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Naslov prispevka: Comparison between knee joint kinematics in laboratory skiing simulation and in real skiing while using skies of different width
Avtor:  Martin Zorko
Vrsta predstavitve: Ustna predstavitev
Strokovno srečanje:  7th International Scientific Conference on Kinesiology
Kraj in čas srečanja: Opatija (Hrvaška), 22.-25. december 2013

Povzetek

Aim: The aim of the study was to determine the knee joint kinematics in the ski turn while using skies of different waist width. We captured the kinematic parameters both in laboratory skiing simulation by using optical motion capture system and in real skiing situation by using inertial motion capture suit and Global Navigation Satellite System. It was discovered that the abduction and external rotation increase with increasing ski width in simulated skiing in laboratory environment. In real skiing, we noticed only increase of external rotation with increase of the ski width. The arbitrary knee joint flexion in real skiing makes some difficulty in the direct comparison of the results obtained in simulated environment and real skiing, respectively.

Introduction: Alpine skiing is a complex sport in an outdoor environment. Recent evolution of the skis resulted in an increase of the ski waist width, i.e. the width of the skis under the ski boot. In a turn, the ski is in a contact with the snow primarily on the inside edge. Therefore, there is a shift of the point of application of the ground reaction force to the more loaded/external leg inward compared to skiing in a straight line. The wider the ski the bigger the mediolateral shift of the ground reaction force application can be expected. Therefore, the evolution of skis that have increased the waist width would either change the torques that affects the knee joint or more probably the kinematics of the knee. The aim of the study was to analyse how the kinematics of the knee joint change while using the skies of different waist width both in simulated skiing conditions and in the real skiing situation.

Methods: The methodology was divided into two parts: laboratory test and alpine skiing field measurements. The first part of the measurements took place in the laboratory, where we simulated the ski slope inclination and the external forces acting on the skier. One physically well prepared subject without alpine skiing experience was measured standing on specially built setup that simulated skiing on the edge of a ski. The first position was with the ski set flat on the ground following the ski set on the inside edge that simulated ski waist width of 6, 8 and 10cm, respectively. The knee flexion angle was set on 50° and the subject maintained this position by using real time visual feedback system. Knee flexion, relative abduction and relative external rotation were calculated in real time based on position measurements using a contactless motion capture system (NDI Optotrak 3D Investigator). For the analysis, averages of 6s long measurements at a sampling rate of 10 Hz were used.

In the second part of the experiment, three study subjects performed three runs using skis with the width under the boot of 6.5, 8.8 and 11 cm. Each ski had the same declared side cut. Each run included ten equal giant slalom turns. The skiers wore an inertial motion capture suit (MVN BIOMECH, Xsens Technologies) which directly measured 3D accelerations, 3D angular velocities and 3D orientation at a sampling frequency of 120 Hz. The reference trajectory of the skier was measured using the Real Time Kinematics Global Navigation Satellite System (RTKGNSS; Leica Geosystems, series 1200) as explained in more detail elsewhere (M.  Supej, 2010). Four standard phases of the turn were defined: initiation, steering 1, steering 2 and completion phase based on the previous studies (Müller et al., 1998; M. Supej & Holmberg, 2010). The angles in the knee joint were measured in the sagittal, frontal and transversal planes (flexion/extension, abduction/adduction, internal/external rotation) according to International Society of Biomechanics.

Results: In laboratory conditions, the external rotation and abduction increased while the subject changed the position from the ski set flat on the ground to the ski set on the inside edge that simulated the ski width of 6cm. The external rotation increased further with ski width of 8 cm and remained on the same level when the ski width increased to its maximum of 10 cm. In this position the external rotation was about 6° higher than at the starting (ski flat) position. The abduction increased even more constantly with the lowest value at the ski flat position and the highest value at the inside edge position with the maximum width of 10 cm. In this position the abduction was about 27° higher than at the starting (ski flat) position.

In real skiing the knee flexion in the turn was largest with the most narrow ski. In comparison to skiing straight the abduction increased in the turn with each of the skies with the most increment with the narrowest ski. The internal rotation increased in the turn with each of the skies as well but again the most so with the narrowest ski.

Discussion: The increment of abduction while changing the position from skiing flat to setting the ski on the edge both in the laboratory and in the real skiing situation was most probably the result of the active skier's effort to move the knee joint inward and towards the ground reaction force vector. The increment of external rotation with wider ski, as it was discovered in the laboratory settings, most probably serves to the same purpose. In the real skiing conditions the increment of internal rotation in the turn apparently contrasts the laboratory findings. However, the knee joint flexion angle was arbitrary in the real skiing condition while it was fixed in the laboratory conditions. From this point of view one must take in to the account that the abduction and internal rotation of the knee joint are partly function of its flexion (Lu, Tsai, Kuo, Hsu, & Chen, 2008). Without mediolateral force application both parameters decrease while flexion increases. From this point of view the less internal rotation of knee joint with wide skies could be interpreted as the actual appearance of external rotation. The appearance of external rotation in the ski turn was confirmed also in other studies (Yoneyama, Kagawa, Okamoto, & Sawada, 2000).

Conclusion: The results indicate that wider skis may cause larger external rotation and abduction angles in the knee joint. This could lead to an increased injury risk particularly in dynamic situations such as skiing. The comparison of knee joint kinematics between laboratory and real skiing situations is somewhat difficult because in real skiing it is not possible to control the knee joint flexion angle and there might also be influence of some additional parameters (for example vibrations) that are hard to reconstruct in laboratory environment.