# The Influence of Ski Waist-Width and Fatigue on Knee-Joint Stability and Skier's Balance  
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1 Clinical Institute of Occupational, Traffic and Sports Medicine, University Medical Centre Ljubljana, 1000 Ljubljana, Slovenia 1  
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2 Faculty of Sport, University of Ljubljana, 1000 Ljubljana, Slovenia; [EMAIL_ADDRESS]  
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3 Faculty of Health Sciences, University of Primorska, Polje 42, 6310 Izola, Slovenia 3  
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Static equilibrium is defined as the ability to maintain the center of mass (CoM) above the support surface [14]. When the center of pressure (CoP) of the ground reaction force is outside the support surface, the body loses balance or an appropriate human action (e.g., a step) occurs in order to maintain or restore equilibrium [15]. In upright standing, the body uses two main strategies to compensate for challenged balance. In anterior-posterior disturbances, an ankle strategy occurs in which most compensatory movements are performed by the ankle and foot [16]. In disturbances that act in the medial-lateral direction, the body responds with a hip strategy, in which more complex movements occur, especially in the hip joint and torso [16,17]. The contribution of the hip increases with a reduced support surface and with larger and faster disturbances. In skiing, the strategy of the ankle is not expressed because the ankle joint is in a stiff ski-boot and, therefore, does not possess much freedom of movement. Thus, the skier uses predominantly knee and hip joint movements to maintain balance and to regulate the angle of the ski against the snow, from which the turning radius is determined (and, consequently, radial forces) in connection with carved turns [18]. Recently, skis have appeared on the market that are much wider than ordinary skis in the part under the ski boot (waist-width above 100 mm compared to 60 mm on classic skis). Such skis were originally designed for skiing off-piste. However, current skis with waist-widths between 80 and 90 mm are considered \"allride skis\" for on- and off-piste skiing, consequently often being used on hard or icy snow. In powder (off-piste) skiing, such skis have a wider support base and better flow on the snow. When wide skis are being used on icy/hard snow conditions, the outside and more loaded ski's point of application of the ground reaction force is farther away from the middle of the foot and shifted medically compared to when using narrower skis [19]. It was found that the knee-joint kinematics is consequently different on wider skis than on narrower ones, with knee rotation being more affected than knee abduction/adduction. In a study that simulated a quasi-static equilibrium position in a ski turn, it was found that the kinematic changes in the knee were such that the torque in the joint remained unchanged, regardless of the width of the ski [20]. The possible explanation for this was that, by keeping the external torques relatively low, there was also less muscle effort. From studies analyzing human gait, it is known that, as the antigravity muscles get fatigued, the total speed of movement of the CoM, the amplitude of movement in the mediolateral and anteroposterior directions, and the total range of motion of the CoM increase [21,22]. The purpose of the current research was to investigate the functional stability of the knee joint and balance in a quasi-static simulation of a ski turn when using skis of different waist-widths in connection with fatigue, as the lower-limb muscle fatigue might be an injury risk factor in skiing [23]. In a broader context, the study examined hitherto unknown factors that could affect knee-joint injury, which was proven to be the most commonly injured joint in both recreational and competitive skiing [24,25]. The following hypotheses were set:  
**Hypotheses H1a.