Letter to the Editor

Abstract

We read with interest the article by Vel Lace K.L. and Błażkiewicz M. "How does the ski boot affect human gait and joint loading?" (2021, vol. 13, no. 1, pp. 163-169). The article studies the effects of a ski boot on the lower extremity joint kinematics and kinetics during gait. The authors collected marker-based motion and ground reaction forces measurements on ten female participants who performed gait trials with and without a ski boot. Subsequently, they used these data as input into a musculoskeletal model that estimated joint reaction forces. Based on the computed joint reaction forces, the authors concluded that walking with a ski boot is not destructive in terms of joint reaction forces. However, we believe that the study suffers from a major methodological error, which has a significant impact on the computed joint reaction forces. The model does not correctly account for the interaction forces and moments between the lower extremity and ski boot and thus, the conclusions drawn are most likely inaccurate.In general, musculoskeletal models are used to address, among others, rehabilitation [1], injury prevention [2, 3] and ergonomics [4, 5] related research questions. Such models implement the interaction of the human body with its environment by means of contact forces and moments [1][2][3][4][5]. These contact forces and moments can be measured with force plates [1-5] or pressure mats [2,3]. Alternatively, when such measurements are not available, a model of the interaction forces and moments must be made [1]. This interaction model affects the quality of the implementation approach; additionally, it affects the muscle and joint reaction forces estimations. In the article under discussion, no measurements of the interaction forces and moments of the ski boot and the lower extremity were provided. Moreover, in the musculoskeletal model, no attempt was made to implement a model of this interaction. Therefore, the employed computational model for studying the gait trials performed with the ski boot does not describe the physics of the task under investigation. We do not consider this as a study limitation, but rather a fundamental error. The gait trials with the ski boot are hence not simulated correctly and thus, the reported joint reaction forces are inaccurate, leading the authors to draw questionable conclusions.The article under discussion, in its current version, could mislead the readers regarding the use of ski boots. Furthermore, it could lead more researchers to adapt similar methodological procedures for investigating the effects of externally applied mechanical restrictions on human joints via computational modelling. Therefore, we would like to recommend the authors to correct their computational model with the implementation of the interaction between the lower extremity and the ski boot. This interaction can be implemented into the model by means of contact forces, possibly by taking into account the compliance of the ski boot, as no measurements of the interaction forces were provided. Subse...