Towards a Balance Algorithm to Determine Forward and Backward Leans in Skiing
Verena Venek, Richard Brunauer, Michael Buchecker, Rüdiger Jahnel, Thomas Stöggl (2019): Towards a Balance Algorithm to Determine Forward and Backward Leans in Skiing In: ECSS Book of Abstracts 2019
Keeping balance is a crucial skill of alpine skiing (Lesnik et al., 2017; Hrysomallis, 2011). Skiers are advised to prepare with balance exercises to prevent self-inflicted falls and injury (Cigrovski et al., 2017). To assess tendencies towards unwanted backward and forward leans, the influence of the ski boot on balance performance must be considered (Mildner et al., 2010). We present a study on a novel concept for balance assessment by comparing pressure sensors integrated into ski boots towards a force plate system. We aimed at developing a balance algorithm displaying back and forward leans based on pressure distribution over the feet to provide a proof-ofconcept for in-store application and a pre-study for balance on-ski.
Readings from two pressure sensors positioned at the ski boots’ forefoot and heel were weighted to result in balance values ranging –1 and +1 reflecting maximal backward and forward lean. To determine reference data, two expert skiers performed a sequence of alternating forward and backward leans according to the natural range of motion during skiing on a force plate (AMTI) with boots clipped on skis. By placing markers onto each ski boot and the force plate edges, the center position of the ski boots on the plate was obtained. The center of pressure (CoP) course in anterior-posterior axis from the force plate was adapted to the center position of the boots. Then the maximal values of the forward and the backward lean were determined to normalize the CoP signal to the range of –1 and +1. Finally, the sensor-based balance results were optimized using the reference data to align the weights of the pressure sensors to detect the leans.
The comparison of the sensor-based balance values with the reference data revealed that there are not common weights to map the pressure sensor values to the whole balance range since there are two ways for how to perform forward and backward leans. One subject performed as expected; the extreme leaning positions coincided with the maximal pressure on the corresponding feet area, i.e. forward lean was produced by maximal pressure on the forefoot and backward lean vice versa. The other subject applied a torque in the opposite direction, i.e. the maximal pressure was applied over the heel during forward leans instead of over the forefoot and during backward leans vice versa.
During the study, an unexpected result occurred because there are at least two patterns for extreme forward and backward lean performances. This already overexerted the sensor-based balance algorithm. The identification of subtypes of forward and backward lean in skiing supports the further development of the algorithm and the application on more subjects to confirm these two types. Simultaneously, a further development of the reference data generation (e.g. force binding system) and balance understanding for on-piste conditions is warranted to monitor the CoP during skiing and compare to the pressure distribution over the feet.