We used the framework of motor program adaptability to examine how unilateral above-knee (AK) or below-knee (BK) amputee subjects organize the global and local biomechanical processes of generation of the propulsive forces during gait initiation to overcome the segmental and neuro-muscular asymmetry. The organization of the global biomechanical process refers to the kinematics behavior of the couple center of foot pressure (CoP) and center of mass (CoM); the organization of the local biomechanical process refers to the propulsive forces generated by the prosthetic or intact limb during the anticipatory postural adjustment phase and the step execution phase. Specifically, we examined: i) the strategy to regulate the progression velocity, i.e., to maintain it comparably when the leading limb changed from the prosthetic limb to the intact limb; and ii) the strategy to modulate the progression velocity, i.e., to increase it when gait was initiated with the prosthetic limb vs. intact limb. The kinematics of the CoM and CoP in the amputees showed the same global biomechanical organization that is typically observed in able-bodied subjects, i.e., the production of the forward disequilibrium torque was obtained by a backward shift of the CoP, followed by a forward acceleration of the CoM. However, gait initiation was achieved by using a different local strategy depending on which limb was used to initiate the step. For the regulation of the CoM progression velocity, when the gait was initiated with the intact limb, the slope of the progression velocity during the anticipatory postural adjustment phase (APA) was steeper and lasted longer, the step execution duration was shorter, and the variation of the CoM speed was lower. In other words, to regulate the speed of progression, the amputee subjects controlled the spatial and temporal parameters of the propulsive forces. In the modulation of the CoM progression velocity, when the gait was initiated with the intact limb, the amputees controlled only the intensity of the propulsive forces during both the APA and step execution phases. In contrast, when the gait was initiated with the prosthetic limb, the modulation resulted mainly from the propulsive forces generated during the step execution phase. These different strategies are discussed in terms of the subject's capacity to adapt the motor program for gait initiation to new constraints.
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