# The Influence of Ski Waist-Width and Fatigue on Knee-Joint Stability and Skier's Balance
## 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  
Figure 10: The mean frequency of the power spectrum of CoP in the mediolateral direction (MFML) in a prefatigued state (before F) and at different times after fatigue (after F) with two different ski waist-widths. + depicts statistically significant difference compared to prefatigued state (\(p<0.05\)); * depicts statistically significant difference between all-narrow against all-wide waist-width measurements. 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, 36, 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 effect was most significantly expressed immediately after the fatigue procedure, in accordance with a previous study conducted on an isokinetic dynamometer [12].