european college of sport science

Saturday, June 27 th , 2009
08:30 - 10:00
Oral presentations
OP-ST06 Sports 6
OP-ST06 Sports 6
THE EXTERNAL FORCES ACTING IN THE DIRECTION OF TRAVEL AND THEIR RELATION TO MECHANICAL ENERGY DISSI-
PATION IN SLALOM.
GILGIEN, M., REID, R., HAUGEN, P., SMITH, G.
NORWEGIAN SCHOOL FOR SPORT SCIENCES
Introduction:
The literature quantifying the external forces acting on an alpine skier is limited in general, but particularly in the slalom discipline. This
study therefore describes the size and time course of the external forces acting on a skier parallel to the direction of travel based on the
kinematics of a group of highly skilled slalom racers. Subsequently, the braking forces of ski-snow friction and wind drag are related to
mechanical energy dissipation.
Method:
The derivation of the external forces was based on the kinematics of 6 members of the Norwegian Europa Cup team (aged 17-20). Skier
kinematics were captured during 2 turns of a slalom race simulation by 4 panning and synchronized camcorders [1]. The centre of mass
(COM) position calculation was based on the Zatsiorsky body segment parameter model and the de Leva adjustments. The component
of gravity accelerating the skier downhill and the resultant force were both calculated from the COM position data. To calculate the air
drag force, frontal area was estimated using a kinematics-based procedure, drag coefficients were adapted from a study of cross country
skiers [2], and COM velocity was calculated using finite central differences. The remaining frictional force in the ski snow interaction
was calculated as the difference between the resultant force, the air drag force and the component of gravity accelerating the skier
downhill.
Skier mechanical energy dissipation, defined as the amount of total mechanical energy dissipated to the surroundings per meter altitude
skied [3], was derived from the COM position data. The braking forces were then correlated to mechanical energy dissipation.
Results:
The air drag force acting in slalom is rather small and constant (25 - 50N), while the frictional force between ski and snow alters rapidly
during a turn cycle with peak values as high as 500N occurring at about gate passage.
Energy dissipation showed a strong inverse statistical correlation with the frictional force in the ski-snow interaction (R2 = -0.96, p < 0.001),
while the relation between energy dissipation and the air drag force was not significant.
[1] Nachbauer, W., et al. (1996). J Appl Biomechanics, 12, 104-115
[2] Spring, E., et al. (1988). Int. J. Sport Biomechanics, 4
[3] Supej, M. (2008). J. Appl Biomechanics, 24, 121–129
PHYSIOLOGICAL RESPONSE OF ELDERLY ALPINE SKIERS DEPENDS ON INDIVIDUAL MOVEMENT PATTERN.
SCHEIBER, P., SEIFERT, J., MÜLLER, E.
UNIVERSITY OF SALZBURG, DEPARTMENT OF SPORT SCIENCE & KINESIOLOGY
Introduction: It has been demonstrated that there are skiers who have a low physiological response when skiing, while other skiers have
greater physiological responses at similar skiing velocities (Scheiber et al. 2009). Interestingly, the aerobic capacity of these skiers seemed
to have little effect on the physiological responses of these technically homogenous skiers while skiing. It has also been reported that
blood lactate is a valid predictor for cumulative physiological stress in alpine skiing (Seifert et al. 2008). It can be speculated that the interindividual
variation in kinematical variables of subjects skiing technique may also influence physiological response. Thus, the purpose of
this study was to quantify the influence of kinematic variables on the physiological response in recreational alpine skiing.
Method: Nineteen older alpine skiers (61.1±5.7 years) performed two different ski-instructor paced (LP and HP) and one self paced (SP)
skiing mode on a medium pitch, well prepared slope. Blood lactate (LA), HR, blood pressure (BP), and knee angle were determined.
Mean knee angle (MKA), as well as the range of knee angle (RKA) of twenty double turns were calculated for each run. Pearson correlations
were obtained between kinematical and physiological variables. Independent t-tests were performed to calculate mean group
differences in kinematic and physiological variables of low (LO) vs. high lactate (HI) skiers.
Results: The most interesting physiological findings were found for LA. Mean LA of the whole group was 1.40±0.34 mmol.L-1 for LP runs,
1.94±0.59 mmol.L-1 for HP runs, and 2.34±0.87 mmol.L-1 for SP runs. MKA of LP runs was 123.8±6.1°, for HP runs 117.5±6.0° and for SP
runs 117.1±6.0°. The RKA of LP runs was 19.0±3.9°, for HP runs 20.3±3.7° and for SP runs 17.7±3.6°. A significant correlation was found
only for RKA and LA during SP runs, (r²=.465; p