Three-Rigid-Body-Particle Modeling and Optimization of Trajectory and Posture for Alpine Skiing

This paper presents a new study on modeling and optimization of trajectory and posture for the super-giant(SG)slalom of alpine skiing.It is the first time that a Three-Rigid-Body-Particle model based on rigorous derivations and stability analysis is established to represent skiers trajectory and posture characteristics,as it is more accurate than the single-rigid-body model which is commonly used in existing studies.In addition,the Radau pseudospectral method is applied to solve the trajectory and posture optimization problem in order to obtain better skiing trajectory,skiing posture,and some key kinematic parameters of skiers.Moreover,this paper analyzes the effects of different body types,minimum turning radii,and flexor and extensor strength of knees and hip joint on skiing performance.Finally,based on the findings of the study,some advice about how to improve the performance of the SG slalom in view of science and technology is given to skiers and coaches for reference.

“When you’re down, stay down”:A lesson for all competitive alpine skiers supported by an ACL rupture measured in vivo

Background:During an experiment,a ski racer equipped with various measurement devices suffered an anterior cruciate ligament(ACL)rupture in his right knee.The aim of this study was to describe the underlying injury mechanism from a functional perspective.Methods:Eight giant slalom turns(i.e.,4 left turns),followed by 1 left turn at which the ACL injury occurred,were recorded by 2 video cameras,electromyography of 4 relevant muscle groups,inertial measurement units to measure knee and hip angles,and pressure insoles to determine ground reaction forces.Results:Due to a loss of balance,the ski racer began to slide sideways at the apex of a left turn.During sliding,his right(outside)leg was actively abducted upward without touching the ground.The ski racer then attempted to stand up again by dropping his leg back towards the snow surface.The end of this dropping was accompanied by a decrease in electromyographic activity in the knee stabilizing muscles.Once the inside edge of the outer ski caught the snow surface,a rapidly increasing peak force,knee flexion,and an aggressive sudden activation of the vastus medialis muscle were observed,while biceps femoris and rectus femoris further decreased their activation levels.This likely resulted in excessive anterior translation of the tibia relative to the femur,causing damage to the ACL.Conclusion:Our example emphasizes that ski racers should not get up until they stop sliding.Remember:“When you’re down,stay down.”

Limb symmetry index in competitive alpine ski racers:Reference values and injury risk identification according to age-related performance levels

Purpose： The aims of this study were to assess differences of limb symmetry index （LSI） in strength- and coordination-related tasks between high-level, competitive, noninjured ski racers of different age-related performance levels and to prospectively assess limb differences as a possible risk factor for traumatic and overuse injury in youth ski racers. Methods： The study （Study 1） included 285 high-level competitive ski racers （125 females, 160 males） of 3 age-related performance levels and based on the school system： 95 youth （10-14 years, secondary modem school）, 107 adolescent （15-19 years, grammar school）, and 83 elite athletes （20-34 years）. To investigate the second aim （Study 2）, 67 of the 95 youth athletes were included and any traumatic or overuse injuries were prospectively recorded over 2 seasons. All athletes performed 4 unilateral tests （strength related： one-leg counter movement jump （OL-CMJ） and one-leg isometric/isokinetic press strength test （OL-ILS）; coordination related： one-leg stability test （OL-ST） and one-leg speedy jump test （OL-SJ））. The LSI was calculated by dividing the dominant leg by the nondominant leg and multiplying by 100. Kruskal-Wallis H tests and binary logistic regression analyses were conducted. Results： There were significant differences between the LSI of the 3 age-related performance-level groups only in the strength-related tests： the OL-CMJ （X^2（2, 285） = 9.09; p = 0.01） and the OL-ILS （X^2（2,285） = 14.79; p 〈 0.01）. The LSI for OL-ILS was found to be a significant risk factor for traumatic injury in youth ski racers （Wald = 7.08; p 〈 0.01）. No significant risk factors were found for overuse injuries. Conclusion： Younger athletes display slightly greater LSI values only in the strength-related tests. The cut-off value of limb differences of 〈 10% for return to sport decisions seems to be appropriate for elite athletes, but for youth and adolescent athletes it has to be critically discussed. It seems to be necessary to define thresholds based on specific performance tasks （strength vs. coordination related） rather than on generalizations, and age-related performance levels must be considered. Limb differences in unilateral leg extension strength represent a significant injury risk factor in youth ski racers.2018 Published by Elsevier B.V. on behalf of Shanghai University of Sport. This is an open access article under the CC BY-NC-ND license. （http：//creativecommons.org/licenses/by-nc-nd/4.0/）.

The Influence of Ski Type on Muscle Activity, Performance, and Self-Efficacy in Young Alpine Ski Racers

Wide skis are the most popular ski sold in the United States and are skied routinely in all types of terrain,especially by young skiers.This study investigated the influence of slalom(SL,64 mm underfoot width)and wide skis(WS,97.7 mm underfoot width)on perceptual responses,timed performance,edge inclination angles,and EMG in young alpine ski racers.Thirteen subjects,mean age 12.8±0.5 years,completed one run on each of two courses which were set to approximate the different turning radii of each ski.They completed a Likert-type questionnaire to assess components of self-efficacy after each run.EMG activity was recorded on the gluteus medius(GM),vastus lateralis,tibialis anterior and peroneus longus(PL).Per-ceptions of confidence,aggressiveness,speed,and skiing on line were significantly greater,by about 31%,for SL than WS regardless of course.Turn times were significantly less by 4% for SL than WS,peak edge inclination angle was 7%greater for SL than WS,and PL activity was 40% less for SL compared to WS.Young racers are developing skills to improve their racing techniques and are still forming technique strategies to deal with the many factors present in skiing which may disrupt learning progressions.With lower perceptual and performance responses on WS,young ski racers who ski on groomed snow with WS could be interfering with the adaptation of fundamental racing technique at a critical time in their development.

Trunk Flexion to Extension Strength Ratio of 11- to 18- Year Old Youth Ski Racers: Data from 15 Years of Talent Development in Austria

Purpose The aim of the present study was to evaluate the trunk strength capacity of alpine ski racers aged 10-18 years,who were tested during the last 15 years,to identify reference values for trunk flexor to extensor strength ratios according to age and sex.Methods In total,2841 participants(1605 males,1236 females;10-18 years)were included,who were pupils of a famous skiing-specific secondary modern school or members of the provincial ski team between 2006 and 2020.The maximum isometric trunk flexion and extension strength was measured using the slightly modified Back Check.Sex-specific differ-ences were assessed with Student’s t test or Mann-Whitney-U test.Univariate analyses of variance or Kruskal-Wallis-H tests were used to assess differences between age groups.Descriptive sex-and age-specific reference values were calculated(norm area:mean±½standard deviation).Results Sex-specific differences were found for both flexion(starting at 11 years)and extension strength(starting at 12 years)(P<0.001).Lower flexion to extension strength ratios were identified for males(0.89±0.18)compared with females(0.82±0.15),but the ratios remained constant across age groups for both sexes.Conclusion The present study provides age-and sex-specific reference values for trunk flexion to extension strength ratios for 10-to 18-year old youth and adolescent ski racers.The data of the present study represent a large data pool of youth ski racers at a high-performance level;thus,coaches can use the reference values for comparing the ratios of their athletes.