatigue). Therefore, typical short breaks along the descent appear not to be su fficient to level out fatigue effects. These results in terms of skiing safety put into question long chair lifts or gondolas when skiers are not taking long enough breaks during their descents. In other studies that investigated the fatigue e ffect on the deterioration of muscle force production [12,38] and CoP movement [30], most of the force-producing functions and the balance returned to normal after 6 to 10 min. Such longer resting periods typically only occur in alpine skiing between runs, waiting for lifts, and travelling (back) up the mountain/slope. Nevertheless, previous studies reported that the body sway increased proportionally to the developing fatigue when the subjects ran on a treadmill [39]. In contrast, Bryanton and Bilodeau [40] observed that CoP movement started to increase with but plateaued or possibly even decreased during their fatigue protocol, consisting of a sit–stand exercise. It remains unknown how repeated bouts of high-intensity skiing throughout the training session/skiing day a ffect postural control. For future research, the e ffect of additional repetitive fatiguing should be examined to elucidate what is expected to happen with postural stability on a typical skiing day consisting of several consecutive runs.

The main limitation of this study was probably that it simulated skiing and was not conducted during on-snow skiing. On the other hand, in this way, the experiment was significantly more controlled. Moreover, forceful fatigue, applied in this study, would most likely pose a high risk of injury during experiments if it were to be performed in real skiing. Undoubtedly, such measurements should be performed in situations to minimize the risk of injury, and this was provided by the fatigue and skiing simulation in the laboratory. Future research incorporating less forceful (to decrease the risk of injury during the experiment) but repetitive fatigue followed by a resting period would further elucidate the e ffects of real skiing fatigue on balance and knee-joint stability.
