A biomechanical field testing approach in snow sports: Case studies toward a detailed analysis of joint loading

In this study, kinematic and kinetic measurements were combined to assess the effects of removing the stiff shaft from a ski boot. It was hypothesized that joint flexion at the ankle, knee and hip increase and reduce joint loading specifically at the knee. A previously developed force sensor was combined with a high-speed camera system for data collection of 6 degrees of freedom ground reaction forces and three-dimensional marker data in the field on a wave slope. The collected data were used as input to a musculoskeletal model for the estimation of joint kinematics and joint moments and contact forces in the ankle and knee. The force sensor, which was previously used for skiing, had experienced wear and tear and was thus prone to breakage. As a result, joint loading could only be analyzed for two skiers. These two skiers did not use the added range of ankle flexion to its full extent, but showed substantial reductions in joint moments and joint contact forces (e.g. knee compression force from 85 to 57 N/kg). Only one of the five experienced skiers tested was able to adopt the anticipated movement pattern by substantially increased maximum ankle joint flexion angle (from 10° to 37°) and knee joint flexion angle (from 93° to 105°) and the respective ranges of motion when skiing through a wave course. The study provides information on possible individual adaptations to ski boot modifications. The mechanical construction of the force sensor will need to be modified to withstand the high forces expected during freestyle skiing. The study also supports the future use of this measurement setup for comprehensive studies in snow sports, provided that a sufficient training period is given.