Unconstrained Segmental Biomechanics Applied to Gait Initiation: A Conceptual Approach

Traditional biomechanical models of locomotion describe the human body as an articulated chain with constrained degrees of freedom, where movements are represented as rotations around fixed joint axes. They assume joint constraints as fixed points in space, thereby nullifying the forces acting at those points and reducing the ability of the model to represent the dynamic forces within the system, particularly during the non-cyclical phases of locomotion. This theoretical study introduces Unconstrained Segmental Biomechanics (USB), a conceptual approach that represents movement as the temporal activation of basic motor units, each composed of a pair of segments articulated by a non-fixed joint constraint. This representation enables the simultaneous analysis of dynamic and kinematic interactions and highlights the specificity of transitional phases, such as gait initiation, compared to cyclical locomotion. To examine how these models are currently transmitted in education, we investigated how gait is taught through a review of the literature and of teaching practices in leading sport science institutions. This analysis revealed that the adopted models remain exclusively kinematic, with no application of integrated dynamic representations such as USB. This work is proposed as a conceptual approach to reframe the foundations of locomotion biomechanics and to stimulate new perspectives.