Skiing Forces and Moments at the Knee and Boot Top: Boot Stiffness Effects and Modeling

The goals of this study are (1) to quantify the contribution of boot stiffness to the maximum forces and moments predicted at the boot top and knee, (2) to identify generalized regression models predicting the moment components at the boot top and knee for a skiing population, and (3) to quantify the contribution of inertial forces to the forces and moments predicted at the boot top and knee. Six subjects skiing a slalom course alternated test ski runs with lowest boot stiffness and highest boot stiffness settings in a boot with adjustable stiffness. Average (over all subjects and test runs) maximum ankle flexion is significantly greater (P = 0.005) by 11.5 degrees for the soft boot compared to the stiff boot. Average maximum compression force along the tibia is significantly smaller (P = 0.024) by 178N for the soft boot runs compared to the stiff boot runs. No other maximum force or moment component at the boot top and knee is significantly different between soft and stiff boot ski runs. Bending moments at boot top and knee are not clearly related to ankle flexion near neutral boot flexion (20 deg). Maximum bending moments at the boot top do not necessarily occur at maximum ankle flexion.

Generalized regression models predicting moments at the boot top and knee are developed for the subject population, using regression coefficients from individual ski runs normalized by subject height and weight. For the soft boot setting, average regression model correlation coefficients for A-P bending at the boot top, varus-valgus moment at the knee, and torsional moment along the tibia are 0.980, 0.916, and 0.867, respectively. Corresponding values for the stiff boot setting are 0.979, 0.889, and 0.882.