The biomechanics of cycling has been examined extensively from different perspectives. It had been shown that a non-seated position increases force production capability of the lower limb and more effectively transfers the force developed by the hip extensors and ankle plantar flexors to the crank, which, in return, reduces mechanical loads on the knee joint (Caldwell et al. 1999). Questions remain about the application of those studies done on a static ergometer in the field. Indeed, in a non-seated posture, cyclists sway their bike laterally underneath their body and this may affect the power generating profile of different joints (Soden et al. 1979). In addition, slope inclination also influences kinematic, kinetics and muscles coordination while pedalling (Duc et al. 2008). Overall, traditional cycling has been widely studied in comparison to BMX. The BMX standing start has several specificities (e.g., the non-seated position, the downhill slope and rolling movement of the bike), and no information is available on the joint moment and power of the lower limb during the BMX start and in the general standing start action. Yet to date, all kinetic bicycle analysis has been focused on a sagittal plan analysis. Moreover, specific joint-power analysis is complex as there is no consensus on how negative and positive power should be summed in a single joint or regarding the distribution and repartition of power between each joint (van der Kruk et al. 2018). In 3D, the interpretation is even more complex as power is a scalar quantity and it cannot be decomposed into three axes. To help interpreting the 3D joint power in gait analysis, Dumas and Cheze (2008), proposed a new approach that characterizes joint power as driving, stabilising or resisting whether joint moment vector is aligned with joint angular velocity vector or not. The purpose of this study is to describe hip joint dynamics during the BMX start using 3D moments, 3D joint power and the 3D angle between the joint moment and joint angular velocity vectors. This information may highlight specific pedalling technique and dynamic strategies, which would be valuable coaching tools for training.
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